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COLLEGE  OF  PHYSICIANS 
AND   SURGEONS 


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A  TEXT-BOOK 

OF 

HUMAN  PHYSIOLOGY 


BRUBAKER 


A  TEXT-BOOK 


OF 


HUMAN   PHYSIOLOGY 


INCLUDING  A  SECTION  ON 


PHYSIOLOGIC  APPARATUS 


BY 

ALBERT  P.  BRUBAKER,  A.M.,M.  D. 

PROFESSOR   OF    PHYSIOLOGY    AND    MEDICAL   JURISPRUDENCE  IN  THE   JEFFERSON  MEDICAL    COL- 
LEGE;   FORMERLY  PROFESSOR  OF  PHYSIOLOGY'  IN  .THE  PENNSYLVANIA    COLLEGE    OF 
DENTAL     surgery;      LECTURER     ON     PHYSIOLOGY'     AND     HYGIENE      IX 
THE    DREXEL    INSTITUTE    OF    ART,    SCIENCE,    AND   INDUSTRY' 


FOURTH  EDITION.     REVISED  AND  ENLARGED 
WITH   1  COLORED  PLATE  AND  377  ILLUSTRATIONS 


PHILADELPHIA 

P.    BLAKiSTON'S   SON   &   CO 

1012   WALNUT   STREET 
1912 


Copyright,  1912,  by  P.  Blakiston's  Son  &  Co. 


THE.MAPLE.PRES.S.TORK.PA 


TO 

KENNETH  M.  BLAKISTON 

LOYAL   FRIEND    COLTITEOUS   GENTLEMAN 

GENEROUS   PUBLISHER 

THE  PRESENT  EDITION   OF  THIS   WORK 

IS 

AFFECTIONATELY    DEDICATED 


PREFACE  TO  FOURTH  EDITION. 


The  preparation  of  a  fourth  edition  of  this  Text  Book  of  Physiology  has 
furnished  the  opportunity  for  revision  and  the  incorporation  of  new  matter. 
Through  condensation  and  elimination  it  has  become  possible  to  insert  addi- 
tional matter  equivalent  to  some  fifty  pages  without  increasing  the  size  of 
the  volume.  The  new  paragraphs  which  have  been  inserted  in  the  various 
chapters  contain  facts  relating  to  the  mechanic  movements  of  the  stomach 
and  intestines  and  the  nerve  mechanisms  regulating  them;  the  digestion  and 
absorption  of  the  proteins;  the  viscosiLy,  specific  gravity  and  coagulability 
of  the  blood;  the  physiologic  mechanism  of  the  heart  and  the  properties  of 
the  cardiac  muscle;  the  venous  pulse;  the  auscultatory  method  of  determin- 
ing blood  pressure;  the  modifications  of  the  respiratory  rhythm;  the  physio- 
logic action  of  the  pituitary  gland  and  the  adrenals,  etc.  These  additions, 
it  is  believed,  will  enhance  its  value  to  the  medical  student  and  practitioner. 

In  the  preparation  of  this,  as  of  preceding  editions  the  aim  has  been  to 
present  the  facts  in  such  a  form  as  to  familiarize  students  with  the  essential 
problems  of  physiology. 

To  those  teachers  and  students  who  have  recommended  and  used  this 
work  and  to  whom  I  am  indebted  for  generous  praise,  kind  criticisms,  and 
helpful  suggestions,  I  wish  to  express  my  sincere  thanks  and  trust  that  in  its 
improved  form  it  will  continue  to  meet  their  approval. 

Once  again  I  desire  to  express  my  appreciation  of  the  unwearied  and 
invaluable  assistance  of  Mr.  I.  A.  Hagy  in  preparing  the  manuscript  for  the 
press. 

I  am  also  indebted  to  Dr.  Lucius  Tuttlc,  demonstrator  of  physiology, 
for  the  reading  of  the  proof. 


PREFACE  TO  FIRST  EDITION, 


The  object  in  \dew  in  the  preparation  of  this  volume  was  the  selection  and 
presentation  of  the  more  important  facts  of  physiology,  in  a  form  which  it  is 
believed  will  be  helpful  to  students  and  to  practitioners  of  medicine.  Inas- 
much as  the  majority  of  students  in  a  medical  college  are  preparing  for  the 
practical  duties  of  professional  life,  such  facts  have  been  selected  as  will  not 
only  elucidate  the  normal  functions  of  the  tissues  and  organs  of  the  body,  but 
which  will  be  of  assistance  in  understanding  their  abnormal  manifestations 
as  they  present  themselves  in  hospital  and  private  work.  Both  in  the  selec- 
tion of  facts  and  in  the  method  of  presentation  the  author  has  been  guided  by 
an  experience  gained  during  twenty  years  of  active  teaching. 

The  description  of  physiologic  apparatus  and  the  methods  of  investiga- 
tion, other  than  those  having  a  clinical  interest,  have  been  largely  excluded 
from  the  text,  for  the  reason  that  both  are  more  appropriately  considered  in 
works  devoted  to  laboratory  methods  and  laboratory  instruction,  and  for  the 
further  reason  that  the  student  receives  this  infoi:mation  while  engaged  in  the 
practical  study  of  physiology  in  the  laboratory,  now  an  established  feature 
in  the  curriculum  of  the  majority  of  medical  colleges.  For  those  who  have 
not  had  laboratory  opportunities  a  brief  account  of  some  essential  forms  of 
apparatus  and  the  purposes  for  which  they  are  intended  will  be  found  in  an 
appendix. 

I  wish  to  acknowledge  my  indebtedness  to  Professor  Colin  C.  Stewart  for 
many  valuable  suggestions  in  the  preparation  of  different  sections  of  the 
volume;  to  Dr.  Carl  Weiland  for  assistance  in  the  chapter  on  vision;  to  Dr. 
Joseph  P.  Bolton  for  excellent  suggestions  on  questions  relating  to  physiologic 
chemistry. 


IX 


TABLE  OF  CONTENTS. 


Page 
CHAPTER   I. 
Introduction i 

CHAPTER  II. 
Chemic  Composition  of  the  Human  Body      6 

CIL\PTER  III. 
Physiology  of  the  Cell 23 

CHAPTER  IV. 

Histology  of  the  Epithelul  and  Connective  Tissues 30 

CH.\PTER  V. 
The  Physiology  of  Movement 38 

CH.\PTER  VI. 
The  Physiology  of  the  Skeleton 44 

CHAPTER  VII. 
General  Physiology  of  Muscle-tissue 48 

CHAPTER  VIII. 
The  General  Physiology  of  Nerve-tissue 87 

CHAPTER  IX. 

Foods 115 

CHAPTER  X. 
Digestion 133 

CIL\PTER  XI. 
Absorption 205 

CHAPTER  XII. 

The  Blood 227 

CHAPTER  XHI. 
The  Circulation  of  the  Blood     261 

CHAPTER  JvIV. 

The  Circulation  of  the  Blood  (Continued) 319 

xi 


xii  CONTENTS. 

Page 
CHAPTER  XV. 

Respiration 377 

CHAPTER  XVI. 

Animal  Heat      429 

CHAPTER  XVH. 

Secretion 438 

CHAPTER  XVIII. 
Excretion 470 

CHAPTER  XIX. 
The  Central  Organs  of  the  Ner\te  System  and  their  Nervfs 498 

CHAPTER  XX. 

The  Medulla  Oblongata;  the  Isthmus  OF  the  Encephalon;  the  Basal  Ganglia    .    .  519 

CHAPTER  XXI. 
The  Cerebrum 537 

CHAPTER  XXII. 
The  Cerebellum      565 

CHAPTER  XXI 1 1. 

The  Cr.\nial  Nerves 572 

CHAPTER  XXIV. 

The  Autonomic  or  Sympathetic  N^erve  System 608 

CHAPTER  XXV. 
Pronation;  Articul.ate  Speech 620 

CHAPTER  XXVI. 

The  Special  Senses 631 

CHAPTER  XXVII. 

The  Sense  of  Sight 642 

CHAPTER  XXVIII. 
The  Sense  of  Hearing     677 

CHAPTER  XXIX. 

Reproduction 688 

APPENDIX. 

Physiologic  Apparatus 704 

Index 727 


TEXT-BOOK  OF  PHYSIOLOGY. 


CHAPTER  I. 
INTRODUCTION. 


An  animal  organism  in  the  living  condition  exhibits  a  series  of  phe- 
nomena which  relate  to  growth,  movement,  mentality,  and  reproduction. 
During  the  period  preceding  birth,  as  well  as  during  the  period  included 
between  birth  and  adult  life,  the  individual  grows  in  size  and  complexity 
from  the  introduction  and  assimilation  of  material  from  without.  Through- 
out its  life  the  animal  exhibits  a  series  of  movements,  in  virtue  of  which  it 
not  only  changes  the  relation  of  one  part  of  its  body  to  another,  but  also 
changes  its  position  relatively  to  its  environment.  If,  in  the  execution  of 
these  movements,  the  parts  are  directed  to  the  overcoming  of  opposing 
forces,  such  as  gravity,  friction,  cohesion,  elasticity,  etc.,  the  animal  may 
be  said  to  be  doing  work.  The  result  of  normal  growth  is  the  attainment 
of  a  physical  development  that  will  enable  the  animal,  and,  more  especially, 
man,  to  perform  the  work  necessitated  by  the  nature  of  its  environment  and 
the  character  of  its  organization.  In  man,  and  probably  in  lower  animals 
as  well,  mentality  manifests  itself  as  intellect,  feeling,  and  volition.  At  a 
definite  period  in  the  life  of  the  animal  it  reproduces  itself,  in  consequence 
of  which  the  species  to  which  it  belongs  is  perpetuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality,  and 
reproduction  constitutes  the  science  of  animal  physiology.  But  as  these 
general  activities  are  the  resultant  of  and  dependent  on  the  special  activities 
of  the  individual  structures  of  which  an  animal  body  is  composed,  physi- 
ology in  its  more  restricted  and  generally  accepted  sense  is  the  science  which 
investigates  the  actions  or  functions  of  the  individual  organs  and  tissues  of 
the  body  and  the  physical  and  chemic  conditions  which  underlie  and  deter- 
mine them. 

This  may  naturally  be  divided  into: 

1.  Special  physiology,  the  object  of  which  is  a  study  of  the  vital  phenomena 
or  functions  exhibited  by  the  organs  of  any  individual  animal. 

2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of  the 
vital  phenomena  or  functions  exhibited  by  the  organs  of  two  or  more 
animals  of  different  species,  with  a  view  to  unfolding  their  points  of 
resemblance  or  dissimilarity. 

Human  physiology  is  that  department  of  physiologic  science  which 
has  for  its  object  the  study  of  the  functions  of  the  organs  and  tissues  of 
the  human  body  in  a  state  of  health. 

Inasmuch  as  the  study  of  function,  or  physiology,  is  associated  with 
and  dependent  on  a  knowledge  of  structure,  or  anatomy,  it  is  essential  that 


2  TEXT-BOOK  OF  PHYSIOLOGY. 

the  student  should  have  a  general  acquaintance  not  only  with  the  structure 
of  man,  but  with  that  of  typical  forms  of  lower  animal  life  as  well. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be  composed 
of  a  number  of  well-defined  structures,  such  as  heart,  lungs,  stomach,  brain, 
eye,  etc.,  to  w^hich  the  term  organ  was  originally  applied,  for  the  reason 
that  they  were  supposed  to  be  instruments  capable  of  performing  some 
important  act  or  function  in  the  general  activities  of  the  body.  Though  the 
term  organ  is  usually  employed  to  designate  the  larger  and  more  familiar 
structures  just  mentioned,  it  is  equally  appHcable  to  a  large  number  of 
other  structures  which,  though  possibly  less  obvious,  are  equally  important 
in  maintaining  the  Hfe  of  the  individual — e.g.,  bones,  muscles,  nerves,  skin, 
teeth,  glands,  blood-vessels,  etc.  Indeed,  any  complexly  organized  struc- 
ture capable  of  performing  a  given  function  may  be  described  as  an  organ. 
A  description  of  the  various  organs  which  make  up  the  body  of  an  animal, 
their  external  form,  their  internal  arrangement,  their  relations  to  one  another, 
constitutes  the  science  of  animal  anatomy. 

This  may  naturally  be  divided  into : 

1.  Special  anatomy,  the  object  of  which  is  the  investigation  of  the  construc- 
tion, form,  and  arrangement  of  the  organs  of  any  individual  animal. 

2.  Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the  organs 
of  two  or  more  animals  of  different  species,  with  a  view  to  determining 
their  points  of  resemblance  or  dissimilarity. 

If  the  organs,  however,  are  subjected  to  a  further  analysis,  they  can  be 
resolved  into  simple  structures,  apparently  homogeneous,  to  which  the 
name  tissue  has  been  given — e.g.,  epithelial,  connective,  muscle,  and  nerve 
tissue.  When  the  tissues  are  subjected  to  a  microscopic  analysis,  it  is 
found  that  they  are  not  homogeneous  in  structu-re,  but  composed  of  still 
simpler  elements,  termed  cells  and  fibers.  The  investigation  of  the  inter- 
nal structure  of  the  organs,  the  physical  properties  and  structure  of  the 
tissues,  as  well  as  the  structure  of  their  component  elements,  the  cells  and 
fibers,  constitutes  a  department  of  anatomic  science  known  as  histology,  or 
as  it  is  prosecuted  largely  with  the  microscope,  microscopic  anatomy. 

Human  anatomy  is  that  department  of  anatomic  science  which  has 
for  its  object  the  investigation  of  the  construction  of  the  human  body. 

GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY. 

The  body  of  every  animal,  from  fish  to  man,  may  be  divided  into — 

1.  An  axial  portion,  consisting  of  the  head,  neck,  and  trunk;  and — 

2.  An  appendicular  portion,  consisting  of  the  anterior  and  posterior  limbs 
or  extremities. 

The  axial  portion  of  all  mammals,  to  which  class  man  zoologically 
belongs,  as  well  as  of  all  birds,  reptiles,  amphibians,  and  osseous  fish,  is 
characterized  by  the  presence  of  a  bony,  segmented  axis,  which  extends  in  a 
longitudinal  direction  from  before  backward,  and  which  is  known  as  the 
vertebral  column  or  backbone.  In  virtue  of  the  existence  of  this  column 
all  the  classes  of  animals  just  mentioned  form  one  great  division  of  the 
animal  kingdom,  the  Vertehrata. 

Each  segment,  or  vertebra,  of  this  axis  consists  of — 


INTRODUCTION.  3 

1.  A  solid  portion,  known  as  the  body  or  centrum,  and 

2.  A  bony  arch  arising  from  the  dorsal  aspect  and  surmounted  by  a  spine- 
like process. 

At  the  anterior  extremity  of  the  body  of  the  animal  the  vertebrae  are 
variously  modified  and  expanded,  and,  with  the  addition  of  new  elements, 
form  the  skull;  at  the  posterior  extremity  they  rapidly  diminish  in  size, 
and  terminate  in  man  in  a  short,  tail-like  process.  In  many  animals,  how- 
ever, the  vertebral  column  extends  for  a  considerable  distance  beyond  the 
trunk  into  the  tail.  The  vertebral  column  may  be  regarded  as  the  founda- 
tion element  in  the  plan  of  organization  of  all  the  higher  animals  and  the 
center  around  which  the  rest  of  the  body  is  developed  and  arranged  with  a 
certain  degree  of  conformity.  In  all  vertebrate  animals  the  bodies  of  the 
segments  of  the  vertebral  column  form  a  partition  which  serves  to  divide 
the  trunk  of  the  body  into  two  cavities — -viz.,  the  dorsal  and  the  ventral. 

The  dorsal  cavity  is  found  not  only  in  the  trunk,  but  also  in  the  head. 
Its  walls  are  formed  partly  by  the  arches  which  arise  from  the  posterior 
or  dorsal  surface  of  the  vertebras  and  partly  by  the  bones  of  the  skull.  If  a 
longitudinal  section  be  made  through  the  center  of  the  vertebral  column, 
and  including  the  head,  the  dorsal  cavity  will  be  observed  running  through 
its  entire  extent.  Though  for  the  most  part  it  is  quite  narrow,  at  the  anterior 
extremity  it  is  enlarged  and  forms  the  cavity  of  the  skull.  This  cavity  is 
lined  by  a  membranous  canal,  the  neural  canal,  in  which  are  contained  the 
brain  and  the  neural  or  spinal  cord.  Through  openings  in  the  sides  of  the 
dorsal  cavity  nerves  pass  out  which  connect  the  brain  and  spinal  cord  with 
all  the  structures  of  the  body. 

The  ventral  cavity  is  confined  mainly  to  the  trunk  of  the  body.  Its 
walls  are  formed  by  muscles  and  skin,  strengthened  in  most  animals  by  bony 
arches,  the  ribs.  Within  the  ventral  cavity  is  contained  a  musculo-mem- 
branous  tube  or  canal  known  as  the  alimentary  or  food  canal,  which  begins 
at  the  mouth  on  the  ventral  side  of  the  head,  and,  after  passing  through  the 
neck  and  trunk,  terminates  at  the  posterior  extremity  of  the  trunk  at  the 
anus.  It  may  be  divided  into  mouth,  pharynx,  esophagus,  stomach,  small 
and  large  intestines. 

In  all  mammals  the  ventral  cavity  is  divided  by  a  musculo-membranous 
partition  into  two  smaller  cavities,  the  thorax  and  abdomen.  The  former 
contains  the  lungs,  heart  and  its  great  blood-vessels,  and  the  anterior  part  of 
the  alimentary  canal,  the  gullet  or  esophagus;  the  latter  contains  the  con- 
tinuation of  the  alimentary  canal — that  is,  the  stomach  and  intestines 
— and  the  glands  in  connection  with  it,  the  liver  and  pancreas.  In  the 
posterior  portion  of  the  abdominal  cavity  are  found  the  kidneys,  ureters, 
and  bladder,  and  in  the  female  the  organs  of  reproduction.  The  thoracic 
and  abdominal  cavities  are  each  lined  by  a  thin  serous  membrane,  known, 
respectively,  as  the  pleural  and  peritoneal  membranes,  which,  in  addition, 
are  reflected  over  the  surfaces  of  the  organs  contained  within  them.  The 
alimentary  canal  and  the  various  cavities  connected  with  it  are  lined  through- 
out by  a  mucous  membrane. 

The  surface  of  the  body  is  covered  by  the  skin.  This  is  composed 
of  an  inner  portion,  the  derma,  and  an  outer  portion,  the  epidermis.  The 
former  consists  of  fibers,  blood-vessels,  nerves,   etc.;    the  latter  of  layers 


4  TEXT-BOOK  OF  PHYSIOLOGY. 

of  scales  or  cells.  Embedded  within  the  skin  are  numbers  of  glands,  which 
exude,  in  the  different  classes  of  animals,  sweat,  oily  matter,  etc.  Project- 
ing from  the  surface  of  the  skin  are  hairs,  bristles,  feathers,  claws.  Beneath 
the  skin  arc  found  muscles,  bones,  blood-vessels,  nerves,  etc. 

The  appendicular  portion  of  the  body  consists  of  two  pairs  of  symmetric 
limbs,  w^hich  project  from  the  sides  of  the  trunk,  and  which  bear  a  determinate 
relation  to  the  vertebral  column.  They  consist  fundamentally  of  bones, 
surrounded  by  muscles,  blood-vessels,  nerves  and  lymphatics.  The  limbs, 
though  having  a  common  plan  of  organization,  are  modified  in  form  and 
adapted  for  prehension  and  locomotion  in  accordance  with  the  needs  of  the 
animal. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have  certain 
peculiarities  of  structure  in  common  are  classified  by  anatomists  into 
systems — e.g.,  the  bones,  collectively,  constitute  the  bony  or  osseous  system; 
the  muscles,  the  nerves,  the  skin,  constitute,  respectively,  the  muscle,  the 
nerve,  and  the  tegumentary  systems. 

Physiologic  Apparatus. — More  important  from  a  physiologic  point 
of  view  than  a  classification  of  organs  based  on  similarities  of  structure 
is  the  natural  association  of  two  or  more  organs  acting  together  for  the 
accomplishment  of  some  definite  object,  and  to  which  the  term  physiologic 
apparatus  has  been  applied.  While  in  the  community  of  organs  which 
together  constitute  the  animal  body  each  one  performs  some  definite  function, 
and  the  harmonious  cooperation  of  all  is  necessary  to  the  life  of  the  individual, 
everywhere  it  is  found  that  two  or  more  organs,  though  performing  totally 
distinct  functions,  are  cooperating  for  the  accomplishment  of  some  larger 
or  compound  function  in  which  their  individual  functions  are  blended — e.g., 
the  mouth,  stomach,  and  intestines,  with  the  glands  connected  with  them, 
constitute  the  digestive  apparatus,  the  object  or  function  of  which  is  the  com- 
plete digestion  of  the  food.  The  capillary  blood-vessels  and  lymphatic 
vessels  of  the  body,  and  especially  those  in  relation  to  the  villi  of  the  small 
intestine,  constitute  the  absorptive  apparatus,  the  function  of  which  is  the 
introduction  of  new  material  into  the  blood.  The  heart  and  blood-vessels 
constitute  the  circulatory  apparatus,  the  function  of  which  is  the  distribution 
of  blood  to  all  portions  of  the  body.  The  lungs  and  trachea,  together  with 
the  diaphragm  and  the  walls  of  the  chest,  constitute  therespiratory  apparatus, 
the  function  of  which  is  the  introduction  of  oxygen  into  the  blood  and  the 
elimination  from  it  of  carbon  dioxid  and  other  injurious  products.  The 
kidneys,  the  ureters,  and  the  bladder  constitute  the  urinary  apparatus.  The 
skin,  with  its  sweat-glands,  constitutes  the  perspiratory  apparatus,  the  func- 
tions of  both  being  the  excretion  of  waste  products  from  the  body.  The 
liver,  the  pancreas,  the  mammary  glands,  as  well  as  other  glands,  each  form 
a  secretory  apparatus  which  elaborates  some  specific  material  necessary 
to  the  nutrition  of  the  individual.  The  functions  of  these  different  physio- 
logic apparatus — e.g.,  digestion,  absorption  of  food,  elaboration  of  blood, 
circulation  of  blood,  respiration,  production  of  heat,  secretion,  and  excretion 
— are  classified  as  nutritive  functions,  and  have  for  their  final  object  the 
preservation  of  the  individual. 


INTRODUCTION.  5 

The  nerves  and  muscles  constitute'  the  nervo-muscle  apparatus,  the 
function  of  which  is  the  production  of  motion.  The  eye,  the  ear,  the  nose, 
the  tongue,  and  the  skin,  with  their  related  structures,  constitute,  respec- 
tively, the  visual,  auditory,  olfactory,  gustatory,  and  tactile  apparatus,  the 
function  of  which,  as  a  whole,  is  the  reception  of  impressions  and  the  trans- 
mission of  nerve  impulses  to  the  brain,  where  they  give  rise  to  visual,  audi- 
tory, olfactory,  gustatory,  and  tactile  sensations  and  volitional  impulses. 

The  brain,  in  association  with  the  sense  organs,  forms  an  apparatus 
related  to  mental  processes.  The  lar}mx  and  its  accessory  organs — the 
lungs,  trachea,  respiratory  muscles,  the  mouth  and  resonant  cavities  of  the 
face — form  the  vocal  and  articulating  apparatus,  by  means  of  which  voice 
and  articulate  speech  are  produced.  The  functions  exhibited  by  the  ap- 
paratus just  mentioned — viz.,  motion,  sensation,  language,  mental  and 
moral  manifestations — are  classified  SiS  functions  of  relation,  as  they  serve  to 
bring  the  individual  into  conscious  relationship  with  the  external  world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs,  the 
former  producing  the  germ-cell,  the  latter  the  sperm  element.  Together 
with  their  related  structures — the  fallopian  tubes,  uterus,  and  vagina  in  the 
female,  and  the  urogenital  canal  in  the  male — they  constitute  the  repro- 
ductive apparatus  characteristic  of  the  two  sexes.  Their  cooperation  results 
in  the  union  of  the  germ-cell  and  sperm  element  and  the  consequent  develop- 
ment of  a  new  being.  The  function  of  reproduction  serves  to  perpetuate 
the  species  to  which  the  individual  belongs. 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but  one 
composed  of  a  large  number  of  widely  dissimilar  but  related  organs.  As 
all  vertebrate  animals  have  the  same  general  plan  of  organization,  there  is  a 
marked  similarity  both  in  form  and  structure  among  corresponding  parts 
of  different  animals.  Hence  it  is  that  in  the  study  of  human  anatomy  a 
knowledge  of  the  form,  construction,^  and  arrangement  of  the  organs  in 
different  types  of  animal  life  is  essential  to  its  correct  interpretation;  it 
follows  also  that  in  the  investigation  and  comprehension  of  the  complex 
problems  of  human  physiology  a  knowledge  of  the  functions  of  the  organs 
as  they  manifest  themselves  in  the  different  types  of  animal  life  is  indispen- 
sable. As  many  of  the  functions  of  the  human  body  are  not  only  complex, 
but  the  organs  exhibiting  them  are  practically  inaccessible  to  investigation, 
we  must  supplement  our  knowledge  and  judge  of  their  functions  by  analogy, 
by  attributing  to  them,  within  certain  limits,  the  functions  revealed  by 
experimentation  upon  the  corresponding  organs  of  lower  animals.  This 
experimental  knowledge,  corrected  by  a  study  of  the  clinical  phenomena  of 
disease  and  the  results  of  post-mortem  investigations,  forms  the  basis  of 
modern  human  physiology. 


CHAPTER  II. 

CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY. 

Since  it  has  been  demonstrated  that  every  exhibition  of  functional 
activity  is  associated  with  changes  of  structure,  it  has  been  apparent  that  a 
knowledge  of  the  chemic  composition  of  the  body,  not  only  when  in  a  state 
of  rest,  but  to  a  far  greater  degree  when  in  a  state  of  activity,  is  necessary  to 
a  correct  understanding  of  the  intimate  nature  of  physiologic  processes. 
Though  the  analysis  of  the  dead  body  is  comparatively  easy,  the  determina- 
tion of  the  successive  changes  in  composition  of  the  living  body  is  attended 
with  many  difficulties.  The  living  material,  the  bioplasm,  is  not  only 
complex  and  unstable  in  composition,  but  extremely  sensitive  to  all  physical 
and  chemic  influences.  The  methods,  therefore,  which  are  employed  for 
analysis  destroy  its  composition  and  vitality,  and  the  products  which  are 
obtained  are  peculiar  to  dead  rather  than  to  living  material. 

Chemic  analysis,  therefore,  may  be  directed — 

1.  To  the  determination  of  the  composition  of  the  dead  body. 

2.  To  the  determination  of  the  successive  changes  in  composition  which 
the  living  bioplasm  undergoes  during  functional  activity. 

A  chemic  analysis  of  the  dead  body,  with  a  view  to  disclosing  the  sub- 
stances of  which  it  is  composed,  their  properties,  their  intimate  structure, 
their  relationship  to  one  another,  constitutes  what  might  be  termed  chemic 
anatomy.  An  investigation  of  the*  living  material  and  of  the  successive 
changes  it  undergoes  in  the  performance  of  its  functions  constitutes  what 
has  been  termed  chemic  physiology  or  physiologic  chemistry. 

By  chemic  analysis  the  animal  body  can  be  reduced  to  a  number  of 
liquid  and  solid  compounds  which  belong  to  both  the  inorganic  and  organic 
worlds.  These  compounds,  resulting  from  a  proximate  analysis,  have 
been  termed  proximate  principles.  That  they  may  merit  this  term,  how- 
ever, they  must  be  obtained  in  the  form  under  which  they  exist  in  the  living 
condition.  The  organic  compounds  consist  of  representatives  of  the  carbo- 
hydrate, fatty,  and  protein  groups  of  organic  bodies;  the  inorganic  compounds 
consist  of  water,  various  acids,  and  inorganic  salts. 

The  compounds  or  proximate  principles  thus  obtained  can  be  further 
resolved  by  an  ultimate  analysis  into  a  small  number  of  chemic  elements 
which  are  identical  with  elements  found  in  many  other  organic  as  well  as 
inorganic  compounds.  The  different  chemic  elements  which  are  thus 
obtained,  and  the  percentages  in  which  they  exist  in  the  body,  are  as  follows 
— viz.,  oxygen,  72  per  cent.;  hydrogen,  9.10;  nitrogen,  2.5;  carbon,  13.50; 
phosphorus,  1.15;  calcium,  1.30;  sulphur,  0.147;  sodium,  o.io;  potassium, 
0.026;  chlorin,  0.085;  Auorin,  iron,  silicon,  magnesium,  in  small  and  variable 
amounts. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  7 

THE  CARBOHYDRATES. 

The  carbohydrate  compounds,  which  enter  into  the  composition  of  the 
animal  body,  are  mainly  starches  and  sugar.  In  many  respects  they  are 
closely  related,  and  by  appropriate  means  are  readily  converted  into  one 
another.  In  composition  they  consist  of  the  elements  carbon,  hydrogen, 
and  oxygen.  As  their  name  implies,  the  hydrogen  and  oxygen  are  present  in 
these  compounds  in  the  proportion  in  which  they  exist  in  water,  or  as  2:1. 
The  molecule  of  the  carbohydrates  above  mentioned  consists  of  either  six 
atoms  of  carbon  or  a  multiple  of  six;  in  the  latter  case  the  quantity  of 
hydrogen  and  oxygen  taken  up  by  the  carbon  is  increased,  though  the 
ratio  remains  unchanged. 

The  carbohydrates  may  be  divided  into  three  groups — viz.:  (i)  amyloses, 
including  starch,  dextrin,  glycogen,  and  cellulose;  (2)  dextroses,  including 
dextrose,  levulose,  galactose;  (3)  saccharoses,  including  saccharose,  lactose, 
and  maltose.  According  to  the  number  of  carbon  atoms  entering  into  the 
second  group  (six),  they  are  frequently  termed  monosaccharids;  those  of 
the  third  group,  disaccharids — twice  six;  those  of  the  first  group,  poly- 
saccharids — multiples  of  six. 

Though  but  few  of  the  members  of  the  carbohydrate  group  are  con- 
stituents of  the  human  body,  many  are  constituents  of  the  foods;  on 
account  of  their  importance  in  this  respect,  and  their  relation  to  one 
another,  the  chemic  features  of  the  more  generally  consumed  carbohydrates 
will  be  stated  in  this  connection. 

I.  AMYLOSES,  (CeHioOJn. 

Starch  is  widely  distributed  in  the  vegetable  world,  being  abundant 
in  the  seeds  of  the  cereals,  leguminous  plants,  and  in  the  tubers  and  roots 
of  many  vegetables.  It  occurs  in  the  form  of  microscopic  granules  which 
vary  in  size,  shape,  and  appearance,  according  to  the  plant  from  which 
they  are  obtained  Each  granule  presents  a  nucleus,  or  hilum,  around 
which  is  arranged  a  series  of  eccentric  rings,  alternately  light  and  dark. 
The  granule  consists  of  an  envelope  and  stroma  of  cellulose,  containing 
in  its  meshes  the  true  starch  material — granidose.  Starch  is  insoluble  in 
cold  water  and  alcohol.  When  heated  with  water  up  to  70°  C,  the  granules 
swell,  rupture,  and  liberate  the  granulose,  which  forms  an  apparent  solution; 
if  present  in  sufficient  quantity,  it  forms  a  gelatinous  mass  termed  starch 
paste.  On  the  addition  of  iodin,  starch  strikes  a  characteristic  deep  blue 
color;  the  compound  formed — iodid  of  starch — is  weak,  the  color  dis- 
appearing on  heating,  but  reappearing  on  cooling. 

Boiling  starch  with  dilute  sulphuric  acid  (25  per  cent.)  converts  it  into 
dextrose.  In  the  presence  of  vegetable  diastase  or  animal  ferments,  starch 
is  converted  into  maltose  and  dextrose,  two  forms  of  sugar. 

Dextrin  is  a  substance  formed  as  an  intermediate  product  in  the  trans- 
formation of  starch  into  sugar.  There  are  at  least  two  principal  varieties 
— erythrodextrin,  which  strikes  a  red  color  with  iodin,  and  achroddextrin, 
which  is  without  color  when  treated  with  this  reagent.  In  the  pure  state 
dextrin  is  a  yellow-white  powder,  soluble  in  water.  In  the  presence  of 
animal  ferments  ervthrodextrin  is  converted  into  maltose. 


8  TEXT-BOOK  OF  PHYSIOLOGY. 

Glycogen  is  a  constituent  of  the  animal  liver,  and,  to  a  slight  extent, 
of  muscles,  0.5  to  0.9  per  cent.,  and  of  tissues  generally.  In  the  tissues  of  the 
embryo  it  is  especially  abundant.  When  obtained  in  a  pure  state  it  is  an 
amorphous,  white  powder.  It  is  soluble  in  water,  forming  an  opales- 
cent solution.  With  iodin  it  strikes  a  port-wine  color.  In  some  respects 
it  resembles  starch,  in  others  dextrin.  Like  vegetable  starch,  glycogen  or 
animal  starch  can  be  converted  by  dilute  acids  and  ferments  into  sugar 
(dextrose) . 

Cellulose  is  the  basic  material  of  the  more  or  less  solid  framework 
of  plants.  It  is  soluble  in  ammoniacal  solution  of  cupric  oxid,  from  which 
it  can  be  precipitated  by  acids.  It  is  an  amorphous  powder;  dilute  acids  can 
convert  it  into  dextrose. 

2.  DEXTROSES,  C.U.^O,. 

Dextrose,  glucose,  or  grape-sugar  is  found  in  grapes,  most  sweet 
fruits,  and  honey,  and  as  a  normal  constituent  of  liver,  blood,  muscles, 
and  other  animal  tissues.  In  the  disease  diabetes  melHtus  it  is  found  also 
in  the  urine. 

When  obtained  from  any  source,  it  is  soluble  in  water  and  in  hot  alcohol, 
from  which  it  crystallizes  in  six-sided  tables  or  prisms.  As  usually  met  with, 
it  is  in  the  form  of  irregular,  warty  masses.  It  is  sweet  to  the  taste.  W^hen 
examined  with  the  polariscope,  it  will  be  found  that  dextrose  turns  the  plane 
of  polarized  light  to  the  right.     It  is  therefore  termed  dextro-rotatory. 

It  has  for  a  long  time  been  known  that  when  sugar,  cupric  hydroxid,  and 
an  alkali — e.g.,  sodium  or  potassium — are  present  in  solution,  the  sugar 
will  abstract  from  the  cupric  hydroxid  a  portion  of  its  oxygen,  thus  reducing 
it  to  a  lower  stage  of  oxidation  giving  rise  to  cuprous  oxid.  Sugar  has  a 
similar  action  on  both  silver  and  bismuth.  On  this  property  of  sugar  a 
standard  solution  of  cupric  hydroxid  w'as  suggested  by  Fehling  which  may 
be  employed  for  both  qualitative  and  quantitative  tests  for  the  presence  of 
sugar  in  solution, 

Fehling' s  Test  Solution. — This  is  a  solution  of  cupric  hydroxid  made 
alkaline  by  an  excess  of  sodium  or  potassium  hydroxid  with  the  addition 
of  sodium  and  potassium  tartrate.  It  is  made  by  dissolving  cupric  sulphate 
34.64  grams,  potassium  hydroxid  125  grams,  sodium  and  potassium  tartrate 
173  grams,  in  distilled  water  sufficient  to  make  one  liter. 
The  reaction  is  expressed  by  the  following  equation: 
CuSO,  +  2KOH  =  Cu(OH)2  -F  K,SO,. 

The  object  of  the  sodium  and  potassium  tartrate  is  to  dissolve  the  cupric 
hydroxid  and  hold  it  in  solution. 

For  qualitative  analysis  it  is  only  necessary  to  boil  a  few  cubic  centi- 
meters of  this  solution  in  a  test-tube;  then  add  the  suspected  solution  and 
again  heat  to  the  boiling-point.  If  sugar  be  present,  the  cupric  hydroxid 
is  reduced  to  the  condition  of  a  cuprous  oxid,  which  shows  itself  as  a  red 
or  orange-yellow  precipitate.  The  color  of  the  precipitate  depends  on  the 
relative  excess  of  either  copper  or  sugar,  being  red  with  the  former,  orange 
or  yellow  with  the  latter.  The  delicacy  of  this  test  is  shown  by  the  fact  that 
a  few  minims  of  this  solution  will  detect  in  i  c.c.  of  water  the  yV  o^  ^  milli- 
gram of  sugar. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  9 

For  quantitative  analysis,  10  c.c.  of  Fehling's  solution,  diluted  with 
40  c.c.  of  water,  are  heated  in  a  porcelain  capsule,  to  which  the  suspected 
solution  is  cautiously  added  from  a  buret  until  the  blue  color  entirely  dis- 
appears. The  strength  of  this  solution  is  such  that  10  c.c.  is  decolorized 
by  50  milligrams  of  sugar  (dextrose).  Thus  if  0.8  c.c.  of  the  suspected  solu- 
tion, e.g.,  urine,  decolorizes  10  c.c.  of  Fehling's  solution,  then  it  contains  50 
milligrams  of  sugar,  from  which  the  percentage  of  sugar  in  the  urine  can  be 
determined. 

The  Fermentation   Test. — All  the  sugars  with  the  exception  of  lactose 

undergo  reduction  to  simpler  compounds,  mainly  alcohol  and  carbon  dioxid, 

under  the  action  of  the  yeast  plant,  Saccharomyces  cerevisice.     The  change 

with  dextrose  is  expressed  in  the  following  equation: 

C„Hi,0„  =  2C,H,0  +  2C02. 

Dextrose    =  Alcohol    +  Carbon 

Dioxid. 

About  95  per  cent,  of  the  dextrose  is  so  changed,  the  remaining  5  per 
cent,  yielding  secondary  products — succinic  acid,  glycerin,  etc.  As  a 
means  of  testing  any  solution  for  the  presence  of  sugar  this  method  may 
be  adopted.  It  is  generally  very  satisfactory.  From  the  quantity  of 
carbon  dioxid  and  alcohol  thus  produced  the  quantity  of  sugar  in  the  solu- 
tion may  be  determined. 

Levulose,  or  fruit-sugar,  is  found  in  association  with  dextrose  as 
a  constituent  of  many  fruits.  It  is  sweeter  than  dextrose  and  more  soluble 
in  both  w^ater  and  dilute  alcohol.  From  alcoholic  solutions  it  crystallizes 
in  fine,  silky  needles,  though  it  usually  occurs  in  the  form  of  a  syrup. 

Levulose  is  distinguished  from  dextrose  by  its  property  of  turning 
the  plane  of  polarized  light  to  the  left;  the  extent  to  w^hich  it  does  so,  how- 
ever, varies  with  the  temperature  and  concentration  of  the  solution.  For 
this  reason  it  is  turned  levulo-rotatory. 

Under  the  influence  of  the  yeast  plant  it  slowly  undergoes  fermen- 
tation, yielding  the  same  products  as  dextrose.  It  also  has  a  reducing 
action  on  cupric  hydroxid. 

Galactose  is  obtained  by  boiling  milk-sugar  (lactose)  wdth  dilute  sul- 
phuric acid.  In  many  chemic  relations  it  resembles  dextrose.  It  is  less 
soluble  in  water,  however,  crystallizes  more  easily,  and  has  a  greater  dextro- 
rotatory power.     It  also  undergoes  fermentation  with  the  yeast  plant. 

3.  SACCHAROSES,  C,,'E.,,0^,. 

Saccharose,  or  cane-sugar,  is  widely  distributed  throughout  the  vege- 
table world,  but  is  especially  abundant  in  sugar-cane,  sorghum  cane,  sugar- 
beet,  Indian  corn,  etc.  It  crystallizes  in  large  monoclinic  prisms.  It  is 
soluble  in  water  and  in  dilute  alcohol.  Saccharose  has  no  reducing  power  on 
cupric  hydroxid,  and  hence  its  presence  cannot  be  detected  by  Fehling's 
solution.  It  is  dextro-rotator}\  Boiled  with  dilute  mineral,  as  well  as 
with  organic  acids,  saccharose  combines  with  water  and  undergoes  a  change 
in  virtue  of  which  it  rotates  the  plane  of  polarized  light  to  the  left,  and  hence 
the  product  was  termed  invert  sugar.  This  latter  has  been  shown  to  be  a 
mixture  of  equal  quantities  of  levulose  and  dextrose.  This  inversion  of 
saccharose  through  hydration  and  decomposition  is  expressed  in  the  fol- 
lowing equation: 


lo  TEXT-BOOK  OF  PHYSIOLOGY. 

C12H22O11  +  HjO  =C6H,20a  +C8Hi20e 
Saccharose  +  Water  =  Levuloses+ Dextrose 


Invert  Sugar. 

Saccharose  is  not  directly  fermentable  by  yeast,  but  through  the  specific 
action  of  a  ferment,  inverUn  or  invertase,  secreted  by  the  yeast  plant,  or  the 
inverting  ferment  of  the  small  intestine,  it  undergoes  inversion,  as  pre- 
viously stated,  after  which  it  is  readily  fermented,  yielding  alcohol  and 
carbon  dioxid. 

Lactose  is  the  form  of  sugar  found  exclusively  in  the  milk  of  the  mam- 
malia, from  which  it  can  be  obtained  in  the  form  of  hard,  white,  rhombic 
prisms  united  with  one  molecule  of  water.  It  is  soluble  in  water,  insoluble 
in  alcohol  and  ether.  It  is  dextro-rotatory.  It  reduces  cupric  hydroxid, 
but  to  a  less  extent  than  dextrose.  Dilute  acids  decompose  it  into  equal 
quantities  of  dextrose  and  galactose.  Lactose  is  not  fermentable  with 
yeast,  but  in  the  presence  of  the  lactic  acid  bacillus  it  is  decomposed  into 
lactic  acid,  and  finally  into  butyric  acid,  as  expressed  in  the  following 
equation : 

C12H22OH  -r    HoO    =  4C3Hfl03 
Lactose      +  Water    =  Lactic  Acid. 

2C3He03    =      C^HgO,     +     2CO.     -t-     2H,, 

Latic  Acid     =  Buytric  Acid  +     Carbon      +      Free 

Dioxid        Hydrogen. 

Maltose  is  a  transformation  product  of  starch,  and  arises  whenever 
the  latter  is  acted  on  by  malt  extract  or  the  diastatic  ferments  in  saliva  and 
pancreatic  juice.     The  change  is  expressed  by  the  following  equation: 

Starch.  Water.  Maltose. 

Maltose  crystallizes  in  the  form  of  white  needles,  which  are  soluble  in 
water  and  in  dilute  alcohol.  It  is  dextro-rotatory.  In  the  presence  of 
ferments  and  dilute  acids  maltose  undergoes  hydration  and  decomposition, 
giving  rise  to  two  molecules  of  dextrose.  It  has  a  reducing  action  on  cupric 
hydroxid.  Fermentation  is  readily  caused  by  yeast,  but  whether  directly 
or  indirectly  by  inversion  is  somewhat  uncertain. 

Osazones. — All  the  sugars  which  possess  the  power  of  reducing  cupric 
hydroxid  are  capable  of  combining  with  phenyl-hydrazin,  with  the  formation 
of  compounds  termed  osazones.  The  osazones  so  formed  are  crystalline 
in  structure,  but  have  different  melting-points,  varying  degrees  of  solubility 
and  optic  properties,  all  of  which  serve  to  detect  the  various  sugars  and  to 
distinguish  one  from  the  other.  Of  the  different  osazones,  phenyl-gluco- 
sazone  is  the  most  characteristic,  and  occurs  in  the  form  of  long,  yellow 
needles.  It  may  be  obtained  from  dextrose  by  the  following  method: 
To  50  c.c.  of  a  dextrose  solution  add  2  gm.  of  phenyl-hydrazin  and  2  gm. 
of  sodium  acetate,  and  boil  for  an  hour.  On  cooling,  the  osazone  crystal- 
lizes in  the  form  of  long,  yellow  needles. 

THE  FATS. 

The  fats  constitute  a  group  of  organic  bodies  found  in  the  tissues  of 
both  vegetables  and  animals.  In  the  vegetable  world  they  are  largely 
found  in  fruits,  seeds,  and  nuts,  where  they  probably  originate  from  a 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  ii 

transformation  of  the  carbohydrates.  In  the  animal  body  the  fats  are 
found  largely  in  the  subcutaneous  tissue,  in  the  marrow  of  bones,  in  and 
around  various  internal  organs  and  in  milk.  In  these  situations  fat  is 
contained  in  small,  round  or  polygon-shaped  vesicles,  which  are  united  by 
areolar  tissue  and  surrounded  by  blood-vessels.  At  the  temperature  of  the 
body  the  fat  is  liquid,  but  after  death  it  soon  solidifies  from  the  loss  of  heat. 

The  fats  are  compounds  consisting  of  carbon,  hydrogen,  and  oxygen. 
The  percentage  composition  of  fat  (stearin)  is  as  follows:  Carbon,  76.86; 
hydrogen,  12.36;  oxygen,  10.78.  The  fat  found  in  animals  is  a  mixture,  in 
varying  proportions  in  different  animals,  of  three  neutral  fats — stearin, 
palmitin,  and  olein.  Each  fat  is  a  derivative  of  glycerin  and  the  particular 
acid  indicated  by  its  name — e.g.,  stearic  acid,  in  the  case  of  stearin,  etc. 
The  reaction  which  takes  place  in  the  combination  of  glycerin  and  the 
acid  is  expressed  in  the  following  equation : 

C3H,(HO)3     +     sHjsCj.HO,     =     C,ll,{C,,Ii,,0,),     +     2,B.,0. 

Glycerin.  Stearic  Acid.  Stearin.  Water. 

Hence,  strictly  speaking,  the  fats  are  compound  ethers,  in  which  the 
hydrogen  of  the  organic  acid  is  replaced  by  the  trivalent  radicle,  tritenyl, 
C3H5. 

Stearin,  C3H5(C^8H3gO,)3,  is  the  chief  constituent  of  the  more  solid 
fats.  It  is  solid  at  ordinary  temperatures,  melting  at  55°  C,  then  solidify- 
ing again  as  the  temperature  rises,  until  at  71°  C.  it  melts  permanently.  It 
crystallizes  in  square  tables. 

Palmitin,  C3H5(Ci6H3j02)3  is  a  semifluid  fat,  solid  at  45°  C.  and 
melting  at  62°  C.     It  crystallizes  in  fine  needles,  and  is  soluble  in  ether. 

Olein,  C3H5(Cj8H3302)3,  is  a  colorless,  transparent  fluid,  liquid  at 
ordinary  temperatures,  only  solidifying  at  0°  C.  It  possesses  marked 
solvent  powers,  and  holds  stearin  and  palmitin  in  solution  at  the  temperature 
of  the  body. 

Saponification. — -When  subjected  to  the  action  of  superheated  steam, 
a  neutral  fat  is  saponified — i.e.,  decomposed  into  glycerin  and  the  particular 
acid  indicated  by  the  name  of  the  fat  used:  e.g.,  stearic,  palmitic,  or  oleic. 
The  reaction  is  expressed  as  follows: 

C3H,(C,3H330,),-f-3H,0=C3H5(HO)3  +  3C,sH3,02 
Olein.  Water.  Glycerin.         Oleic  Acid. 

The  fat  acids  thus  obtained  are  characterized  by  certain  chemic  fea- 
tures, as  follows: 

Stearic  acid  is  a  firm,  white  solid,  fusible  at  69°  C.  It  is  soluble  in 
ether  and  alcohol,  but  not  in  water. 

Palmitic  acid  occurs  in  the  form  of  white,  glistening  scales  or  needles, 
melting  at  62°  C. 

Oleic  acid  is  a  clear,  colorless  liquid,  tasteless  and  odorless  when  pure. 
It  crystallizes  in  white  needles  at  0°  C. 

If  this  saponification  takes  place  in  the  presence  of  an  alkali — e.g., 
potassium  hydroxid  or  sodium  hydroxid — the  acid  produced  combines 
at  once  with  the  alkali  to  form  a  salt  known  as  a  soap,  while  the  glycerin 
remains  in  solution.     The  reaction  is  as  follows: 

3KHO   +   sCjgHsiOs   =    3KC,3H330o     +     3H2O 
Potassium.        Oleic  Acid.  Potassium  Oleate,       Water. 


12  TEXT-BOOK  OF  PHYSIOLOGY. 

All  soaps  arc,  therefore,  salts  formed  by  the  union  of  alkalies  and  fat 
acids.  The  sodium  soaps  are  generally  hard,  while  the  potassium  soaps 
are  soft.  Those  made  with  stearin  and  palmitin  are  harder  than  those 
made  with  olein.  If  the  soap  is  composed  of  lead,  zinc,  copper,  etc.,  it  is 
insoluble  in  water. 

Emulsification. — When  a  neutral  oil  is  vigorously  shaken  with  water 
or  other  fluid,  it  is  broken  up  into  minute  globules  that  are  more  or  less 
permanently  suspended;  the  permanency  depending  on  the  nature  of  the 
liquid.  The  most  permanent  emulsions  are  those  made  with  soap  solutions. 
The  process  of  emulsification  and  the  part  played  by  soap  can  be  readily 
observed  by  placing  on  a  few  cubic  centimeters  of  a  solution  of  sodium 
carbonate  (0.25  per  cent.)  a  small  quantity  of  a  perfectly  neutral  oil  to  which 
has  been  added  2  or  3  per  cent,  of  a  fat  acid.  The  combination  of  the  acid 
and  the  alkali  at  once  forms  a  soap.  The  energy  set  free  by  this  combination 
rapidly  divides  the  oil  into  extremely  minute  globules.  A  spontaneous 
emulsion  is  thus  formed. 

THE  PROTEINS. 

The  proteins  constitute  a  group  of  organic  bodies  which  are  found  in  both 
vegetable  and  animal  tissues.  Though  present  in  all  animal  tissues,  they 
are  especially  abundant  in  muscles  and  bones,  where  they  constitute  20 
per  cent,  and  30  per  cent,  respectively.  Though  genetically  related,  and 
possessing  many  features  in  common,  the  different  members  of  the  protein 
group  are  distinguished  by  characteristic  physical  and  chemic  properties 
which  serve  not  only  for  their  identification,  but  for  their  classification  into 
more  or  less  well-defined  groups. 

Chemic  Composition. — A  chemic  analysis  of  proteins  shows  that 
they  consist  of  carbon,  hydrogen,  oxygen,  nitrogen  and  sulphur,  though 
the  percentage  of  each  of  these  elements  varies  somewhat  in  the  different 
proteins. 

A  certain  number  of  proteins  contain  phosphorus  while  almost  all 
of  them  contain  different  inorganic  salts  in  varying  amounts.  The  average 
percentage  composition  of  several  proteins  is  shown  in  the  following  analyses: 

C.        H.        N.         o.       S. 

Egg-albumin 52.9  7.2  15.6  23.9  0.4    (Wiirtz). 

Serum-albumin 53-°  6.8  16.0  22.29  i  .77  (Hammers  ten). 

Casein 52.3  7-07  15-91  22.03  0.82  (Chittenden  and  Painter). 

Myosin 52.82  7. 11  16.77  21.90  i  .27  (Chittenden  and  Cummins). 

The  molecular  composition  of  the  proteins  is  not  definitely  known 
and  the  formulae  which  have  been  suggested  are  therefore  only  approxi- 
mative. Leow  assigns  to  albumin  the  formula  0^211^2^580228,  while 
Schiitzenberger  raises  the  numbers  to  Cj^oHggjNgjO^Sg,  either  of  which 
shows  that  the  protein  molecule  is  extremely  complex. 

Structure  of  the  Protein  Molecule. — From  the  large  size  of  the  protein 
molecule  as  indicated  by  its  chemic  composition  it  might  be  inferred  that 
its  structure  was  equally  complex.  This  modern  investigation  has  shown 
to  be  the  case. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  13 

When  any  one  of  the  typical  proteins,  found  in  animal  or  vegetable 
tissues,  is  hydrolyzed  by  acids,  alkalies  and  animal  ferments  under  appro- 
priate conditions,  it  can  be  resolved  through  a  series  of  descending  stages 
into  relatively  simple  nitrogen-holding  bodies  termed  amino-acids  and 
diamino-acids,  of  which  somewhat  more  than  twenty  have  been  isolated 
and  their  properties  determined.  The  principal  amino-acids  are  as  follows: 
Glycocoll,  alanin,  leucin,  isoleucin,  amino-isovalerianic  acid,  serin,  aspartic 
acid,  glutamic  acid,  phenylalanin,  tyrosin,  prolin,  tryptophan.  The  principal 
diamino-acids  are  as  follows:     Ornithin,  lysin,  histidin,  arginin,  cystin. 

The  protein  molecule  is  therefore  structurally  complex.  The  manner 
in  which  these  elementary  compounds  are  arranged,  united  or  grouped 
in  any  given  protein,  is  practically  unknown.  More  or  less  successful 
attempts  have  been  made  at  the  reconstruction  of  the  protein  molecule  by 
synthetic  methods,  by  the  union  of  two  or  more  of  the  amino-acids.  A 
number  of  such  compounds  have  been  formed  by  the  union  of  from  two  to 
ten  or  more  amino-acids,  all  of  them  exhibiting  many  of  the  protein  reac- 
tions. Such  bodies  are  termed,  according  to  their  complexity,  peptids  and 
polypeptids. 

Physical  Properties. — As  a  class  the  proteins  are  characterized  by  the 
following  properties: 

1.  Indiffusibility. — None  of  the* proteins  normally  assume  the  crystalline 

form,  and  hence  they  are  not  capable  of  diffusing  through  parchment 
or  an  animal  membrane.  Peptone,  a  product  of  the  digestion  of 
proteins,  is  an  exception  as  regards  its  diffusibility.  As  met  with  in 
the  body,  all  proteins  are  amorphous,  but  vary  in  consistence  from  the 
liquid  to  the  solid  state.  The  colloid  character  of  the  proteins  permits 
of  their  separation  and  purification  from  crystalloid  diffusible  com- 
pounds by  the  process  of  dialysis. 

2 .  Solubility. — Some  of  the  proteids  are  soluble  in  water,  others  in  solutions 

of  the  neutral  salts  of  varying  degrees  of  concentration,  in  strong  acids 
and  alkalies.     All  are  insoluble  in  alcohol  and  ether. 

3.  Coagulability. — Under   the   influence   of  heat   and   animal   ferments, 

some  of  the  proteins  readily  pass  from  the  soluble  liquid  state  to  the 
insoluble  solid  state,  attended  by  a  permanent  alteration  in  their  chemic 
composition.  To  this  change  the  term  coagulation  has  been  given. 
The  various  proteins,  however,  coagulate  at  different  temperatures. 
Proteins  are  capable  of  precipitation  without  losing  their  solubility 
by  ammonium  sulphate,  sodium  chlorid,  and  magnesium  sulphate. 

4.  Fermentability. — In  the  presence  of  specific  microorganisms — bacteria 

— the  proteins,  owing  to  their  complexity  and  instability,  are  prone 
to  undergo  disintegration  and  reduction  to  simpler  compounds.  This 
decomposition  or  putrefaction  occurs  most  readily  when  the  conditions 
most  favorable  to  the  growth  of  bacteria  are  present — \'iz.,  a  temperature 
varying  from  25°  C.  to  40°  C,  moisture,  and  oxygen.  The  intermediate 
as  well  as  the  terminal  products  of  the  decomposition  of  the  proteins  are 
numerous,  and  vary  with  the  composition  of  the  protein  and  the  specific 
physiologic  action  of  the  bacteria.  Among  the  intermediate  products 
is  a  series  of  alkaloid  bodies,  some  of  which  possess  marked  toxic 


14  TEXT-BOOK  OF  PHYSIOLOGY. 

properties,  known  as  ptomains.  The  toxic  symptoms  which  frequently 
follow  the  ingestion  of  foods  in  various  stages  of  putrefaction  are  to 
be  attributed  to  these  compounds.  The  terminal  products  are  repre- 
sented by  hydrogen  sulphid,  ammonia,  carbon  dioxid,  fats,  phosphates, 
nitrates,  etc. 

Classification. — The  animal  proteins  by  virtue  of  their  structural 
composition,  their  physical  and  chemic  properties,  permit  of  a  provisional 
arrangement  into  three  groups  as  follows:  Simple  proteins,  conjugate 
proteins  and  protein  derivatives. 

SIMPLE  PROTEINS. 

The  simple  proteins  are  so  called  because  of  the  fact  that  when  they 
are  hydrolyzed  they  yield  only  amino  and  diamino-acids.  The  members 
of  this  group  are  as  follows: 

PROTAMINS. 

These  proteins  are  derived  for  the  most  part  from  the  heads  of  the  sper- 
matozoa of  fish.  They  take  their  names  from  the  species  of  fish  from 
which  they  are  obtained,  e.g.,  salmin  (salmon),  sturin  (sturgeon),  scom- 
brom  (mackerel),  etc.  Inasmuch  as  they  respond  to  Piotrowski's  test  in  a 
characteristic  way  they  are  regarded  as  true  proteins.  When  subjected  to 
hydrolysis  they  can  be  resolved  into  the  diamino  bodies,  lysin,  arginin  and 
histidin,  of  which  they  constitute  about  90  per  cent.,  and  a  small  number  of 
the  mono-amino-acids.  Because  of  the  fact  that  the  diamino  bodies,  lysin, 
histidin  and  arginin  contain  6  atoms  of  carbon  they  are  known  as  the  hexone 
bases.  Inasmuch  as  the  protamins  contain  practically  but  these  three 
bodies,  they  are  regarded  as  the  simplest  of  all  the  proteins.  Since  a  typical 
protein  always  yields  on  hydrolysis  the  hexone  bases,  in  addition  to  a  variable 
number  of  mono-amino-acids,  it  is  believed  that  the  usual  protein  is  com- 
posed of  a  nucleus  of  the  hexone  bases  to  which  is  attached  a  variable 
number  of  mono-amino-acids.  The  proportions  in  which  the  bases  exist 
in  the  nucleus  and  the  proportions  in  which  the  amino-acids  are  united  to 
the  nucleus,  vary  in  different  proteins. 

HISTONS. 

The  proteins  embraced  in  this  class  comprise  a  series  of  compounds 
which  are  somewhat  more  complex  than  the  protamins  and  less  complex 
than  the  typical  proteins;  for  on  hydrolysis  they  not  only  yield  the  hexone 
bases  but  in  addition  a  certain  number  of  amino-acids.  They  are,  there- 
fore, intermediate  in  structural  composition  between  the  protamins  and  the 
usual  proteins.  Their  protein  character  is  indicated  by  their  reaction  to 
Millon's  reagent  and  to  Piotrowski's  test.  The  histons  are  usually  found 
in  combination  with  nucleic  acid,  in  the  spermatozoa  of  most  animals  and 
especially  in  fish,  and  in  the  coloring  matter  (the  hemoglobin)  of  the  red 
corpuscles.  The  proteins  of  the  tissues  usually  contain  from  25  to  30  per 
cent,  of  histons. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  15 

ALBUMINS. 

The  members  of  this  group  are  soluble  in  water,  in  dilute  saline  solu- 
tions, and  in  saturated  solutions  of  sodium  chlorid  and  magnesium  sulphate. 
They  are  coagulated  by  heat,  and  when  dried  form  an  amber-colored  mass. 
(a)  Senim-albumin. — This  most  important  protein  is  found  in  blood, 
lymph,  chyle,  and  some    tissue    fluids.     It   is  obtained  readily  by 
precipitation  from  blood-serum,  after  the  other  proteins  have  been 
removed,   on   the   addition   of    ammonium   sulphate.     When   freed 
from  saline  constituents,  it  presents  itself  as  a  pale,  amorphous  sub- 
stance, soluble  in  water  and  in  strong  nitric  acid.     It  is  coagulated 
at  a  temperature  of  73°  C,  as  well  as  by  various  acids — e.g.,  citric, 
picric,  nitric,  etc.     It  has  a  rotator}'  power  of  — 62.6°. 
{b)  Egg-albumin. — Though  not  a  constituent  of  the  human  body,  egg- 
albumin  resembles  the  foregoing  in  many  respects.     When  obtained 
in  the  solid  form  from  the  white  of  the  egg,  it  is  a  yellow  mass  without 
taste  or  odor.     Though  similar  to  serum-albumin,  it  differs  from 
it  in  being  precipitated  by  ether,  in  coagulating  at  54°  C,  and  in 
having  a  lower  rotatory  power,  — 35-5°. 

(c)  Lact-albumin. — As  its  name  impUes,  this  protein  is  found  in  milk. 
It  can  be  precipitated  from  milk-plasma  by  sodium  sulphate  after 
the  precipitation  of  the  other  proteids  by  half  saturation  with  am- 
monium sulphate.     It  slowly  coagulates  at  77°  C. 

(d)  Myo-albumin. — This  protein  is  found  in  muscle-plasma  from 
which  it  subjects  the  plasma  to  fractional  heat  coagulation.  At 
73°  C.  myo-albumin  coagulates. 

GLOBULINS. 

(a)  Serum-globulin  or  Paraglobulin. — ^This  protein,  as  its  name 
implies,  is  found  in  blood-serum,  though  it  is  present  in  other  animal 
fluids.  When  precipitated  by  magnesium  sulphate  or  carbon  dioxid, 
it  presents  itself  as  a  flocculent  substance,  insoluble  in  water,  soluble 
in  dilute  acids  and  alkalies,  and  coagulating  at  75°  C. 

(b)  Fibrinogen. — This  protein  is  found  in  blood-plasma  in  association 
with  serum-globulin  and  serum-albumin.  It  is  also  present  in 
lymph-tissue  fluids  and  in  pathologic  transudates.  It  can  be  ob- 
tained from  blood-plasma  which  has  been  previously  treated  with 
magnesium  sulphate  on  the  addition  of  a  saturated  solution  of  sodium 
chlorid.  It  is  soluble  in  dilute  acids  and  alkalies,  and  coagulates 
at  56°  C. 

(c)  Paramyosinogen  or  Myosin. — This  protein  is  a  constituent  of  the 
muscle-plasma  from  which  it  can  be  precipitated  bv  a  temperature 
of  47°  C. 

(d)  Myosinogen  or  Myogen. — This  protein  is  the  chief  constituent  of 
the  muscle-plasma  and  is  of  great  nutritive  value.  During  the 
living  condition  it  is  liquid,  but  after  death  it  readily  undergoes  a 
chemic  change  and  contributes  to  the  formation  of  an  insoluble 
protein  known  as  myogen  fibrin.  It  is  soluble  in  dilute  hydrochloric 
acid  and  dilute  alkalies.     It  coagulates  at  56°  C. 

(e)  Crystallin  or  Globulin. — This  is  obtained  by  passing  a  stream 
of  CO2  through  a  watery  extract  of  the  crystalline  lens. 


i6  TEXT-BOOK  OF  PHYSIOLOGY. 

SCLERO-PROTEINS  (ALBUMINOIDS). 

The  sclcro-protcins  ronstitule  a  group  ot  substances  similar  to  the  pro- 
teins in  many  respects,  though  differing  from  them  in  others.  When  ob- 
tained from  the  tissues,  in  which  they  form  an  organic  basis,  they  are  found 
to  be  amorphous,  colloid,  and  when  decomposed  yield  products  similar 
to  those  of  the  true  proteins.  The  principal  members  of  this  group  are  as 
follows: 

(a)  Collagen,  Ossein. — These  are  two  closely  alHed,  if  not  identical, 
substances,  found  respectively  in  the  white  fibrous  connective  tissue 
and  in  bone.  When  the  tendons  of  muscles,  the  ligaments,  or  de- 
calcified bone  are  boiled  for  several  hours,  the  collagen  and  ossein 
are  converted  into  soluble  gelatin,  which,  when  the  solution  cools, 
becomes  solid. 

(b)  Chondrigen. — This  is  supposed  to  be  the  organic  basis  of  the  more 
permanent  cartilages.  When  the  latter  are  boiled,  they  yield  a 
substance  which  gelatinizes  on  cooling,  and  to  which  the  name 
chondrin  has  been  given.  Chondrin,  however,  is  not  a  pure  gelatin, 
but  has  associated  with  it  a  compound  protein  known  as  chrondro- 
mucoid. 

(c)  Elastin  is  the  name  given  to  the  substance  composing  the  fibers  of 
the  yellow,  elastic  connective  tissue. 

(d)  Keratin  is  the  substance  found  in  all  horny  and  epidermic  tissues, 
such  as  hairs,  nails,  scales,  etc.  It  differs  from  most  proteins  in  con- 
taining a  high  percentage  of  sulphur. 

PHOSPHO-PROTEINS. 

The  two  members  of  this  group  are  distinguished  by  yielding  on  decom- 
position a  protein  which  contains  phosphorus.  It  was  formerly  regarded 
as  a  nuclein. 

(a)  Caseinogen. — This  is  the  principal  protein  of  milk,  in  which  it 
exists  in  association  with  calcium  in  a  form  known  as  calcium- 
caseinogenate.  It  is  precipitated  by  acetic  acid  and  by  magnesium 
sulphate.  It  is  coagulated  by  rennin,  though  the  nature  of  the  process 
is  not  very  clear.  It  was  formerly  taught  that  under  the  action  of 
rennin,  an  enzyme  of  the  gastric  mucous  membrane,  caseinogen  was 
separated  into  a  solid  portion,  casein  or  tyrein,  and  a  soluble  portion. 
The  cleavage  action  of  rennin  thus  indicated  has  not  been  verified  by 
subsequent  investigations.  It  is  more  in  accordance  with  the  facts  to 
assume  that  the  process  is  a  double  one  and  that  the  action  of  rennin  is 
to  change  the  caseinogen  to  a  soluble  form,  termed  paracasein,  after 
which  the  lime  salts  present  react  with  the  paracasein  in  such  a 
manner  as  to  cause  it  to  assume  the  solid  condition.  Calcium 
phosphate  seems  to  be  the  natural  alkali  necessary  to  this  process,  for 
if  it  be  removed  by  dialysis,  or  precipitated  by  the  addition  of  potassium 
oxalate,  coagulation  does  not  take  place. 

(b)  Vitellin. — ^Mtellin  is  a  constituent  of  the  vitellis  or  yolk  of  eggs. 
It  differs  from  other  proteins  in  the  fact  that  it  is  semicrystalline 
in  character.  Though  usually  regarded  as  a  nucleo-protein  it  is 
not  definitely  known  whether  or  not  it  contains  phosphorus  in  its 
composition. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  17 

CONJUGATED  OR  COMBINED  PROTEINS. 

The  conjugated  proteins  are  compounds  in  which  the  protein  molecule  is 
combined  with  some  other  molecule  or  molecules,  the  chemic  nature  of 
which  varies  considerably  in  the  different  members  of  the  group,  e.g., 
coloring  matter,  carbohydrates  and  nuclein.  The  chemic  character  of 
the  non-protein  substance  furnishes  the  basis  for  the  following  classification: 

CHROMO-PROTEINS. 

(a)  Hemoglobin. — Hemoglobin  is  the  coloring  matter  of  the  red  cor- 
puscles, of  which  it  constitutes  about  30  per  cent,  of  the  total  weight. 
It  possesses  the  power  of  absorbing  oxygen  as  it  passes  through  the 
lung  capillaries  and  of  yielding  it  up  to  the  tissues  as  it  passes  through 
the  tissue  capillaries.  In  the  arterial  blood  it  is  known  as  oxy- 
hemoglobin, and  in  the  venous  blood  as  deoxy-  or  reduced-hemoglobin. 
When  hydrolysed  by  acids  or  alkalies,  hemoglobin  undergoes  a 
cleavage  into  a  protein,  globin,  and  a  pigment  hematin. 

(b)  Myohematin. — Myohematin  is  a  protein  supposed  to  be  present  in 
muscle.  It  has  never  been  isolated,  hence  its  chemic  features  are 
unknown.  Spectroscopic  examination  indicates  that  it  is  capable  of 
absorbing  and  again  yielding  up  oxygen.  For  this  reason  it  is  believed 
to  be  a  derivative  of  hemoglobin. 

GLUCO -PROTEINS. 

(a)  Mucin.— Mucin  is  the  protein  which  gives  the  mucus,  secreted  by 
the  epithelial  cells  of  the  mucous  membranes  and  related  glands, 
its  viscid,  tenacious  character.  It  is  also  a  constituent  of  the  inter- 
cellular substance  of  the  connective  tissues.  It  is  readily  precipitated 
by  acetic  acid.  When  heated  with  dilute  acids,  mucin  undergoes  a 
cleavage  into  a  simpler  protein  and  a  carbohydrate  termed  mucose, 
which  is  capable  of  reducing  Fehling's  solution. 

(b)  Mucoids. — The  mucoids  resemble  the  mucins  though  differing  from 
them  in  solubility  and  in  not  being  precipitable  from  alkaline  solutions 
by  acetic  acid.  They  are  found  in  the  vitreous  humor,  white  of  egg, 
cartilage,  and  in  other  situations.  They  dift'er  slightly  one  from  the 
other  in  properties  and  chemic  composition.  They  yield  on  decom- 
position a  carbohydrate. 

NUCLEO-PROTEINS. 

The  nucleo-proteins  are  obtained  from  the  nuclei  and  cell-substance 
of  tissue-cells.  Chemically  they  are  characterized  by  the  presence  of 
phosphorus  in  relatively  large  amounts.  When  hydrolysed,  they 
separate  into  a  protein  and  a  nuclein.  The  nucleins  derived  from 
cell  nuclei  can  be  still  further  separated  into  a  simpler  protein  and 
nucleic  acid,  which  latter  in  turn  yields  phosphoric  acid  and  the  so- 
called  purin  bases,  xanthin,  hypoxanthin,  adenin.  and  guanin.  All 
nucleins  which  yield  the  purin  bases  are  termed  true  nucleins. 

DERIVATIVES  OF  PROTEIN. 

The  protein  derivatives  include  a  variety  of  substances  which  arise 
through  a  process  of  hydrolysis  of  simple  proteins  under  the  action  of  enzymes 


i8  TEXT-BOOK  OF  PHYSIOLOGY. 

and  in  the  presence  of  acids  and  alkalies.  The  number  of  derivatives 
obtained  between  the  first  cleavage  of  the  protein  molecule  and  its  final 
cleavage  to  amino-acids  is  large  and  will  be  presented  at  length  in  the  para- 
graph relating  to  protein  digestion.     The  chief  derivatives  are  as  follows: 

INFRA-PROTEINS. 

{a)  Acid-albumin. — This  is  formed  when  a  native  albumin  is  digested 
with  dilute  hydrochloric  acid  (0.2  per  cent.)  or  dilute  sulphuric  acid 
for  some  minutes.  It  is  precipitated  by  neutralization  with  sodium 
hydroxid  (o.i  percent,  solution).  After  the  precipitate  is  washed, 
it  is  found  to  be  insoluble  in  distilled  water  and  in  neutral  saline  solu- 
tions. In  acid  solutions  it  is  not  coagulated  by  heat. 
.(6)  Alkali-albumin. — This  is  formed  when  a  native  albumin  is  treated 
with  a  dilute  alkali — e.g.,  o.i  per  cent,  of  sodium  hydroxid — for  five 
or  ten  minutes.  On  careful  neutralization  with  dilute  hydrochloric 
acid,  it  is  precipitated.  It  is  also  insoluble  in  distilled  water  and  in 
saline  solutions;  it  is  not  coagulable  by  heat. 

PROTEOSES,  PEPTONES  AND  POLYPEPTIDS. 

During  the  progress  of  the  digestive  process,  as  it  takes  place  in  the  stom- 
ach and  intestines,  there  is  produced  by  the  action  of  the  gastric  and  pan- 
creatic juices,  out  of  the  proteins  of  the  food,  a  series  of  new  proteins, 
known  as  proteoses,  peptones  and  polypeptids.  The  chemic  properties  of 
these  substances  will  be  considered  in  connection  with  the  process  of  digestion. 

COAGULATED  PROTEINS. 

Although  these  proteins  are  not  found  as  constituents  of  the  animal 
organism,  they  possess  much  interest  on  account  of  their  relation  to  prepared 
foods  and  to  the  digestive  process.  They  are  produced  when  solutions  of 
egg-albumin,  serum-albumin,  or  globulins  are  subjected  to  a  temperature 
of  100°  C.  or  to  the  prolonged  action  of  alcohol.  They  are  insoluble  in 
w^ater,  in  dilute  acids,  and  in  neutral  saline  solutions. 

In  this  same  group  may  be  included  also  those  coagulated  proteins 
which  are  produced  by  the  action  of  animal  ferments  on  soluble  proteins — 
e.g.,  fibrin,  myosin,  casein. 

(a)  Fibrin. — Fibrin  is  derived  from  one  of  the  blood  proteins — viz., 
fibrinogen.  It  is  not  present  under  normal  circumstances  in  the 
circulating  blood,  but  makes  its  appearance  after  the  blood  is  with- 
drawn from  the  vessels  and  at  the  time  of  coagulation.  It  can  also  be 
obtained  by  whipping  the  blood  with  a  bundle  of  twigs,  on  which  it 
accumulates.  When  freed  from  blood  by  washing  under  water,  it  is 
seen  to  consist  of  bundles  of  white  elastic  fibers  or  threads.  It  is  in- 
soluble in  water,  in  alcohol,  and  ether.  In  dilute  acids  it  swells,  be- 
comes transparent,  and  finally  is  converted  into  acid  albumin.  In 
dilute  alkalies  a  similar  change  takes  place,  but  the  resulting  pro- 
duct is  an  alkali-albumin.  Fibrin  possesses  the  property  of  decom- 
posing hydrogen  dioxid,  H2O2 — i-e.,  liberating  oxygen,  which  accu- 
mulates in  the  form  of  bubbles  on  the  fibrin.  On  incineration  fi- 
brin yields  an  ash  which  contains  calcium  phosphate  and  magnesium 
phosphate. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  19 

Two  views  are  held  as  to  the  origin  of  fibrin:  first  that  it  is  the  result 
of  the  action  of  a  special  enzyme,  termed  thrombin  on  fibrinogen, 
though  the  nature  of  the  action  is  not  very  clear;  second  that  it  is 
the  result  of  a  definite  combination,  physio-chemic  in  character,  of 
fibrinogen  with  thrombin  which,  however,  is  not  regarded  as  an  enzyme, 
inasmuch  as  it  is  not  destroyed  by  boiling,  but  a  definite  compound 
partaking  of  the  nature  of  an  organic  colloid.  The  amount  of  fibrin 
formed  from  fibrinogen  will  be  proportional  to  the  amount  of  thrombin 
present  (Howell). 

(b)  Myosin  fibrin  and  Myogen  fibrin  are  two  insoluble  proteins  developed 
out  of  the  two  chief  proteins  of  muscle  plasma.  Their  develop- 
ment after  death  is  believed  to  be  the  cause  of  the  stiffening  of  the 
muscles.  It  is  not  definitely  known  whether  this  is  the  result  of  the 
action  of  a  special  enzyme  or  not. 

(c)  Casein. — Casein  is  derived  from  the  chief  protein  of  milk — caseinogen 
— by  the  action  of  a  special  ferment  known  as  rennin  or  chymosin. 
This  ferment  is  a  constituent  of  gastric  juice. 

The  Color  Reactions  of  Proteins. — When  proteins  are  present  in 
solution,   they   may   be   detected   by   the   following   color  reactions — viz.: 

1.  Xanthoproteic.     The   solution   is   boiled   with   nitric   acid    for   several 

minutes,  when  the  protein  assumes  a  light  yellow  color.  After 
the  solution  has  cooled,  the  addition  of  ammonia  changes  the  color 
to  an  orange  or  amber-red,  due  to  the  presence  of  phenylalaninand 
ty  rosin. 

2.  The  rose-red   reaction.     The   solution  is  boiled  with  acid  nitrate  of 

mercury  (Millon's  reagent)  for  a  few  minutes,  when  the  coagulated 
protein  turns  a  purple-red  color.  This  color  is  attributed  to  the 
presence  of  tyrosin. 

3.  The  blue-violet  reaction.     A  few  drops  of  copper  sulphate  solution  are 

first  added  to  the  protein  solution,  and  then  an  excess  of  sodium 

hydroxid.     A  blue-violet  color  is  produced,  w^hich  deepens  somewhat 

on  heating,  but  no  further  change  ensues.     This  is  also  known  as 

.  Piotrowski's  test:     As  this  same  color  is  developed  with  the  substance 

biuret,  it  is  also  known  as  the  biuret  reaction.     Biuret  is  formed  by 

heating  urea  and  driving  off  ammonia. 

Precipitation   Tests. — ^Proteins   in    solution   may   be   precipitated   by 

nitric   acid,   acetic   acid   and   potassium   ferrocyanid,   picric   acid,    copper 

sulphate,  tannin,  alcohol,  etc.     As  stated  in  a  foregoing  paragraph,  certain 

of  the  proteins,  e.g.,  fibrinogen,  caseinogen  and  myosinogen,  will  undergo, 

by  the  action  of  an  animal  ferment  a  change  of  state  by  virtue  of  which 

they  become  solid.     To  this  process  the  term  ferment  coagulation  is  applied. 

The  solidification  of  proteins  by   the   action  of  heat   is   designated   heat 

coagulation. 

INORGANIC  CONSTITUENTS. 

The  inorganic  compounds  and  mineral  constituents  obtained  from  the 
solids  and  fluids  of  the  body  are  very  numerous,  and,  in  some  instances, 
quite  abundant.  Though  many  of  the  compounds  thus  obtained  are 
undoubtedly  derivatives  of  the  tissues  and  necessary  to  their  physical  and 


20  TEXT-BOOK  OF  PHYSIOLOGY. 

physiologic  activity,  others,  in  all  probability,  are  decomposition  products, 
or  transitory  constituents  introduced  with  the  food.  Of  the  inorganic 
compounds,  the  following  are  the  most  important: 

WATER. 

Water  is  the  most  important  of  the  inorganic  constituents,  as  it  is  in- 
dispensable to  life.  It  is  present  in  all  the  tissues  and  fluids  without  excep- 
tion, varying  from  99  per  cent,  in  the  saliva  to  80  per  cent,  in  the  blood,  75 
per  cent,  in  the  muscles  to  2  per  cent,  in  the  enamel  of  the  teeth.  The  total 
quantity  contained  in  a  body  weighing  75  kilograms  (165  pounds)  is  52.5 
kilograms  (115  pounds).  Much  of  the  water  exists  in  a  free  condition,  and 
forms  the  chief  part  of  the  fluids,  giving  to  them  their  characteristic  degree  of 
fluidity.  Possessing  the  capability  of  holding  in  solution  a  large  number  of 
inorganic  as  well  as  some  organic  compounds,  and  being  at  the  same  time 
diffusible,  it  renders  an  interchange  of  materials  between  all  portions  of  the 
body  possible.  It  aids  in  the  absorption  of  new  material  into  the  blood  and 
tissues,  and  at  the  same  time  it  transfers  waste'products  from  the  tissues  to 
the  blood,  from  which  they  are  finally  eliminated,  along  with  the  water  in 
which  they  are  dissolved.  A  portion  of  the  water  is  chemically  combined 
with  other  tissue  constituents  and  gives  to  the  tissues  their  characteristic 
physical  properties.  The  consistency,  elasticity,  and  pliability  are,  to  a 
large  extent,  conditioned  by  the  amount  of  water  they  contain.  The  total 
quantity  of  water  eliminated  by  the  kidneys,  lungs,  and  skin  amounts  to 
about  3  kilograms  (6|  pounds)  daily. 

CALCIUM  COMPOUNDS. 

Calcium  phosphate,  Ca3(POj2'  has  a  very  extensive  distribution 
throughout  the  body.  It  exists  largely  in  the  bones,  teeth,  and  to  a  slight  ex- 
tent in  cartilage,  blood,  and  other  tissues.  Milk  contains  0.27  per  cent. 
The  solidity  of  the  bones  and  teeth  is  almost  entirely  due  to  the  presence  of 
this  salt,  and  is,  therefore,  to  be  regarded  as  necessary  to  their  structure. 
It  enters  into  chemic  union  with  the  organic  matter,  as  shown  by  the  fact 
that  it  cannot  be  separated  from  it  except  by  chemic  means,  such  as  immer- 
sion in  hydrochloric  acid.  Though  insoulble  in  water,  it  is  held  in- solution 
in  the  blood  and  milk  by  the  protein  constituents,  and  in  the  urine  by  the 
acid  phosphate  of  soda.  The  total  quantity  of  calcium  phosphate  which 
enters  into  the  formation  of  the  body  has  been  estimated  at  2.5  kilograms. 
The  amount  eliminated  daily  from  the  body  has  been  estimated  at  0.4  gm., 
a  fact  which  indicates  that  nutritive  changes  do  not  take  place  with  much 
rapidity  in  those  tissues  in  which  it  is  contained. 

Calcium  carbonate,  CaCOg,  is  present  in  practically  the  same  situa- 
tions in  the  body  as  the  phosphate,  and  plays  essentially  the  same  r61e.  It 
is,  however,  found  in  the  crystalline  form,  aggregated  in  small  masses  in  the 
internal  ear,  forming  the  otoliths,  or  ear  stones.  Though  insoluble,  it  is  held 
in  solution  by  the  carbonic  acid  diffused  through  the  fluids. 

Calcium  fluorid,  CaF,,  is  found  in  bones  and  teeth. 
SODIUM  COMPOUNDS. 

Sodium  chlorid,  XaCl,  is  present  in  all  the  tissues  and  fluids  of  the 
body,  but  especially  in  the  blood,  0.6  per  cent.,  lymph,  0.5,  and  pancreatic 
juice,  0.25  per  cent.     The  entire  quantity  in  the  body  has  been  estimated 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY.  21 

at  about  200  gm.  Sodium  chlorid  is  of  much  importance  in  the  body  as  it 
determines  and  regulates  to  a  large  extent  the  phenomena  of  diffusion 
which  are  there  constantly  taking  place.  The  ingested  water  is  absorbed 
into  the  blood  largely  in  consequence  of  the  percentage  of  this  salt  which  it 
contains.  The  normal  percentage  of  sodium  chlorid  in  the  blood-plasma 
assists  in  maintaining  the  shape  and  structure  of  the  red  blood-corpuscles 
by  determining  the  amount  of  water  entering  into  their  composition.  The 
same  is  true  of  other  tissue  elements. 

Sodium  chlorid  also  influences  favorably  the  general  nutritive  process, 
though  the  manner  in  which  it  acts  is  not  very  clear.  During  its  existence 
in  the  body  it  undergoes  chemic  transformations  or  decompositions,  yielding 
its  chlorin  to  form  the  potassium  chlorid  of  the  blood-corpuscles  and  muscles 
and  to  form  the  hydrochloric  acid  of  the  gastric  juice. 

Sodium  phosphate,  Na2HP04,  is  found  in  all  solids  and  fluids  of  the 
body,  to  which,  with  but  few  exceptions,  it  imparts  an  alkaline  reaction. 
This  is  especially  true  of  blood,  lymph,  and  tissue  fluids  generally.  It  is 
essential  to  physiologic  action  that  all  tissue  elements  should  be  bathed  by 
an  alkaline  medium. 

Sodium  carbonate,  Na^COg,  is  generally  found  in  association  with  the 
preceding  salt.  As  it  is  an  alkaline  compound,  it  also  assists  in  giving  to 
the  blood  and  lymph  their  characteristic  alkalinity.  In  carnivorous  animals 
the  sodium  phosphate  is  the  more  abundant,  while  in  the  herbivorous  animals 
the  sodium  carbonate  is  the  more  abundant. 

Sodium  sulphate,  Na,SO^,  is  present  in  many  of  the  tissues  and  fluids, 
especially  in  the  urine.  Though  introduced  in  the  food,  it  is  also,  in  all 
probability,  formed  in  the  body  from  the  decomposition  and  oxidation  of 
the  proteids. 

POTASSIUM  COMPOUNDS. 

Potassium  chlorid,  KCl,  is  met  with  in  association  with  sodium  chlorid 
in  almost  all  situations  in  the  body.  It  preponderates,  however,  in  the 
tissue  elements,  especially  in  the  muscle  tissue,  ner\'e  tissue,  and  red  cor- 
puscles. The  plasma  with  which  these  structures  are  bathed  contains  but  a 
very  small  amount  of  this  salt,  but,  as  previously  stated,  a  relatively  large 
quantity  of  sodium  chlorid.  Though  introduced  to  some  extent  in  the  food, 
it  is  very  Hkely  that  it  is  also  formed  through  the  decomposition  of  the  so- 
dium chlorid. 

Potassium  phosphate,  KjHPO^,  is  found  in  association  with  sodium 
phosphate  in  all  the  fluids  and  solids.  As  it  has  similar  chemic  properties, 
its  functions  are  practically  the  same. 

Potassiimi  carbonate,  K2CO3,  is  generally  found  with  the  preceding 
salt. 

MAGNESIUM  COMPOUNDS. 

Magnesium  phosphate,  Mg3(POj2>  is  found  in  all  tissues,  in  associa- 
tion with  calcium  phosphate,  though  in  much  smaller  quantity. 

Magnesium  carbonate,  MgCOg,  occurs  only  in  traces  in  the  blood. 

IRON  COMPOUNDS. 

Iron  is  a  constituent  of  the  coloring-matter  of  the  blood.  Traces,  how- 
ever, are  also  found  in  lymph,  bile,  gastric  juice,  and  in  the  pigment  of  the 


22  TEXT-BOOK  OF  PHYSIOLOGY. 

eyes,  skin  and  hair.  The  amount  of  iron  contained  in  a  body  weighing  70 
kilograms  is  about  2.2  gm.  It  exists  under  various  forms — e.g.,  ferric  oxid, 
and  in  combination  with  organic  compounds. 

Chemic  analysis  thus  shows  that  the  chemic  elements  into  which  the 
compounds  may  be  resolved  by  an  ultimate  analysis  do  not  exist  in  the 
body  in  a  free  state,  but  only  in  combination,  and  in  characteristic  pro- 
portions, to  form  compounds  whose  properties  are  the  resultant  of  those  of 
the  elements.  Of  the  four  principal  elements  which  make  up  97  per  cent, 
of  the  body,  O,  H,  N  are  extremely  mobile,  elastic,  and  possessed  of  great 
atomic  heat.  C,  H,  N  are  distinguished  for  the  narrow  range  of  their 
affinities,  and  for  their  chemic  inertia.  C  possesses  the  great  atomic  cohe- 
sion.    O  is  noted  for  the  number  and  intensity  of  its  combinations. 

As  the  properties  of  th  compounds  formed  by  the.  union  of  elements 
must  be  the  resultants  of  the  properties  of  the  elements  themselves,  it  follows 
that  the  ternary  compounds,  starches,  sugars,  and  fats  must  possess  more 
or  less  inertia,  and  at  the  same  time  instability;  while  in  the  more  complex 
proteids,  in  which  sulphur  and  phosphorus  are  frequently  combined  with 
the  four  principal  elements,  molecular  instability  attains  its  maximum. 
As  all  the  foregoing  compounds  possess  in  varyyig  degrees  the  properties  of 
inertia  and  instability,  it  follows  that  living  matter  must  possess  correspond- 
ing properties,  and  the  capabiHty  of  undergoing  unceasingly  a  series  of 
chemic  changes,  both  of  composition  and  decomposition,  in  response  to  the 
chemic  and  physical  influences  by  which  it  is  surrounded,  and  which  underlie 
all  the  phenomena  of  life. 

PRINCIPLES  OF  DISSIMILATION. 

In  addition  to  the  previously  mentioned  compounds — ^viz.,  carbo- 
hydrates, fats,  proteids,  and  inorganic  salts — there  is  obtained  by  chemic 
analysis  from  the  tissues  and  fluids  of  the  body: 

1.  A  number  of  organic  acids,  such  as  acetic,  lactic,  oxalic,  butyric,  pro- 
pionic, etc.,  in  combination  with  alkaline  and  earthy  bases. 

2.  Organic  compounds,  such  as  alcohol,  glycerin,  cholesterin. 

3.  Pigments,  such  as  those  found  in  bile  and  urine. 

4.  Crystallizable  nitrogenized  bodies,   such  as  urea,  uric  acid,   xanthin, 
hippuric  acid,  creatin,  creatinin,  etc. 

While  some  few  of  these  compounds  may  possibly  be  regarded  as  neces- 
sary to  the  physiologic  integrity  of  the  tissues  and  fluids,  the  majority  of 
them  are  to  be  regarded  as  products  of  dissimilation  of  the  tissues  and  foods 
in  consequence  of  functional  activity,  and  represent  stages  in  their  reduction 
to  simpler  forms  previous  to  being  eliminated  from  the  body. 


CHAPTER  III. 
PHYSIOLOGY  OF  THE  CELL. 

A  histologic  analysis  of  the  organs  and  tissues  of  the  animal  body  shows 
that  they  can  be  resolved  into  ultimate  elements,  termed  cells,  which  may, 
therefore,  be  regarded  as  the  primary  units  of  structure.  Though  cells 
vary  considerably  in  shape,  size,  and  chemic  composition  in  the  different 
tissues  of  the  adult  body,  they  are,  nevertheless,  descendants  from  typical 
cells,  known  as  embryonic  or  undifferentiated  cells,  the  first  offspring  of  the 
fertilized  ovum.  Ascending  the  line  of  embryonic  development,  it  will  be 
found  that  every  organized  body  originates  in  a  single  cell — the  ovum.  As 
the  cell  is  the  elementary  unit  of  all  tissues,  the  function  of  each  tissue  must 
be  referred  to  the  function  of  the  cell.  Hence  the  cell  may  be  defined  as  the 
primary  anatomic  and  physiologic  unit  of  the  organic  world,  to  which  every 
exhibition  of  life,  whether  normal  or  abnormal,  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and  internal 
structure  in  different  portions  of  the  body,  a  typical  cell  may  be  said  to  con- 
sist mainly  of  a  gelatinous  substance  forming  the  body  of  the  cell,  termed 
cytoplasm  or  bioplasm,  in  which  is  embedded  a  smaller  spheric  body,  the 
nucleus.  Within  the  nucleus  there  is  frequently  seen  a  still  smaller  body, 
the  nucleolus.  The  shape  of  the  adult  cell  varies  according  to  the  tissue  in 
which  it  is  found;  when  young  and  free  to  move  in  a  fluid  medium,  the  cell 
assumes  a  spheric  form,  but  when  subjected  to  pressure,  may  become 
cylindric,  fusiform,  polygonal,  or  stellate.  Cells  vary  in  size  within  wide 
limit,  ranging  from  7.7/f  (ttoit  of  an  inch,  the  diameter  of  a  red  blood- 
corpuscle),  to  135/Z  (^-^-Q-  of  an  inch,  the  diameter  of  the  large  cells  in  the 
gray  matter  of  the  spinal  cord).     (See  Fig.  i.) 

The  cytoplasm  consists  of  a  soft,  semifluid,  gelatinous  material,  varying 
somewhat  in  appearance  in  different  tissues.  Though  frequently  homogene- 
ous, it  often  exhibits  a  finely  granular  appearance  under  medium  powers 
of  the  microscope.  Young  cells  consist  almost  entirely  of  clear  cytoplasm. 
Mature  cells  contain,  according  to  the  tissue  in  which  they  are  found, 
material  of  an  entirely  different  character — e.g.,  small  globules  of  fat, 
granules  of  glycogen,  mucigen,  pigments,  digestive  ferments,  etc.  Under 
high  powers  of  the  microscope  the  cytoplasm  is  found  to  be  pen-adedby 
a  network  of  fibers,  termed  spongio plasm,  in  the  meshes  of  which  is  con- 
tained a  clearer  and  more  fluent  substance,  the  hyaloplasm.  The  relative 
amount  of  these  two  constituents  varies  in  different  cells,  the  proportion  of 
hyaloplasm  being  usually  greater  in  young  cells.  The  arrangement  of  the 
fibers  forming  the  spongioplasm  also  varies,  the  fibers  having  sometimes  a 
radial  direction,  in  others  a  concentric  disposition,  but  most  frequently  being 
distributed  evenly  in  all  directions.  In  many  cells  the  outer  portion  of 
the  cell  protoplasm  undergoes  chemic  changes  and  is  transformed  into  a 
thin,    transparent,    homogeneous    membrane — the    cell   membrane — which 

23 


24 


TEXT-BOOK  OF  PHYSIOLOGY. 


completely  incloses  the  cell  substance.  The  cell  membrane  is  permeable 
to  water  and  watery  solutions  of  various  inorganic  and  organic  substances. 
It  is,  however,  not  an  essential  part  of  the  cell. 

The  nucleus  is  a  small  vesicular  body  embedded  in  the  cytoplasm  near 
the  center  of  the  cell.  In  the  resting  condition  of  the  cell  it  consists  of  a 
distinct  membrane,  composed  of  amphipyrenin,  inclosing  the  nuclear  con- 
tents. The  latter  consists  of  a  homogeneous  amorphous  substance — the 
nuclear  matrix — in  which  is  embedded  the  nuclear  network.  It  can  often 
be  seen  that  a  portion  of  one  side  of  the  nucleus,  called  the  pole,  is  free  from 
this  network.  The  main  cords  of  the  network  are  arranged  as  V-shaped 
loops  about  it.  These  main  cords  send  out  secondary  branches  or  twigs, 
which,  uniting  with  one  another,  complete  the  network.  The  nuclear  cords 
are  composed  of  granules  of  chromatin — so  called  because  of  its  affinity  for 


Nuclear  membrane.    » 


Linin. 


Nuclear  fluid  (matrix). 


Nucleolus. 


Chromatin-cords 
(nuclear  network). 


Nodal  enlargements  . 
of  the  chromatin. 


"  ■  "  ■  "Cell  membrane. 


Exoplasm. 

Microsomes. 

Centrosoma. 

Spongioplasm. 
Hyaloplasm. 


Foreign  inclosures. 
Fig.  I. — DiAGR.'\M  of  a  Cell.     Microsomes  and  spongioplasm  are  only  partly  drawn. — (Slokr.) 

certain  staining  materials — held  together  by  an  achromatin  substance 
known  as  limn.  Besides  the  nuclear  network,  there  are  embedded  in  the 
nuclear  matrix  one  or  more  small  bodies  composed  of  pyrenin,  known  as 
nucleoli.  At  the  pole  of  the  nucleus,  either  within  or  just  without  in  the 
cytoplasm,  is  a  small  body,  the  centrosome,  or  pole  corpuscle. 

Chemic  Composition  of  the  Cell. — The  composition  of  living  bioplasm 
is  difficult  of  determination,  for  the  reason  that  all  chemic  and  physical 
methods  employed  for  its  analysis  destroy  its  vitality,  and  the  products 
obtained  are  pecuHar  to  dead  rather  than  to  living  matter.  Moreover,  as 
bioplasm  is  the  seat  of  extensive  chemic  changes,  it  is  not  easy  to  determine 
whether  the  products  of  analysis  are  crude  food  constituents  or  cleavage  or 
disintegration  products.  Nevertheless,  chemic  investigations  have  shown 
that  even  in  the  living  condition  bioplasm  is  a  highly  complex  compound — 
the  resultant  of  the  intimate  union  of  many  different  substances.  About 
75  per  cent,  of  bioplasm  consists  of  water  and  25  per  cent,  of  solids,  of 
which  the  more  important  compounds  are  various  nucleo-proteins  (char- 
acterized by  their  large  percentage  of  phosphorus),  globulins,  lipoids,  such 
as  lecithin  fa  phosphorized  fat)  and  cholesterin  (a  mo'natomic  alcohol)  and 


PHYSIOLOGY  OF  THE  CELL.  25 

possibly  fat  and  carbohydrates.  Inorganic  salts,  especially  the  potassium, 
sodium,  and  calcium  chlorids  and  phosphates,  are  almost  invariable  and 
essential  constituents. 

MANIFESTATIONS  OF  CELL  LIFE. 

Growth,  the  Maintenance  of  Nutrition,  and  Reproduction. — All 

cells  exhibit  three  fundamental  properties  of  life — viz.,  growth,  the  mainte- 
nance of  their  nutrition,  and  reproduction.  Growth  is  an  increase  in  size. 
When  newly  reproduced  all  cells  are  extremely  small,  but  in  consequence  of 
their  organization  and  the  character  of  their  surrounding  medium,  they 
gradually  grow  until  they  attain  the  size  characteristic  of  the  adult  state. 

Nutrition  may  be  defined  as  the  sum  of  the  processes  concerned  in  the 
maintenance  of  the  physiologic  condition  of  the  cell  and  includes  both  growth 
and  repair.  So  long  as  this  is  accomplished,  the  cells  and  the  tissues  which 
are  formed  by  them  continue  to  exhibit  their  functions  or  their  characteristic 
modes  of  activity.  Both  growth  and  nutrition  are  dependent  on  the  power 
which  living  material  possesses  of  not  only  absorbing  nutritive  material  from 
the  surrounding  medium,  the  lymph,  but  of  subsequently  assimilating  it, 
organizing  it,  transforming  it  into  material  like  itself  and  endowing  it  with 
its  own  physiologic  properties. 

In  the  physiologic  condition  the  living  material  of  the  cell,  the  bioplasm, 
is  the  seat  of  a  series  of  chemic  changes  which  vary  in  degree  from  moment  to 
moment  in  accordance  with  the  degree  of  functional  activity,  and  on  the 
continuance  of  which  all  life  phenomena  depend.  Some  of  these  chemic 
changes  are  related  to  or  connected  with  the  molecules  of  the  living  material, 
while  others  are  connected  with  the  food  material  supplied  to  them.  Of 
the  chemic  changes  occurring  within  the  molecules  some  are  destructive, 
dissimilative  or  disintegrative  in  character,  whereby  the  molecule  is  in  part 
eventually  reduced  through  a  series  of  descending  chemic  stages  to  simpler 
compounds  which,  apparently  of  no  use  in  the  cell,  are  eliminated  from  it. 
It  is,  therefore,  said  that  the  living  material  undergoes  molecular  disintegra- 
tion as  a  result  of  functional  activity.  To  these  changes  the  term  katabolism 
is  also  applied.  Other  of  these  changes  are  constructive,  assimilative  or 
integrative  in  character,  whereby  a  part  at  least  of  the  food  material  furnished 
by  the  blood  plasma  is  transformed  through  a  series  of  ascending  chemic 
stages  into  living  material,  and  whereby  it  is  repaired  and  its  former  physio- 
logic condition  restored.  It  is,  therefore,  said  that  the  living  material  under- 
goes molecular  integration  as  a  preparation  for  functional  activity.  To  these 
changes  the  term  anaholism  is  also  applied.  During  the  course  of  its  physio- 
logic activities  the  cell  bioplasm  produces  materials  of  an  entirely  different 
character  which  vary  with  the  cell,  such  as  fat,  glycogen,  mucigen,  pigments, 
ferments,  etc.,  which  are  generally  spoken  of  as  metabolic  products. 

Living  material  has  also  a  temperature  varying  in  degree  in  different 
species  of  animals  as  well  as  in  different  parts  of  the  same  animal.  Here  as 
elsewhere  the  temperature  is  due  to  heat  liberated  from  organic  compounds 
through  disruption  and  subsequent  oxidation  to  simpler  compounds. 
Though  some  of  the  heat  liberated  may  come  from  the  tissue  molecules,  the 
larger  part  by  far  comes  from  the  food  molecules — sugar,  fat,  and  protein. 


26  TEXT-BOOK  OF  PHYSIOLOGY. 

constituents  of  the  fluids  circulating  in  the  tissue  spaces.  These  foods  carry 
into  the  body  potential  energy,  ultimately  derived  from  the  sun.  When  they 
are  disrupted  and  oxidized  the  potential  energy  is  transformed  into  kinetic 
energy  which  manifests  itself  for  the  most  part  as  heat.  To  the  sum  total 
of  all  the  chemic  changes  occurring  in  tissues  and  foods  the  term  metabolism 
is  given. 

There  is,  however,  much  difference  of  opinion  as  to  the  extent  to  which 
the  living  material  is  metabolized  and  to  the  actual  disposition  of  the  food 
materials,  and  especially  the  proteins.  Thus  Voit  contended  that  the  tissue 
molecules  are  comparatively  stable  in  composition  and  under  ordinary  con- 
ditions of  nutrition  do  not  undergo  any  material  change  during  either  rest 
or  activity,  and  that  metabolism  is  confined  to  the  food  materials  occupying 
spaces  in  and  around  the  living  cell.  The  cause  which  initiates  this  metab- 
olism is  unknown,  but  is  supposed  to  reside  in  the  cell,  if  it  is  not  a  property 
of  the  cell  itself.  Because  of  the  fact  that  but  a  very  small  amount,  if  any, 
of  sugar  or  fat  enters  into  the  composition  of  bioplasm  it  is  generally  admitted 
that  these  foods  are  metabolized  in  the  tissue  spaces  and  in  the  manner  just 
alluded  to.  The  problem,  however,  is  different  in  the  case  of  the  proteins. 
Voit  contended,  as  previously  stated,  that  the  proteins  of  the  tissue  molecules, 
which  he  distinguished  as  tissue  proteins,  do  not  metabolize  and  confined  all 
protein  metabolism  to  the  food  proteins  circulating  in  the  tissue  spaces  and 
which  he  distinguished  as  circulating  proteins.  Even  in  starvation  the  tissue 
proteins,  as  such,  do  not  metabolize  until  they- have  been  dissolved  in  con- 
sequence of  chemic  changes  and  transformed  into  circulating  protein. 

PflUger,  however,  asserted  that  the  circulating  proteins  cannot  be  metab- 
olized but  that  they  must  first  be  built  up  into  tissue  proteins.  The  metab- 
olism of  protein  is,  therefore,  confined,  in  this  view,  to  the  molecules  of 
the  living  material.  It  is  possible,  however,  that  both  views  are  correct  and 
that  in  the  physiologic  condition  the  activity  of  the  tissues  is  attended  by  a 
partial  destruction  of  the  tissue  molecules  which  is  followed  in  turn  by  con- 
struction during  the  subsequent  rest,  but  that  the  greater  part  of  the  protein 
metabolism  takes  place  outside  the  cell,  though  in  contact  with  it. 

Though  the  cell  is,  therefore,  the  seat  of  two  opposing  processes,  assimila- 
tion and  dissimilation,  it  retains  under  normal  conditions  an  average  physio- 
logic state,  and  so  long  as  this  is  the  case  it  is  in  a  condition  of  mitritive 
equilibrium  and  capable  of  performing  its  various  functions. 

Though  the  foregoing  statements  are  applied  to  the  individual  cell  they 
are  equally  applicable  to  the  body  as  a  whole,  inasmuch  as  the  organs  and 
tissues  of  which  it  consists  are  composed  of  cells.  The  body  grows  in  size 
and  maintains  its  nutrition,  by  the  introduction  of  food  materials  which  are 
utilized  in  part,  for  the  repair  of  the  tissues  which  have  undergone  molecular 
disintegration  in  consequence  of  activity,  and  in  part  for  the  liberation  of 
energy.  As  a  result  of  the  disintegration  or  the  metabolism  of  tissue  and 
food  materials,  products  such  as  carbon  dioxid,  urea,  etc.,  are  formed  which, 
apparently  of  no  further  use,  are  discharged  from  the  body  by  eliminating 
organs  as  the  kidney,  lungs,  skin,  etc.  Assimilation  and  dissimilation  are 
constantly  taking  place.  If  the  food  assimilated  and  metabolized  exactly 
replaces  the  tissues  dissimilated  and  the  food  metabolized  the  body  will  retain 
a  .condition  of  nutritive  equilibrium. 


PHYSIOLOGY  OF  THE  CELL. 


27 


Reproduction. — Cells  reproduce  themselves  in  the  higher  animals  in 
two  ways — by  direct  division  and  by  indirect  division,  or  karyokinesis.  In 
the  former  the  nucleus  becomes  constricted,  and  divides  without  any  special 
grouping  of  the  nuclear  elements.  It  is  probable  that  this  occurs  only  in 
disintegrating  cells,  and  never  in  a  physiologic  multiplication.  In  division 
by  karyokinesis  (Fig.  2)  there  is  a  progressive  rearranging  and  definite 
grouping  of  the  nucleus,  the  result  of  which  changes  is  the  division  of  the 
centrosome,  the  chromatin,  and  the  rest  of  the  nucleus  into  two  equal  por- 
tions, which  form  the  nuclei.  Following  the  division  of  the  nuclei,  the 
protoplasm  divides.     The  process  may  be  divided  into  three  phases: 


Close  Skein 
(viewed  from 
the  side). 
Polar  field. 


Loose  Skein  (viewed 
from  above — i.  e.,  from 

the  pole).  Mother  Stars  (viewed  from  the  side). 


'  ^'  ■: 


Polar 
radia- 
JS        tion. 

-V  =~    Spindle. 


*     f 


^>^ 


Mother  Star  (viewed      Daughter  Star         Begmnmg  Completed 

from  above).  Division  of  the  Protoplasm. 

Fig.  2. — Karyokinetic  Figures  Observed  in  the  Epithelium  of  the  Oral  Cavity  of 
A  Salamander.  The  picture  in  the  upper  right-hand  corner  is  from  a  section  through  a  dividing 
egg  of  Siredon  pisciformis.  Neither  the  centrosomes  nor  the  first  stages  of  the  development  of 
the  spindle  can  be  seen  by  this  magnification.      X560. — (Stdhr.) 

I.  Prophase. — The  centrosome,  at  first  small  and  lying  within  the  nucleus, 
increases  in  size  and  moves  into  the  protoplasm,  where  it  lies  near  the 
nucleus,  surrounded  by  a  clear  zone,  from  which  delicate  threads 
radiate  through  an  area  known  as  the  attraction  sphere.  The  nucleus 
enlarges  and  becomes  richer  in  chromatin.  The  lateral  twigs  of  the 
chromatin  cords  are  drawn  in,  while  the  main  cords  become  much 
contorted.  These  cords  have  a  general  direction  transverse  to  the  long 
axis  of  the  cell,  and  parallel  to  the  plane  of  future  cleavage.  They  are 
seen  as  V-shaped  segments  or  loops,  chromosomes,  having  their  closed 
ends  directed  toward  a  common  center,  the  polar  field,  while  the  other 
ends  interdigitate  on  the  opposite  side  of  the  nucleus — the  antipole. 
Th  polar  field  corresponds  to  the  area  occupied  by  the  centrosome.  This 
arrangement  is  known  as  the  close  skein;  but  as  the  process  goes  on,  the 
chromosomes  become  thicker,  shorter  and  less  contorted,  producing  a 
much  looser  arrangement,  known  as  the  loose  skein.  During  the 
formation  of  the  loose  skein,  the  centrosome  divides  into  two  portions. 


28  TEXT-BOOK  OF  PHYSIOLOGY. 

which  move  apart  to  positions  at  the  opposite  ends  of  the  long  axis  of 
the  nucleus.  At  the  same  time  delicate  achromatin  fibers  make  their 
appearance,  arranged  in  the  form  of  a  double  cone,  the  apices  of  which 
correspond  in  position  to  the  centrosomes.  This  is  known  as  the 
■nuclear  spindle.  During  the  prophase  the  nuclear  membrane  and  the 
nucleoli  disappear. 

2.  The  Metaphase. — The  two  centrosomes  are  at  opposite  ends  of  the  long 

axis  of  the  nucleus,  each  surrounded  by  an  attraction  sphere,  now 
called  the  polar  radiation.  The  chromosomes  become  yet  shorter  and 
thicker,  and  move  toward  the  equator  of  the  nucleus,  where  they  lie 
with  their  closed  ends  toward  the  axis,  presenting  the  appearance, 
when  seen  from  the  poles,  of  a  star — the  so-called  mother  star,  or  mon- 
aster. While  moving  toward  the  equator  of  the  nucleus,  and  often 
earlier,  each  chromosome  undergoes  longitudinal  cleavage,  the  sister 
loops  remaining  together  for  a  time.  Upon  the  completion  of  the 
monaster,  one  loop  of  each  pair  passes  to  each  pole  of  the  nucleus, 
guided,  and  perhaps  drawn  by  the  threads  of  the  nuclear  spindle.  The 
separation  of  the  sister  segments  begins  at  their  apices,  and  as  the  open 
ends  are  drawn  apart  they  remain  connected  by  delicate  achromatin 
filaments  drawn  out  from  the  chromosomes.  This  separation  of  the 
daughter  chromosomes,  and  their  movement  toward  the  daughter 
centrosomes,  is  called  metakinesis.  As  they  approach  their  destination, 
we  have  the  appearance  of  two  stars  in  the  nucleus — the  daughter  stars, 
or  diasters. 

3.  Anaphase.— The  daughter  stars  undergo,  in  reverse  order,  much  the 

same  changes  that  the  mother  star  passed   through.     The  chromo- 
somes become  much  convoluted,  and  perhaps  united  to  one  another, 
the  lateral  twigs  appear,  and  the  chromatin  resumes  the  appearance 
of  the  resting  nucleus.     The  nuclear  spindle,  with  most  of  the  polar 
radiation,   disappears,   and  the  nucleoli  and  the  nuclear  membrane 
reappear,   thus  forming  two  complete  daughter  nuclei.     Meanwhile 
the  protoplasm  becomes  constricted  midway  between  the  young  nuclei. 
This  constriction  gradually  deepens  until  the  original  cell  is  divided, 
with  the  formation  of  two  complete  cells. 
Physiologic  Properties  of  Bioplasm. — All  living  bioplasm  possesses 
properties  which  serve  to  distinguish  and  characterize  it — ^viz.,  irritability, 
conductivity,  and  motility. 

Irritability ,  or  the  power  of  reacting  in  a  definite  manner  to  some  form 
of  external  excitation,  whether  mechanic,  chemic,  or  electric,  is  a  funda- 
mental property  of  all  living  bioplasm.  The  character  and  extent  of  the 
reaction  will  vary,  and  will  depend  both  on  the  nature  of  the  bioplasm  and 
the  character  and  strength  of  the  stimulus.  If  the  bioplasm  be  muscle, 
the  response  will  be  a  contraction;  if  it  be  gland,  the  response  will  be  a 
secretion;  if  it  be  nerve,  the  response  will  be  a  sensation  or  some  other  form 
of  nerve  activity. 

Conductivity,  or  the  power  of  transmitting  molecular  disturbances  arising 
at  one  point  to  all  portions  of  the  irritable  material,  is  also  a  characteristic 
feature  of  all  bioplasm.  This  power,  however,  is  best  developed  in  that  form 
of  bioplasm  found  in  nerves,  which  serv^es  to  transmit,  with  extreme  rapidity. 


PHYSIOLOGY  OF  THE  CELL.  29 

molecular  disturbances  arising  at  the  periphery  to  the  brain,  as  well  as  from 
the  brain  to  the  periphery.  Muscle  bioplasm  also  possesses  the  same  power 
in  a  high  degree. 

Motility,  or  the  power  of  executing  apparently  spontaneous  movements, 
is  exhibited  by  many  forms  of  cell  bioplasm.  In  addition  to  the  molecular 
movements  which  take  place  in  certain  cells,  other  forms  of  movement  are 
exhibited,  more  or  less  constantly,  by  many  cells  in  the  animal  body — e.g., 
the  waving  of  cilia,  the  ameboid  movements  and  migrations  of  white  blood- 
corpuscles,  the  activities  of  spermatozooids,  the  projection  of  pseudopodia, 
etc.  These  movements,  arising  without  any  recognizable  cause,  are  fre- 
quently spoken  of  as  spontaneous.  Strictly  speaking,  however,  all  proto- 
plasmic movement  is  the  resultant  of  natural  causes,  the  true  nature  of 
which  is  beyond  the  reach  of  present  methods  of  investigation. 


CHAPTER  IV 


HISTOLOGY  OF  THE  EPITHELIAL  AND  CONNECTIVE  TISSUES. 

I.  EPITHELIAL  TISSUE. 

The  epithelial  tissue  consists  of  one  or  more  layers  of  cells  resting  on  a 
homogeneous  membrane,  the  other  side  of  which  is  abundantly  supplied 
with  blood-vessels  and  nerves.  The  form  of  the  epithelial  cell  varies  in 
different  situations,  and  may  be  flattened,  cuboid,  spheroid  or  columnar. 
(See  Figs.  4,  5,  and  6.)  The  form  of  the  cell  in  all  instances  is  related  to 
some  specific  function.  When  arranged  in  layers  or  strata,  the  cells  are 
cemented  together  by  an  intercellular  substance. 

The  epithelial  tissue  forms  a  continuous  covering  for  the  surfaces  of  the 
body.  The  external  investment  (the  skin)  and  the  internal  investment  (the 
mucous  membrane,  which  lines  the  entire  ahmentary  canal  as  well  as  as- 
sociated body  cavities)  are  both  formed,  in  all  situations,  by  the  homogeneous 
basement  membrane,  covered  with  one  or  more  layers  of  cells.     The  glands 


Fig.  3. — Epithelial  Cells  of  Rabbit,  Isolated.  X  560.  i.  Squamous  cells  (mucous 
membrane  of  mouth).  2.  Columnar  cells  (corneal  epithelium).  3.  Columnar  cells,  with  cuticular 
border,  5  (intestinal  epithelium).     4.  Ciliated  cells;  h,  cilia  (bronchial  epithelium). — {Stohr.) 

of  the  skin,  the  lungs  and  the  glands  in  connection  with  the  alimentary  canal 
and  the  uro-genital  apparatus  are  formed  of  the  same  elemental  structures. 
All  materials,  therefore,  whether  nutritive,  secretory,  or  excretory,  must  pass 
through  epithelial  cells  before  they  can  enter  into  the  formation  of  the  blood 
or  be  eliminated  from  it.  The  nutrition  of  the  epithelial  tissue  is  maintained 
by  the  nutritive  material  derived  from  the  blood  diffusing  itself  into  and 
through  the  basement  membrane.  Chemically,  the  epithelial  cells  of  the 
epidermis— hair,  nails,  etc. — are  composed  of  a  sclero-protein  (keratin),  a 
small  quantity  of  water,  and  inorganic  salts.  In  other  situations,  especially 
on  the  mucous  membranes,  the  cells  consist  largely  of  mucin,  in  association 
with  other  proteins.  The  consistency  of  epithelium  varies  in  accordance  with 
external  influences,  such  as  the  presence  or  absence  of  moisture,  pressure, 
friction,  etc.  This  is  well  seen  in  the  skin  of  the  palms  of  the  hands  and  the 
soles  of  the  feet — situations  where  it  acquires  its  greatest  density.  In  the 
ahmentary  canal,  in  the  lungs,  and  in  other  cavities,  where  the  reverse  condi- 
tions prevail,  the  epithelium  is  extremely  soft.     Epithelial  tissues  also  possess 

30 


THE  COXXECTR'E  TISSUES. 


31 


varying  degrees  of  cohesion  and  elasticity — physical  properties  which  enable 
them  to  resist  considerable  pressure  and  distention  without  hax-ing  their 
physiologic  integrity  destroyed.  Inasmuch  as  these  tissues  are  poor  con- 
ductors of  heat,  they  assist  in  preventing  too  rapid  radiation  of  heat  from 
the  body,  and  cooperate  with  other  mechanisms  in  maintaining  the  normal 
temperature.  The  physiologic  activity  of  all  epithelial  tissue  depends  on 
a  due  supply  of  nutritive  material  derived  from  the  blood,  which  not 
only  maintains  its  nutrition,  but  affords  those  materials  out  of  which  are 
formed  the  secretions  of  the  glands,  whether  of  the  skin  or  mucous 
membrane. 

The     cells    lining    the    blood- 
^  vessels,  the  lymph-vessels,  the  peri- 

^5  toneal,    pleural,    pericardial,    and 

i  other   closed    cavities    are  usuallv 


3- 


3-¥ 


Fig.  4. —  Stratified  Squ.\mox:s 
Epithelium  (Laryxx  of  Max). 
X  240.  I.  Columnar  cells.  2.  Prickle- 
cells.     3.  Squamous  cells. — (Siohr.) 


Fig.  5. —  Stratified  Ciliated 
Epithelhtm.  X  560.  From  the  res- 
piratory nasal  mucous  membrane 
of  man.  i.  Oval  cells.  2.  Spindle- 
shaped  cells.  3.  Columnar  cells. — 
(Stohr.) 


termed  endothelial  cells.     The  cells  in  these  situations  are  flat,  irregular  in 
shape,  with  borders  more  or  less  wavy  or  sinuous  in  outline. 

Functions  of  Epithelial  Tissue. — In  succeeding  chapters  the  form, 
chemic  composition,  and  functions  of  epithelial  cells  will  be  considered  in 
connection  with  the  functions  of  the  organs  of  which  they  constitute  a  part. 
In  this  connection  it  may  be  stated  in  a  general  way  that  the  functions  of 
the  epithelial  tissues  are : 

1.  To  sers'e  on  the  surface  of  the  body  as  a  protective  covering  to  the  under- 

lying structures  which  collectively  form  the  true  skin,  thus  protecting 
them  from  the  injurious  influences  of  moisture,  air,  dust,  microorgan- 
isms, etc.,  which  would  other^'ise  impair  their  vitality.  Wherever  con- 
tinuous pressure  is  applied  to  the  skin,  as  on  the  palms  of  the  hands  and 
soles  of  the  feet,  the  epithelium  increases  in  thickness  and  density,  and 
thus  prevents  undue  pressure  on  the  ner\^es  of  the  true  skin.  The 
density  of  the  epidermis  enables  it  to  resist,  within  limits,  the  injurious 
influence  of  acids,  alkalies,  and  poisons. 

2.  To  promote  absorption.     Inasmuch  as  the  skin  and  mucous  membranes 

cover  the  surfaces  of  the  body,  it  is  obvious  that  all  nutritive  material 
entering  the  body  must  first  traverse  the  epithelial  tissue.  Owing  to 
their  density,  however,  the  epitheHal  cells  covering  the  skin  play  but  a 


32  TEXT-BOOK  OF  PHYSIOLOGY. 

feeble  role  as  absorbing  agents  in  man  and  the  higher  animals.  The 
epithelium  of  the  mucous  membrane  of  the  alimentary  canal,  particu- 
larly that  of  the  small  intestine,  is  especially  adapted,  from  its  situa- 
tion, consistency,  and  properties,  to  play  the  chief  rdle  in  the  absorp- 
tion of  new  materials  from  the  canal.  The  epithelium  lining  the  air- 
vesicles  of  the  lungs  is  engaged  in  promoting  the  absorption  of  oxygen 
and  the  exhalation  of  carbon  dioxid. 
3.  To  form  secretions  and  excretions.  Each  secretory  gland  connected 
with  the  surfaces  of  the  body  is  lined  by  epithelial  cells,  which  are  actively 
concerned  in  the  formation  of  the  secretion  peculiar  to  the  gland. 
Each  excretory  organ  is  similarly  provided  with  epithelial  cells,  which 
are  engaged  either  in  the  production  of  the  constituents  of  the  excretion 
or  in  their  removal  from  the  blood. 

2.  THE  CONNECTIVE  TISSUES. 

The  connective  tissues,  in  their  collective  capacity,  constitute  a  frame- 
work which  per\^ades  the  body  in  all  directions,  and,  as  the  name  impHes, 
serve  as  a  bond  of  connection  between  the  individual  parts,  at  the  same  time 
affording  a  basis  of  support  for  the  muscle,  nerve,  and  gland  tissues.  The 
connective-tissue  group  includes  a  number  of  varieties,  among  which  may 
be  mentioned  the  areolar,  adipose,  retiform,  white  fibrous,  yellow  elastic, 
cartilaginous  and  osseous.  Notwithstanding  their  apparent  diversity,  they 
possess  many  points  of  similarity.  They  have  a  common  origin,  developing 
from  the  same  embryonic  material;  they  have  much  the  same  structure, 
passing  imperceptibly  into  one  another,  and  perform  practically  the  same 
functions. 

Areolar  Tissue. — This  variety  is  found  widely  distributed  throughout 
the  body.  It  serves  to  unite  the  skin  and  mucous  membrane  to  the  struct- 
ures on  which  they  rest;  to  form  sheaths  for  the  support  of  blood-vessels, 
nerv^es,  and  lymphatics;  to  unite  into  compact  masses  the  muscular  tissue 
of  the  body,  etc.  Examined  with  the  naked  eye,  it  presents  the  appearance 
of  being  composed  of  bundles  of  fine  fibers  interlacing  in  every  direction. 
In  the  embryonic  state  the  elements  of  this  form  of  connective  tissue  are 
united  by  a  ground  substance,  gelatinous  in  character.  In  the  adult  state  this 
substance  shrinks  and  largely  disappears,  leaving  intercommunicating 
spaces  of  varying  size  and  shape,  from  which  the  tissue  takes  its  name. 
When  subjected  to  the  action  of  various  reagents,  and  examined  micro- 
scopically, the  bundles  can  be  shown  to  consist  of  extremely  delicate,  color- 
less, transparent,  wavy  fibers,  which  are  cemented  together  by  a  ground 
substance  composed  largely  of  mucin.  Other  fibers  are  also  observed, 
which  are  distinguished  by  a  straight  course,  a  sharp,  well-defined  out- 
line, a  tendency  to  branch  and  unite  with  adjoining  fibers,  and  to  curl  up 
at  their  extremities  when  torn.  From  their  color  and  elasticity  they  are 
known  as  yellow  elastic  fibers.  Distributed  throughout  the  meshes  of  the 
areolar  tissue  are  found  flattened,  irregularly  branched,  or  stellate  corpus 
cles,  connective-tissue  corpuscles,  plasma  cells,  and  granule  cells. 

Adipose  Tissue. — This  tissue,  which  exists  very  generally  throughout 
the  body,  though  found  most  abundantly  beneath  the  skin,  around  the 


THE  CONNECTR'E  TISSUES. 


33 


kidneys,  and  in  the  bones,  is  practically  but  a  modification  of  areolar  tissue. 
In  these  situations  it  presents  itself  in  small  masses  or  lobules  of  varying 
size  and  shape,  surrounded  and  penetrated  by  the  fibers  of  connective 
tissue.  (See  Fig.  6.)  Microscopic  examination  shows  that  these  masses 
consist  of  small  vesicles  or  cells,  round,  elliptical  or  polyhedral  in  shape, 
depending  somewhat  on  pressure.  (See  Fig.  7.)  Each  vesicle  consists  of  a 
thin,  colorless,  protoplasmic  membrane,  thickened  at  one  point,  in  which  a 
nucleus  can  usually  be  detected.  This  membrane  incloses  a  globule  of  fat, 
which  during  life  is  in  the  liquid  state.  It  is  composed  of  olein,  stearin,  and 
palmitin.  The  origin  of  the  fat  is  to  be  referred  to  a  retrograde  change  in 
the  protoplasmic  material  of  the  connective-tissue  cells.  When  this  proto- 
plasm becomes  rich  in  carbon  and  hydrogen,  it  is  speedily  converted  into 
fat,  which  makes  its  appearance  in  the  form  of  minute  drops  in  different 


In 
super- 
posed 
layers. 


Fig.  6. — .\dipose  Tissue. — (Stohr.) 


7. — Fat-cells  from  the 
OF  M.AX.  I.  The  equator 
cell  in  focus.  2.  The  ob- 
somewhat  elevated.  3,  4. 
changed  by  pressure,  p. 
Traces  of  protoplasm  in  the  vicinity 
of  the  flat  nucleus  k. — (Stolir.) 


portions  of  the  cell.  As  the  drops  accumulate,  at  the  expense  of  the  cell 
protoplasm,  they  gradually  coalesce,  until  there  remains  but  a  thin  stratum 
of  the  protoplasm,  which  forms  the  wall  of  the  vesicle.  Adipose  tissue 
may,  therefore,  be  regarded  as  areolar  tissue,  in  which,  and  at  the  expense 
of  some  of  its  elements,  fat  is  stored  for  the  future  needs  of  the  organism. 
A  diminution  of  food,  especially  of  fat  and  carbohydrates,  is  promptly 
followed  by  an  absorption  of  fat  by  the  blood-vessels  and  by  its  transference 
to  the  tissues,  where  it  is  either  utilized  for  tissue  construction  or  for  oxida- 
tion purposes.  In  the  situations  in  which  adipose  tissue  is  found  it  serves, 
by  its  chemic  and  physical  properties,  to  assist  in  the  prevention  of  a  too 
rapid  radiation  of  heat  from  the  body,  to  give  form  and  roundness,  and 
to  diminish  angularities,  etc. 

Retiform  and  adenoid  tissue  are  also  modifications  of  areolar  tissue. 
The  meshes  of  the  former  contain  but  little  ground  substance,  its  place  being 
taken  by  iiuids;  the  meshes  of  the  latter  contain  large  numbers  of  lymph 
corpuscles. 

Fibrous  Tissue. — This  variety  of  connective  tissue  is  widely  distributed 
throughout  the  body.  It  constitutes  almost  entirely  the  ligaments  around 
the  joints,  the  tendons  of  the  muscles,  the  membranes  covering  organs  such 
as  the  heart,  liver,  nen-e  system,  bones,  etc.     All  fibrous  tissue,  wherever 


34 


TEXT-BOOK  OF  PHYSIOLOGY. 


found,  can  be  resolved  into  elementary  bundles,  which  on  microscopic  exam- 
ination are  seen  to  consist  of  delicate,  wavy,  transparent,  homogeneous 
libers,  which  pursue  an  independent  course,  neither  branching  nor  uniting 
with  adjoining  fibers.  (See  Fig.  8.)  A  small  amount  of  ground  substance 
serves  to  hold  them  together.  Fibrous  tissue  is  tough  and  inextensible,  and 
in  consequence  is  admirably  adapted  to  fulfil  various  mechanical  functions 
in  the  body.  It  is,  however,  quite  pliant,  bending  easily  in  all  directions. 
When  boiled,  fibrous  tissue  yields  gelatin,  a  derivative  of  collagen. 

Elastic  Tissue. — The  fibers  of  elastic  tissue  are  usually  associated  in 
varying  proportions  with  the  white  fibrous  tissue;  but  in  some  structures — 
as  the  ligamentum  nuchse,  the  ligamenta  subflava,  the  middle  coat  of  the 
larger  blood-vessels — the  elastic  fibers  are  almost  the  only  elements  present. 


Fig.  8. — Connective-tissue 
Bundles  of  Various  Thick- 
nesses OF  the  Intermuscular 
Connective  Tissue  of  Man. 
X  240. — {Stohr.) 


Fig.  9. — Elastic  Fibers  of  the 
Subcutaneous  Areolar  Tissue  of 
a  Rabbit. — (AiterSchdfer.) 


and  give  to  these  structures  a  distinctly  yellow  appearance.  The  fibers 
throughout  their  course  give  off  many  branches,  which  unite  with  adjoining 
branches  to  form  a  more  or  less  close  network.  As  the  name  implies,  these 
fibers  are  highly  elastic,  and  are  capable  of  being  extended  as  much  as  60 
per  cent,  of  their  length  before  breaking.     (See  Fig.  9.) 

Cartilaginous  Tissue. — This  form  of  connective  tissue  differs  from  the 
preceding  varieties  chiefly  in  its  density.  As  a  rule,  it  is  firm  in  consistency, 
though  somewhat  elastic.  It  is  opaque,  bluish-white  in  color,  though  in  thin 
sections  translucent.  All  cartilaginous  tissues  consist  of  connective-tissue 
cells  embedded  in  a  solid  ground  substance.  According  to  the  amount 
and  texture  of  the  ground  substance,  three  principal  varieties  may  be 
distinguished: 

I.  Hyaline  cartilage,  in  which  the  cells,  relatively  few  in  number,  are  embeded 
in  an  abundant  quantity  of  ground  substance  (Fig.  10, a.)  The  body  of 
the  cells  is  in  many  instances  distinctly  marked  off  from  the  surround- 
ing substance  by  concentric  lines  of  fibers,  which  form  a  capsule  for  the 
cell.  Repeated  division  of  the  cell  substance  takes  place,  until  the  whole 
capsule  is  completely  occupied  by  daughter  cells.  The  ground  sub- 
stance is  pervaded  by  minute  channels,  which  communicate  on  one  hand 


THE  CONNECTIVE  TISSUES. 


35 


2. 


with  the  spaces  around  the  cells,  and  on  the  other  with  lymph-spaces  in 
the  connective  tissue  surrounding  the  cartilage.  By  means  of  these 
channels,  nutritive  fluid  can  permeate  the  entire  structure.  Hyaline 
cartilage  is  found  on  the  ends  of  the  long  bones,  where  it  enters  into  the 
formation  of  the  joints;  between  the  ribs  and  sternum,  forming  the  costal 
cartilage,  as  well  as  in  the  nose  and  larynx. 
White  fibro-cartilage,  the  ground  substance  of  which  is  pervaded  by  white 
fibers,  arranged  in  bundles  or  layers,  between  which  are  scattered  the 
usual    encapsulated    cells.     (See   Fig.  io,c.)     White  fibro-cartilage  is 


■^    .    ^        -^    •       ^  ^ 


^ 


>  -.1 


^ 


'.1^ 


A  B  e 

Fig  io. — The  Three  Types  of  Cartilage:  A,  Hyaline;  B,  Elastic;  C,  Fibrous. — {Rad- 
asch).  a,  b,  Outer  and  inner  layers  of  perichondrium;  c,  young  cartilage  cells;  d,  older  cartilage 
cells;  e,  f,  capsule;  g,  lacuna. 

tough,  resistant,  but  flexible,  and  is  found  in  joints  where  strength  and 
fixedness  are  required.     Hence  it  is  present  between  the  vertebrae, 
forming  the  intervertebral  discs,  between  the  condyle  of  the  lower  jaw 
and  the  glenoid  fossa,  in  the  knee-joint,  around  the  margins  of  the  joint 
cavities,  etc.     In  these  situations  it  assists  in  maintaining  the  apposition 
of  the  bones,  in  giving  a  certain  degree  of  mobility  to  the  joints,  and  in 
diminishing  the  effects  of  shock  and  pressure  imparted  to  the  bones. 
3.    Yellow  fibro-cartilage,  the  ground  substance  of  which  is  perA'aded  by 
opaque,  yellow  elastic  fibers,  which  form,  by  the  interlacing  of  then- 
branches,  a  complicated  network,  in  the  meshes  of  which  are  to  be  found 
the  usual  corpuscles.     (See  Fig.  io,b.)     As  these  fibers  are  elastic,  they 
impart  to  the  cartilage  a  very  considerable  degree  of  elasticity.     Yellow 
fibro-cartilage  is  well  adapted,  therefore,  for  entering  into  the  formation  of 
the  external  ear,  epiglottis.  Eustachian  tube,  etc. — structures  which 
require  for  their  functional  activity  a  certain  degree  of  flexibility  and 
elasticity. 
Osseous  Tissue. — Osseous  tissue,   as  distinguished  from  bone,   is  a 
member  of  the  connective-tissue  group,  the  ground  substance  of  which  is 
permeated  with  insoluble  lime  salts,  of  which  the  phosphate  and  carbonate 
are  the  most  abundant.     Immersed  in  dilute  solutions  of  hydrochloric  acid, 
they  can  be  converted  into  soluble  salts  and  dissolved  out.     The  osseous 
matrix  left  behind  is  soft  and  pliable.     When  boiled,  it  yields  gelatin. 


36  TEXT-BOOK  OF  PHYSIOLOGY. 

A  thin,  transverse  section  of  a  decalcified  bone,  when  examined  micro- 
scopically, reveals  a  number  of  small,  round,  or  oval  openings,  which  repre- 
sent transverse  sections  of  canals  which  run  through  the  bone,  for  the  most 
part  in  a  longitudinal  direction,  though  frequently  anastomosing  with  one 
another.  These  so-called  Haversian  canals  in  the  living  state  contain  blood- 
vessels and  lymphatics.     (See  Fig.  ii.) 

Around  each  Haversian  canal  is  a  series  of  concentric  laminae,  composed 
of  white  fibers.  Between  every  two  lamina;  are  found  small  cavities  (lacunae), 
from  which  radiate  in  all  directions  small  canals  (canaliculi),  which  com- 
municate freely  with  one  another.  The  Haversian  canals,  with  their  associ- 
ated lacunae  and  canaliculi,  form  a  system  of  intercommunicating  passages, 
which  circulate  lymph  destined  for  the  nourishment  of  bone.  Each  lacuna 
contains  the  bone  corpuscle,  which  bears  a  close  resemblance  to  the  usual 
branched  connective-tissue  corpuscle,  and  whose  function  appears  to  be  the 
maintenance  of  the  nutrition  of  the  bone. 

^-^^  Periosteum. 
^2;-^  Outer  ground  lamella. 
Haversian  canals. 

Haversian  lamellae. 


Interstitial  lamella. 
Inner  ground  lamellae. 


Marrow. 


Fig.  II. — From  a  Cross-section  of  a  Metacarp  of  Man.     X  50.     The  Haversian  canals 
contain  a  little  marrow  (fat-cells).     Respiration  line  at  h. — {Stbhr). 

The  surface  of  every  bone  in  the  living  state  is  invested  with  a  fibrous 
membrane,  the  periosteum,  except  where  it  is  covered  with  cartilage.  The 
inner  surface  of  this  membrane  is  loose  in  texture,  and  supports  a  fine  plexus 
of  capillary  blood-vessels  and  numerous  protoplasmic  cells — the  osteoblasts. 
As  this  layer  is  directly  concerned  in  the  formation  of  bone,  it  is  spoken  of 
as  the  osteogenetic  layer. 

A  section  of  a  bone  shows  that  it  is  composed  of  two  kinds  of  tissue — 
compact  and  cancellated.  The  compact  is  dense,  resembling  ivory,  and  is 
found  on  the  outer  portion  of  the  bone;  the  cancellated  is  spongy,  and  appears 
to  be  made  up  of  thin,  bony  plates,  which  intersect  one  another  in  all  direc- 
tions, and  is  found  in  greatest  abundance  in  the  interior  of  the  bones.  The 
shaft  of  a  long  bone  is  hollow.  This  central  cavity,  which  extends  from  one 
end  of  the  bone  to  the  other,  as  well  as  the  interstices  of  the  cancellated  tissue, 
is  filled  in  the  living  state  with  marrow.  The  marrow  or  medulla  is  composed 
of  a  connective-tissue  framework  supporting  blood-vessels.  In  its  meshes 
are  to  be  found  characteristic  bone  cells  or  osteoblasts,  the  function  of  which 
is  supposed  to  be  the  formation  of  bone.  In  the  long  bones  the  marrow  is 
yellow,  from  the  presence  in  the  connective-tissue  corpuscle  of  fat  globules. 


THE  CONNECTIVE  TISSUES.  37 

which  arise  through  the  transformation  of  the  cell  protoplasm.  In  the 
cancellated  tissue,  near  the  extremities  of  the  long  bones,  this  fatty  transfor- 
mation does  not  take  place  to  the  same  extent,  and  the  marrow  appears  red. 
The  cells  of  the  red  marrow  are  believed  to  give  birth  indirectly  to  the  red 
blood-corpuscles. 

Physical  and  Physiologic  Properties  of  Connective  Tissues. — 
Among  the  physical  properties  may  be  mentioned  consistency,  cohesion,  and 
elasticitv.  Their  consistency  varies  from  the  semiliquid  to  the  solid  state, 
and  depends  on  the  quantity  of  water  which  enters  into  their  composition. 
Their  cohesion,  except  in  the  softer  varieties,  is  very  considerable,  and  offers 
great  resistance  to  traction,  pressure,  torsion,  etc.  In  all  the  movements  of 
the  body,  in  the  contraction  of  muscles,  in  the  performance  of  work,  the 
consistence  and  cohesion  of  these  tissues  play  most  important  roles.  Wher- 
ever the  various  forms  of  connective  tissue  are  found,  their  chemic  composi- 
tion and  structure  are  in  relation  to  their  functions.  If  traction  be  the  pre- 
ponderating force,  the  structure  becomes  fibrous  as  in  ligaments  and  tendons, 
and  the  cohesion  greatest  in  the  longitudinal  direction.  If  pressure  be 
exerted  in  all  directions,  as  upon  membranes,  the  fibers  interlace  and  offer 
a  uniform  resistance.  When  pressure  is  exerted  in  a  definite  direction,  as 
on  the  extremities  of  the  long  bones,  the  tissue  becomes  expanded  and  can- 
cellated. The  lamellae  of  the  cancellated  tissue  arrange  themselves  in 
curves  w^hich  correspond  to  the  direction  of  the  greatest  pressure  or  traction. 
Extensibility  is  not  a  characteristic  feature,  except  in  those  forms  containing 
an  abundance  of  yellow  elastic  fibers.  The  elasticity  is  an  essential  factor 
in  many  physiologic  actions.  It  not  only  opposes  and  limits  forces  of  trac- 
tion, pressure,  torsion,  etc.,  but  on  their  cessation  returns  the  tissues  or 
organs  to  their  original  condition.  Elasticity  thus  assists  in  maintaining 
the  natural  form  and  position  of  the  organs  by  counterbalancing  and  oppos- 
ing temporarily  acting  forces. 

The  Skeleton. — The  connective  tissues  in  their  entirety  constitute  a 
framework  which  presents  itself  under  two  aspects:  (i)  As  a  solid,  bony 
skeleton,  situated  in  the  trunk  and  limbs,  affording  attachment  for  muscles 
and  viscera;  (2)  as  a  fine,  fibrous  skeleton,  found  everywhere  throughout 
the  body,  connecting  the  various  viscera  and  affording  support  for  the 
epithelial,  muscle,  and  nerve  tissues. 


CHAPTER  V. 

THE  PHYSIOLOGY  OF  MOVEMENT. 

Of  the  four  phenomena  presented  by  an  animal,  that  which  more  im- 
mediately interests  the  physiologist  is  movement,  for  the  reason  that  it  is 
not  only  the  animal's  most  characteristic  form  of  activity,  and  that  which 
serves  to  distinguish  it  in  the  main  from  forms  of  vegetable  life,  but  its 
solution  affords  an  explanation  of  many  physiologic  processes  occurring 
within  the  human  body.  It  is  also  for  this  reason  that  movement  constitutes 
for  the  most  part  the  subject-matter  of  physiologic  experimentation. 

The  movements  of  the  body  may  for  convenience  be  divided  into  two 
groups,  viz.,  external  and  internal. 

The  external  movements  are  exhibited  mainly  by  the  head  and  extremities 
and  may  be  either  special  as  when  the  animal  changes  the  relation  of  one 
part  of  the  body  to  an  other,  or  general  as  when  it  changes  its  position 
relatively  to  the  environment  as  in  the  various  acts  of  locomotion.  The 
external  movements  are  the  result  of  the  cooperation  of  the  skeletal  muscles 
and  the  bones  of  the  skeleton  to  which  they  are  attached.  The  muscles 
possess  the  power  of  suddenly  shortening  or  contracting  and  by  virtue  of 
their  relation  to  the  bones  impart  to  them  all  the  external  movements  char- 
acteristic of  the  animal.  The  change  of  relation  of  the  bones  and  hence  of 
the  parts  of  the  animal  of  which  they  form  a  part,  are  dependent  on  the 
construction  of  the  joints. 

In  the  execution  of  the  movements  the  animal  of  necessity  meets  with 
various  forms  of  resistance,  viz.,  gravity,  cohesion,  friction,  etc.  When  its 
different  parts  are  appHed  or  directed,  either  volitionally  and  in  a  determinate 
manner,  or  non-volitionally  and  in  an  indeterminate  or  reflex  manner,  to 
the  overcoming  of  these  opposing  forces  in  the  environment,  the  animal 
may  be  said  to  be  doing  work. 

In  the  animal  as  in  the  physical  machine,  work  is  accomplished  by  the 
intermediation  of  levers.  In  the  animal  machine,  the  levers  are  found  in 
the  bones  of  the  skeleton  and  more  particularly  in  the  long  bones  of  the 
extremities,  the  fulcra  of  which,  the  points  around  w^hich  they  move,  lie  in 
the  joints. 

That  a  lever  may  be  effective  as  an  instrument  for  the  accomplishment  of 
work  it  must  not  only  be  capable  of  moving  around  its  fulcrum,  but  it  must 
at  the  same  time  be  acted  on  by  two  opposing  forces,  one  passive,  the  other 
active.  In  the  movements  of  the  bony  levers  of  the  animal  body,  the 
passive  forces  to  be  overcome  are  largely  those  connected  with  the  environ- 
ment, e.g.,  gravity,  cohesion,  friction,  etc.,  the  active  forces  by  which 
these  are  opposed  and  overcome  through  the  mediation  of  the  bony  levers, 
are  found  in  the  muscles  attached  to  them.  The  muscles  are  therefore  to 
be  regarded  as  the  seat  of  those  active  energies  that  impart  movement  to 
the  levers. 

38 


THE  PHYSIOLOGY  OF  MOVEMENT.  39 

■    The  internal  movements  are  exhibited  by  the  \dscera,  the  vascular  appa- 
ratus and  by  glands,  and,  though  less  obvious,  are  no  less  characteristic. 

The  viscera,  by  virtue  of  the  presence  of  non-striated  muscle-fibers  in 
their  walls,  are  capable  of  changing  their  caliber  from  moment  to  moment 
either  in  the  way  of  an  increase  or  decrease  and  thus  regulate  and  control 
the  passage  of  their  contents  through  them. 

The  vascular  apparatus,  and  its  adjunct,  the  lymph-vessel  apparatus,  is 
engaged  in  the  distribution  of  blood  and  nutritive  material  throughout  the 
body.  The  heart  drives  the  blood  through  the  vessels  in  opposition  to  the 
friction  presented  by  their  walls,  while  the  vessels  themselves  and  especially 
the  arteries,  by  virtue  of  the  non-striated  muscle-fibers  in  their  walls,  increase 
and  decrease  in  caliber  from  moment  to  moment  and  thus  regulate  the 
amount  of  blood  flowing  through  them  in  accordance  with  the  physiologic 
needs  of  the  organ  to  which  they  are  distributed. 

The  glands  and  more  especially  their  epithelial  investments  are  the  seat 
of  certain  molecular  movements  the  result  of  which  is  the  production  and 
discharge  of  a  secretion  destined  to  play  a  more  or  less  important  part  in  the 
maintenance  of  the  acti\dties  of  the  body. 

When  these  various  organs  are  applied  to  the  overcoming  of  various 
resistances  or  forces,  as  they  are  in  the  performance  of  their  functions,  it  can 
also  be  said  that  they  too  are  doing  work.  The  cooperation  of  external  and 
internal  organs  is  necessary,  however,  not  only  for  the  maintenance  of  the 
life  of  the  animal  but  also  for  the  accomplishment  of  external  work. 

The  various  tissues  of  the  body,  mentioned  in  foregoing  paragraphs, 
though  irritable  do  not  possess  spontaneity  of  action,  but  require  for  the 
manifestation  of  their  characteristic  forms  of  activity  the  application  of  a 
stimulus. 

Thus  the  skeletal  muscles  and  glands  though  capable  of  being  excited 
to  activity  by  various  artificial  stimuli,  require  for  the  exhibition  of  their 
normal  activity  the  arrival  of  the  physiologic  stimulus,  the  nerve  impulse, 
developed  in  and  transmitted  to  them  by  the  nerve  tissue. 

The  visceral  and  vascular  muscles  though  apparently  capable  of  being 
excited  to  activity  by  agencies  other  than  the  nerv^e  impulse  are  nevertheless 
augmented  or  inhibited  in  their  activity  from  moment  to  moment  by  nerve 
impulses. 

It  is  evident  therefore  that  the  activities  of  the  organs  and  tissues  which 
are  engaged  in  promoting  the  work  of  the  body  are  excited  to  action  and 
controlled  by  the  nerve  tissue,  a  fact  which  presupposes  an  anatomic  con- 
nection between  them. 

For  an  understanding  of  the  mode  of  excitation  of  the  motor  organs  and 
the  manner  in  which  they  cooperate  in  the  performance  of  any  given  move- 
ment, a  brief  preliminary  account  of  the  general  arrangement  and  mode  of 
action  of  the  nerve  tissue  wdll  be  found  helpful. 

The  General  Relation  of  the  Nerve  Tissue  to  the  Motor  Organs. — 
The  nerve  tissue  is  arranged  partly  in  masses  contained  within  the  cavities 
of  the  head  and  spinal  column  (the  encephalon  or  brain  and  spinal  cord), 
forming  the  central  organs  of  the  nerve  system,  and  partly  in  the  form  of 
cords  or  nerves,  (the  cranial  and  spinal  nerves),  forming  the  peripheral  organs 
of  the  nerve  svstem.     The  latter  connect  the  former  not  onlv  with  muscles, 


40 


TEXT-BOOK  OF  PHYSIOLOGY. 
c.  s.c. 


Fig.  12. — Diagram  Showing  the  Relaton  of  Skeletal,  Muscle  and  Nerve 
Tissues.  (C.  Bachman.)  f.a.  Bones  of  the  forearm  representing  the  skeletal  tissue;  e.j. 
the  elbow  joint,  the  fulcrum  of  the  lever  formed  by  the  bones  of  the  forearm;  W.  a  weight 
acting  in  a  downward  direction  and  representing  the  passive  force  of  gravity;  sk.m.  a 
skeletal  muscle  acting  in  an  upward  direction  and  the  source  of  the  active  power  to  be  ap- 
plied to  the  lever;  sp.c.  transection  of  the  spinal  cord  showing  the  relation  of  the  white  and 
the  gray  matter:  in.c.  a  motor  cell  in  the  anterior  horn  of  the  gray  matter;  ef.n.  an  effer- 
ent nerve-fiber  connecting  the  motor  cell  from  which  it  arises  with  the  skeletal  muscle  and 
contained  in  the  ventral  roots  of  the  spinal  nerves;  a/.n.  an  afferent  nerve-fiber  arising  from 
the  ganglion  cell  along  its  course  and  connecting  the  skin,  5.,  on  the  one  hand  with  the  spinal 
cord  on  the  other  hand  and  contained  in  the  dorsal  roots  of  the  spinal  nerves;  c.s.c. 
coronal  section  of  the  cerebrum  showing  the  relation  of  the  gray  to  the  white  matter;  v.c. 
a  volitional  or  motor  cell;  d.a.  a  descending  axon  or  nerve-fiber  connecting  the  volitional 
cell  from  which  it  arises  with  the  motor  cell  in  the  spinal  cord;  s.c.  a  sensor  cell;  a. a.  an 
ascending  axon  or  nerve-fiber  connecting  a  receptive  cell  from  which  it  arises  (not  shown  in 
the  diagram)  with  the  sensor  cell  in  the  gray  matter  of  the  cerebrum.  The  nerve-fibers 
which  pass  outward  from  the  spinal  cord  to  the  glands,  blood-vessels,  and  the  muscle 
walls  of  the  viscera,  have  for  the  sake  of  simplicity  been  omitted  from  the  diagram. 


THE  PHYSIOLOGY  OF  MOVEMENT. 


41 


glands,  blood-vessels,  and  viscera,  but  with  the  skin,  mucous  membranes, 
etc.,  as  well. 

(The  relation  of  the  ner\'e  tissue  to  the  skeletal  muscles,  to  glands,  to 
blood-vessels,  and  viscera  are  shown  in  Figs.  12,  13.) 

The  spinal  cord  is  more  especially  the  seat  of  origin  of  the  nerve  energy 
that  immediately  excites  and  controls  the  activity  of  the  motor  organs,  a 
knowledge  of  its  structure,  of  its  relations  to  these  organs,  and  the  manner 
in  which  it  is  excited  to  activity  is  necessary  to  an  understanding  of  the  prob- 
lem of  movement. 

The  spinal  cord  is  narrow  and  cylindric  in  shape  and  occupies  the  spinal 
canal  from  the  level  of  the  first  vertebra  as  far  down  as  the  second  or  third 


,sp.c. 


Fig.  13. — DLA.GRAM  Showing  the  Structures  Involved  in  the  Production  of  Reflex 
Actions.— (G.  Bachman.)  r.s.  Receptive  surface;  af.n.  afferent  nerve;  ex.  emissive  or  motor 
cells  in  the  anterior  horn  of  the  gray  matter  of  the  spinal  cord,  sp.c;  ef.n.  efferent  nerves  distributed 
to  responsive  organs,  e.g.,  directly  to  skeletal  muscles,  sk.n.,  and  indirectly  through  the  inter- 
mediation of  sympathetic  ganglia,  sym.g.,  to  blood-vessels,  b.v.,  and  to  glands,  g.  The  nerves 
distributed  are  not  represented. 

lumbar  vertebra.  It  presents  both  on  its  ventral  and  dorsal  surfaces  a 
deep  longitudinal  fissure  which  partly  divide  the  cord  into  halves,  a  right 
and  a  left.  To  each  side  of  the  cord  there  is  attached  thirty-one  nerves, 
which  as  they  pass  out  through  foramnia  in  the  walls  of  the  spinal  column  are 
termed  spinal  nerves.  Each  spinal  nerve  is  connected  with  the  spinal  cord 
by  two  roots,  termed  from  their  relation  to  the  ventral  and  dorsal  surfaces, 
the  ventral  and  dorsal  roots. 

Experimental  investigation  has  demonstrated  that  the  ventral  roots  are 
connected  peripherally  with  the  motor  organs  and  transmit  to  them  ner\'e 
energy  developed  in  the  spinal  cord;  that  the  dorsal  roots  are  connected 
peripherally  with  the  skin,  mucous  membranes,  etc.,  and  transmit  nerve 
energy  developed  in  their  terminations  to  the  spinal  cord.  The  ventral 
and  dorsal  roots  are  therefore  termed  from  their  function,  efferent  and  affere7it 
nerves  respectively. 

A  transverse  section  of  the  spinal  cord  shows  that  each  half  is  composed 
externally  of  white  matter,  and  internally  of  gray  matter.     The  gray  matter 


42  TEXT-BOOK  OF  PHYSIOLOGY. 

in  each  half  is  arranged  in  the  form  somewhat  of  a  crescent  united  in  the 
median  line  by  a  transverse  band  or  commissure,  the  whole  forming  a 
figure  resembling  the  letter  H.  Though  varying  in  shape  in  different 
regions  of  the  cord,  the  gray  matter  in  all  situations  presents  on  cither  side 
an  anterior  or  ventral  and  a  posterior  or  dorsal  horn. 

In  the  ventral  horns  of  the  gray  matter  are  located  large  nerve-cells 
which  give  origin  to  ncr\'e-fibcrs;  these  fibers  in  their  growth  pass  forward 
through  the  cord  and  emerge  as  ventral -roots;  continuing  to  grow,  these 
fibers  gradually  reach  and  become  connected  with  the  motor  organs  to 
which  they  are  by  heredity  directed. 

It  has  been  experimentally  demonstrated  that  each  nerv-e-cell  not  only 
generates  but  under  given  conditions  discharges  a  form  of  energy  termed 
a  nerve  impulse,  which  is  transmitted  by  the  nerve-fiber  arising  from  it  and 
by  way  of  the  ventral  roots  of  the  spinal  nerv^es  directly  to  skeletal  muscles 
and  indirectly  through  the  ganglia  of  the  sympathetic  nerve  system  and  their 
branches  to  glands,    blood-vessels  and    walls  of   viscera.     (See  Fig.   13.) 

The  arrival  of  the  nerve  impulse  at  once  calls  forth  the  form  of  activity 
characteristic  of  the  structure  stimulated.  Thus  the  muscle,  for  example, 
passes  from  the  passive  to  the  active  state,  that  is,  the  muscle  becomes  shorter 
and  thicker,  and  the  bone  to  which  it  is  attached  is  moved.  This  is  at 
once  followed  by  a  return  of  the  muscle  to  the  passive  state;  that  is,  it 
lengthens,  becomes  narrower,  and  resumes  its  original  form;  the  bone  at 
the  same  time  returns  to  its  former  position.  Coincident  with  this  change 
of  shape  there  is  a  liberation  of  heat  and  electricity.  The  nerve  impulse 
which  occasions  this  transformation  of  potential  into  kinetic  energy  is  the 
normal  or  the  physiologic  stimulus.  The  glands  in  response  to  the  nerve 
impulse  pour  out  a  secretion,  the  blood-vessels  and  viscera  change  their 
caliber;  all  these  tissues  responding  to  the  nerve  impulse  in  a  characteristic 
manner  are  said  to  be  irritable. 

The  ner\^e-cells  in  the  ventral  horns  of  the  gray  matter  of  the  spinal  cord 
are  therefore  the  sources  of  the  energy  requisite  for  the  physiologic  excitation 
of  the  motor  organs.  If  they  are  destroyed  either  experimentally  or  by 
pathologic  processes,  the  energy  is  no  longer  discharged  and  the  motor  organs 
become  incapable  of  performing  their  functions  in  a  physiologic  manner. 

The  nerve-cells,  though  extremely  irritable,  do  not  possess  spontaneity 
of  action,  but  require  for  their  excitation  the  arrival  and  stimulating  action 
of  other  nerv^e  impulses.  These  may  come  (i)  from  the  periphery  through 
afferent  nerv^e- fibers  by  way  of  the  dorsal  roots  of  the  spinal  nerves;  and 
(2)  from  motor  nen-e-cells  in  the  cortex  of  the  cerebral  portion  of  the  brain, 
through  descending  axons  or  ner\^e-fibers. 

In  the  first  instance  the  resulting  movements  taking  place  in  response 
to  a  peripheral  or  surface  stimulation  and  independently  of  volitional  or 
emotional  activity  are  termed  reflex  movements;  in  the  second  instance  the 
resulting  movements  taking  place  in  response  to  volitional  or  emotional 
activities  are  termed  volitional  or  emotional  movements. 

The  only  organ  that  can  be  properly  said  to  be  excited  to  action  by  a 
volitional  act  is  the  skeletal  muscle;  the  glands,  blood-vessels,  and  viscera  are 
apparently  only  influenced  in  their  activity  by  emotional  states. 

In  the  case  of  reflex  movements,  the  nerve  impulses  are  primarily  devel- 


THE  PHYSIOLOGY  OF  MOVEMENT.  43 

oped  in  specialized  organs  located  in  the  skin  or  mucous  membranes  and  as 
a  result  of  the  impact  of  various  external  agents,  which  for  this  reason  are 
termed  stimuli.  The  nerve  impulses  thus  developed  are  transmitted  by  the 
afferent  nerves  to  the  nerve-cells  which  are  in  turn  excited  to  activity. 

In  the  case  of  skeletal-muscle  movements,  the  nerve  impulses  which 
cause  the  movements  are  discharged  from  certain  motor  or  efferent  nerve- 
cells  in  the  gray  matter  of  the  cortex  of  the  cerebrum  and  transmitted  by 
descending  axons  or  nerve-fibers  direct  to  the  ners'e-cells  in  the  spinal  cord, 
by  which  they  in  turn  are  excited  to  activity.     Fig.  12. 

The  movements  due  to  cerebral  or  psychic  activity  are,  however,  the 
immediate  or  the  more  or  less  remote  effects  of  sensations  which  have  been 
evoked  in  the  sense  areas  of  the  brain,  by  the  arrival  of  ner\^e  impulses  coming 
through  ascending  axons,  or  nerve-fibers  from  peripheral  sense  organs,  e.g., 
skin,  eye,  ear,  nose,  tongue,  and  which  have  been  developed  by  the  impact  of 
objects  in  the  external  world. 

The  ner\'e-cells  and  their  related  ner\-e-fibers,  responding  by  the  develop- 
ment and  conduction  of  nerve  impulses  are  also  said  to  be  irritable.  The 
transformation  of  energy,  however,  manifests  itself  mainly  as  electricity  and 
molecular  motion.  The  animal  body  in  its  entirety  may  therefore  be  regarded 
as  a  machine  for  the  transformation  of  potential  energy  into  kinetic 
energy,  viz.:  heat  and  electricity,  movements  of  muscles  and  bony  levers, 
secretion,  sensation  and  other  forms  of  nerve  activity.  When  muscles  and 
bones  are  applied  to  the  overcoming  of  opposing  forces,  mechanic  work  is 
accomplished.  In  the  following  chapters  some  of  the  problems  connected 
with  the  activities  of  the  primary  mechanisms,  the  skeletal,  muscle  and 
nerve  tissues  will  be  first  considered  and  subsequently  some  of  the  problems 
connected  with  the  activities  of  the  secondarv  mechanisms. 


CHAPTER  VI. 
THE  PHYSIOLOGY  OF  THE  SKELETON. 

The  skeleton  in  its  entirety  determines  the  plan  of  organization  of  the 
animal  body.  Its  axial  portion  is  the  foundation  element  and  the  center 
around  which  the  appendicular  portions  are  developed  and  arranged  with 
a  certain  degree  of  conformity.  The  character  and  the  arrangement, of  the 
bones  of  the  axial  portion  endow  the  animal  mechanism  with  a  certain  degree 
of  fixity,  combined  with  slight  mobility,  while  the  character  and  arrangement 
of  the  bones  of  the  appendicular  portions  endow  it  with  extreme  mobility. 
The  bones  collectively  constitute  a  system  of  levers,  the  fulcra  of  which  lie 
in  the  points  of  union  of  the  bones,  and  with  which  the  animal  is  enabled  to 
execute  a  variety  of  movements,  to  change  its  position  relatively  to  its  environ- 
ment and  overcome  opposing  forces.  The  structure  and  the  chemic  com- 
position of  the  bones,  consisting  as  they  do  of  inorganic  matter  67  per  cent, 
and  of  organic  matter  33  per  cent,  endow  them  with  both  rigidity  and  elastic- 
ity, physical  properties  which  admirably  adapt  them  to  the  character  of  the 
work  necessitated  by  the  environment  and  the  organization  of  the  animal. 
The  rigidity  of  bone  is  considerable  as  compared  with  other  hard  and  rigid 
materials.  The  breaking  limit,  in  terms  of  the  weight  in  kilos  required  to 
tear  across  a  rod  one  square  millimeter  in  cross-section  of  various  materials  is 
as  follows:  Cast  iron  13;  bone  12;  oak  6.5;  granite  1.9.  The  elasticity  is 
about  one-sixth  that  of  wrought  iron  and  twice  that  of  oak  parallel  to  the 
grain  (MacAlister).  In  youth  bones  are  quite  elastic;  in  old  age  they  are 
fragile  because  of  a  diminution  of  tissue  and  an  increased  porosity,  and, 
therefore,  at  both  periods  less  capable  of  functionating  as  effectively  as  in 
the  middle  period  of  life.  The  skeleton  also  serves  for  the  attachment  of 
muscles  and  affords  support  and  protection  to  viscera. 

For  the  manifestation  of  the  activities  of  the  animal  it  is  essential  that  the 
relation  of  the  various  portions  of  the  bony  skeleton  to  one  another  shall  be 
such  as  to  permit  of  movement  while  yet  retaining  close  apposition.  This 
is  accomplished  by  the  mechanical  conditions  which  have  been  evolved  at 
the  points  of  union  of  bones,  and  which  are  technically  known  as  articulations 
or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of  ( i )  the 
static  conditions,  or  those  states  of  equilibrium  in  which  the  body  is  at  rest 
— e.g.,  standing,  sitting;  (2)  the  dynamic  conditions,  or  those  states  of 
activity  characterized  by  movement — e.g.,  walking,  running,  etc.  In  this 
connection,  however,  only  those  physical  and  physiologic  peculiarities  of  the 
skeleton,  especially  in  its  relation  to  joints,  will  be  referred  to,  which  underlie 
and  determine  both  the  static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation  of 
joints  are: 

I.  Bones,  the  articulating  surfaces  of  which  are  often  more  or  less  expanded, 
especially  in  the  case  of  long  bones,  and  at  the  same  time  variously 

44 


THE  PHYSIOLOGY  OF  THE  SKELETON.  45 

modified  and  adapted  to  one  another  in  accordance  with  the  character 
and  extent  of  the  movements  which  there  take  place. 

2.  Hyaline  cartilage,  which  is  closely  applied  to  the  articulating  end  of  each 

bone.  The  smoothness  of  this  form  of  cartilage  facilitates  the  move- 
ments of  the  opposing  surfaces,  while  its  elasticity  diminishes  the  force 
of  shocks  and  jars  imparted  to  the  bones  during  various  muscular  acts. 
In  a  number  of  joints,  plates  or  discs  of  white  fibro-cartilage  are  inserted 
between  the  surfaces  of  the  bones. 

3.  .4  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline  cartilage, 

entirely  inclosing  the  cavity  of  the  joint.  This  membrane  is  composed 
largely  of  connective  tissue,  the  inner  surface  of  which  is  lined  by  endo- 
thelial cells,  which  secrete  a  clear,  colorless,  viscid  fluid — the  synovia. 
This  fluid  not  only  fills  up  the  joint-cavity,  but,  flowing  over  the 
articulating  surfaces,  diminishes  or  prevents  friction. 

4.  Ligaments — tough,  inelastic  bands,  composed  of  white  fibrous  tissue — 

which  pass  from  bone  to  bone  in  various  directions  on  the  different 
aspects  of  the  joint.     As  white  fibrous  tissue  is  inextensible  but  pliant, 
ligaments  assist  in  keeping  the  bones  in  apposition,  and  prevent  dis- 
placement while  yet  permitting  of  free  and  easy  movements. 
Classification  of  Joints. — All  joints  may  be  divided,  according  to  the 

extent  and  kind  of  movements  permitted  by  them,  into  (i)  diarthroses;  (2) 

amphiarthroses;  (3)  synarthroses. 

I.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those  which 
permit  of  free  movement.  In  the  majority  of  instances  the  articulating 
surfaces  are  mutually  adapted  to  each  other.  If  the  articulating  sur- 
face of  one  bone  is  convex,  the  opposing  but  corresponding  surface  is 
concave.  Each  surface,  therefore,  represents  a  section  of  a  sphere  or 
a  cylinder,  which  latter  arises  by  rotation  of  a  line  around  an  axis  in 
space.  According  to  the  number  of  axes  around  which  the  movements 
take  place  all  diarthrodial  joints  may  be  divided  into: 

I.  Uniaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 
segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface  more  or 
less  completely  corresponds.  In  such  a  joint  the  single  axis  of  rotation, 
though  nearly,  is  not  exactly  at  right  angles  to  the  long  axis  of  the  bone, 
and'' hence  the  movements — flexion  and  extension — which  take  place 
are  not  confined  to  one  plane.  Joints  of  this  character — e.g.,  the  elbow, 
knee,  ankle,  the  phalangeal  joints  of  the  fingers  and  toes — are,  therefore, 
termed  ginglymi,  or  hinge-joints.  Owing  to  the  obliquity  of  their 
articulating  surfaces,  the  elbow  and  ankle  are  cochleoid  or  screw- ginglymi. 
Inasmuch  as  the  axes  of  these  joints  on  the  opposite  sides  of  the  body 
are  not  coincident,  the  right  elbow  and  left  ankle  are  right-handed 
screws;  the  left  elbow  and  right  ankle,  left-handed  screws.  In  the 
knee-joint  the  form  and  arrangement  of  the  articulating  surfaces  are 
such  as  to  produce  that  modification  of  a  simple  hinge  known  as  a 
spiral  hinge,  or  helicoid.  As  the  articulating  surfaces  of  the  condyles 
of  the  femur  increase  in  convexity  from  before  backward,  and  as  the 
inner  condyle  is  longer  than  the  outer,  and,  therefore,  represents  a 
spiral  surface,  the  line  of  translation  or  the  movement  of  the  leg  is  also 
a  spiral  movement.     During  flexion  of  the  leg  there  is  a  simultaneous 


46  TEXT-BOOK  OF  PHYSIOLOGY. 

inward  rotation  around  a  vertical  axis  passing  through  the  outer  condyle 
of  the  femur;  during  extension  a  reverse  movement  takes  place.  More- 
over, the  slightly  concave  articulating  surfaces  of  the  tibia  do  not  revolve 
around  a  single  fixed  transverse  axis,  as  in  the  elbow-joint,  for  during 
flexion  they  slide  backward,  during  extension  forward,  around  a  shifting 
axis,  which  varies  in  position  with  the  point  of  contact. 

In  some  few  instances  the  axis  of  rotation  of  the  articulating  surface 
is  parallel  with  rather  than  transverse  to  the  long  axis  of  the  bone,  and 
as  the  movement  takes  place  around  a  more  or  less  conic  surface, 
the  joint  is  termed  a  trochoid  or  pulley — e.g.,  the  odonto-atlantal  and 
the  radio-ulnar.  In  the  former  the  collar  formed  by  the  atlas  and  its 
transverse  ligament  rotates  around  the  vertical  odontoid  process  of  the 
axis.  In  the  latter  the  head  of  the  radius  revolves  around  its  own  long 
axis  upon  the  ulna,  giving  rise  to  the  movements  of  pronation  and 
supination  of  the  hand.  The  axis  around  which  these  two  movements 
take  place  is  continued  through  the  head  of  the  radius  to  the  styloid 
process  of  the  ulna. 

2.  Biaxial  Joints. — In  this  group  the  articulating  surfaces  are  unequally 

curved,  though  intersecting  each  other.  When  the  surfaces  lie  in  the 
same  direction,  the  joint  is  termed  an  ovoid  joint — e.g.,  the  radio-carpal 
and  the  atlanto-occipital.  As  the  axes  of  these  surfaces  are  vertical  to 
each  other,  the  movements  permitted  by  the  former  joint  are  flexion, 
extension,  adduction,  and  abduction,  combined  with  a  slight  amount 
of  circumduction;  the  latter  joint  permits  of  flexion  and  extension  of  the 
head,  with  inclination  to  either  side.  When  the  surfaces  do  not  take  the 
same  direction,  the  joint,  from  its  resemblance  to  the  surfaces  of  a 
saddle,  is  termed  a  saddle-joint — e.g.,  the  trapezio-metacarpal.  The 
movements  permitted  by  this  joint  are  also  flexion,  extension,  adduction, 
abduction,  and  circumduction. 

3.  Polyaxial   Joints. — In  this  group  the  convex  articulating  surface  is  a 

segment  of  a  sphere,  which  is  received  by  a  socket  formed  by  the 
opposing  articulating  surface.  In  such  a  joint,  termed  an  enarthrodial 
or  ball-and-socket  joint — e.g.,  the  shoulder-joint,  hip-joint — the  distal 
bone  revolves  around  an  indefinite  number  of  axes,  all  of  which  intersect 
one  another  at  the  center  of  rotation.  For  simplicity,  however,  the 
movement  may  be  described  as  taking  place  around  axes  in  the  three 
ordinal  planes — viz.,  a  transverse,  a  sagittal,  and  a  vertical  axis.  The 
movements  around  the  transverse  axis  are  termed  flexion  and  extension; 
around  the  sagittal  axis,  adduction  and  abduction;  around  the  vertical 
axis,  rotation.  When  the  bone  revolves  around  the  surface  of  an 
imaginary  cone,  the  apex  of  which  is  the  center  of  rotation  and  the  base 
the  curve  described  by  the  hand,  the  movement  is  termed  circumduction. 

2.  Amphiarthroses. — In  this  division  are  included  all  those  joints  which 

permit  of  but  slight  movement — e.g.,  the  intervertebral,  the  interpubic, 
and  the  sacro-iliac  joints.  The  surfaces  of  the  opposing  bones  are 
united  and  held  in  position  largely  by  the  interv^ention  of  a  firm,  elastic 
disc  of  fibro-cartilage.     Each  joint  is  also  strengthened  by  ligaments. 

3.  Synarthroses. — In  this  division  are  included  all  those  joints  in  which  the 

opposing  surfaces  of  the  bones  are  immovably  united,  and  hence  do  not 


THE  PHYSIOLOGY  OF  THE  SKELETON.  47 

permit  of  anv  movement — e.g.,  the  joints  between  the  bones  of  the 
skull. 

The  Vertebral  Column. — In  all  static  and  dynamic  states  of  the  body 
the  vertebral  column  plays  a  most  essential  role.  Situated  in  the  middle  of 
the  back  of  the  trunk,  it  forms  the  foundation  of  the  entire  skeleton.  It  is 
composed  of  a  series  of  superimposed  bones,  termed  vertebrae,  which  increase 
in  size  from  above  downward  as  far  as  the  brim  of  the  pelvic  cavity.  Superi- 
orly, it  supports  the  skull;  laterally,  it  affords  attachment  for  the  ribs,  which 
in  turn  support  the  weight  of  the  upper  extremities;  below,  it  rests  upon  the 
pelvic  bones,  which  transmit  the  weight  of  the  body  to  the  inferior  extremities. 
The  bodies  of  the  vertebrae  are  united  one  to  another  by  tough  elastic  discs 
of  fibro-cartilage,  which,  collectively,  constitute  about  one-quarter  of  the 
length  of  the  vertebral  column.  The  vertebrae  are  held  together  by  ligaments 
situated  on  the  anterior  and  posterior  surfaces  of  their  bodies,  and  by  short, 
elastic  ligaments  between  the  neural  arches  and  processes.  These  structures 
combine  to  render  the  vertebral  column  elastic  and  flexible,  and  enable  it  to 
resist  and  diminish  the  force  of  shocks  communicated  to  it. 

The  amphiarthrodial  character  of  the  inter\-ertebral  joints  endows  the 
entire  column  with  certain  forms  of  movement  which  are  necessary  to 
the  performance  of  many  body  activities.  While  the  range  of  movement 
between  any  two  vertebrae  is  slight,  the  sum  total  of  movement  of  the  entire 
series  of  vertebrae  is  considerable.  In  different  regions  of  the  column  the 
character,  as  well  as  the  range  of  movement,  varies  in  accordance  with  the 
form  of  the  vertebrae  and  the  inclination  of  their  articular  processes.  In  the 
cervical  and  lumbar  regions  extension  and  flexion  are  freely  permitted, 
though  the  former  is  greater  in  the  cerv'ical,  the  latter  in  the  lumbar  region, 
especially  between  the  fourth  and  fifth  vertebrae.  Lateral  flexion  takes  place 
in  all  portions  of  the  column,  but  is  particularly  marked  in  the  cervdcal 
region.  A  rotatory  movement  of  the  column  as  a  whole  takes  place  through 
an  angle  of  about  twenty-eight  degrees.  This  is  most  evident  in  the  lower 
cervical  and  dorsal  regions. 


CHAPTER  VII. 
GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 

The  Muscle-tissue. — The  muscle-tissue,  which  closely  invests  the 
bones  of  the  body  and  which  is  familiar  to  all  as  the  flesh  of  animals,  is  the 
immediate  cause  of  the  active  movements  of  the  body.  This  tissue  is  grouped 
in  masses  of  varying  size  and  shape,  which  are  technically  known  as  muscles. 
The  majority  of  the  muscles  of  the  body  are  connected  with  the  bones  of  the 
skeleton  in  such  a  manner  that,  by  an  alteration  in  their  form,  they  can 
change  not  only  the  position  of  the  bones  with  reference  to  one  another,  but 
can  also  change  the  individual's  relation  to  surrounding  objects.  They  are 
therefore,  the  active  organs  of  both  motion  and  locomotion,  in  contradistinc- 
tion to  the  bones  and  joints,  which  are  but  passive  agents  in  the  performance 
of  the  corresponding  movements.  In  addition  to  the  muscle  masses  which 
are  attached  to  the  skeleton,  there  are  also  other  collections  of  muscle- 
tissue  surrounding  cavities  such  as  the  stomach,  intestine,  blood-vessels,  etc., 
which  impart  to  their  walls  motility,  and  so  influence  the  passage  of  material 
through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are  attached 
by  the  property  with  which  they  are  endowed  of  changing  their  shape, 
shortening  or  contracting  under  the  influence  of  a  stimulus  transmitted  to 
them  from  the  nerve  system.     Muscles  are  divided  into: 

1.  Voluntary  muscles,   comprising   those   the  activity  of  which  is   called 

forth  by  an  act  or  effort  of  volition. 

2.  Involuntary  muscles,  comprising  those  the  activity  of  which  is  entirely 
independent  of  the  volition. 

The  voluntary  muscles  are  also  known  from  their  attachment  to  the 
skeleton  as  skeletal,  and  from  their  microscopic  appearance  as  striped  or 
striated  muscles.  Though  for  the  most  part  these  muscles  are  red,  there 
are  certain  muscles  in  man  and  other  animals  which  are  pale  in  color  and 
in  many  muscles  pale  fibers  are  extensively  distributed  among  the  red  fibers. 
The  involuntary  muscles,  from  their  relation  to  the  viscera  of  the  body,  are 
known  also  as  visceral,  and  from  their  microscopic  appearance  as  plain, 
smooth,  or.  non-striated  muscles. 

THE  VOLUNTARY  OR  SKELETAL  MUSCLE. 

All  skeletal  muscles  consist  of  a  central  fleshy  portion,  the  body  or  belly, 
provided  at  either  extremity  with  a  tendon  in  the  form  of  a  cord  or  mem- 
brane. The  body  is  the  active,  contractile  region,  the  source  of  the  move- 
ment; the  tendon  is  the  inactive  region,  the  passive  transmitter  of  the  move- 
ment to  the  bones. 

A  skeletal  muscle  is  a  complex  organ  consisting  of  a  framework  of 
connective    tissue,    supporting    muscle-fibers,    blood-vessels,    nerves,    and 

48 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


49 


lymphatics.  The  general  body  of  the  muscle  is  covered  by  a  dense  layer 
of  connective  tissue,  the  epimysium,  which  blends  with  and  partly  forms 
the  tendon.  From  the  under  surface  of  this  covering,  septa  of  connective 
tissue  pass  inward,  dividing  and  grouping  the  fibers  into  larger  and  smaller 
bundles,  termed  fasciculi.  The  fasciculi,  invested  by  a  special  sheath,  the 
perimysium,  are  prismatic  in  shape  and  on  cross-section  present  an  irregular 
outline.  The  muscle-fibers  composing  the  fasciculi  are  separated  one  from 
another  and  supported  by  a  very  delicate  connective  tissue,  the  endomysium. 
The  connective  tissue  thus  surrounding  and  penetrating  the  muscle  binds 
the  fibers  into  a  distinct  organ  and 
affords  support  to  all  remaining  struc- 
tures (Fig.  14). 

Histology  of  the  Skeletal  Mus- 
cle-fiber.— The  muscle-fiber  is  the 
ultimate  anatomic  unit  of  the  muscle 
system.  The  fibers  for  the  most  part 
are  arranged  parallel  one  to  another 
and  in  a  direction  corresponding  to 
the  long  axis  of  the  muscle.  They 
vary  in  length  from  30  to  40  millimeters 
and  in  breadth  from  20  to  30  micro- 
millimeters.  There  are  exceptional 
fibers,  however,  which  have  a  much 
greater  length.  As  the  fibers  have 
but  a  limited  length  in  the  vast  major- 
ity of  muscles,  each  end,  more  or  less 
pointed  or  beveled,  is  united  to  adjoin : 
ing  fibers  by  cement.  In  this  way  the 
length  of  the  muscles  is  built  up. 

When  examined  with  the  micro- 
scope, the  muscle-fiber  is  seen  to  be 
cylindric  or  prismatic  in  shape  and 
to  consist  of  a  thin  transparent  mem- 
brane, the  sarcolemma,  in  which  is 
contained  the  true  muscle  substance  or  sarcous  substance.  The  sarco- 
lemma is  elastic  and  adapts  itself  to  all  changes  of  form  the  sarcous  sub- 
stance undergoes.  Beneath  the  sarcolemma  there  are  several  nuclei 
surrounded  by  granular  material;  a  muscle-fiber  may  therefore  be  re- 
garded as  a  large  multinucleated  cell.  Each  fiber  also  presents  a  series 
of  transverse  bands  alternately  dim  and  bright  which  give  to  it  a  striated 
appearance.  If  the  bright  bands  are  examined  with  high  magnifying 
powers,  each  one  is  seen  to  be  crossed  by  a  fine  dark  line  which  at  the 
time  of  its  discovery  by  Krause  was  regarded  as  the  optic  expression  of  a 
membrane  attached  laterally  to  the  sarcolemma.  According  to  Rollet, 
it  is  composed  of  a  series  of  granules  so  closely  applied  as  to  give  rise  to  the 
appearance  of  a  continuous  line  (Fig.  15). 

The  muscle-fiber  also  presents  a  longitudinal  striation  which  indicates 
that  it  is  composed  of  finer  elements  placed  side  by  side,  termed  fibrillas. 
The  fibrillas  extend  throughout  the  entire  length  of  the  fiber,  though  they 
4 


Fig.  14. — From  a  Cross-sectiox  of  the 
Adductor  Muscle  of  a  Rabbit.  P.  Peri- 
mysium, containing  two  blood-vessels,  at  g; 
m,  muscle-fibers;  many  are  shrunken  and  be- 
tween them  the  endomysium,  p,  can  be  seen ; 
at  .V  the  section  of  muscle-fiber  has  fallen  out. 
X  ()o.—{Stdhr.) 


so 


TEXT-BOOK  OF  PHYSIOLOGY 


are  not  of  uniform  thickness.  That  portion  of  the  librilla  correspond- 
ing in  position  to  the  dim  band  is  thick,  prismatic,  or  rod-Hke  in  shape,  and 
termed  a  sarcostyle;  that  portion  corresponding  in  position  to  the  bright 
band  is  extremely  thin  and  narrow  and  presents  at  its  middle  a  slight  en- 
largement or  nodule.  The  fibrillae  are  embedded  in  a  clear  transparent  fluid 
which,  from  its  supposed  nutritive  character,  is  termed  sarcoplasm,  or 
interfibrillar  substance.  The  diminution  in  caliber  of  the  fibrillae  at  different 
levels  would  permit  of  the  accumulation  and  storage  of  a  larger  amount  of 
this  nutritive  material  than  could  otherwise  be  the  case.  It  is  for  this 
reason  that  the  fiber  at  these  points  presents  a  brighter  appearance. 

When  the  muscle-fiber  is  examined 
by  polarised  light,  the  dim  band  ap- 
pears bright  and  the  bright  band  appears 
dim  against  a  dark  background,  indicat- 
ing that  the  former  is  doubly  refracting 
or  anisotropic,  the  latter  singly  refracting 
or  isotropic. 


Fig.  i6 — A.  Diagram  of 
arrangement  of  the  contrac- 
tile substance  according  to 
the  view  of  Rollett;  the 
granular  figures  represent 
the  contractile  elements,  the 
intervening  light  areas  the 
sarcoplasm.  B.  Small 
muscle-fiber  of  man,  the 
corresponding  parts  in  the 
two  figures  are  indicated; 
t,  i,  I,  respectively  the  trans- 
verse, the  intermediate,  and 
lateral  discs.  ;?.  Muscle 
nuclei.— (Piersol.) 


Fig.  15. — Muscle-fiber 
OF  A  Rabbit.  a.  Dark 
band.  b.  Light  band.  c.  In- 
termediate line.  n.  Nucleus. 
— {Landois  and  Stirling.) 


This  interpretation  of  the  structure  of  the  muscle-fiber  has  been  subjected 
to  criticism  in  recent  years  by  Heidenhain.  This  observer  regards  the  trans- 
verse line  in  the  bright  band  as  did  its  discoverer  Krause  as  a  true  membrane 
which  is  attached  laterally  to  the  sides  of  the  sarcolemma.  The  fibrillae  he 
also  regards  as  continuous  but  of  uniform  thickness,  and  passing  directly 
through  the  transverse  membrane  by  which  they  are  supported  and  main- 
tained in  their  normal  relation.  In  this  view  the  fibrilla  consists  of  alternate 
regions  of  a  doubly  refracting  and  a  singly  refracting  material.  The  sarco- 
plasm is,  therefore,  confined  to  the  interfibrillar  spaces.    Fig.  17. 

The  fiber  of  the  pale  muscle  is  similar  histologically  to  the  fiber  of  the 
red  muscle.  It,  however,  does  not  contain  so  much  granular  protoplasm  as 
does  the  fiber  of  the  red  muscle  and  hence  does  not  intercept  the  light  to  the 
same  extent.  The  greater  the  quantity  of  granular  protoplasm  the  darker 
the  muscle. 

The  Blood-supply. — Muscles  in  the  physiologic  condition  require  for 
the  maintenance  of  their  activity  a  large  amount  of  nutritive  material.     This 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


51 


is  obtained  directly  from  the  lymph  and  indirectly  from  the  blood  furnished 
by  the  blood-vessels.  The  vascular  supply  to  the  muscles  is  very  great  and 
the  disposition  of  the  capifiary  vessels  with  reference  to  the  muscle-fiber  is 
very  characteristic.  The  arterial  vessels,  after  entering  the  muscle,  are 
supported  by  the  peri-mysium;  in  this  situation  they  give  off  short  transverse 
branches,  which  immediately  break  up  into  a  capillary  network  of  rectangu- 
lar shape  within  which  the  muscle-fibers  are  contained. 

The  muscle-fiber,  in  intimate  relation  with  the  capillary,  is  bathed  with 
lymph  derived  from  it.     Its  contractile  substance,  how- 
ever,  is   separated  from  the  lymph  by  its  own  investing 
membrane,  through  which  all  interchange  of  nutritive  and 
waste  material  must  take  place.  "/ 

The  nutritive  material  passes  through  the  capillary 
wall  into  the  lymph-space,  then  through  the  sarcolemma 
into  the  interior  of  the  fiber,  where  it  comes  into  relation  j,i 
with  the  living  muscle  material.  The  waste  products 
arising  in  the  muscle  as  a  resiilt  of  nutritive  changes  pass 
in  the  reverse  direction  first  into  the  lymph  and  then  into 
the  blood,  by  which  they  are  carried  away  to  eliminating 
organs.  Lymphatics  are  present  in  muscle,  but  confined 
to  the  connective  tissue,  in  the  spaces  of  which  they  take 
their  origin.  "' 

The  Nerve-supply. — The  nerv^es  which  carry  the 
stimuli  to  a  muscle  enter  near  its  middle  point.  Many 
of  the  fibers  pass  directly  to  the  muscle-fibers  with  which  : 

they  are  connected;  others  are  distributed  to  blood-vessels. 
Every  muscle-fiber  is  supplied  with  a  special  ner\-e-fiber  b^^SBisi^ 

except  in  those  instances  where  the  nerve-trunks  entering  fjljljjuillj 

a  muscle  do  not  contain  as  many  fibers  as  the  muscle.  In  p-j^  j-  _dia- 
such  cases  the  nerve-fibers  divide  near  their  termination  gram  of  Muscle 
until  the  number  of  branches  equals  the  number  of  muscle-  ?J^^/°^^'~^P'^''f" 
fibers.  The  individual  muscle-fiber  is  penetrated  near  Histology.")  The 
its  center  by  the  nerve  where  it  terminates;  the  ends  fibrillae  consists  of 
being  practically  free  from  ner\^e  influence.  The  stimulus  ^^randUgh^bandJ' 
that  comes  to  the  muscle-fiber  acts  primarily  upon  its  Lb. '  l.b.  is  crossed 
center,  the  effect  of  which  then  travels  in  both  directions  ^y  Krause's  mem- 
to  the  ends.  The  manner  in  which  the  nerv^e-fibers  termi-  membrane  ^  crosses 
nate  in  muscle  will  be  more  fully  described  in  connection  the  dim  band  accord- 
with  the  histology  of  the  nerve  tissue.  ^"g  *°  Heidenliain. 


1  h  il  '1 II  !i!|. 

[iMIlJllJlj 

■\W\  |l 

1    '!  1     !       ll 

i[    ll 

j 

1 

CHEMIC  COMPOSITION  OF  MUSCLE. 


The  chemic  composition  of  living  muscle  is  but  imperfectly  understood 
owing  to  the  fact  that  shortly  after  death  some  of  its  constituents  undergo  a 
spontaneous  coagulation  and  for  the  reason  that  the  methods  employed  for 
analysis  also  tend  to  alter  its  composition.  To  human  muscle,  the  following 
average  percentage  composition  has  been  given : 


5^  TEXT-BOOK  OF  PHYSIOLOGY. 

Water,.... 73.5 

Proteins,    including    those    of    sarcolemma,    connective    tissue, 

pigments, i8 .02 

Gelatin, i  -99 

Fat, 2.27 

Extractives, 0.22 

Inorganic  salts, 312   (Halliburton.) 

When  fresh  muscle  is  freed  from  fat  and  connective  tissue,  frozen,  rubbed 
up  in  a  mortar,  and  expressed  through  Hnen,  a  slightly  yellow  syrupy  alkaline 
or  neutral  liquid  is  obtained  which  has  been  termed  muscle-plasma.  This 
fluid  at  normal  temperatures  coagulates  spontaneously,  the  phenomena 
resembling  in  many  respects  those  observed  in  the  coagulation  of  blood- 
plasma.  The  coagulum  subsequently  contracts  and  squeezes  out  an  acid 
muscle-serum.  The  coagulated  protein  partakes  of  the  nature  of  fibrin  and 
belongs  to  the  class  of  globulins.  Inasmuch  as  it  is  not  present  in  living 
muscle  and  only  makes  its  appearance  under  conditions  not  strictly  physio- 
logic, it  is  regarded  as  a  derivative  of  pre-existing  proteins.  An  analysis  of 
muscle-plasma  has  shown  the  presence  of  at  least  two  proteins  which  are 
distinguished  by  their  varying  solubilities  in  different  salt  solutions,  and  by 
the  varying  temperatures  at  which  they  coagulate.  One  of  these  proteins 
coagulates  at  about  47°  C.  and  because  of  its  chemic  relations  has  been 
termed  myosin  or  paramyosinogen;  the  other  coagulates  at  about  56°  C. 
and  for  similar  reasons  has  been  termed  myogen  or  myosinogen.  The 
latter  is  three  or  four  times  more  abundant  than  the  former.  If  the  tempera- 
ture of  the  cooled  plasma  be  permitted  to  rise,  both  myosin  and  myogen 
undergo  a  change  of  state  termed  coagulation.  The  substances  resulting 
are  known  as  myosin  fibrin  and  myogen  fibrin.  It  is  not  known  whether 
these  changes  are  due  to  the  action  of  an  enzyme  or  not.  A  similar  change 
in  myosin  and  myogen  occurs  after  death,  giving  rise  to  the  condition  known 
as  death  stiffening  or  rigor  mortis.  The  coagulation  of  these  proteins  in  this 
instance  is  probably  caused  by  the  presence  and  accumulation  of  metabolic 
products.  From  the  muscle-serum,  according  to  Halliburton,  may  also  be 
obtained  at  68°  C.  a  globulin  body  termed  myoglobulin  and  a  small  quantity 
of  myoalbumin.  Among  the  proteins  may  be  mentioned  hemoglobin,  which 
gives  the  color  to  the  muscles.  Spectroscopic  investigation  reveals  the 
presence  of  a  special  pigment,  myohematin,  which  is  supposed  to  have  a 
respiratory  function,  inasmuch  as  its  spectral  absorption  bands  change  by 
oxidation  and  reduction. 

Among  the  extractives  containing  nitrogen  may  be  mentioned  creatin, 
creatinin,  xanthin,  carnin,  urea,  uric  acid,  carnic  acid,  etc.  Among  the 
extractives  free  of  nitrogen,  glycogen,  dextrose,  inosite,  lactic  acid  and  fat,  are 
the  most  important.  Inorganic  salts  are  relatively  abundant,  of  which 
potassium  is  the  most  abundant  among  the  bases,  and  phosphoric  acid 
among  the  acids. 

THE  PHYSICAL  AND  PHYSIOLOGIC  PROPERTIES  OF  MUSCLE -TISSUE. 

Consistency. — The  consistency  of  muscle-tissue  during  life  varies 
considerably  in  accordance  with  different  states  of  the  muscle.  In  a  state  of 
tension  it  is  hard  and  resistant;  in  the  absence  of  tension  it  is  soft  and  fluctu- 
ating to  the  sense  of  touch.     Tension  alone  efives  rise  to  hardness. 


GENERAL  PHYSIOLOGY  OF  MUSCLE  TISSUE. 


53 


iS. — Extension   Curve  tdf 
Muscle. — {Gad.) 


Cohesion. — The  cohesion  of  a  muscle  is  largely  dependent  on  the  quan- 
tity of  connective  tissue  it  contains.  A  band  of  fresh  human  muscle  one 
square  centimeter  in  cross-section  has  been  found  able  to  resist  a  weight  of 
14  kilograms  without  rupture  (MacAlister).  Cohesion  resists  the  forces  of 
traction  and  pressure. 

Elasticity. — Muscle,  in  common  with  many  other  organic  as  well  as 
inorganic  substances,  is  capable  of  being  ex- 
tended beyond  its  normal  length  by  the  action 
of  external  forces  and  of  resuming  the  normal 
length  when  these  forces  cease  to  act.  All  such 
bodies  are  said  to  be  elastic;  and  the  greater  the 
variations  between  the  natural  and  acquired 
lengths,  the  greater  is  their  elasticity  said  to 
be.  Muscles,  therefore,  possessing  extensibility 
and  retractility  are  said  to  be  elastic.  If  the 
muscle  of  a  frog,  preferably  the  sartorius,  the 
fibers  of  which  are  arranged  in  a  practically 
parallel  manner,  be  fastened  at  one  extremity 
by  a  clamp,  and  then  extended  by  a  series  of 
successive  weights  which  differ  by  a  common 
increment,  it  will  be  found  that  the  extensi- 
bility of  muscle  does  not  follow  the  law  of 
elasticity  as  determined  for  inorganic  bodies;  p-j^, 
i.e.,  directly  proportional  to  the  weight  and  to  the 
length  of  the  body  extended;  but  that  while  in- 
creasing in  length  with  each  successive  weight,  the  increase  is  always  in  a 
diminishing  ratio.  Thus,  for  example,  as  shown  in  Fig.  18:  The  exten- 
sion produced  by  5  grams  is  5  millimeters,  that  produced  by  10  grams  is 
only  4  millimeters  more,  and  so  on  with  additional  weights  until  the  in- 
'crease  in  passing  from  25  to  30  grams  is  only  i  millimeter.  The  exten- 
sibility is  thus  shown  to  be  proportionately 
greater  with  small  than  with  larger  weights.  It 
is,  however,  actually  greater  with  the  larger 
weights.  The  extension  cun-e  A  B  formed  by 
joining  the  ends  of  the  muscle  approximates  that 
of  a  parabola.  The  behavior  of  the  muscle 
in  returning  to  its  original  length  also  shows  a 
variation  from  the  behavior  of  inorganic  bodies. 
With  the  successive  removal  of  the  weights,  the 
elasticity  of  the  muscle  asserts  itself  with  gradu- 
ally increasing  energy  until  its  normal  length  is 
nearly,  if  not  entirely,  regained  (Fig.  19).  It  is 
usually  stated  that  the  elasticity  of  muscle  is  in- 
complete, but  it  must  be  borne  in  mind  that  the  experiments  have  usually 
been  made  on  muscles  removed  from  the  body,  deprived  of  blood  and 
ner\^e  influences,  and  hence  under  abnormal  conditions.  It  is  highly  prob- 
able that  in  the  living  body  muscles  possess  perfect  elasticity  which  enables 
them  completely  to  return  to  their  normal  length  after  extension.  The 
extension  and  retraction  or  elastic  recoil  of  muscle  depends  on  the  main- 


FiG.  19. — Curve  of  Elas- 
ticity Produced  by  Continu- 
ous Extension  and  Recoil 
OF  a  Frog's  Muscle,  o  x.  Ab- 
scissa before;  x',  after  exten- 
sion.— {Landois  and  Stirling.) 


54  TEXT-BOOK  OF  PHYSIOLOGY. 

tenance  of  physiologic  conditions.  If  the  nutrition  is  impaired  by  fatigue, 
deficient  blood-supply,  or  any  pathologic  condition,  the  elasticity  is  at  once 
impaired. 

Tonicity. — Muscle  tonus  may  be  defined  as  a  state  of  tension  of  a 
muscle  due  to  a  slight  but  continuous  contraction  of  its  individual  fibers  in 
consequence  of  which  it  tends  to  become  shorter,  and  would  do  so,  were  it 
not  restrained  by  its  attachments.  As  a  result  of  this  tension  its  efficiency 
as  a  quickly  responsive  motor  organ  is  increased.  Though  the  skeletal 
musculature  of  the  body  as  a  whole  is  in  a  state  of  tonus,  individual  muscles 
vary  in  the  degree  of  their  tonus  from  time  to  time  in  consequence  of  varia- 
tions in  the  causes  that  give  rise  to  it.  That  such  a  tonus  or  tension  exists 
is  apparently  show^n  by  the  fact  that  when  a  muscle  in  a  living  animal  is 
divided  the  two  portions  will  retract  and  separate  a  certain  distance.  This 
would  indicate  that  the  muscle  even  in  a  state  of  relative  rest  is  in  a  slight 
degree  of  contraction. 

This  condition  of  tonus  is  attributed  to  the  continuous  arrival  of  nerve 
impulses  through  efferent  nerves  discharged  by  nerve-cells  in  the  spinal  cord. 
The  tonus  was  therefore  at  one  time  attributed  to  an  automatic  activity  of 
the  spinal  cord.  Brondgeest,  however,  showed  that  this  was  not  the  case, 
but  that  the  activity  of  the  spinal  cord  and  hence  the  tonus  of  the  muscles  is 
partly  reflex  in  origin  inasmuch  as  it  largely  disappears  on  division  of  the 
posterior  or  dorsal  roots  of  the  spinal  nerves.  The  afferent  impulses  excit- 
ing the  cord  refiexly  may  come  from  the  skin  in  which  they  are  developed 
by  the  impressions  made  by  external  stimuli,  or  from  the  tendons  and 
muscles  themselves  in  which  they  are  developed  by  the  slight  degree  of 
extension  and  variations  in  extension  to  which  these  are  subjected  from 
moment  to  moment.  That  this  latter  is  a  considerable  factor  in  the  pro- 
duction of  the  tonus  is  shown  by  the  effects  which  follow  division  of  the 
afferent  nerves  coming  from  any  given  muscle  group;  with  the  division  of 
the  nerves  the  muscles  relax  and  lose  their  usual  tone.  It  is  also  probable 
that  the  activity  of  the  cord  is  partly  the  result  of  impulses  descending  the 
cord  in  consequence  of  cerebral  and  sense  organ  activities. 

Muscle  tonus  or  elastic  tension  plays  an  important  role  in  muscle  con- 
traction; being  always  on  the  stretch  the  muscle  loses  no  time  in  acquiring 
that  degree  of  tension  necessary  to  immediate  action  on  the  bone  to  which 
it  is  attached.  The  working  power  of  a  muscle  is  also  considerably  increased 
by  the  presence  within  limits  of  some  resistance  to  the  act  of  contraction. 
According  to  Marey,  the  amount  of  work  is  considerably  increased  when  the 
muscle  energy  is  transmitted  by  an  elastic  body  to  the  mass  to  be  moved, 
while  at  the  same  time  the  shock  of  the  contraction  is  lessened.  The  posi- 
tion of  a  passive  limb  is  the  resultant  also  of  the  elastic  tension  of  antago- 
nistic groups  of  muscles.  Again  as  a  result  of  the  slight  but  continuous 
stimulation  from  the  spinal  cord  the  metaboHc  changes  in  the  muscle  material 
are  maintained  at  a  certain  level,  with  a  corresponding  production  of  heat. 
A  function  of  the  tonicity  would  thus  be  the  production  of  heat,  other  functions 
which  the  tone  subser\'es  being  more  or  less  secondary. 

Irritability,  Contractility. — These  are  terms  employed  to  denote 
that  property  of  muscle-tissue  by  virtue  of  which  it  responds  by  a  change  of 
form,  becoming  shorter  and  thicker  on  the  application  of  a  stimulus.     On 


GENEIL\L  PHYSIOLOGY  OF  MUSCLE  TISSUE.  55 

the  withdrawal  of  the  stimulus  the  muscle  again  undergoes  a  reverse  change 
of  form,  becoming  longer  and  narrower,  and  returning  to  its  original  condi- 
tion. All  muscles  which  possess  this  capability  are  said  to  be  irritable  and 
contractile;  and  all  agents  which  call  forth  this  response  of  the  muscle  are 
termed  stimuli.  The  rapid  change  of  form  which  a  highly  irritable  muscle 
undergoes  in  response  to  the  action  of  a  stimulus  of  short  duration  is  usually 
termed  a  twitch  or  pulsation.  With  appropriate  apparatus  it  can  be  shown 
that  the  muscle  at  the  time  of  the  twitch  becomes  warmer  and  exhibits  electric 
phenomena.  The  muscle  is  therefore  an  apparatus  for  the  conversion  of 
potential  into  kinetic  energy:  viz.,  heat,  electricity,  and  mechanic  motion. 

Though  usually  associated  with  the  activity  of  the  nen^e  system,  and  to 
some  extent  dependent  on  it,  irritability  is  nevertheless  an  independent 
endowment  of  the  muscle  and  persists  for  a  longer  or  shorter  period,  as 
shown  by  many  experiments,  after  all  nerve  connections  have  been  destroyed. 
Among  the  proofs  which  may  be  presented  in  support  of  this  view  are  the 
following:  The  introduction  of  the  drug,  curara,  into  the  body  of  an  animal 
produces  in  a  short  time  complete  paralysis.  Experiment  has  shown  that 
curara  suspends  the  conductivity  of  the  intramuscular  terminations  of  the 
nerv^e-fiber  and  thus  separates  the  muscle  entirely  from  the  nerve.  Though 
the  animal  is  incapable  of  executing  a  single  movement,  its  muscles  respond 
promptly  on  the  application  of  a  stimulus.  Moreover,  portions  of  muscles 
exhibit  irritability  although  containing  no  trace  of  nerve  structure.  This  is 
the  case  with  the  ends  of  the  sartorius  muscle  of  the  frog  and  the  anterior 
end  of  the  retractor  muscle  of  the  eyeball  of  the  cat.  These  and  other 
facts  demonstrate  the  independence  of  muscle  irritability. 

In  the  living  body  irritability  and  nutritive  activity,  with  which  it  is 
closely  associated,  are  maintained  by  a  due  supply  of  oxygen,  and  of  nutritive 
material,  the  removal  of  waste  products,  and  a  normal  temperature.  The 
muscles  of  the  cold-blooded  animals,  for  example  the  frog,  retain  their 
irritability  for  a  much  longer  period  after  death  than  the  muscles  of  the 
warm-blooded  animals.  This  is  the  case  also  with  the  individual  muscles 
after  removal  from  the  body  of  the  animal.  The  reason  for  this  is  found  in 
all  probability  in  the  difference  in  the  rate  of  their  nutritive  activities  and  in 
the  quantity  of  nutritive  material  stored  up  in  their  cells.  The  duration  of 
the  irritability  of  isolated  muscles  can  be  considerably  prolonged  by  keeping 
them  in  a  moist  atmosphere. 

Conductivity. — All  muscle  protoplasm  possesses  conductivity.  The 
change  excited  in  a  muscle-fiber  by  the  arrival  of  a  nerve  impulse  is  at  once 
conducted  with  great  rapidity  in  opposite  directions  to  the  ends  of  the  fiber; 
the  advance  of  the  excitation  process  is  immediately  succeeded  by  the  con- 
traction process,  the  change  of  form  which  constitutes  the  contraction. 
With  the  disappearance  of  the  former,  the  latter  also  disappears  and  the 
muscle  resumes  its  previous  passive  condition.  There  is  no  evidence,  how- 
ever, that  the  excitation  process  travels  transversely — that  is,  into  adjoining 
fibers — being  prevented  from  doing  so  by  the  presence  of  the  .  limiting 
membranes,  the  sarcolemmata.  The  fact  that  each  muscle-fiber  receives 
its  own,  or  at  least  a  branch  of  a  nerve-fiber,  and  hence  its  own  nerve  impulse 
or  stimulus,  would  also  indicate  that  the  excitation  process  cannot  be  con- 
ducted longitudinally  into  adjoining  fibers,  or  at  least  with  sufficient  rapidity 


56  TEXT-BOOK  OF  PHYSIOLOGY. 

for  the  purposes  of  ordinary  muscle  actions.  Nevertheless  if  a  long  muscle, 
such  as  the  sartorius,  from  a  curarized  frog  be  stimulated  at  one  end  with 
an  induced  electric  current,  the  excitation  and  the  contraction  processes  will 
be  conducted  with  extreme  rapidity  to  the  opposite  end  of  the  muscle.  The 
rapidity  of  conduction  in  human  muscles  has  been  estimated  at  from  lo  to  13 
meters  per  second,  and  in  frog's  muscle  at  from  3  to  4.5  meters  per  second. 
The  contraction  process,  the  thickening  of  the  muscle,  is  termed  the  con- 
traction wave.  As  it  is  the  result  of  the  excitation  process  and  immediately 
succeeds  it,  its  rate  of  conduction  must  be  the  same  as  that  given  above. 
With  appropriate  apparatus  the  duration  of  the  wave  at  any  given  point  has 
been  shown  to  be,  in  the  frog's  muscle,  about  one-tenth  of  a  second  and  its 
three-tenths  of  a  meter. 

Muscle  Stimuli. — Though  consisting  of  a  highly  irritable  tissue,  muscles 
do  not  possess  spontaneity  of  action.  They  rec[uire  for  the  manifestation 
of  their  characteristic  activity  the  application  of  a  stimulus.  In  the  living 
body  all  contractions,  at  least  of  the  skeletal  muscles,  occurring  under 
normal  or  physiologic  conditions  are  caused  by  the  action  of  "nerve  im- 
pulses" transmitted  by  the  nerves  from  the  central  nen^e  system  to  the 
muscles.  The  nerve  impulse  is  the  normal  or  physiologic  stimulus.  After 
removal  from  the  body  and  freed  from  nerve  connections  muscles  can  be 
excited  to  action  by  various  agents  of  a  mechanic,  chemic,  thermic,  or  electric 
nature.     These  are  artificial  or  non-physiologic  stimuli. 

1.  Mechanic  Stimuli. — Cutting,  pinching,  sharply  tapping  the  muscle  will 

cause  it  to  contract,  providing  the  stimulus  has  sufficient  intensity. 
With  each  stimulation  a  short,  fleeting  contraction  ensues.  If  repeated 
with  sufficient  rapidity,  a  series  of  continuous  but  irregular  pulsations 
are  produced. 

2.  Chemic  Stimuli. — Various  chemic  substances  in  solution  will  excite  single 

or  continuous  pulsations  if  the  strength  of  the  solution  is  not  such  as  to 
destroy  at  once  the  irritability.  They  owe  their  efficiency  as  stimuli  to 
the  rapidity  with  which  they  alter  the  composition  of  the  muscle-sub- 
stance. Among  these  may  be  mentioned  solutions  of  potassium  and 
sodium  salts,  weak  solutions  of  the  mineral  and  organic  acids,  ammo- 
nium vapor,  distilled  water,  glycerin,  and  sugar. 

3.  Thermic  Stimuli. — The  application  of  a  heated  object,  such  as  a  hot 

wire,  causes  the  muscle  to  contract  rapidly. 

4.  Electric  Stimuli. — The  most  efficient  stimulus  and  the  one  least  injurious 

to  the  tissue  is  the  electric  current.     Either  the  constant  or  the  induced 

current  may  be  used.^ 
The  Constant  Current. — If  the  ends  of  the  wires  in  connection  with  an 
electric  cell  be  provided  with  non-polarizable  electrodes  and  the  latter  placed 
on  opposite  ends  of  a  freshly  prepared  sartorius  muscle  of  a  frog  which  has 
been  previously  curarized,  it  will  be  found  on  closing  or  making  the  circuit 
that  the  muscle  will  exhibit  a  short,  quick  pulsation.  During  the  actual 
passage  of  the  current,  especially  if  it  is  weak,  there  may  be  no  apparent 

'  Since  the  study  of  the  physiologic  properties  of  both  muscle-tissue  and  nerve-tissue 
involves  the  employment  of  electricitv  as  a  stimulus,  it  is  necessary  for  the  student  to 
familiarize  himself  with  certain  forms  of  apparatus  by  which  it  is  generated,  controlled,  and 
applied.  To  avoid  interrupting  the  continuity  of  the  text  this  information  is  embodied 
in  an  appendix.     The  facts  therein  contained  should  be  mastered  at  this  time  by  the  student. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  57 

change  in  the  muscle.  If  the  current  is  strong,  the  muscle  may,  on  the 
contrary,  remain  in  a  state  of  continuous  contraction.  With  the  opening  or 
breaking  of  the  current  the  muscle  at  once  relaxes,  or  perhaps  again  contracts 
and  then  relaxes.  The  extent  of  the  contraction  depends  mainly  on  the 
strength  of  the  current,  being  greater  with  strong,  less  with  weak  currents. 
When  the  current  is  sufficiently  strong  to  elicit  both  making  and  breaking 
contractions,  it  is  found  that  the  contraction  occurring  on  the  make  or  closure 
of  the  circuit  is  regularly  greater  than  that  occurring  on  the  break  or  opening 
of  the  circuit.  Moreover,  it  has  been  shown  in  many  ways  that  the  con- 
traction occurring  on  the  closure  of  the  circuit  has  its  origin  at  the  point 
where  the  current  is  leaving  the  muscle — i.e.,  in  the  immediate  neighborhood 
of  the  negative  pole  or  cathode — and  propagates  itself  to  the  opposite  extrem- 
ity; while  the  contraction  occurring  on  the  opening  of  the  circuit  has  its 
origin  at  the  point  where  the  current  is  entering  the  muscle,  i.e.,  in  the 
neighborhood  of  the  positive  pole  or  anode. 

These  facts  can  be  readily  demonstrated  by  destroying  the  irritability 
and  contractility  of  one  extremity  of  a  muscle  with  parallel  fibers  such  as  the 


A.  B. 

Fig.  20. — Diagram  to  Show  the  Effect  of  Local  Injtjry  ox  the  Irritability  of  a 
Muscle. — {After  Starling.)  C  Z  an  electric  cell  from  which  wires  pass  to  non-polarizable  elec- 
trodes, anode  and  kathode,  in  contact  with  a  muscle,  the  injured  end  of  which  is  more  deeply 
shaded.     The  arrows  indicate  the  direction  of  the  current. 

sartorius.  On  applying  non-polarizable  electrodes  to  the  muscle  as  in  Fig. 
20,  A,  it  will  be  found  that  when  the  circuit  is  made  a  contraction  occurs 
which  must,  of  course,  have  developed  at  the  irritable  cathodic  region,  for 
on  the  break  of  the  circuit  the  muscle  remains  at  rest.  When  the  electrodes 
are  applied  as  in  Fig.  20,  B,  and  the  circuit  made  the  muscle  remains  at 
rest,  but  on  the  break  of  the  circuit  a  contraction  occurs  which  must  have 
developed  at  the  irritable  anodic  region. 

The  Induced  Current. — If  the  primary  coil  of  the  inductorium  be  con- 
nected with  an  electric  cell  and  the  secondar}'  coil  be  connected  with  a 
muscle,  it  wall  be  found  that  the  current  induced  in  the  secondary  circuit, 
both  on  the  make  and  break  of  the  primary,  will  also  cause  the  muscle  to 
pulsate  sharply  and  rapidly  if  the  two  coils  are  sufficiently  near  each  other. 
Obser\'ation,  however,  makes  it  evident  that  the  pulsation  occurring  with 
the  break  of  the  primary  circuit  is  more  energetic  than  that  occurring  with 
the  make,  a  result  the  opposite  of  that  obtained  with  the  constant  current. 
This  is  not  due  to  any  difference  in  the  electricity,  however,  but  to  peculiari- 
ties in  the  construction  of  the  inductorium.  When  the  primary  circuit  is 
interrupted  with  sufficient  frequency,  as  it  can  be  by  throwing  into  the  circuit 
Neef's  hammer  or  some  other  form  of  interrupter,  the  contractions  excited 


58  TEXT-BOOK  OF  PHYSIOLOGY. 

by  the  induced  currents  may  be  made  to  succeed  one  another  so  rapidly  that 
they  become  fused  together,  producing  a  spasm  or  tetanus  of  the  muscle. 
The  rapidity  with  which  the  induced  current  appears  and  disappears,  its 
brief  duration,  the  ease  with  which  its  strength  can  be  regulated,  combine 
to  render  it  a  most  efficient  stimulus  for  either  muscle  or  nerve. 

PHENOMENA  FOLLOWING  A  MUSCLE  STIMULATION. 
PHYSICAL  PHENOMENA. 

Physiologic  investigation  has  made  it  apparent  that  when  a  nerve  impulse 
reaches  a  muscle,  it  occasions  a  disruption  of  certain  complex  energy- 
holding  compounds  and  their  subsecjuent  oxidation  to  simpler  compounds. 
Coincidently  with  the  chemic  changes  there  is  a  transformation  of  the 
potential  energy  of  the  molecules  into  kinetic  energy  which  manifests 
itself  under  three  forms,  heat,  electricity  and  mechanic  motion,  or  a  change 
of  shape  of  the  muscle.  These  phenomena  vary  in  extent  in  accordance  with 
the  intensity  of  the  impulse  as  well  as  the  frequency  of  its  repetition. 
Though  the  chemic  changes  are  the  first  effects  of  the  action  of  the  nerve 


.:-i;J 


%i. 


Fig.  21.— Showing  the  Changes  in  a  Muscle  and  Muscle-fiber  during 

Contraction. 

impulse  and  the  ones  on  which  other  phenomena  depend,  it  will  be  found 
convenient  to  consider  the  most  evident  efTect,  the  physical  change  in  the 
shape  of  the  muscle,  first. 

Change  of  Shape. — The  most  obvious  change  in  a  muscle  following  the 
arrival  of  a  nerve  impulse  is  that  relating  to  its  form.  The  muscle  not  only 
becomes  shorter,  but  at  the  same  time  thicker.  The  extent  to  which  it  may 
shorten  when  unopposed  may  amount  to  30  per  cent,  or  more  of  its  original 
length.  The  increase  in  thickness  practically  compensates  for  the  diminu- 
tion in  length,  for  tkere  is  no  observable  diminution  in  volume.  The  change 
in  form  of  the  entire  muscle  results  from  a  corresponding  change  of  form  of 
its  individual  fibers  as  determined  by  microscopic  examination,  each  of 
which  becomes  shorter  and  thicker.  The  successive  changes  in  both  the 
muscle  and  the  individual  fibers  are  represented  in  Fig.  21. 

When  the  contraction  begins  both  the  dim  and  bright  bands  diminish  in 
length,  but  at  the  same  time  increase  in  breadth.  This  continues  until  the 
contraction  reaches  its  maximum.     The  diminution  in  the  length  of  the 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


59 


bright  band  is  greater  proportionally  than  the  diminution  in  the  length  of  the 
dim  band,  a  fact  which  gave  rise  to  the  supposition  on  the  part  of  Englemann 
that  there  is  at  the  time  of  the  contraction  a  passage  of  fluid  material  from 
the  bright  into  the  dim  band  or  from  the  sarcoplasm  into  the  sarcostyles. 
When  the  relaxation  begins,  a  reverse  change  in  the  dim  and  bright  bands 
sets  in  and  continues  until  they  regain  their  former  shape  and  volume.  Coin- 
cidently  there  is  a  passage  of  fluid  material  from  the  sarcostyles  to  the 
sarcoplasm.  As  the  contraction  wave  reaches  its  maximum  the  optic  proper- 
ties of  the  bright  and  dim  bands  change.  The  former  now  becomes  darker 
and  less  transparent  until  at  the  crest  of  the  wave  it  assumes  the  appearance 
of  a  distinct  dark  band;the  latter  now  becomes  clear  and  bright  in  compari- 
son. This  change  in  the  appearance  of  the  fiber  is  due  to  an  increase  in 
refrangibihty  of  the  bright,  and  a  decrease  in  the  refrangibility  of  the  dim 
band,  coincident  with  the  passage  of  the  fluid  from  the  former  into  the  latter. 
There  is  at  the  height  of  the  contraction  a  complete  reversal  in  the  positions 
of  the  striations.  At  a  certain  stage  between  the  beginning  and  the  crest  of 
the  wave  the  striae  almost  entirely  disappear,  giving  to  the  fiber  an  appearance 


0            5 

lO" 

s 

1 

5               2 

0              2 

5             3 

0 

""> 

"    -    -. 

--,.^ 

^ 

P 

Fig.  22. — Extension  Curves:     B  B',  of  the  resting;  b  B'.  of  the  contracting  muscle. 

of  homogeneity.  There  is,  however,  no  change  in  refractive  power  as  shown 
by  the  polarizing  apparatus.  When  the  contraction  w^ave  has  reached  the 
stage  of  greatest  intensity,  there  is  a  reversal  of  the  above  phenomena  as  the 
fiber  returns  to  its  former  condition,  that  of  relaxation. 

Change  of  Elasticity. — During  the  contraction  of  a  muscle  there  is  a 
greater  or  less  alteration  in  its  elasticity,  as  shown  by  the  fact  that  it  is  ex- 
tended to  a  greater  degree  by  the  same  weight  in  the  active  than  in  the  passive 
condition.  The  degree  to  which  the  extensibihty  is  increased  and  the  elas- 
ticity decreased  is  dependent  on  the  amount  of  the  resisting  force.  These 
facts,  as  determined  experimentally,  are  represented  in  Fig.  22.  Let  A  B 
and  A  b  represent  the  length  of  the  normal  unweighted  muscle,  passive  and 
active  states  respectively;  the  line  B  B',  the  extension  curve  of  the  passive 
muscle  produced  by  successive  weights,  5,  10, 15,  20,  25, 30  grams,  differing  by 
a  common  increment;  the  line  b  B',  the  extension  curve  of  the  active  con- 
tracted muscle  when  weighted  with  the  same  weights;  A'  B'  the  length  of 
the  muscle  when  the  weight  is  sufiiciently  great  to  prevent  shortening. 
It  will  be  obsen-ed  from  these  facts  that  while  the  muscle  is  extended  in 


6o  TEXTBOOK  OF  PHYSIOLOGY. 

both  the  passive  and  active  states  by  corresponding  weights,  the  extension 
during  the  latter  is  progressively  greater,  until  with  a  given  weight  the  length 
of  the  muscle  is  the  same.  Under  such  circumstances,  there  being  no  short- 
ening of  the  muscle,  the  force  of  its  contraction  manifests  itself  physically 
merely  as  tension.  In  the  successive  actions  of  the  muscle  represented  in 
the  same  figure  there  is  to  be  observed  also  a  combination  of  a  change  of 
length  and  a  change  of  tension,  the  ratio  of  the  one  to  the  other  being  deter- 
mined by  the  amount  of  the  supported  weights.  When  the  weight  is  slight 
in  amount,  the  shortening  of  the  muscle  reaches  a  maximum  and  the  tension 
a  minimum;  when  the  weight  is  large  in  amount,  the  reverse  conditions  obtain. 

GRAPHIC  REPRESENTATION  OF  THE  CHANGE  OF  SHAPE. 

The  contraction  of  a  muscle  as  it  takes  place  in  the  living  body  and 
under  normal  physiologic  conditions  is  a  complex  act  persisting  for  a  variable 
length  of  time  in  accordance  with  the  number  of  stimuli  transmitted  to  it 
in  a  given  unit  of  time,  and  as  determined  experimentally  is  the  resultant 
of  the  fusion  of  a  greater  or  less  number  of  separate  and  individual  contrac- 
tions or  pulsations.  To  this  enduring  contraction  the  term  tetanus  has  been 
given.  With  the  aid  of  appropriate  apparatus  it  has  become  possible  to 
obtain  and  record  single  muscle  contractions,  to  analyze  and  decompose  them 
into  their  constituent  elements,  or  to  combine  them  in  such  a  manner  as  to 
produce  practically  a  normal  physiologic  tetanus.  As  in  the  experimental 
study  of  the  phenomena  of  muscle  contraction  it  frecjuently  becomes  neces- 
sary to  remove  the  muscle  from  the  body  of  the  animal,  the  muscle  of  warm- 
blooded animals  are  not  well  adapted  for  this  purpose,  owing  to  the  rapid 
alteration  in  composition  they  undergo,  with  a  consecjuent  loss  of  irritability, 
when  deprived  of  their  normal  blood-supply.  The  excised  muscles  of  cold- 
blooded animals,  such  as  the  frog — in  which,  owing  to  the  relatively  slow 
rate  of  the  nutritive  activities,  the  irritability  and  contractility  endure  for 
a  relatively  long  period  of  time,  even  though  deprived  of  blood — are  particu- 
larly valuable  for  experimental  studies.  The  muscles  generally  employed 
are  the  gastrocnemius,  the  sartorius,  and  the  hyoglossus.  If  kept  at  a  normal 
temperature  and  moistened  with  0.6  per  cent,  solution  of  sodium  chlorid, 
such  a  muscle  will  contract  for  a  long  period  of  time  on  the  application  of 
any  form  of  stimulus,  but  especially  the  electric. 

Method  of  Recording  a  Muscle  Contraction. — Inasmuch  as  the 
changes  in  the  form  of  a  muscle  during  a  single  contraction  take  place 
with  extreme  rapidity,  their  succession,  peculiarities,  and  time  relations 
cannot  be  determined  with  any  degree  of  accuracy  by  the  unaided  eye. 
This  difficulty  can  largely  be  overcome  by  the  employment  of  the  graphic 
method,  the  principle  of  which  consists  in  recording  the  movements  by 
means  of  a  pen  on  some  appropriate  moving  and  receiving  surface.  To 
accomplish  this  object  the  muscle  is  attached  at  one  extremity  by  a  clamp 
to  a  firm  support,  and  at  the  other  extremity  to  a  weighted  lever,  which  is, 
however,  sufficiently  light  to  enable  it  to  take  up,  reproduce,  and  magnify 
its  movements.  The  end  of  the  lever  provided  with  a  point  is  applied  to 
a  smooth  surface,  such  as  glazed  paper  on  a  cylinder  or  plate,  covered 
with  lampblack.  If  the  surface  is  stationary,  the  contraction  is  recorded  as  a 
vertical  line;  if  it  is  put  in  movement  at  a  uniform  rate  by  clockwork,  the 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


6i 


contraction  is  recorded  in  the  form  of  a  curv^e,  the  width  of  the  arms  of  which 
will  depend  on  the  rate  of  movement.  The  time  relations  of  the  phases  of 
the  contraction  can  be  obtained  by  placing  beneath  the  lever  a  writing  point 
attached  to  an  electro-magnet  thrown  into  action  by  a  tuning-fork  vibrating 
in  hundredths  of  a  second.  In  order  to  determine  the  rapidity  with  which 
the  contraction  follows  the  stimulation,  it  is  essential  that  the  moment  of 
the  latter  be  also  recorded.  This  is  accomplished  by  an  automatic  key,  the 
opening  or  closing  of  which  develops  the  stimulus  which  excites  the  muscle. 
A  combination  of  these  different  appliances  constitutes  a  myograph  and  the 
curv'e  of  contraction  a  myogram.     (See  Fig.  23.) 


Fig.  23. — Myograph.     K.  Recording  cylinder.     M.  Moist  chamber.     L. 
Rccordino;  lever.     ^\^  Weight.     I.  Induction  coil. 


It  is  necessary  for  the  purpose  of  placing  the  excised  muscle  under  me- 
chanic conditions  similar  to  those  found  in  the  body  and  for  the  registration  of 
its  movements  under  varying  conditions  to  give  the  lever  mass.  This  is  accom- 
plished by  attaching  weights  to  it  beneath  the  muscle. 

The  Isotonic  Myogram. — With  the  object  of  obtaining  a  curve  of 
the  successive  changes  in  the  length  of  a  muscle  during  a  single  contraction 
and  at  the  same  time  avoiding  changes  in  tension  and  therefore  an  accelera- 
tion of  the  lever,  the  weight  attached  to  the  lever  should  be  applied  close  to  its 
axis,  in  accordance  with  the  isotonic  method.  The  curve  of  contraction  thus 
obtained  is  known  as  an  isotonic  myogram.^ 

^  The  weighting  of  the  lever  or  the  loading  of  the  muscle  is  accomplished  in  several  ways:  (i) 
The  weight  is  attached  to  the  lever  just  beneath  or  in  the  immediate  neighborhood  of  the  point  of 
attachment  of  the  muscle.  This  is  known  as  the  "  loaded  method  "  and  has  the  effect  of  extending 
the  muscle  beyond  its  normal  length  previous  to  the  moment  of  its  stimulation  and  contraction. 
(2)  The  weight  is  attached  to  the  lever  at  the  same  point  as  in  the  foregoing  method,  but  by 
means  of  a  support  beneath  the  lever,  the  weight  is  prevented  from  extending  the  muscle  previous 
to  the  moment  of  its  stimulation  and  contraction.  This  is  known  as  the  "after-loaded"  method. 
In  either  case  a  certain  momentum  is  imparted  to  the  weight,  which  continues  after  the  muscle 
has  ceased  to  act,  both  when  shortening  and  relaxing,  and  so  imparts  to  the  recording  lever  addi- 
tional movements  which  vitiate  the  true  character  of  the  curve.  (3)  The  weight  is  attached  to  a 
small  pulley  on  the  axis  of  the  lever  and  therefore  at  some  distance  from  the  point  of  attachment 
of  the  muscle.  The  advantage  of  this  method  lies  in  the  fact  that  the  initial  tension  of  the  muscle 
induced  by  the  load  remains  practically  constant  throughout  the  contraction  period  and  hence 
acceleration  of  the  movement  of  the  lever  is  prevented.     This  is  known  as  the  "isotonic  method." 


62  TEXT-BOOK  OF  PHYSIOLOGY. 

With  the  muscle  arranged  as  previously  described  and  stimulated 
directly  with  a  single  induced  electric  current,  the  contraction  will  be  re- 
corded in  the  form  of  a  cur\T  similar  to  that  represented  in  Fig.  24,  in  which 
the  horizontal  line  represents  the  abscissa  of  time;  a,  the  moment  of  stimula- 
tion; and  bed,  the  degree  of  shortening  and  the  subsequent  relaxation  at  each 
successive  moment.  The  undulating  line  shows  the  time  relations,  the 
distance  from  crest  to  crest  representing  hundredths  of  a  second.  The 
curve  may  be  divided  into  three  portions : 

I.  A  short  but  measurable  portion  between  the  point  of  stimulation  and 
the  first  evidence  of  the  shortening,  a  b,  known  as  the  "latent  period." 
The  duration  of  this  period  for  the  skeletal  muscle  of  the  frog  was 
originally  determined  to  be  0.0 1  second,  but  with  the  employment 
of  more  accurate  apparatus  it  has  been  reduced  to  0.002  5  to  0.004  second. 
During  this  period  it  is  supposed   that  certain  chemic  changes  are 


Fig.  24. — The  Isotonic  Mvogkam. 

taking  place  preparatory  to  the  exhibition  of  the  movement.  The 
duration  of  the  latent  period  in  influenced  by  a  variety  of  conditions, 
e.g.,  temperature,  fatigue,  strength  of  stimulus,  etc. 

2.  An  ascending  portion,  b  c,  the  contraction  or  period  of  increasing  energy. 

The  contraction  as  shown  by  the  character  of  the  curve  begins  slowly, 
then  proceeds  rapidly,  and  again  slowly  as  the  shortening  reaches  its 
maximum.  The  contraction  may  be  said  to  end  when  the  tangent 
to  the  curve  becomes  parallel  with  the  abscissa. 

3.  A  descending  portion,  c  d,  the  relaxation  or  period  of  decreasing  energy. 

The  relaxation  as  shown  by  the  character  of  the  curve  begins  slowly, 
then  proceeds  rapidly,  and  again  slowly  as  the  muscle  attains  its 
original  length.  The  termination  of  the  relaxation  is  at  the  point  where 
the  curv^e  cuts  the  abscissa.  The  curve  beyond  this  point  may  be 
complicated  by  the  presence  of  one  or  more  residual  or  after-vibrations, 
which  are  probably  due  to  the  inertia  of  the  lever  as  well  as  to  changes 
in  the  muscle  elasticity. 

The  duration  of  the  period  of  shortening  is  about  0.04  second,  and 
of  the  period  of  relaxation  0.05  second.  A  single  pulsation  of  the  isolated 
muscle  of  the  frog  therefore  occupies,  from  the  moment  of  stimulation  to 
termination,  the  tenth  of  a  second.  Muscles  of  many  other  animals  have 
a  contraction  period  the  duration  of  which  varies  considerably  from  this. 
Thus,  in  man  the  time  of  a  single  contraction  is  one-twentieth  of  a  second, 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TLSSUE.  63 

in  some  insects  one  three-hundredth  of  a  second,  and  in  the  turtle  one  second. 
Pale  muscles  have  a  shorter  period  than  the  red. 

Influences  Modifying  the  Effect  of  the  Stimulus. — The  contraction 
process  in  its  entirely  as  well  as  in  its  individual  parts  is  considerably  modi- 
fied by  both  external  and  internal  conditions,  among  which  may  be  mentioned 
the  following: 

I.  Character  0}  the  Stimulus. — As  the  contraction  is  the  response  of  the 
muscle  to  a  stimulus,  it  may  be  inferred  that  the  vigor  of  the  former  is 
proportional,  within  limits,  to  the  strength  of  the  latter.  Thus  using 
as  a  stimulus  the  single  induced  current,  it  has  been  found  that  if  the 
strength  of  the  current  is  progressively  increased,  the  height  of  the  con- 
traction will  correspondingly  increase  until  a  certain  maximum  height  is 
attained  (Fig.  25,  A,  a  C) ;  then  notwithstanding  a  continued  increase 
in  the  strength  of  the  stimulus,  this  height  will  not  be  exceeded  for  some 
time.     But  if  the  strength  of  the  stimulus  be  yet  further  increased,  there 


A.  B. 

Fig.  25. — Tr-acixg  Showing  the  Effects  of  a  Gradual  Increase  in  the  Strength 
OF  the  Stimulus  on  the  Height  of  the  Contraction,  a.  Minimal  contraction;  a  b.  pro- 
gressive increase  in  the  height;  b  c.  first  ma.ximum  (a  number  of  contractions  have  been  omitted 
for  economy  of  space) ;  d  e.  second  maximum. 

comes  a  moment  when  the  contractions  again  increase  in  vigor  and  a 
second  maximum  height  is  attained  (Fig.  25,  B,  d  e).  Beyond  this  no 
further  increase  in  height  is  observed.  The  second  maximum  has  been 
attributed  to  the  presence  in  the  muscle  of  two  different  substances 
differently  affected  by  changes  in  temperature,  by  fatigue  and  by 
various  drugs. 

It  has  also  been  shown  that  the  rate  at  which  the  muscle  is  stimulated 
with  a  given  stimulus  of  uniform  strength  will  influence  the  char- 
acter of  the  contraction  process.  If  the  inten'als  between  the  successive 
stimulations  be  such  as  permit  the  muscle  to  recover  from  the  effects  of 
the  contraction,  it  may  contract  as  many  as  a  thousand  times  without 
showing  any  particular  variation  from  the  normal  form;  but  if  the 
intervals  are  shorter  than  that  just  stated  it  is  found  that  from  the 
beginning  of  the  stimulation  each  succeeding  contraction  slightly  exceeds 
in  height  the  preceding  contraction,  until  a  certain  maximum  is  reached 
and  maintained,  indicating  that  for  some  reason  the  irritability  and  the 
energy  of  the  contraction  have  been  increased.     This  gradual  increase 


64  TEXT-BOOK  OF  PlIVSIOLOGV. 

in  the  height  of  the  contraction  has  been  termed  the  staircase  effect,  or 
the  treppe.  In  the  beginning  of  the  period  of  stimulation  there  is  some- 
times observed  a  decrease  in  the  height  of  the  contraction  following 
several  stimulations  before  the  staircase  effect  develops,  indicating 
a  temporary  decrease  in  the  irritability.  These  staircase  contractions 
have  been  observed  in  the  muscle  of  both  warm-blooded  and  cold- 
blooded animals.  The  cause  for  this  increase  in  irritability  upon  which 
the  effect  depends  is  attributed  to  the  presence  of  certain  chemic  sub- 


F1G.26. — Single  Contractions  of  the  Gastrocnemius  Muscle  at  Different  Tempera- 
tures.    Time  tracing  200  per  second. — {Bradie.) 

stances  in  the  muscle  arising  as  a  result  of  its  katabolism,  such  as 
carbon  dioxid,  mono-potassium  phosphate,  and  paralactic  acid.  These 
compounds,  when  present  in  small  amounts  or  in  larger  amounts  for  a 
short  time,  augment  the  action  of  the  muscle  and  give  rise  to  the  treppe 
effect.  (Lee.)  In  time,  however,  if  the  stimulation  be  continued,  the 
irritability  declines,  the  height  of  the  contraction  diminishes  and 
finally  the  muscle  ceases  to  respond  to  any  stimulus. 


Fig. 


27. — Contractions  of   a   Gastrocnemius   Muscle 
WITH  Different  Loads. — {Brodie.) 


Variations  in  the  Temperature. — The  temperature  at  which  all  phases  of 
the  contraction  process,  as  represented  by  the  myogram,  attain  their 
physiologic  maximum  value  is  about  30°  C.  If  the  temperature 
of  the  muscle  falls  to  20°  C.  there  is  a  corresponding  decline  in  activity, 
as  shown  by  an  increase  of  the  latent  period,  a  decrease  in  the  height 
of  curve — i.e.,  in  the  shortening  of  the  muscle — an  increase  both  in 
the  contraction  and  relaxation  periods.  As  the  temperature  approaches 
0°  C,  the  height  of  the  curve  again  suddenly  increases,  indicating,  for 
some  unknown  reason,  an  increase  in  the  irritability.  This  is,  however, 
scarcely  a  physiologic  condition.     At  a  temperature  of  40°  C.  to  50°  C. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  65 

the  muscle  suddenly  contracts  and  passes  into  the  condition  of  heat 
rigor  or  rigor  caloris.  The  protein  constituents  of  the  muscle  are 
coagulated  and  the  irritabihty  destroyed.       (Fig.  26.) 

Variations  in  the  Load. — The  extent  to  which  a  muscle  is  loaded  or 
weighted  will  not  only  determine  the  height  of  the  contraction,  but  also 
the  time  relations  of  all  its  phases.  This  is  apparent  from  an  exami- 
nation of  Fig.  27,  in  which  it  is  shown  that  with  an  increase  in  load 
there  is  a  decrease  in  the  height  of  the  contraction,  an  increase  in  the 
latent  period,  and  a  general  increase  in  the  duration  of  both  the  periods 
of  rising  and  falling  energy. 

Rapidly-repeated  Stimulation. — Prolonged  or  excessive  activity  of  our  own 
muscles  is  accompanied  by  a  feeling  of  stiffness  or  soreness  and  lassi- 
tude. There  is  at  the  same  time  a  diminution  in  the  speed  and  vigor 
of  the  contractions  and  the  power  of  doing  work.  To  this  condition 
the  term  fatigue  has  been  given.  The  cause  of  the  fatigue  is  attributed 
to  a  diminution  in  the  amount  of  the  energy-yielding  compounds  as 
well  as  to  the  production  and  accumulation  of  waste  products  resulting 
from  katabolic  activity.  Among  the  waste  products,  mono-potassium 
phosphate,  paralactic  acid,  and  carbon  dioxid  are  the  most  important. 
These  substances,  when  present  in  small  amounts  or  in  larger  amounts 
for  a  short  time,  increase  the  irritability  of  the  muscle,  but  when  they 
accumulate  more  rapidly  than  they  are  removed,  as  is  the  case  during 
excessive  activity,  they  exert  a  depressive  influence  on  the  irritability  of 
the  muscle  and  thus  diminish  its  contractile  power  and  its  capacity 
for  doing  work.  The  more  rapidly  they  are  removed,  the  sooner  is 
a  fatigued  muscle  restored  to  its  normal  condition.  The  condition 
of  fatigue  with  its  attendant  phenomena  is  shown  by  stimulating 
through  its  nerve  an  excised  frog  muscle  with  induced  electric  currents 


Fig.  28.— Fatigue  Curves.     Every  Twentieth  Contraction  Recorded. 

at  interv'als  of  one  second.  In  a  variable  period  of  time  the  muscle 
shows  an  increase  in  the  duration  of  the  latent  period,  a  diminution 
of  the  height  of  the  contraction,  in  the  power  of  doing  work,  and  an 
increase  in  the  time  required  for  relaxation.  (Fig.  28.)  If  the  stimu- 
lation is  continued  the  contractions  gradually  decline  as  the  muscle 
becomes  exhausted.  When  a  muscle  will  no  longer  respond  to  stimu- 
lation through  its  related  nerve,  it  can  be  made  to  respond  to  direct 
stimulation  with  the  electric  current.  This  taken  in  connection  with 
the  fact  that  stimulation  of  a  nerv-e-trunk  even  for  several  hours  does 
not  fatigue  it,  leads  to  the  inference  that  the  cause  of  the  cessation  of 
contraction  does  not  He  wholly  in  the  muscle  but  partly  in  the  nerve 
endings  in  the  muscle.  These  structures  it  is  believed  fatigue  more 
readily  than  the  muscle  structures,  and  hence  fail  to  conduct  the  nerve 

5 


66  TEXT-BOOK  OF  PHYSIOLOGY. 

impulse  to  the  muscle.     By  this  means  it  is  protected  from  absolute 
exhaustion. 
5.  Nutrition. — The   irritability    of    a    muscle   which    conditions    the    con- 
traction process  is  dependent  on  the  maintenance  of  its  nutrition; 
hence  a  continuous  and  sufficient  supply  of  nutritive  material  and 
a  rapid  removal  of  waste  products  arc  essential  conditions  for  the  ex- 
hibition of  normal  contractions.     A  diminution  of  blood  supply  or 
an  accumulation  of  waste  products  sooner  or  later  impairs  the  irritability 
and   diminishes   the   vigor   and   extent   of   the   contraction.     Various 
drugs — e.g..  veratrin,  barium,  etc. — introduced  into  the  circulation  and 
finding  their  way  into  the  muscle  modify  the  contraction  process,  as 
shown  by  a  very  great  increase  in  the  duration  of  the  relaxation  period. 
The  Isometric  Myogram. — With  the  object  of  obtaining  a  cmrvt  of 
the  increase  and  decrease  in  the  tension  of  a  muscle  during  a  single  con- 
traction, with  the  exclusion  as  far  as  possible  of  a  change  in  length,  the  muscle 
may  be  made  to  contract  against  a  strong  spring  or  similar  resistance  practically 
though  not  absolutely  sufficient  to  prevent  shortening.     To  this  method 
the  term  isometric  has  been  given,  and  the  curve  so  obtained  is  an  isometric 
myogram  or  a  tonogram.     The  recording  portion  of  the  lever  is  prolonged 

some  distance  so  that  the  very  slight 
upward  movement  at  its  axis,  close  to 
which  the  muscle  is  attached,  will  be 
considerably     magnified.       That     the 
ordinate  value   of  an  isometric  curve 
may  be  known,  the  apparatus  must  be 
graduated  by  subjecting  the  spring  to 
a  series  of  weights  playing  over  a  pul- 
ley supported  by  the  muscle  clamp. 
The  curve  of  the  variation  in  ten- 
FiG.  29.— a.  Diagram  of  Isotonic;  b,   gion  obtained  by  the  isometric  method 
OF  Isometric  Muscle  Curves. — (Landois    •      1  •     r?-  i,    •        \.-  x.  ^\.     j. 

and  Stirling.)  IS  shown  m  Fig.  29,  b,  m  which  the  two 

curv^es  are  contrasted.  The  form  of  the 
curA^e  indicates  that  the  muscle  attains  its  maximum  of  tension  more 
rapidly  than  its  maximum  of  shortening;  that  the  tension  endures  for  a 
certain  period  of  time  unchanged;  that  the  fall  in  tension  takes  place  more 
rapidly  than  the  muscle  lengthens. 

The  Myogram  Due  to  the  Make  and  the  Break  of  a  Galvanic  Current. 
— The  contraction  of  the  muscle  which  has  heretofore  been  recorded  has 
been  caused  by  the  momentary  action  of  an  induced  current.  The  con- 
traction of  the  muscle  which  is  caused  by  the  action  of  a  constant  or  galvanic 
current  presents  features  which  are  somewhat  different  and,  as  it  sers^es  to 
illustrate  the  difference  in  the  effects  of  a  constant  or  galvanic  and  an  induced 
or  interrupted  current,  a  myogram  of  a  contraction  due  to  the  make  and 
break  of  a  galvanic  current  is  introduced  at  this  place.  The  effects  which 
are  observed  in  a  muscle  during  the  passage  of  both  feeble  and  strong 
currents  have  been  alluded  to  in  a  previous  section.  (See  page  57.)  In 
Fig.  30  these  effects  are  graphically  represented.  It  will  be  observed  that 
on  the  closure  of  the  circuit  at  c  the  muscle  at  once  contracted  and  so  long  as 
the  current  was  flowing,  the  muscle  remained  in  a  more  or  less  contracted 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


67 


state  known  as  galvanotonus;  on  opening  the  circuit  at  o  the  muscle  again  con- 
tracted, after  which  it  gradually  relaxed  and  returned  to  its  original 
condition.  The  record  shows  also  that  during  the  actual  passage  of  the  cur- 
rent the  muscle  substance  was  being  stimulated  by  it. 

The  Work  Accomplished  by  a  Muscle  during  the  Time  of  a  Single 
Contraction. — -By   work   is   meant   the    overcoming   of   opposing   forces. 


Fig. 


50. — Myogram  Due  to  the  Action  of  a  Galvanic  Current,  Applied  Directly  to  a 
Muscle,  \a\hen  the  Circuit  was  Closed  (c)  and  when  it  was  Opened  (o). 


In  the  physiologic  activities  of  the  body  the  muscles  at  each  contraction  not 
only  overcome  the  resistances  of  antagonistic  muscles,  the  weight  of  the 
limbs,  the  friction  of  joints,  etc.,  but  in  addition  overcome  various  external 
resistances  connected  with  the  evironment — e.g.,  gravity,  cohesion,  friction, 
elasticity,  etc.  The  muscles  may  therefore  be  regarded  as  machines  for 
the  accomplishment  of  work.  Experimentally  the  work  done  by  an  iso- 
lated muscle  may  be  calculated  if  the  height  of  the  contraction  is  first  obtained 
and  then  multiplied  by  the  weight 
raised.  The  influence  of  the 
weight  on  the  height  of  the  con- 
traction is  shown  in  Fig.  31,  in 
which  only  the  height  of  the  con- 
traction or  the  degree  of  shorten- 
ing and  hence  the  lift  of  the  weight 
is  represented.  From  this  tracing 
it  will  be  observed  that  the  extent 
to  which  a  muscle  will  shorten  in 
response  to  a  maximal  stimulus 
is  greatest  when  it  is  unweighted ; 
but  as  weights  differing  by  a  com- 
mon increment  are  added,  the 
height  of  the  contraction  dimin- 
ishes until  with  a  given  weight  it 
is  nil. 

A  careful  study  of  the  results  of  this  experiment  will  show  that  the 
work  done  gradually  increased  as  the  load  was  increased  from  o  to  70 
grams,  when  it  amounted  to  210  gram-millimeters;  but  that  .after  this, 
even  though  the  weight  lifted  was  greater,  the  height  to  which  it  was  lifted 
was  less,  and  hence  the  work  done  gradually  decreased,  until  it  amounted 
to  nothing. 

The  following  table  will  also  show  the  work  done  by  a  frog's  muscle 
according  to  Rosenthal. 


Fig.  31. — Tracing  Showing  the  Gradual 
Diminution  in  the  Height  of  the  Contrac- 
tion AS  THE  Weight  was  Increased  by  a  Com- 
mon Increment  of  10  Grams  from  o  to  180 
Gr-AMS.     Magnification  of  the  Lever,  4. 


68  TEXT-BOOK  OF  PHYSIOLOGY. 


Weight. 

Height. 

\\'ork.  Done. 

o  grains 

14  mm. 

0  gram-millimeters. 

50  grains 

9  mm. 

450  gram-millimeters, 

100  grams 

7  mm. 

700  gram-millimeters. 

150  grams 

5  mm. 

730  gram-millimeters. 

200  grains 

2  mm. 

400  gram-millimeters. 

230  grams 

0  mm. 

0  gram-millimeters. 

From  the  preceding  figures  it  is  evident  that  the  mechanical  work  of  a 
muscle  increases  with  increasing  weights  up  to  a  certain  maximum,  and 
then  dechnes  to  zero.  Equally  when  the  muscle  contracts  to  its  maxi- 
mum without  being  weighted,  and  when  it  does  not  contract  at  all  from 
being  overweighted,  no  work  is  done.  Between  these  two  extremes  the 
muscle  performs  varying  amounts  of  work. 

Absolute  Muscle  Force. — The  maximum  amount  of  force  which  a 
muscle  puts  forth  during  a  contraction  is  naturally  measured  by  the  amount  of 
work  done;  but  as  this  varies  with  the  degree  to  which  the  muscle  is  weighted, 
another  measure  has  been  adopted,  to  which  the  term  absolute  muscle 
force  or  static  force  has  been  given.  The  absolute  force  is  measured  by 
the  weight  which  is  just  sufficient  to  prevent  the  muscle  from  shortening 
when  stimulated.  This  is  best  determined  by  the  method  of  after-loading  in 
which  the  muscle  is  not  extended  by  the  weight  previous  to  the  contraction. 
It  has  been  found  that  the  absolute  force  of  a  muscle  is  directly  dependent 
on  the  number  and  not  the  length  of  the  fibers  it  contains  and  proportional 
to  the  physiologic  transverse  section  of  the  muscle.  The  transverse  section 
of  a  muscle  is  obtained  by  dividing  its  volume  (obtained  by  di\dding  its 
actual  weight  by  the  specific  weight  of  muscle-tissue,  1.058)  by  the  average 
length  of  the  fibers.  Assuming  that  the  muscle  weighs  609  grams,  its  volume 
would  be  576  c.c. ;  and  if  it  be  further  assumed  that  the  fibers  have  an  average 
length  of  4  centimeters  the  transverse  section  would  contain  144  sq.  centi- 
meters each  of  which  would  have  a  length  of  4  centimeters. 

For  purposes  of  comparison  it  is  customary  to  refer  the  absolute 
force  to  the  units  of  area — viz.,  one  square  centimeter.  Rosenthal  esti- 
mates the  force  for  the  square  centimeter  of  the  muscle  of  the  frog  at  from 
2  to  8  kilograms;  for  the  muscles  of  man  at  6  to  8  kilograms;  Koster  at 
about  10  kilograms  for  the  muscles  of  the  leg  and  7  to  8  kilograms  for  the 
muscles  of  the  arm. 

Summation  Effects. — If  a  series  of  successive  stimuli  be  applied 
to  a  muscle,  the  effect  will  vary  according  to  the  rapidity  with  which  they 
follow  one  another.  As  previously  stated,  if  the  interval  preceding  each 
stimulus  be  sufficiently  long  to  enable  the  muscle  to  recover  from  the  effects 
of  the  previous  contraction,  there  will  be  no  change  in  the  form  or  the  char- 
acter of  the  contraction  for  a  long  time  except  a  slight  increase,  in  the  early 
period,  of  the  irritability  as  shown  by  the  increased  height  of  the  curve 
or  shortening  of  the  muscle.  If,  however,  a  second  stimulus  be  applied 
to  a  muscle  during  the  period  of  relaxation,  a  second  contraction  immediately 
follows  which  is  added  to  or  superposed  on  the  first;  the  effect  produced  will 
be  greater  than  that  produced  by  either  stimulus  separately.     (See  Fig.  32.) 

A  third  stimulus  applied  during  the  relaxation  of  the  second  contraction 
produces  a  third  contraction  which  adds  itself  to  the  second,  and  so  on. 
The  increment  of  increase  in  the  extent  of  the  successive  contractions  grad- 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  69 

ually  diminishes,  however,  until  the  muscle  reaches  a  maximum  of  contrac- 
tion. The  superposition  of  the  second  contraction  on  the  first,  the  third  on 
the  second,  and  so  on,  is  termed  summation  of  contractions  or  effects.  Experi- 
ment has  shown  that  the  greatest  effect  of  a  second  stimulus — that  is,  the 
highest  contraction — is  produced  when  the  stimulus  is  applied  during  the 
last  third  of  the  period  of  rising  energy,  when  the  sum  of  the  two  contractions 
is  almost  twice  as  great  as  the  first  contraction  would  have  been.  (Fig.  32.) 
The  effects  following  both  maximal  and  submaximal  stimuli  indicate  that 
the  muscle  cannot  attain  its  maximum 
of  shortening  except  through  a  summa- 
tion of  several  stimuli.  If  a  second  max- 
imal stimulus  enters  a  muscle  during  the 
latent  period  following  the  first,  the  effect 
produced  will  be  no  greater  than  that 
produced  by  a  single  stimulus.  The 
muscle  during  this  period  is  said  to  be 
refractory  or  non-responsive  to  a  second 
stimulus.  If,  however,  the  stimuli  are 
submaximal  they  add  themselves  to- 
gether, and  though  the  effect  is  but  a 
single  contraction,  it  is  larger  than  either 
would  have  produced  separately.  This 
is  termed  the  summation  of  stimuli. 

Still  further,  if  a  series  of  submini- 
mal stimuli,  each  of  which  is  alone  in- 
sufficient to  produce  a  contraction  of 
the  muscle,  be  applied  in  rapid  succes- 
sion, a  contraction  frequently  results. 
This  is  termed  the  summation  of  submini- 
mal stimuli. 

Tetanus. — Tetanus  may  be  defined 
as  a  more  or  less  continuous  contraction 
of  a  muscle  which  arises  when  the  time 
intervals  between  the  stimuli  are  shorter 
than  the  time  of  the  contraction  proc- 
ess. Tetanus  will  be  incomplete  or  Fig.  32.— Tracing  Showing  the  Ef- 
COmplete  according  to  the  number  of  ^^cxs  of  Two  Successive  Stimuli,  a.  a' 
.•        T   4.-U    t.  ^    i-x.  1     •  WITH    Gradually    Diminishing    Inter- 

i,timuli  that  reach  the  muscle  m  a  sec-  ..^^  on  a  Muscle  Contraction.  To  be 
Ond  of  time.      When  a  muscle   is    Stimu-  read  from  below  upward. 

lated     directly     or,     better,     indirectly 

through  its  related  nerv^e  by  a  series  of  induced  currents  at  the  rate  of  four 
or  six  per  second,  it  undergoes  a  corresponding  number  of  contractions 
of  about  equal  extent.  If  the  rate  of  stimulation  is  increased  up  to  the 
point  when  the  interval  between  each  stimulus  is  less  than  the  duration  of 
the  entire  contraction  process,  the  muscle  does  not  have  time  to  relax  com- 
pletely before  the  arrival  of  the  succeeding  stimulus,  and  hence  remains  in 
a  more  or  less  contracted  state,  during  which  it  exhibits  a  series  of  alternate 
partial  contractions  and  relaxations.     To  this  condition  of  muscle  activity 


70  TEXT-BOOK  OF  PHYSIOLOGY. 

the  term  incomplete  tetanus  or  clonus  is  applied.     A  graphic  record  of  an 
incomplete  tetanus  is  given  in  Fig.  33. 

In  such  a  tracing  it  is  observed  that  the  second  stimulation,  occurring 
before  the  muscle  had  time  to  relax,  gave  rise  to  a  second  contraction, 
which  was  superposed  on  the  first;  the  same  result  followed  the  third  stimu- 
lus, the  fourth,  the  fifth,  and  so  on.  Owing  largely  to  this  summation 
of  the  contractions  there  is  a  gradual  rise  in  the  height  of  the  contraction 
curve.     This  condition  of  the  muscle,  viz.,  continued  contraction,  com- 


FiG.  33. — Curves  Showing  the  Analysis  of  Tetanus  of  a  Frog's  Muscle  (Gastroc- 
nemius). The  numbers  under  the  curves  indicate  the  number  of  shocks  per  second  applied  to 
the  muscle.  There  is  almost  complete  tetanus  with  twenty-five  per  second,  and  it  is  a  little  lower 
than  the  previous  one  because  the  muscle  was  slightly  fatigued. — {Stirling.) 

bined  with  diminished  power  of  relaxation,  is  termed  contracture.  The 
tracing  also  shows  that  as  the  stimulus  continues,  the  base  line,  that  con- 
necting the  lowest  points  of  the  contractions,  gradually  rises  and  takes  the  form 
of  a  curve  which  increases  in  height  as  the  stimulus  continues.  The  apex 
line,  that  connecting  the  highest  points  of  the  contractions,  also  rises  at  the 
same  time,  indicating  a  continuous  increase  in  the  height  of  the  contractions. 
The  length  of  time  a  muscle  will  exhibit  incomplete  tetanus  depends  on  a 
variety  of  circumstances,  e.g.,  character  of  muscle,  rate  and  strength  of 


Fig.  34. — Development  of  Fatigue  and  Contraction.    Muscle  stimulated  once  a  second  by 

a  strong  induced  current. 

Stimulation,  etc.,  but  mainly  on  the  rapidity  with  which  the  muscle  becomes 
fatigued.  With  the  oncoming  of  fatigue  the  muscle  begins  to  relax,  and 
ultimately  returns  to  its  normal  condition,  notwithstanding  the  continued 
stimulation.  If  the  stimulation  be  withdrawn,  the  muscle  does  not  at  once 
return  to  its  original  length  but  remains  more  or  less  contracted  for  a  variable 
time.  This  contraction  after  stimulation  is  known  as  the  contraction- 
remainder. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  71 

If  the  stimulation  be  still  further  increased  in  frequency,  the  individual 
contractions  become  fused  together  and  the  curve  described  by  the  lever 
becomes  a  continuous  line.  (See  Fig.  33.)  Notwithstanding  the  fact 
that  the  individual  contractions  are  no  longer  visible,  it  can  be  shown  by 
other  methods  that  the  muscle  is  undergoing  a  series  of  slight  alternate  con- 
tractions and  relaxations  or  vibrations  at  least.  After  a  varying  length  of 
time  the  muscle  becomes  fatigued,  relaxes,  and  returns  to  its  natural  con- 
dition even  though  the  stimulation  be  continued.  The  number  of  stimuU 
per  second  necessary  to  develop  complete  tetanus  will  depend  under  normal 
circumstances  on  the  period  of  duration  of  the  individual  contractions. 
The  longer  this  period,  the  less  the  number  of  stimuli  required,  and  the 
reverse.  Hence  the  number  of  stimuli  will  vary  for  different  classes  of  animals 
and  for  different  muscles  in  the  same  animal,  e.g.,  2  or  3  for  the  muscles  of 
the  tortoise,  10  for  the  muscles  of  the  rabbit,  15  to  20  for  the  frog,  70  to 
80  for  birds,  330  to  340  for  insects. 

An  effect  which  follows  frequent  stimulation  of  a  muscle,  e.g.,  50  to 
60  times  per  minute,  and  especially  when  the  muscle  is  somewhat  fatigued 
or  cold  is  shown  in  Fig.  34.  It  is  evidently  a  combination  of  contracture  and 
fatigue.  It  will  be  observed  that  at  the  beginning  of  the  stimulation  there 
is  a  staircase  effect,  a-b,  combined  with  diminished  relaxation.  This  in 
turn  is  followed  by  a  decHne  in  the  height  of  the  contractions,  b-c,  and  a  fall 
of  the  base  line  which  may  be  attributed  to  fatigue  conditions.  After  a  short 
time  there  is  a  second  rise  of  the  base  line,  d,  and  a  rapid  development  of 
contracture.  The  muscle  at  this  period  is  in  a  condition  of  incomplete 
tetanus  which  gradually  passes  into  complete  tetanus  attended  by  fatigue. 

The  tetani  of  muscles  may  be  classified  in  accordance  with  their  causes 
as  follows:— 

-ni-     -1     •    f  Volitional. 

1.  Physiologic  I  ^^^^^^ 

2.  Experimental. 

3.  Pharmacologic. 

■n  .1-  1     •     f  Bacterial. 

4.  Pathologic  I  ^^^^^_ 

I.  Physiologic  Tetanus,  i.  Volitional.— Becd^wsQ  of  the  fact  that 
during  the  continuance  of  a  volitional  movement  the  muscle  is  in  a  state  of 
continuous  contraction,  it  may  be  accepted  that  vohtional  contractions  are 
states  of  tetanus,  more  or  less  complete;  for  the  shortest  possible  volitional 
contraction,  however  quickly  it  takes  place,  has  always  a  longer  duration 
than  a  single  contraction  caused  by  an  induced  electric  current.  As  the 
volitional  contraction  is  similar  to  that  observed  when  a  muscle  or  its  related 
nerve  is  stimulated  by  rapidly  repeated  induced  currents,  it  is  assumed  that 
the  nerve-cells  in  the  spinal  cord  are  discharging  in  a  rhythmic  manner  a 
certain  number  of  nerve  impulses  per  second  in  consequence  of  the  arrival 
of  nerv'e  impulses  coming  from  the  cerebral  cortex,  the  result  of  volitional 
acts.  In  other  words  the  volitional  tetanus  is  the  result  of  a  discontinuous 
stimulation.  The  number  of  stimuli  transmitted  to  a  muscle  during  a 
volitional  tetanus  has  been  estimated  by  the  employment  of  the  graphic 
method  at  from  8  to  13  per  second,  10  being  about  the  average.     When  a 


72  TEXT-BOOK  OF  PHYSIOLOGY. 

volitional  contraction  is  recorded  the  myogram  not  infrequently  exhibits  a 
series  of  small  wave-Hke  elevations  which  indicate  that  the  muscle  is  not  in 
a  state  of  complete  tetanus  but  is  undergoing  slight  alternate  contractions 
and  relaxations.  Unless  the  contraction  process  in  human  muscle  differs 
from  that  of  frogs  it  is  difficult  to  see  how  lo  or  even  20  stimuli  per  second 
can  give  rise  to  even  an  incomplete  tetanus  when  the  single  contraction  is  -^ 
of  a  second  in  duration. 

2.  Re/lex. — A  tetanus  of  muscle,  physiologic  in  character,  arises  during 
the  performance  of  many  muscle  movements  in  consequence  of  peripherally 
acting  causes  and  may  therefore  be  termed  a  reflex  tetanus.  The  duration 
of  a  tetanus  thus  induced,  like  the  duration  of  a  volitional  tetanus,  will  vary 
with  the  duration  of  the  exciting  cause.  Reflex  tetani  are  presented  by  the 
muscles  of  the  lower  jaw  during  mastication,  by  the  intercostal  muscles 
during  breathing,  by  the  muscles  of  the  limbs  during  walking,  etc.  In  these 
and  other  instances  there  are  reasons  for  believing  that  for  a  variable  period 
of  time  the  muscles  are  in  a  state  of  continuous  contraction  from  the  dis- 
charge of  nerv^e  impulses  from  the  nerve  cells  in  the  spinal  cord  as  the  result 
of  the  arrival  of  nerve  impulses  coming  from  a  peripheral  surface. 

2.  Experimental  Tetanus. — The  tetanus  of  muscle  developed  in 
accordance  with  the  method  described  in  foregoing  paragraphs,  i.e.,  by  the 
employment  of  instrumental  procedures,  may  be  termed  experimental 
tetanus.  Its  mode  of  development  serves  to  illustrate  and  explain  the 
method  by  w^hich  individual  contractions  are  summated  and  continuous 
contractions  made  possible  for  the  performance  of  volitional  acts. 

3.  Pharmacologic  Tetanus. — The  administration  of  certain  drugs,  e.g., 
strychnin,  in  sufficient  amounts,  is  followed  in  a  short  time  by  a  series  of 
intermittent  spasms  in  which  all  the  muscles  of  the  body  are  involved.  At 
the  beginning  of  the  spasms  the  muscles  are  thrown  into  tonic  or  complete 
tetanus,  during  the  continuance  of  which  the  muscles  are  hard  and  firm.  In 
a  short  time  this  tonic  state  begins  to  subside,  giving  way  to  tremors  or  a 
series  of  irregular  contractions  resembling  incomplete  tetanus  or  clonus.  A 
tetanus  of  this  character  may  be  termed  pharmacologic.  Though  the  onset 
of  the  tetanus  is  occasioned  largely  by  peripheral  stimulation,  the  seat  of 
action  of  strychnin  is  central  and  for  the  most  part  focalized  in  the  spinal 
cord.  The  exact  seat  of  its  action  is  not  definitely  determined  but  there  are 
reasons  for  believing  that  it  is  on  the  end-tufts  of  afferent  nerves  in  the  spinal 
cord  or  on  the  intercalated  neuron  between  them  and  the  nerve-cells  in  the 
anterior  horns  of  the  gray  matter,  the  irritability  of  which  is  raised  and  the 
resistance  to  the  transmission  of  nerve  impulses  coming  from  the  periphery 
diminished.  As  a  result  the  nerve  impulses  are  transmitted  to  the  nerve- 
cells  more  readily,  not  only  in  a  horizontal  but  also  in  a  longitudinal  direction, 
and  the  effects  they  produce  enormously  increased. 

4.  Pathologic  Tetanus,  i.  Bacterial. — The  introduction  of  a  specific 
bacillus  into  a  wound  in  any  region  of  the  body  is  followed  after  a  period  of 
incubation  of  from  three  or  four  days  to  a  week  by  a  tetanus  in  which  nearly 
all  the  muscles  of  the  body  are  involved,  characterized  by  a  tonic  contraction 
and  clonic  exacerbations.  A  tetanus  of  this  character  may  be  termed 
pathologic.  The  persistent  tonic  contraction  is  the  result  of  a  more  or  less 
continuous  discharge  of  nerv'e  impulses  from  the  nerve-cells  of  the  spinal  cord 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  73 

which  have  been  rendered  abnormally  irritable  by  the  action  of  a  toxin, 
produced  by  the  bacilli,  and  having  a  selective  action  on  these  structures. 
The  clonic  exacerbations  are  evoked  from  time  to  time  by  various  forms  of 
peripheral  stimulation. 

2.  Reflex. — A  tetanus  of  individual  muscles  more  or  less  continuous  in 
character  is  occasionally  the  result  of  peripheral  irritations  of  a  pathologic 
character.  A  tonic  contraction  of  the  masseter  muscles,  for  example,  firmly 
closing  the  jaws  for  weeks  and  months  at  a  time  is  caused  in  some  instances 
by  an  impacted  wisdom  tooth  or  an  ulcerative  condition  of  the  mouth. 
Since  the  removal  of  the  cause  is  followed  by  a  relaxation  of  the  muscle,  this 
form  of  tetanus,  known  as  trismus,  may  be  regarded  as  pathologic  in  char- 
acter and  reflex  in  origin. 

The  Muscle  Sound. — If  a  stethoscope  or  a  myophone  with  telephone 
connections  be  placed  on  a  muscle  while  in  a  condition  of  volitional  tetanus 
and  at  the  same  time  kept  in  a  certain  degree  of  tension,  there  will  be  devel- 
oped in  the  observer  a  sensation  of  sound  or  tone  which  is  spoken  of  as  a 
muscle  sound  or  tone.  It  is  also  readily  heard  in  the  masseter  muscle  when 
the  side  of  the  face  is  placed  on  a  receiving  body  such  as  a  pillow,  and  the 
masseter  muscles  made  to  contract  volitionally.  This  tone  is  attributed  to 
a  vibration  or  an  alternate  contraction  or  relaxation  of  the  muscle  or  to  an 
intermittent  rhythmic  variation  in  tension,  the  result  of  the  rate  of  stimula- 
tion. This  tone  corresponds  to  a  vibration  frequency  of  from  18  to  20  per 
second  and  is  accepted  as  one  of  the  proofs  that  the  physiologic  volitional 
tetanus  is  not  continuous  but  discontinuous  in  character.  If  a  muscle  is 
tetanized  with  induced  currents,  the  tone  increases  in  pitch  for  a  limited 
time  as  the  frequency  of  the  current  per  second  increases  up  to  a  certain 
maximum,  which  for  frogs  is  about  200  and  for  mammals  about  1000. 

CHEMIC  PHENOMENA. 

The  chemic  changes  which  underlie  the  transformation  of  energy  in  the 
living  muscle  even  when  in  a  state  of  relative  rest  are  active  and  complex, 
though  but  little  is  known  as  to  their  exact  character.  As  shown  by  an 
analysis  of  the  blood  flowing  to  and  from  the  resting  muscle,  it  has,  while 
flowing  through  the  capillaries,  lost  oxygen  and  gained  carbon  dioxid.  The 
amount  of  oxygen  absorbed  by  the  muscle  (9  per  cent.)  is  greater  than  the 
amount  of  carbon  dioxid  (6.7  per  cent.)  given  off.  Notwithstanding  the 
relation  of  the  oxygen  absorbed  to  the  carbon  dioxid  produced,  there  is  no 
parallelism  between  these  two  processes,  as  the  carbon  dioxid  will  be  given 
off  in  the  absence  of  free  oxygen  or  in  an  atmosphere  of  nitrogen. 

If  the  muscle  be  stimulated  through  its  related  nerve  all  the  chemic 
changes  are  increased  as  shown  both  by  an  increased  absorption  of  oxygen 
and  an  increased  production  of  carbon  dioxid,  though  the  ratio  existing 
between  them  differs  considerably  from  that  of  the  resting  muscle.  Thus, 
according  to  Ludwig,  an  active  muscle  absorbs  12.26  per  cent,  of  oxygen 
and  gives  oft"  10.8  per  cent,  carbon  dioxid.  At  the  same  time  the  muscle- 
tissue  changes  from  a  neutral  to  an  acid  reaction,  from  the  development  of 
sarcolactic  acid  and  possibly  phosphoric  acid.  The  degree  of  the  acidity 
depends  partly  on  the  duration  of  the  contraction  periods.     Chemic  analysis 


74  TEXT-BOOK  OF  PHYSIOLOGY. 

of  a  tetanizcd  muscle  shows  that  it  contains  less  glycogen  than  a  resting 
muscle,  and  that  it  contains  a  larger  amount  of  water.  Coincident  with 
the  muscle  contraction,  the  blood-vessels  become  widely  dilated,  leading  to  a 
large  increase  in  the  blood-supply  and  a  rapid  removal  of  the  products  of 
decomposition. 

Rigor  Mortis. — A  short  time  after  death  the  muscles  pass  into  a  condi- 
tion of  extreme  rigidity  or  contraction  known  as  death  stiffening  or  rigor 
mortis,  which  lasts  from  one  to  five  days.  In  this  state  they  offer  great 
resistance  to  extension.  At  the  same  time  their  tonicity  disappears,  their 
cohesion  diminishes,  and  their  irritability  ceases.  The  time  of  the  appear- 
ance of  this  post-mortem  rigidity  varies  from  a  quarter  of  an  hour  to  seven 
hours.  Its  onset  and  duration  are  influenced  by  the  condition  of  the  muscle 
irritability  at  the  time  of  death.  When  the  irritability  is  impaired  from  any 
cause,  such  as  chronic  disease  or  defective  blood-supply,  the  rigidity  appears 
promptly  but  is  of  short  duration.  After  death  from  acute  diseases  it  is  apt 
to  be  delayed,  but  will  continue  for  a  longer  period.  The  rigidity  first 
appears  in  the  muscles  of  the  lower  jaw  and  neck;  next  in  the  muscles 
of  the  abdomen  and  upper  extremities;  finally  in  the  trunk  and  lower  ex- 
tremities. It  disappears  in  practically  the  same  order.  Chemic  changes 
of  a  marked  character  accompany  this  process.  The  muscle  becomes  acid 
in  reaction  from  the  development  of  sarcolactic  acid  and  there  is  a  large 
increase  in  the  amount  of  carbon  dioxid  given  off.  The  immediate  cause 
of  the  rigidity  appears  to  be  coagulation  of  the  myosin  and  myogen  within 
the  sarcolemma  with  the  formation  of  two  insoluble  proteins,  myosin  fibrin 
and  myogen  fibrin.  In  the  early  stages  of  the  coagulation  restitution  is 
possible  by  the  circulation  of  arterial  blood  through  the  vessels.  The  final 
disappearance  of  this  post-mortem  rigidity  is  due  probably  to  the  action  of 
acids  which  render  the  myosin  and  myogen  fibrins  soluble,  and  possibly 
to  the  action  of  various  microorganisms  which  give  rise  to  putrefactive 
changes. 

Source  of  the  Muscle  Energy. — Notwithstanding  many  investigations, 
the  nature  of  the  materials  which  are  the  immediate  source  of  the  muscle 
energy  is  not  known.  The  absence  of  any  noticeable  increase  in  the  quan- 
tity of  urea  or  other  nitrogen-holding  compounds  excreted  renders  it  prob- 
able that  the  energy  does  not  come  from  the  metabolism  of  protein  materials. 
The  marked  production  of  carbon  dioxid  and  sarcolactic  acid  points  to  the 
decomposition  of  some  unstable  compound,  of  a  carbohydrate  character, 
rich  in  carbon  and  oxygen.  It  has  been  suggested  that  glycogen  furnishes 
the  energy,  inasmuch  as  this  substance,  generally  present  in  muscle,  dis- 
appears during  activity.  A  muscle  which  has  been  tetanized  contains  less 
glycogen  than  the  corresponding  muscle  at  rest.  A  muscle  which  has  been 
separated  from  the  nervous  system  by  division  of  its  nerves  and  thus  pre- 
vented from  contracting  accumulates  glycogen.  Bunge  is  of  the  opinion 
that  though  the  carbohydrates  are  the  main,  they  are  not  the  only  sources  of 
muscle  energy.  If  there  is  a  deficiency  or  absence  of  carbohydrate  food, 
the  muscle  will  utilize  fat  and  protein,  for  experiment  has  shown  that  the 
available  glycogen  is  entirely  consumed  by  the  second  or  third  day.  The 
mechanism  by  which  the  energy  is  liberated,  whether  by  direct  oxidation  or 
decomposition  is  uncertain.     The  general  trend  of  experimental  investigation 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  75 

points  to  the  disruption  of  some  carbohydrate,  perhaps  sugar,  derived  from 
the  stored  glycogen  and  the  oxidation  of  the  intermediate  products  to  carbon 
dioxid  and  water.  The  oxidizable  compound  appears  to  be  lactic  acid. 
For  if  the  muscle  be  made  to  contract  in  an  atmosphere  deficient  in  oxygen, 
the  amount  of  lactic  acid  produced  is  relatively  large  and  the  amount  of 
carbon  dioxid  relatively  small.  If  the  surrounding  atmosphere  be  rich 
in  oxygen,  the  reverse  conditions  obtain.  Under  physiological  conditions, 
when  the  muscle  is  supplied  with  blood  containing  its  customary  percentage 
of  oxygen,  it  is  probable  that  the  products  set  free  by  the  disruption  of  the 
sugar  molecule  are  rapidly  oxidized  to  CO,  and  HgO,  with  the  liberation  of 
their  contained  energy.  But  the  fact  that  muscle  will  contract  in  an  atmos- 
phere free  of  oxygen,  that  no  free  oxygen  can  be  obtained  from  muscle, 
would  support  the  idea  that  the  mechanism  is  one  of  decomposition.  Her- 
mann suggests  that  the  energy  of  a  contraction  is  liberated  by  the  splitting 
and  subsequent  re-formation  of  a  complex  body  belonging  neither  to  the 
carbohydrates  nor  fats,  but  to  the  proteins — to  this  hypothetic  body  the 
term  inogen  is  given.  This  complex  molecule,  the  product  of  the  nutritive 
activity  of  the  muscle-cell  in  undergoing  decomposition,  would  yield  carbon 
dioxid,  sarcolactic  acid,  and  a  protein  residue  resembling  myosin.  On  the 
cessation  of  the  contraction  the  muscle-cell  recombines  the  protein  residue 
with  oxygen,  carbohydrates,  and  fats,  and  again  forms  the  energy-holding 
compound,  inogen.  The  phenomena  of  rigor  mortis  support  this  view. 
At  the  moment  of  this  contraction  the  muscle  gives  off  CO2  in  large  amount, 
develops  sarcolactic  acid  and  myosin.  There  is  thus  a  close  analogy  be- 
tween the  two  processes;  in  other  words,  a  contraction  is  a  partial  death  of 
the  muscle.  If  this  view  is  correct,  then  the  oxygen  is  required  mainly  for 
heat  production  through  oxidation  processes. 

THERMIC  PHENOMENA. 

The  potential  energy  liberated  in  a  muscle  on  the  arrival  and  subsequent 
action  of  a  nerve  impulse,  manifests  itself  partly  as  heat  and  partly  as 
mechanic  motion  or  a  change  of  shape  of  the  muscle.  Though  heat  pro- 
duction is  taking  place  even  during  the  passive  condition,  it  is  largely  in- 
creased by  muscle  activity.  The  amount  of  heat  produced  will  vary  however 
with  a  variety  of  conditions,  as  strength  of  stimulus,  tension,  work  done,  etc. 

Stimulus, — It  has  been  experimentally  determined  that  the  skeletal 
muscle  of  the  frog,  the  gastrocnemius,  shows  after  a  single  contraction  a  rise 
in  temperature  of  from  0.001°  C.  to  0.005°  C.  and  after  tetanization  an 
increase  of  from  o.  14°  C.  to  o.  18°  C.  It  has  also  been  shown  that  an  increase 
in  the  strength  of  the  stimulus  from  a  minimal  to  a  maximal  value  increases 
the  amount  of  heat  liberated.  This  is  the  direct  result  of  increased  chemic 
change  naturally  following  increased  stimulation. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other  condi- 
tions being  the  same,  is  the  amount  of  heat  liberated.  If  the  muscle  is 
securely  fastened  at  both  extremities  so  that  shortening  is  practically  im- 
possible during  the  stimulation,  the  maximum  of  heat  production  is  reached. 
In  the  tetanic  state  the  great  increase  in  temperature  is  due  to  the  ten- 
sion of  antagonistic  and  strongly  contracted  muscles.     In  both  instances, 


76  TEXT-BOOK  OF  PHYSIOLOGY. 

mechanic  motion  being  prevented,  the  liberated  energy  is  transformed  into 
heat. 

Mechanic  Work. — If  the  muscle  is  permitted  to  shorten  and  raise  a 
weight,  some  of  the  energy  liberated  takes  the  form  of  mechanic  motion.  If 
the  weight  is  removed  at  the  height  of  the  contraction,  external  work  is 
accomplished.  The  greater  the  weight  raised,  within  limits,  the  greater  is 
the  percentage  of  energy  which  takes  the  direction  of  mechanic  motion.  The 
percentage  of  the  total  energy  liberated  which  is  thus  utilized,  has  been 
estimated  at  from  25  to  40  per  cent.  In  accordance  with  the  law  of  the  con- 
servation of  energy,  the  heat  produced,  stated  in  calories,  plus  the  energy 
required  in  the  raising  of  the  weight,  expressed  in  kilogrammeters  of  work, 
must  equal  the  potential  energy  transformed. 

A  muscle  during  a  tetanic  contraction  of  short  duration  accomplishes 
more  work  than  during  a  single  contraction,  the  weight  in  each  case  being 
the  same.  In  the  former  condition  the  height  of  contraction  through  sum- 
mation, and  hence  the  work  done,  is  greater  than  in  the  latter.  The  work 
done  by  a  short  tetanic  contraction  may  be  two  or  three  times  that  of  a  single 
contraction,  but  after  the  muscle  reaches  its  maximum  degree  of  shortening 
and  then  continues  in  a  state  of  tetanus,  no  further  work  is  done.  Internal 
work  is  done,  however,  i.e.,  the  continuous  liberation  of  energy,  as  shown  by 
an  increase  in  the  temperature. 

When  a  weight  which  is  hfted  by  a  muscle  during  a  single  contraction  is 
allowed  to  act  on  the  muscle  during  the  relaxation,  no  external  work  is 
accomplished.  All  the  energy  set  free  manifests  itself  as  heat.  Internal 
work  is  done,  as  shown  by  the  fact  that  the  muscle  becomes  fatigued. 

Work  Done  Daily. — The  muscle  system  in  its  entirety  is  to  be  regarded 
as  a  machine  for  the  transformation  of  potential  into  kinetic  energy,  and  in 
so  doing  accomplishes  work.  Through  the  intermediations  of  the  bones  of 
the  skeleton  which  play  the  part  of  levers  the  individual  not  only  changes  his 
position  in  space,  but  overcomes  to  some  extent  the  resistances  offered  by 
the  environment.  The  employment  of  artificial  levers,  tools,  as  distinguished 
from  natural  levers,  bones,  materially  adds  to  the  effectiveness  of  the  muscle 
machine.  The  amount  of  work  which  a  man  of  average  physical  develop- 
ment weighing  72  kilos  can  perform  in  eight  hours  has  been  variously  esti- 
mated. It  will  naturally  vary  according  to  the  character  of  the  occupation. 
If  the  work  done  be  calculated  from  the  number  of  kilograms  raised  one 
meter,  the  average  laboring-man  performs  about  300,000  kilogrammeters  of 
work. 

ELECTRIC  PHENOMENA. 

Electric  Currents  from  Injured  Muscles. — The  energy  liberated  as 
the  result  of  the  action  of  a  nerve  impulse  is  not  only  transformed  into  heat 
and  mechanic  motion,  but  to  some  extent  also  into  electric  energy.  The 
presence  of  points  of  different  potential  on  the  surface  of  the  muscle,  the 
necessary  condition  for  the  development  of  electric  currents,  is  tested  by 
means  of  non-polarizable  electrodes  connected  by  wires  with  a  sensitive 
galvanometer  or  capillary  electrometer.  When  such  electrodes  are  brought 
in  contact  with  a  muscle  properly  prepared,  there  is  at  once  developed  and 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


77 


conducted  to  the  galvanometer  an  electric  current  the  intensity  and  direction 
of  which  are  indicated  by  the  deflection  of  the  galvanometer  needle.  The 
existence  of  this  current  is  most  conveniently  demonstrated  with  single 
muscles  the  fibers  of  which  are  parallel— e.g.,  the  sartorius,  or  the  semimem- 
branosus of  the  frog.  If  the  tendinous  ends  of  either  of  these  muscles  be 
removed  by  a  section  made  at  right  angles  to  the  long  axis,  a  muscle  prism  is 
obtained  which  presents  a  natural  longitudinal  surface  and  two  artificial 
transverse  surfaces.  A  line  drawn  around  the  surface  of  such  a  muscle 
prism  at  a  point  midway  between  the  two  transverse  sections  constitutes  the 
equator. 

When  the  natural  longitudinal  and  artificial  transverse  surfaces  are 
connected  with  the  wires  of  a  galvanometer  the  terminals  of  which  are  pro- 
vided with  non-polarizable  electrodes,  an  elec- 
tric current  is  at  once  developed.  In  all  in- 
stances the  current,  as  shown  by  the  deflec- 
tion of  the  needle,  originates  at  the  transverse 
surface,  passes  through  the  muscle  to  the 
longitudinal  surface,  thence  through  the  gal- 
vanometer to  the  transverse  surface.  The 
longitudinal  surface  is.  therefore,  electroposi- 
tive, the  transverse  surface  electronegative. 
The  two  points  exhibiting  the  greatest  differ- 
ence of  potential,  and  hence  the  most  power- 
ful current,  lie  in  the  equator  and  in  the  cen- 
ter of  the  transverse  surface.  Currents  of 
gradually  diminishing  intensity  are  obtained 
when  the  electrode  placed  on  the  longitudinal 
surface  is  removed  toward  either  end.  Feeble 
currents  are  developed  when  two  points  situ- 
ated at  unequal  distances,  either  on  correspond- 
ing or  opposite  sides  of  the  equator,  are  con- 
nected; in  either  case  the  current  flows  from 
the  point  lying  nearest  the  equator  to  the  point 
farthest  from  it.  Similar  currents  are  obtained 
when  two  points  on  the  cross-section  situated 
at  unequal  distances  from  the  central  axis 
are  connected,  in  which  case  the  direction  of 
the  currents  will  be  from  the  point  lying  near- 
est the  periphery  toward  the  center.     On  the 

contrary,  no  current  is  developed  when  two  points  on  the  longitudinal  surface 
equally  distant  from  the  equator,  or  two  points  on  the  transverse  surface 
equally  distant  from  the  central  axis,  are  connected.  Such  points  are  said  to 
be  isoelectric.  These  facts  are  shown  in  Fig.  35.  The  natural  ends  of  the 
muscle,  enclosed  by  sarcolemma  and  tendon,  do  not  exhibit,  if  carefully 
preserved  from  injury,  the  negativity  characteristic  of  the  artificial  transverse 
ends. 

Similar  electric  conditions  are  exhibited  by  the  muscles  of  man  and  other 
mammals,  by  the  muscles  of  birds,  reptiles,  amphibia,  etc.  The  currents 
developed  by  connecting  the  equator  on  the  longitudinal  surface  with  the 


Fig.  35. — Diagram  to  Illus- 
trate THE  Current  in  Muscle. 
The  arrowheads  indicate  the  direc- 
tion; the  thickness  of  the  lines  in- 
dicates the  strength  of  the  currents. 
— (Landois  and  Stirling.) 


78 


TEXT-BOOK  OF  PHYSIOLOGY. 


axis  of  the  transverse  surface  have  an  electromotive  force  in  the  frog  muscle 
of  from  0.037  to  0.075  of  a  Daniell  cell. 

The  electric  currents  in  the  muscle  are  intimately  associated  with  the 
chemic  changes  underlying  its  nutrition,  and  hence  their  intensity  rises  and 
falls  with  all  the  conditions  which  maintain  or  impair  muscle  nutrition  and 
irritability.  The  currents  observed  in  the  injured  muscle  during  the  inactive 
state  have  been  termed  currents  of  rest.  Du  Bois-Reymond  regarded  them 
as  pre-existent,  intimately  connected  with  the  living  condition  of  the  muscle, 
and  essential  to  the  performance  of  its  functions,  and  to  be  explained  by  the 
view  that  the  entire  muscle  is  composed  of  molecules  each  of  which  exhibits 
the  same  difference  of  potential  on  its  longitudinal  and  transverse  surfaces 
as  the  muscle  prism  itself.  Hermann  denies  the  existence  of  currents  in 
normal  resting  muscle  and  attributes  them  to  injuries  of  the  surface,  due  to 
methods  of  preparation,  in  consequence  of  which  the  tissue  dies  and  becomes 
electronegative  to  the  uninjured  area,  which  remains  electropositive.  These 
currents  Hermann  terms  "demarcation  currents." 

Negative  Variation  of  the  Muscle  Current, — If  a  muscle  exhibiting  a 
current  of  injury   be   excited   to  activity  by   tetanizing  induced  currents 

applied  to  the  opposite  end  of  the 
muscle,  it  will  be  observed  that  as 
the  contraction  wave  passes  over 
the  muscle  there  is  a  movement  of 
the  galvanometer  needle  toward 
the  zero  point,  indicating  a  dimin- 
ution of  the  potential  on  the  longi- 
tudinal surface.  To  this  dimin- 
ution in  the  strength  of  the  cur- 
rent the  term  negative  variation 
was  given.  On  the  withdrawal  of 
the  stimulus  the  needle  again  re- 
turns in  a  short  time  to  its  former 
position.  The  diminution  of  po- 
tential on  the  longitudinal  surface 
of  the  muscle  is  now  attributed  to  the  passage  of  the  excitation  and  contrac- 
tion processes,  to  a  temporary  disintegration  of  the  muscle  substance  (Fig. 
36).  With  their  disappearance  and  the  subsequent  restoration  of  the 
nutrition  of  the  muscle,  the  former  electric  condition  returns. 

The  primary  deflection  of  the  galvanometer  needle  is  due  to  the  demarca- 
tion current  jvhich  arises  as  a  result  of  the  difference  in  electric  potential 
produced  by  the  destructive  chemic  changes  taking  place  at  the  cut  end  of 
the  muscle.  The  negative  variation  is  caused  by  the  fact  that  the  activity 
of  the  muscle,  with  its  attendant  chemic  changes,  will  always  be  greater  in 
the  uninjured  equatorial  region,  and  hence  will  always  tend  to  counterbalance 
the  original  source  of  difference  in  electric  potential. 

Electric  Currents  from  Non-injured  Muscles. — Though  perfectly 
normal  resting  muscle,  according  to  Hermann,  is  isoelectric,  nevertheless 
electric  currents  are  developed  during  activity  to  which  he  has  given  the  term 
action  currents,  and  which  are  attributed  to  the  propagation  of  the  contrac- 
tion wave. 


FiG.  36. — The  Negative  Vaeiation  cJf  the 
Demarcation  Current.  A.  The  contraction 
wave,  which  as  it  passes  beneath  the  electrode  at  B 
causes  a  diminution  of  potential. 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


79 


Action  Currents. — When  two  isoelectric  points  on  the  longitudinal 
surface  of  a  muscle  are  connected  with  a  galvanometer  and  a  single  stimulus 
applied  directly  to  one  extremity,  it  can  be  shown  that  as  the  contraction 
wave  passes  beneath  A,  Fig.  37,  the  muscle-tissue  at  that  point  becomes 


Fig.   37. — The  Condition  Leading  to  the  Development  of  the  First  Action 

Current. 

electronegative  toward  B  and  a  current  at  once  passes  through  the  galvan- 
ometer from  B  to  A,  as  shown  by  the  deflection  of  the  needle  toward  A.  As 
the  contraction  wave  passes  beneath  B  it  in  turn  becomes  electronegative, 
and  a  temporary  condition  of  equal  potential  is  established  when  the  needle 


Fig.  38. — The  Condition  Leading  to  the  Development  of  the  Second  Action 

Current. 

returns  to  the  zero  point.  In  a  very  short  time  the  nutrition  of  A  is  restored 
and  becomes  electropositive  toward  B,  when  a  current  will  pass  through  the 
galvanometer  in  the  opposite  direction  from  A  to  B,  as  shown  by  the  move- 
ment of  the  needle  toward  B,  Fig.  38.  As  the  contraction  wave  passes 
beyond  B  its  nutrition  is  restored  and  becomes  of  equal  potential   with  A. 


8o  TEXT-BOOK  OF  PHYSIOLOGY. 

The  term  phasic  is  appUed  to  these  currents.  The  first  current  flows  in  the 
muscle  in  the  direction  of  progress  of  the  contraction  wave — first  phase;  the 
second  current  flows  in  the  reverse  direction — second  phase;  the  current  is 
therefore  diphasic.  When  a  muscle  is  tetanized,  there  is  but  a  single  current 
observed,  which,  however,  endures  so  long  as  the  tetanic  contraction  is 
maintained.  To  this  current  the  term  decremential  is  given.  When  a 
muscle  is  excited  to  action  by  the  nerve  impulse  which  enters  at  its  center, 
two  contraction  waves  are  developed,  one  in  each  half  of  the  muscle,  and 
hence  there  are  two  sets  of  diphasic  action  currents. 

The  presence  of  action  currents  in  the  muscle  of  the  living  body  during  a 
single  contraction  was  demonstrated  by  Hermann  in  the  muscles  of  the 
forearm.  The  arrangement  of  the  experiment  was,  briefly,  as  follows: 
The  forearm  was  surrounded  by  two  twine  electrodes  saturated  with  zinc 
solution,  one  being  placed  at  the  physiologic  middle — the  nervous  equator — 
the  other  at  the  wrist.  Both  electrodes  were  then  connected  with  the  galvan- 
ometer. When  the  brachial  plexus  was  stimulated  in  the  axillary  space,  the 
deflections  of  the  galvanometer  needle,  when  analyzed  with  the  repeating 
rheotome,  indicated  phasic  currents  with  a  single  contraction.  In  the  first 
phase — atterminal — the  wrist  became  positive  and  the  current  passed  in  the 
muscle  toward  its  termination;  and  in  the  second — abterminal — it  became 
negative  and  the  current  now  passed  in  the  reverse  direction.  The  action 
currents  which  are  observed  in  the  frog's  muscle  were  thus  shown  to  be 
present  in  the  living  human  muscle,  with  this  difference,  however:  that  the 
second  phase — abterminal — instead  of  being  weaker  in  man,  is  equally 
strong  with  the  atterminal.  This  experiment  also  revealed  the  fact  that  the 
rapidity  of  propagation  of  the  excitation  wave  was  much  greater  in  man, 
amounting  to  about  twelve  meters  per  second.  Hermann  therefore  denies 
the  pre-existence  of  electric  currents  and  regards  them  as  due  to  localized 
temporary  disintegration  of  the  muscle  in  consequence  of  activity,  as  they 
disappear  on  the  restoration  of  the  muscle  to  its  normal  condition. 

SPECIAL  ACTION  OF  MUSCLE  GROUPS. 

The  individual  muscles  of  the  axial  and  appendicular  portions  of  the 
body  are  named  with  reference  to  their  shape,  action,  structure,  etc.;  e.g., 
deltoid,  flexor,  penniform,  etc.  In  different  localities  a  group  of  muscles 
having  a  common  function  is  named  in  accordance  with  the  kind  of  motion 
it  produces  or  to  which  it  gives  rise:  e.g.,  groups  of  muscles  which  alternately 
diminish  or  increase  the  angular  distance  between  two  bones  are  known 
respectively  as  flexors  and  extensors;  such  muscle  groups  are  usually  found 
in  association  with  ginglymus  joints.  Muscles  which  rotate  the  bone  to 
which  they  are  attached  around  its  own  axis  without  producing  any  great 
change  of  position  are  known  as  rotators,  and  are  found  in  association  with 
enarthrodial  or  ball-and-socket  joints.  Muscles  which  impart  an  angular 
movement  to  the  extremities  to  and  from  the  median  line  of  the  body  are 
termed  adductors  and  abductors  respectively. 

In  addition  to  the  actions  of  individual  groups  of  muscles  in  producing 
special  movements,  in  some  regions  of  the  body,  several  groups  of  muscles 
are  coordinated  for  the  accomplishment  of  certain  definite  functions;  e.g., 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  8i 

the  functions  of  respiration,  mastication,  etc.  The  coordination  of  axial 
and  appendicular  muscles  enables  the  individual  to  assume  certain  postures, 
such  as  standing,  sitting,  and  lying;  to  engage  in  various  acts  of  locomotion, 
as  walking,  running,  dancing,  swimming. 

Levers. — The  function  or  special  mode  of  action  of  individual  muscles 
can  be  understood  only  when  the  bones  with  which  they  are  connected  are 
regarded  as  levers  whose  fulcra  or  fixed  points  lie 
in  the  joints  where  the  movement  takes  place,  and  ^ 

the  muscles  as  sources  of  power  for  imparting  move-    ^ 1 i.^i) 

ment  to  the  levers  with  the  object  of  overcoming                ^  . 

resistance.  F  • ^.  , 

In  mechanics  levers  of  three  kinds  or  orders  are    A  W  p '  ^ 

recognized  according  to  the  relative  positions  of  the     •  I      F 

fulcrum  or  axis  of  motion,  the  applied  power,  and     w  p     a^^^ 

the  weight  to  be  moved.     (See  Fig.  .^q.)  _      P^^^  39.-The  Three  Or- 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  ders  of  Levers. 

between  the  weight  or  resistance,  W,  and  the  power 

or  moving  force,  P.  The  distance  P  F  is  known  as  the  power  arm  and 
the  distance  W  F  as  the  weight  arm.  As  examples  of  this  form  of  lever 
found  in  the  human  body  may  be  mentioned: 

1.  The  elevation  of  the  trunk  from  the  flexed  position.     The  axis  of  move- 

ment, the  fulcrum,  lies  in  the  hip-joint;  the  weight,  that  of  the  trunk, 
acting  as  if  concentrated  at  the  center  of  gravity,  which  lies  close  to  the 
tenth  dorsal  vertebra;  the  power,  the  muscles  attached  to  the  tuberosity 
of  the  ischium.  The  opposite  movement  is  equally  one  of  the  first 
order,  but  the  relative  positions  of  P  and  W  are  reversed. 

2.  The  head  in  its  movement  backward  and  forward  on  the  atlas. 

In  levers  of  the  second  order  the  weight  lies  between  the  power  and  the 
fulcrum.     As  illustration  of  this  form  of  lever  may  be  mentioned: 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum  is  the 

temporomaxillary  articulation;  the  resistance,  the  tension  of  the  elevator 
muscles;  the  power,  the  contraction  of  the  depressor  muscles. 

2.  The  raising  of  the  body  on  the  toes,  in  which  movement  the  fulcrum  is 

the  toes,  the  weight  that  of  the  body  acting  through  the  ankle,  the 
power  the  gastrocnemius  muscle  applied  to  the  heel  bone. 
In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying  between 

the  fulcrum  and  the  weight.     As  example  of  this  form  of  lever  may  be 

mentioned: 

1.  The  flexion  of  the  forearm,  in  which  the  fulcrum  is  the  elbow-joint,  the 

power  the  biceps  and  brachialis  anticus  muscles  applied  at  their  points 
of  insertion,  the  weight  that  of  the  forearm  and  hand. 

2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanic  operations,  the  object  aimed  at 
is  the  overcoming  of  a  great  resistance  by  the  application  of  a  small  force 
acting  through  a  great  distance,  so  as  to  obtain  mechanic  advantage.  In 
the  mechanism  of  the  human  body  the  reverse  generally  obtains,  viz.,  the 
overcoming  of  a  small  resistance  by  the  application  of  a  large  force  acting 
through  a  short  distance.  As  a  result  there  is  a  gain  in  the  extent  and  rapid- 
ity of  the  movement  of  the  lever.  The  power,  however,  owing  to  its  point 
6 


82  TEXT-BOOK  OF  PHYSIOLOGY. 

of  application,  acts  at  a  great  mechanic  disadvantage  in  many  instances, 
especially  in  levers  of  the  third  order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles  the  human 
body  can  assume  a  series  of  positions  of  equilibrium,  such  as  standing  and 
sitting,  to  which  the  term  posture  has  been  given.  In  order  that  the  body 
may  remain  in  a  state  of  stable  equilibrium  in  any  posture,  it  is  essential 
that  the  vertical  line  passing  through  its  center  of  gravity  shall  fall  within 
the  base  of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn  through 
the  center  of  gravity  of  the  entire  body  falls  within  the  base  of  support. 
This  position  is  maintained  largely  by  the  mechanical  conditions  of  the 
joints,  apparently  for  the  purpose  of  reducing  to  a  minimum  muscular 
action,  so  that  it  can  be  prolonged  for  some  time  without  giving  rise  to 
fatigue.  In  the  military  position,  which  may  be  assumed  as  the  normal 
position,  all  the  joints  must  be  in  such  a  condition  of  extension  and  fixation 
that  the  body  will  represent  a  rigid  column  resting  on  the  astragalus  and 
supported  by  the  arch  of  the  foot.     This  is  accomplished: 

1.  By  balancing  the  head  on  the  apex  of  the  vertebral  column.     This  is 

done  by  the  action  of  the  muscles  on  the  back  of  the  neck.  The  mus- 
cular effort  is,  however,  very  slight,  as  the  center  of  gravity  of  the  head 
lies  but  a  short  distance  in  front  of  the  articulation. 

2.  By  making  the  vertebral  column  erect  and  rigid.     This  is  brought  about 

by  the  action  of  the  common  extensor  muscles  of  the  trunk.     In  this 
condition  the  center  of  gravity  lies  just  in  front  of  the  tenth  dorsal 
vertebra.     The  head,  trunk,  and  upper  extremities  are  now  supported 
by  the  hip- joints;  and  in  order  that  this  support  may  give  to  the  body 
a  certain  degree  of  stable  equilibrium,  independent  of  muscular  action, 
the  line  of  gravity  falls  behind  the  line  uniting  the  center  of  rotation  of 
the  two  joints.     In  consequence  the  body  would  fall  backward  were  it 
not  prevented  by  the  tension  of  the  iliofemoral  ligament  and  the  fascia 
lata. 
The  line  of  gravity,  continued  downward,  passes  through  the  knee-joint 
posterior  to  the  axis  of  rotation,  and  hence  the  body  would  now  fall  back- 
ward were  it  not  prevented  by  the  tension  of  the  lateral  ligaments  and  the 
contraction  of  the  quadriceps  femoris  muscle.     Though  the  body  is  supported 
by  the  astragalus,  the  line  of  gravity  does  not  pass  through  the  line  uniting 
the  two  joints,  for  in  so  doing  constant  muscular  effort  would  be  required  to 
maintain  stable  equilibrium;  passing  a  short  distance  in  advance  of  this  line, 
there  would  be  a  tendency  of  the  body  to  fall  forward,  which  is  prevented  by 
the  extensor  muscles  of  the  foot.     When  the  body  is  in  the  erect  or  military 
position,  the  center  of  gravity  lies  between  the  sacrum  and  last  lumbar 
vertebra.     Standing  is  thus  an  act  of  balancing,  and  requires  not  only  the 
static  conditions  of  joints,  but  the  dynamic  conditions  of  various  groups  of 
muscles,  and  hence  is  not  a  position  of  absolute  ease  and  cannot  be  main- 
tained for  any  length  of  time  without  experiencing  discomfort  and  fatigue. 
Sitting  erect  is  an  attitude  of  equilibrium  in  which  the  body  is  balanced 
on   the    tubera    ischii,  when    the   head  and  trunk  together  form  a  rigid 
column. 

Locomotion  is  the  act  of  transferring  the  body  as  a  whole  through  space, 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE. 


83 


and  is  accomplished  by  the  combined  action  of  its  own  muscles.  The  acts 
involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act  involving  almost  all  the  voluntary  muscles 
of  the  body  either  for  purposes  of  progression  or  for  balancing  the  head  and 
trunk,  and  may  be  defined  as  a  progression  in  a  forward  horizontal  direction 
due  to  the  alternate  action  of  both  legs.  In  walking  one  leg  becomes  for  the 
time  being  the  active  or  supporting  leg,  carrying  the  trunk  and  head;  the 
other  the  passive  but  progressing  leg,  to  become  in  turn  the  active  leg  when 
the  foot  touches  the  ground.  Each  leg  is  therefore  alternately  in  an  active 
and  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that  at  a  given  moment 
both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 

THE  VISCERAL  MUSCLE. 

The  visceral  muscle,  as  the  name  implies,  is  found  in  the  walls  of  hollow 
viscera,  where  it  is  arranged  in  the  form  of  a  membrane  or  sheet.  It  is 
present  in  the  walls  of  the  alimentary  canal,  blood-vessels,  respiratory  tract, 
ureter,  bladder,  vas  deferens,  uterus,  fallopian  tubes,  iris,  etc.  In  some 
situations  it  is  especially  thick  and  well  developed — e.g..  uterus  and  pyloric 
end  of  the  stomach;  in  others  it  is  thin  and  slightly  developed. 

The  Histology  of  the  Visceral  Muscle-fiber. — When  examined  with 
the  microscope,  the  muscle  sheet  is  seen  to  be  composed  of  fibers,  narrow, 


Fig.  40. — Two  Smooth  Muscle-fibers  from  Small  Intestine  of  Frog.  X  240.  Isolated 
with  35  per  cent,  potash-lye.  The  nuclei  have  lost  their  characteristic  form  through  the  action 
of  the  lye. — {Stbhr.) 

elongated,  and  fusiform  in  shape.     As  a  rule,  they  are  extremely  small, 

measuring  only  from  40  to  250  micromillimeters  in  length  and  from  4  to  8 

micromillimeters  in  breadth.     The  center  of  each  fiber  presents  a  narrow, 

elongated  nucleus.     The  muscle-protoplasm  which  makes  up  the  body  of 

the   fiber  appears   to  be  enclosed  by  a 

delicate  elastic  membrane  resembling  in 

some    respects    the    sarcolemma   of  the 

skeletal  muscle.     In  some  animals   the 

visceral    fiber    presents    a    longitudinal 

striation     suggesting    the    existence    of 

fibrillse  surrounded  by  sarcoplasm  (Fig.  Smooth  muscie-fiber-4^ 

\  '-ni  ri  • .      1     1  • .       1  •      in  transverse  section.       B 

40).  ihe  fibers  are  united  longitudi- 
nally and  transversely  by  a  cement  Fig.  41.— Section  of  the  Circular 
material.  The  muscle'  is  increased  in  Layer  of  the  Muscular  Coat  of  the 
,1  •  1  1,  i.1  -J,-  r  Human  Intestine. — (Stohr.) 
thickness  by  the  superposition  of  succes- 
sive layers.  At  varying  intervals  the  fibers  are  grouped  into  bundles  or 
fasciculi  by  septa  of  connective  tissue  (Fig.  41).  Blood-vessels  ramify  in 
the  connective  tissue  and  furnish  the  necessary  nutritive  material. 

The  visceral  muscle  receives  stimuli  from  the  spinal  cord,  not  directly, 
however,  as  in  the  case  of  the  skeletal  muscle,  but  indirectly  through  the 


Connective-tissu  e. 
septum. 


Nucleus. 


84  TEXT-BOOK  OF  PHYSIOLOGY. 

intermediation  of  ganglion  cells,  which  may  be  located  at  some  distance 
from  the  muscle  or  near  the  walls  of  the  viscera.  Non-medullated  fibers 
from  the  ganglion  pass  directly  into  the  muscle,  where  they  frequently  unite 
to  form  a  general  plexus.  From  this  plexus  fine  branches  take  their  origin 
and  ultimately  become  physiologically  associated  with  the  muscle-fiber. 

Physiologic  Properties. — The  visceral  muscles  which  have  been  sub- 
jected to  experiment  are  mainly  those  of  the  stomach,  intestine,  bladder, 
ureter,  and  iris.  From  the  results  of  the  experiments  which  have  been 
published,  it  is  evident  that  all  visceral  muscles  possess  elasticity,  tonicity, 
irritability,  and  conductivity. 

The  elasticity  of  the  bladder  muscle  of  the  cat  was  strikingly  shown  in 
the  experiments  published  by  Dr.  Colin  C.  Stewart.  When  this  muscle  was 
weighted  with  weights  differing  by  a  common  increment,  it  was  extended  on 
the  addition  of  each  weight,  though  to  a  progressively  less  extent.  On  the 
removal  of  the  weights  the  muscle  eventually  returned  to  its  former  length. 
The  records  of  the  extension  were  similar  to,  if  not  identical  with,  those  of 
the  skeletal  muscle. 

The  tonicity  of  visceral  muscles  is  as  pronounced  in  many  situations  as 
is  the  tonicity  of  skeletal  muscles.  Each  muscle  is  continuously  in  a  state 
of  contraction  intermediate  between  that  of  complete  contraction  and  that 
of  relaxation.  In  how  far  this  is  due  to  local  and  inherent  causes  or  to 
stimuli  reflected  from  the  nen'ous  system  as  a  result  of  peripherally  acting 
causes  is  not  in  individual  instances  readily  determinable.  From  time  to 
time  the  tonicity  varies,  increasing  and  decreasing  in  response  to  these  various 
stimuli  and  in  accordance  with  the  functional  activities  of  the  organs  in 
which  the  muscle  is  found. 

The  irritability  manifests  itself  by  a  change  of  form,  and  doubtless  by 
the  liberation  of  heat  on  the  application  of  any  form  of  stimulus — mechanic, 
chemic,  thermic,  electric. 

The  conductivity  is  less  marked  in  the  visceral  than  in  the  skeletal 
muscle,  and,  contrary  to  what  is  observed  in  the  latter,  the  conduction  extends 
laterally  as  well  as  longitudinally  from  fiber  to  fiber.  This  is  shown  by 
stimulation  of  the  exposed  intestine.  Shortly  after  the  stimulus  is  applied 
the  muscle  contracts  longitudinally — i.e.,  in  a  direction  at  right  angles  to  the 
long  axis  of  the  intestine,  partially  obhterating  its  lumen.  From  this  point 
the  conduction  process  indicated  by  the  contraction  wave  passes  in  opposite 
directions  for  some  distance  along  the  canal.  As  to  whether  this  is  accom- 
plished by  protoplasmic  processes  extending  from  fiber  to  fiber,  or  whether 
the  uniting  membrane  differs  in  conducting  power  from  the  sarcolemma,  is 
as  yet  a  matter  of  doubt.  From  the  fact  that  the  upper  two-thirds  of  the 
ureter,  though  free  of  nerv'e-cells,  exhibits  lateral  conduction,  it  is  evident 
that  it  may  take  place  independent  of  the  nen-ous  system. 

The  Contraction  of  the  Visceral  Muscle. — The  general  character  of 
the  contraction  may  be  witnessed  on  opening  the  abdomen  of  a  recently 
killed  animal,  especially  the  rabbit.  Shortly  after  exposure  to  the  air  the 
walls  of  the  intestine  begin  to  contract  in  a  most  vigorous  manner.  The 
contraction  wave  beginning  at  various  points  is  propagated  in  both  direc- 
tions, running  along  the  intestinal  wall  for  a  variable  distance.  A  succession 
of  similar  waves  mav  be  observ^ed  for  some  minutes.     To  the  alternate 


GENERAL  PHYSIOLOGY  OF  MUSCLE-TISSUE.  85 

contraction  and  relaxation  of  the  muscle-fibers,  which  are  circularly  ar- 
ranged, the  term  peristalsis  is  usually  given.  The  excised  stomach  of  a  dog 
kept  under  suitable  conditions  will  exhibit  similar  movements.  The  same 
holds  true  of  the  bladder  muscle  of  the  cat,  the  muscle  of  the  ureter,  etc. 
Careful  obsen^ation  shows  a  certain  periodicity  in  the  movements.  Inas- 
much as  the  cause  is  not  apparent,  these  contractions  are  termed  spontane- 
ous or  automatic. 

Graphic  Record  of  the  Contraction. — For  experimental  purposes 
narrow  transverse  sections  of  the  stomach  of  the  frog  or  the  entire  bladder 
muscle  of  the  cat,  excised  or  in  situ,  according  to  the  method  of  Prof.  Colin 
C.  Stewart,  may  be  employed.  If  kept  moist,  they  will  retain  their  irritability 
for  some  hours.  The  changes  of  form  may  be  recorded  with  the  usual 
muscle  lever.  When  thus  prepared,  the  muscle  may  exhibit  for  several 
hours  a  series  of  pulsations,  rhythmic  in  character.  With  spontaneously 
acting  mammalian  muscle  the  contraction  and  relaxation  periods  are  of 
equal  duration.  With  the  amphibian  muscle  they 
are  of  unequal  duration,  as  a  rule.  In  both  classes  of 
animals  the  character  of  the  record,  a  succession  of 
large  and  small  contractions,  would  indicate  that  the 
general  rhythmic  movement  is  compounded  of  two  or 
three  secondary  rhythms  which  differ  in  rate  and 
character.  A  single  pulsation  may  be  recorded  by 
stimulating  the  bladder  muscle  with  the  induced  or 
the  make  and  break  of  the  constant  current.  A  curve 
of  such  a  contraction  is  shown  in  Fig.  42.  The  con- 
traction takes  place  more  rapidly  than  the  relaxation ; 
the  two  phases  occupying  five  and  thirty-five  seconds 
respectively.  The  latent  period  covered  0.25  second.  Fig.  42.— The  Curve 
With  other  muscles  the  time  relations  are  slightly  of  Contraction  of  the 
different.  Tetanization  of  the  bladder  muscle  of  the  body-temperature  in 
cat  occurred  when  the  stimuli  succeeded  each  other     Response  to  a  Single 

with  a  certain  rapidity;  the  interval  between  stimuli     i^'duction     Current. 

^      .  •' ,'  ,        ,  ,  The  time  is  indic.\ted 

approximatmg  a  period  somewhat  less  than  two  sec-     ix  seconds.— (Steicart.) 

onds.     This    muscle  responds  to  variations  in  tem- 
perature, to  strength  of  stimulus,  to  the  load,  in  a  manner  similar  to,  if 
not  identical  with,  the  skeletal  muscle. 

The  Function  of  the  Visceral  Muscle. — In  a  general  way  it  may  be 
said  that  the  visceral  muscle  determines  and  regulates  the  passage  through 
the  viscus  or  organ  of  the  material  contained  within  it.  The  food  in  the 
stomach  and  intestines  is  subjected  to  a  churning  process  by  the  muscles,  in 
consequence  of  which  the  digestive  fluids  are  more  thoroughly  incorporated 
and  their  characteristic  action  increased.  At  the  same  time  the  food  is 
carried  through  the  canal,  the  absorption  of  the  nutritive  material  promoted, 
and  the  indigestible  residue  removed  from  the  body.  The  blood  is  delivered 
in  larger  or  smaller  volumes  according  to  the  needs  of  the  tissues  through  a 
relaxation  or  contraction  of  the  muscle-fibers  of  the  blood-vessels.  The 
urine  is  forced  through  the  ureter  and  from  the  bladder  by  the  contraction  of 
their  respective  muscles.  The  mode  of  action  of  the  individual  muscles  will 
be  described  in  successive  chapters. 


.nmri.miiinifiimrinnimmil/lll 


86  TEXT-BOOK  OF  PHYSIOLOGY. 

Ciliary  Movement. — The  free  surface  of  the  epithelium  covering  the 
mucous  membrane  in  certain  regions  of  the  body  is  characterized  by  the 
presence  of  dcUcate  filamentous,  processes  termed  cilia.  (See  Fig.  43.) 
Ciliated  epithelium  is  found  in  man  and  mammals  generally,  in  the  nose, 
Eustachian  tube,  larynx,  with  the  exception  of  the  vocal  membranes,  trachea 
and  bronchial  tubes  as  far  as  the  pulmonary  lobules.  Fallopian  tubes,  uterus, 
and  epididymis.  The  lumen  of  the  central  canal  of  the  spinal  cord  and  the 
cavities  of  the  brain  are  lined,  especially  in  childhood, 
by  cells  provided  with  similar  cilia.  Ciliated  epithe- 
lium is  also  found  in  all  classes  of  animals,  and  especi- 
ally in  the  invertebrates. 

The  cilia  found  in  the  human  body  vary  in  length 
from  0.003  mm.  to  0.005  rnm.  They  are  apparently 
structureless  and  colorless,  and  appear  to  have  their 
origin  in  and  to  be  a  prolongation  of  a  transparent 
material  on  the  outer  surface  of  the  cell  material.     The 

number  of  cilia  present  on  the  surface  of  any  individual 
Fig.  43. — Ciliated  Epi-        ,,  .  'tir  n        4.      4.         iC 

THELiuM.  ^^^^    varies   approximately    from   nve  to  twenty-iive. 

When  ciliated  epithelial  cells,  freshly  removed  from 
the  mucous  membrane  and  moistened  with  normal  saline,  are  examined 
with  the  microscope,  it  will  be  found  that  the  cilia  are  in  continuous  and 
rapid  vibratile  movement,  so  much  so  that  the  individual  cilium  cannot 
be  distinguished.  In  time,  however,  their  vitality  declines  and  the  rapid- 
ity of  movement  diminishes.  When  the  movement  of  the  individual 
cilium  fallstto  about  eight  or  ten  per  second,  its  character  can  be  readily 
determined.  It  will  then  be  seen  that  the  movement  is,  as  a  rule,  alter- 
nately a  backward  and  a  forward  one,  the  cilium  lowering  and  then  rais- 
ing itself,  the  latter  taking  place  more  quickly  and  energetically  than  the 
former.  As  the  cilium  raises  itself  it  becomes  somewhat  flexed  in  a  direc- 
tion corresponding  to  that  of  the  general  movement.  The  movement, 
however,  varies  in  character  in  different  situations  and  in  different  animals. 
The  cause  of  the  movements  and  the  mechanism  of  their  coordination  are 
unknown.  They  are,  as  far  as  known,  independent  of  the  nerve  system. 
The  force  of  ciliary  motion  is  very  great.  A  load  of  twenty  grams  can  be 
supported  and  carried  forward  by  the  cilia  on  the  mucous  membrane  of  the 
mouth  and  esophagus  of  the  frog.  The  activity  of  the  cilia  is  associated  with 
the  nutrition  of  the  cell  of  which  they  are  a  part  and  rises  and  falls  with  it. 
Experimentally  it  has  been  found  that  the  rate  and  energy  of  the  movement 
are  greatest  at  a  temperature  of  about  35°  to  40°  C,  especially  if  they  are 
bathed  with  normal  saline,  rendered  slightly  alkaline.  Low  temperatures, 
acids,  alkalies,  carbon  dioxid,  etc.,  retard  the  movement. 

The  function  of  the  cilia,  though  not  always  apparent,  is  associated  with 
the  function  of  the  passages  in  which  they  are  found.  As  the  surfaces  of 
these  passages  are  swept  by  a  current  of  considerable  power,  it  is  probable 
that  they  assist  in  the  passage  of  the  materials  which  ordinarily  traverse 
them.  Mucus  and  particles  of  dust  are  carried  upward  through  the  air- 
passages;  the  ovarian  cell  is  carried  from  the  ovary  toward  the  uterus;  the 
spermatozoa,  as  well  as  the  fluid  in  which  they  are  contained,  are  carried 
forward  through  the  epididymis  ducts. 


CHAPTER  VIII. 

THE  GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 

The  Nerve-tissue. — The  nen^-tissue,  which  unites  and  coordinates 
the  various  organs  and  tissues  of  the  body  and  brings  the  individual  into 
relationship  with  the  external  world,  is  conventionally  arranged  in  two 
systems,  termed  the  encephalo spinal  or  cerebrospinal  and  the  sympathetic. 

The  encephalospinal  system  consists  of: 

1.  The  brain  and  spinal  cord,  contained  within  the  cavities  of  the  cranium 

and  the  spinal  column  respectively,  and 

2,  The  cranial  and  spinal  ner\-es. 

The  sympathetic  system  consists  of: 

1.  A  chain  of  ganglia  situated  on  each  side  of  the  spinal  column  and  extend- 

ing from  the  base  of  the  skull  to  the  tip  of  the  coccyx. 

2.  Various  collections  of  ganglia  situated  in  the  head,  face,  thorax,  abdomen, 

and  pelvis.  All  these  ganglia  are  united  by  an  elaborate  system  of 
intercommunicating  nerves,  many  of  which  are  connected  with  the 
cerebrospinal  system. 

HISTOLOGY  OF  NERVE -TISSUE. 

The  Neuron. — The  nerve-tissue  has  been  resolved  by  the  investigations 
of  modern  histologists  into  single  morphologic  units,  to  which  the  term 
neurons  has  been  applied.  The  entire  nerve  system  has  been  shown  to  be 
but  an  aggregate  of  an  infinite  number  of  neurons,  each  of  which  is  histologic- 
ally distinct  and  independent.  Though  having  a  common  origin,  as  shown 
by  embryologic  investigations,  they  have  acquired  a  variety  of  forms  in 
different  parts  of  the  nerve  system  in  the  course  of  development.  The  old 
conception  that  the  nerve  system  consisted  of  two  distinct  histologic  elements, 
nerve-cells  and  nerve-fibers,  which  differed  not  only  in  their  mode  of  origin, 
but  also  in  their  properties,  their  relation  to  each  other,  and  their  functions, 
has  been  entirely  disproved. 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the  surface 
of  which  presents  a  greater  or  less  number  of  processes  in  varying  degrees 
of  differentiation.  As  represented  in  Fig.  44,  A,  the  neuron  may  be  said  to 
consist  of:  (i)  The  nerve-cell,  neurocyte,  or  corpus;  (2)  the  axon,  or  nerve 
process;  (3)  the  end-tufts,  or  terminal  branches.  Though  these  three  main 
histologic  features  are  everywhere  recognizable,  they  exhibit  a  variety  of 
secondary  features  in  different  situations  in  accordance  with  peculiarities  of 
function. 

The  Nerve-cell. — The  ner%^e-cell,  or  body  of  the  neuron,  presents  a 
variety  of  shapes  and  sizes  in  different  portions  of  the  ners'e  system. 
Originally  ovoid  in  shape,  it  has  acquired,  in  course  of  development,  pecu- 
liarities of  form  which  are  described  as  pyramidal,  stellate,  pear-shaped, 
spindle-shaped,  etc.     The  size  of  the  cell  varies  considerably,  the  smallest 


TEXT-BOOK  OF  PHYSIOLOGY. 


having  a  diameter  of  not  more  than  lo  to  12  micro-miUimeters,  the  largest 
not  more  than  150  micro-milHmeters.  Each  cell  consists  of  granular, 
striated  cytoplasm,  containing  a  distinct  vesicular  nucleus  and  a  well-de- 
fined nucleolus.  A  characteristic  feature  of  the  cytoplasm  is  the  presence 
of  granules  first  described  by  Nissl,  which  stain  deeply  with  methylene  blue 
and  other  dyes.  For  this  reason  these  granules  are  spoken  of  as  chromo- 
phile  granules.     The  remainder  of  the  cytoplasm  is  penetrated  in  various 


AXON 


NEVP,ILEM}[A  ....4 


AXON 


A.  B. 

Fig.  44. — A.  Efferent  Neuron;  B.  Afferent  Neuron. 

directions  with  nerve  fibrils  which  are  continuous  with  similar  fibrils  run- 
ning through  the  axonic  process  as  well  as  the  dendrites.  The  physiolo- 
gic significance  of  Nissl's  granules  is  unknown.  The  nerve  fibrils  are  prob- 
ably connected  with  the  transmission  of  nerve  impulses.  A  cell  mem- 
brane has  not  been  observed.  From  the  surface  of  the  adult  cell  portions 
of  the  cytoplasm  are  projected  in  various  directions,  which  portions, 
rapidly  dividing  and  subdividing,  form  a  series  of  branches,  termed  den- 
drites or  dendrons.     In  some  situations  the  ultimate  branches  of  the  den- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  89 

drites  present  short  oclateral  presses,  known  as  lateral  buds,  or  gemmules, 
which  impart  to  the  branches  a  feathery  appearance.  This  character- 
istic is  common  to  the  cells  of  the  cortex  of  the  cerebrum  and  of  the 
cerebellum.  The  ultimate  branches  of  the  dendrites,  though  forming  an 
intricate  feltwork,  never  anastomose  with  one  another  nor  unite  with 
dendrites  of  adjoining  cells.  According  to  the  number  of  axons,  nerve-cells 
are  classified  as  monaxonic,  diaxonic,  polyaxonic.  Most  of  the  cells  of 
the  nerve  system  of  the  higher  vertebrates  are  monaxonic.  In  the  ganglia 
of  the  posterior  or  dorsal  roots  of  the  spinal  and  cranial  nerves,  however, 
they  are  diaxonic.  In  this  situation  the  axons,  emerging  from  opposite 
poles  of  the  cell,  either  remain  separate  and  pursue  opposite  directions,  or 
unite  to  form  a  common  stem,  which  subsequently  divides  into  tw^o  branches, 
which  then  pursue  opposite  directions.  (See  Fig.  44,  B.)  The  nerve-cell 
maintains  its  own  nutrition,  and  presides  over  that  of  the  dendrites  and  the 
axon  as  well.  If  the  latter  be  separated  in  any  part  of  its  course  from  the 
cell,  it  speedily  degenerates  and  dies. 

The  Axon. — The  axon,  or  nerve  process,  arises  from  a  cone-shaped  pro- 
jection from  the  surface  of  the  cell,  and  is  the  first  outgrowth  from  its  cyto- 
plasm. At  a  short  distance  from  its  origin  it  becomes  markedly  differentiated 
from  the  dendrites  which  subsequently  develop.  It  is  characterized  by  a  sharp, 
regular  outline,  a  uniform  diameter,  and  a  hyahn  appearance.  In  structure, 
the  axon  appears  to  consist  of  fine  fibrillae  embedded  in  a  clear,  semi-fluid 
material,  the  neuroplasm.  The  axon  varies  in  length  from  a  few  iftillimeters 
to  one  meter.  In  the  former  instance  the  axon,  at  a  short  distance  from 
its  origin,  divides  into  a  number  of  branches,  which  form  an  intricate  felt- 
work  in  the  neighborhood  of  the  cell.  In  the  latter  instance  the  axon 
continues  for  an  indefinite  distance  as  an  individual  structure.  In  its 
course,  however,  especially  in  the  brain  and  spinal  cord,  it  gives  off  a  number 
of  collateral  branches,  which  possess  all  its  histologic  features.  The  long 
axons  serve  to  bring  the  body  of  the  cell  into  direct  relation  with  peripheral 
organs,  or  with  more  or  less  remote  portions  of  the  nerve  system,  thus  con- 
stituting association  or  commissural  fibers.  Physiologic  investigations  have 
established  the  fact  that  the  axon  is  the  conducting  agent  of  the  nerve 
impulses. 

The  Myelin. — At  a  short  distance  from  the  cell  the  more  or  less  elon- 
gated axon  becomes  invested  with  nucleated  oblong  cells,  which  subse- 
quently become  modified  and  constitute  the  medullary  or  myelin  sheath. 
When  fresh  the  myelin  is  clear  and  semi-fluid;  when  treated  with  various 
reagents  it  becomes  opaque  and  imparts  a  white  appearance  to  nerves.  ' 
The  function  of  the  myehn  is  unknown.  All  axons  that  possess  a  myeHn 
investment  are  known  as  myelinated  nerve  fibers. 

The  Neurilemma. — The  myelin  in  many  situations  is  enclosed  by  a 
thin  transparent  elastic  membrane  known  as  the  neurilemma.  In  the 
spinal  cord  and  brain,  the  nerve  fibers  are  for  the  most  part  wanting  in 
this  membrane. 

At  intervals  of  about  seventy-five  times  its  diameter,  the  medullated  nerve- 
fiber  undergoes  a  remarkable  diminution  in  size,  due  to  an  interruption  of 
the  medullary  substance,  so  that  the  neurilemma  lies  directly  on  the  axis- 
cylinder.     These  constrictions,  or  nodes  of  Ranvier,  taking  their  name  from 


90  TEXT-BOOK  OF  PHYSIOLOGY. 

their  discoverer,  occur  at  regular  intervals  along  the  course  of  the  nerve, 
separating  it  into  a  series  of  segments.  The  portion  between  the  nodes  is 
termed  the  internodal  segment.  It  has  been  suggested  that  in  consequence 
of  the  absence  of  the  myelin  at  these  nodes,  a  free  -exchange  of  nutritive 
material  and  decomposition  products  can  take  place  between  the  axis- 
cylinder  and  the  surrounding  plasma.  Beneath  the  neurilemma  in  each 
internodal  segment  there  is  a  large  nucleus  surrounded  by  a  small  amount  of 
granular  protoplasm. 

The  End  Tufts. — The  end-tufts  or  terminal  organs  are  formed  by  the 
splitting  of  the  axon  into  a  number  of  filaments,  which  remain  independent 
of  one  another  and  are  free  from  the  myelin  investment.  The  histologic 
peculiarities  of  the  terminal  organs  vary  in  different  situations,  and  in  many 
instances  are  quite  complex  and  characteristic.  In  peripheral  organs,  as 
muscles,  glands,  blood-vessels,  skin,  mucous  membrane,  the  tufts  are  in 
direct  histologic  and  physiologic  connection  with  their  cellular  elements.  In 
the  brain  and  spinal  cord  the  tufts  are  in  more  or  less  intimate  relation 
with  the  dendrites  of  adjacent  neurons. 

The  neurons  in  their  totality  constitute  the  neuron  or  nen^e  tissue. 
From  the  fact  that  they  are  arranged  both  serially  and  collaterally  into  a 
regular  and  connected  whole,  they  collectively  constitute  a  system  known  as 
the  neuron  or  nerve  system. 

The  neurons  composing  the  spinal  and  cranial  nervTS  are  represented  in 
Fig.  44,  \A4iich  are  connected  peripherally  by  their  terminal  branches  with 
muscles  on  the  one  hand  and  with  epithelium  of  skin,  mucous  membrane,  etc., 
on  the  other  hand.  In  the  spinal  cord  the  terminal  branches  of  the  afferent 
neuron  come  into  histologic  and  physiologic  relation  with  the  dendrites  of  a 
second  neuron,  the  axonic  process  of  which  in  many  instances  ascends  the 
cord  to  different  levels  or  even  as  far  as  the  brain, where  its  terminal  branches 
come  into  relation  with  the  dendrites  of  still  another  neuron,  the  axonic 
process  of  which  is  in  turn  connected  with  neurons  in  the  cortex  of  either 
the  cerebrum  or  cerebellum.  The  surfaces  of  the  body  are  thus  brought 
into  relation  with  the  cerebral  and  cerebellar  neurons.  The  neurons 
arranged  in  this  serial  manner  constitute  the  afferent  side  of  the  nerve 
system. 

In  a  similar  way  the  efferent  neurons  of  the  spinal  and  cranial  nerves  are 
brought  into  relation  with  the  cortex  of  the  cerebrum.  Large  pyramidal- 
shaped  neurocytes  situated  in  specialized  regions  of  the  cortex  of  the  cere- 
brum send  their  axonic  processes  down  through  the  brain  and  cord.  As  they 
'approach  their  destination  the  terminal  branches  become  related  histo- 
logically and  physiologically  with  the  dendrites  of  the  neurons  composing 
the  cranial  and  spinal  nervxs.  The  cortex  of  the  cerebrum  is  thus  brought 
into  relation  with  the  general  musculature  of  the  body.  The  neurons 
arranged  in  this  serial  manner  constitute  the  efferent  side  of  the  nerv^e  system. 

Neurons,  moreover,  are  grouped  into  more  or  less  complexly  organized 
masses,  termed  organs,  which  in  accordance  with  their  locations  may  be 
divided  for  convenience  into  central  and  peripheral  organs. 

The  Central  Organs  of  the  Nerve  System. — The  central  organs  con- 
sist of  the  encephalon  and  spinal  cord,  contained  within  the  cavities  of  the 
head  and  spinal  column  respectively.     They  consist  of  neurons  arranged 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


91 


in  a  very  complex  manner.  In  a  subsequent  chapter  the  anatomic  arrange- 
ment of  their  constituent  parts  will  be  detailed. 

The  Peripheral  Organs  of  the  Nerve  System.— These  consist  of  the 
cranial  and  spinal  nerv^es  and  the  sympathetic  ganglia.  Each  nerve 
consists  of  a  variable  number  of  nerve-fibers  united  into  firm  bundles  by 
connective  tissue  which  supports  blood-vessels  and  lymphatics.  The  bundles 
are  technically  known  as  nerve-trunks  or  nerves. 

The  nerve-trunks  connect  the  brain  and  cord  with  all  the  remaining 
structures  of  the  body.  Each  nerv^e  is  invested  by  a  thick  layer  of  lamel- 
lated  connective  tissue,  known  as  the  epineurium.  A  transverse  section  of  a 
nerve  shows  (see  Fig.  45),  that  it  is  made  up  of  a  number  of  small  bundles  of 


.::::,^^ 


Fig.  45. — Transverse    Section    of  a  Nerve  (Median),     ep.  Epineurium. 
pe.   Perineurium,     ed.  Endoneurium. — {Landois  and  Stirling.) 


fibers,  each  of  which  possesses  a  separate  investment  of  connective  tissue — 
the  perineurium.  Within  this  membrane  the  nerve-fibers  are  supported  by 
a  fine  stroma — the  endoneurium.  After  pursuing  a  longer  or  shorter  course, 
the  nerve-trunk  gives  off  branches,  which  interlace  very  freely  w'ith  neigh- 
boring branches,  forming  plexuses,  the  fibers  of  which  are  distributed  to 
associated  organs  and  regions  of  the  body.  From  their  origin  to  their 
termination,  however,  ne«ve-fibers  retain  their  individuality,  and  never  be- 
come blended  with  adjoining  fibers. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations,  they 
give  off  a  number  of  branches,  each  of  which  becomes  invested  with  a 
lamellated  sheath — an  offshoot  from  that  investing  the  parent  trunk.  This 
division  of  nerve-bundles  and  sheath  continues  throughout  all  the  branchings 
down  to  the  ultimate  nerve-fibers,  each  of  which  i§  surrounded  by  a  sheath 
of  its  own,  consisting  of  a  single  layer  of  endothelial  cells.  This  delicate 
transparent  membrane,  the  sheath  of  Henle.  is  separated  from  the  nerve- 
fiber  by  a  considerable  space,  in  which  is  contained  lymph  destined  for  the 
nutrition  of  the  fiber.     Near  their  ultimate  terminations  the  nerve-fibers 


92  TEXT-BOOK  OF  PHYSIOLOGY. 

themselves  undergo  division,  so  that  a  single  fiber  may  give  origin  to  a  num- 
ber of  branches,  each  of  which  contains  a  portion  of  the  parent  axis-cylinder 
and  myelin. 

Sympathetic  Ganglia. — A  sympathetic  ganglion  consists  essentially  of 
a  connective-tissue  capsule  with  an  interior  framework.  The  meshes  of 
this  framework  contain  nerve-cells  possessing  dendrites  and  branching 
axons.  The  majority  of  the  axons  are  devoid  of  myelin  and  are  therefore 
known  as  non-myelinated  nerve  fibers.  Owing  to  the  absence  of  the  myelin 
they  present  a  rather  pale  or  grayish  appearance.  In  all  instances,  with 
the  exception  of  the  ganglion  cells  of  the  heart,  the  axons  are  distributed  to 
non-striated  muscle  tissue  and  to  the  epithelium  of  glands. 

The  nerve-cells  of  the  ganglia  are  also  in  histologic  connection  with  the 
terminal  branches  of  certain  fine  medullated  nerv^e-fibers  which  leave  the 
spinal  cord  by  way  of  the  anterior  roots  of  the  spinal  nerves.  These 
nerve-fibers  are  designated  pre-ganglionic  fibers,  while  those  emerging 
from  the  cells  are  designated  post-ganglionic  fibers.  (See  Sympathetic 
System.) 

Blood-supply. — Nerves  being  parts  of  living  cells  require  for  the  main- 
tenance of  their  nutrition  a  certain  amount  of  blood.  This  is  furnished  by 
the  blood-vessels  ramifying  in  and  supported  by  the  connective-tissue  frame- 
work. Here  as  elsewhere  there  is  a  constant  exchange,  through  the  capillary 
wall  and  the  neurilemma,  of  nutritive  material  to  the  nerve  proper  and  of 
waste  materials  to  the  blood. 

The  Chemic  Composition  and  Metabolism. — Chemic  analysis  of 
nerve-tissue  has  shown  the  presence  of  water,  proteins  (two  globulins,  a 
nucleo-protein  and  neurokeratin),  certain  lipoids,  e.g.,  (a)  cholesterin  (a 
monotomic  alcohol  free  from  both  nitrogen  and  phosphorus),  (b)  several 
cerebrosides  or  galactosides  (nitrogen-holding  bodies,  free  from  phos- 
phorous, compounds  of  a  glucoside  character,  as  shown  by  their  yielding  on 
hydrolysis  the  reducing  carbohydrate  galactose),  (c)  phosphatids  (com- 
pounds containing  both  nitrogen  and  phosphorus,  e.g.,  lecithin,  kephalin, 
sphingo-myelin) ,  inorganic  salts,  and  a  series  of  nitrogen-holding  bodies 
such  as  creatin,  xanthin,  urea,  leucin,  etc.  As  to  the  metabolism  that  is 
taking  place  in  nerve-cells  and  fibers,  practically  nothing  definite  is  known. 
That  such  changes,  however,  are  taking  place  would  be  indicated  first  by  the 
blood-supply,  and  second  by  the  fact  that  withdrawal  of  the  blood-supply  is 
followed  by  a  loss  of  irritability.  The  metabolism  of  the  central  organs  of 
the  nerve  system  is  more  active  and  extensive.  In  this  situation  any  with- 
drawal of  blood  from  compression  or  occlusion  o[  blood-vessels  is  followed 
by  impairment  of  nutrition  and  loss  of  function. 

THE  RELATION  OF  THE  PERIPHERAL  ORGANS  OF  THE  NERVE 
SYSTEM  TO  THE  CENTRAL  ORGANS. 

Spinal  Nerves. — The  nerves  in  connection  with  the  spinal  cord  are 
thirty-one  in  number  on  "each  side.  If  traced  toward  the  spinal  column,  it 
will  be  found  that  the  nerve-trunk  passes  through  an  inters^ertebral  foramen. 
Near  the  outer  limits  of  the  foramina  each  nerve-trunk  divides  into  two 
branches,  generally  termed  roots,  one  of  which,  curving  slightly  forward  and 
upward,  enters  the  spinal  cord  on  its  anterior  or  ventral  surface,  while  the 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  93 

other,  cun-ing  backward  and  upward,  enters  the  spinal  cord  on  its  posterior 
or  dorsal  surface.  The  former  is  termed  the  anterior  or  ventral  root;  the 
latter,  the  posterior  or  dorsal  root.  Each  dorsal  root  presents  near  its  union 
with  the  ventral  root  a  small  ovoid  grayish  enlargement  known  as  a  ganglion. 
Both  roots  previous  to  entering  the  cord  subdivide  into  from  four  to  six 
fasciculi. 

A  microscopic  examination  of  a  cross-section  of  the  spinal  cord  shows 
that  the  fibers  of  the  ventral  roots  can  be  traced  directly  into  the  body  of  the 
nerve-cells  in  the  ventral  horns  of  the  gray  matter.  The  fibers  of  the  dorsal 
roots  are  not  so  easily  traced,  for  they  diverge  in  several  directions  shortly 
after  entering  the  cord.  In  their  course  they  give  off  collateral  branches 
which,  in  common  wath  the  main  fiber,  end  in  tufts  which  become  associated 
with  nerve-cells  in  both  the  ventral  and  dorsal  horns  of  the  gray  matter. 

Cranial  Nerves. — -The  nerves  in  connection  with  the  base  of  the  brain 
are  knowm  as  cranial  nerves;  some  of  these  nerves  present  a  similar  gangHonic 
enlargement,  and  therefore  may  be  regarded  as  dorsal  nerves,  while  others 
may  be  regarded  as  ventral  nerves.  Their  relations  within  the  medulla 
oblongata  are  similar  to  those  within  the  spinal  cord. 

Efferent  and  Afferent  Nerves. — Nerv^es  are  channels  of  communication 
between  the  brain  and  spinal  cord,  on  the  one  hand,  and  the  skeletal  muscles, 
glands,  blood-vessels,  visceral  muscles,  skin,  mucous  membrane,  etc.,  on 
the  other.  Some  of  the  nerve-fibers  serve  for  the  transmission  of  nerve 
energy  from  the  brain  and  spinal  cord  to  certain  peripheral  organs,  and  so 
accelerate  or  retard,  augment  or  inhibit  their  activities;  others  ser^'e  for  the 
transmission  of  nerve  energy  from  certain  peripheral  organs  to  the  brain  and 
spinal  cord  which  gives  rise  to  sensation  or  other  modes  of  nerve  activity. 
The  former  are  termed  efferent  or  centrifugal,  the  latter  afferent  or  centripetal 
nerves.  Experimentally  it  has  been  determined  that  the  anterior  or  ventral 
roots  contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all  the  afferent 
fibers. 

The  Peripheral  Endings  of  Nerves. — The  efferent  nerves  as  they 
approach  their  ultimate  terminations  lose  both  the  neurilemma  and  myelin 
sheaths.  The  axon  or  axis-cylinder  then  divides  into  a  number  of  branches 
which  become  directly  and  intimately  associated  with  tissue-cells.  The 
particular  mode  of  termination  varies  in  diiierent  situations.  These  termina- 
tions are  generally  spoken  of  as  end-organs,  terminal  organs,  or  end-tufts. 

In  the  skeletal  muscle  the  nerve-fiber  loses  both  neurilemma  and  myelin 
sheath  at  the  point  where  it  comes  in  contact  with  the  muscle-fiber.  After 
penetrating  the  sarcolemma,  the  axon  or  axis-cylinder  divides  into  a  number 
of  small  branches  which  appear  to  be  embedded  in  a  relatively  large  mass  of 
sarcoplasm  and  nuclei,  the  whole  forming  the  so-called  "motor  plate." 
Each  muscle-fiber  possesses  one  such  plate  or  end-organ  in  mammalia, 
several  in  the  frog.     (Fig.  46.) 

In  the  visceral  muscle  the  terminal  nerve-fibers  derived  from  sympathetic 
or  peripheral  neurons  are  primarily  non-medullated.  The  axons  divide  and 
subdivide  and  form  plexuses  which  surround  the  muscle-cell  bundles.  Fine 
fibers  from  the  plexuses  are  given  oft'  which  ultimately  come  into  relation 
with  each  individual  cell,  on  the  surface  of  which  they  terminate  in  the  form 
of  one  or  more  granular  masses. 


94 


TEXT-BOOK  OF  PHYSIOLOGY. 


In  the  glands,  taking  as  an  illustration  the  parotid  and  mammary  glands, 
the  nerve-fibers,  also  derived  from  sympathetic  or  peripheral  neurons,  pass 
into  the  body  of  the  gland  and  ultimately  reach  the  acini,  on  the  outer  surface 
of  which  they  ramify  and  form  a  plexus.     From  this  plexus  fine  fibers  pene- 


Nerve-fiber 
bundle. 


Fig.  46. — Motor  Nerve-exdings  of  Intercostal  Muscle-fibers  of  a  Rabbit.- 

X  ISO.— (Stdhr.) 

trate  the  acinus  wall  and  end  on  the  gland-cell.  The  fibers  present  a  varicose 
appearance  (Fig.  47). 

The  afferent  nerves  as  they  approach  their  ultimate  terminations 
undergo  similar  changes.  The  end-tufts  become  associated,  in  some  situ- 
ations, with  specialized  end-organs  which  are  extremely  complex. 

In  the  skin  and  mucous  membranes  the  mode  of  termination  varies  con- 
siderably.    The  following  are  some  of  the  principal  modes: 

1.  Free  endings  in  the  epithelium. 

2.  Tactile  cells  of  Merkel. 

3.  Tactile  corpuscles  in  the  papillae  of 
the  true  skin. 

4.  Paciniancorpuscles  found  attached  to 
the  nerves  of  the  hands  and  feet,  to 


Fig.  47, — Terminations  of  Nerve- 
FiPERs  IN  the  Gland-cells.  A.  Cell 
of  the  parotid  gland  of  a  rabbit.  B. 
Cells  of  the  mammary  gland  of  a  cat  in 
gestation. — (Doyon  and  Moral.) 


the  intercostal  nerves,  and  to  nerves 
in  other  situations. 
End-bulbs  of  Krause  in  the  conjunc- 
tiva, clitoris,  penis,  etc. 
(A  consideration  of  these  end-organs 
will  be  found  in  the  chapters  devoted  to  the  organs  of  which  they  form 
a  part.) 

In  the  skeletal  muscles  afferent  fibers  become  associated  with  small 
spindle-shaped  structures  known  as  muscle-spindles  or  neuromuscle  end- 
organs.  These  spindles  vary  in  length  from  i  mm.  to  4  mm.  They  consist 
of  a  capsule  of  fibrous  tissue  containing  from  five  to  twenty  muscle-fibers. 
After  penetrating  the  several  layers  of  the  capsule,  the  nerve-fibers  lose  the 
neurilemma  and  mvelin  sheaths.     The  axons  or  axis-cvlinders  then  divide 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


95 


into  several  long  narrow  branches  which  wind  themselves  in  a  spiral  manner 
around  the  contained  muscle-fiber  and  terminate  in  small  oval-shaped  discs. 
Similar  endings  have  been  obser\xd  in  the  tendons  of  muscles. 

Development  and  Nutrition  of  Nerves. — The  efferent  nerve-fibers, 
which  constitute  some  of  the  cranial  nerves  and  all  the  ventral  roots  of  the 
spinal  nerves,  have  their  origin  in  cells  located  in  the  gray  matter  beneath 
the  aqueduct  of  Sylvius,  beneath  the  floor  of  the  fourth  ventricle,  and  in  the 
ventral  horns  of  the  gray  matter  of  the  spinal  cord.  These  cells  are  the 
modified  descendants  of  independent,  oval,  pear-shaped  cells — the  neuro- 
blasts— which  migrate  from  the  medullary  tube.  As  they  approach  the 
surface  of  the  cord  their  axons  are  directed  toward  the  ventral  surface,  which 
eventually  they  pierce.  Emerging 
from  the  cord,  the  axons  continue  to 
grow,  and  become  invested  with  the 
myelin  sheath  and  neurilemma,  thus 
constituting  the  ventral  roots.  (Fig.  48.) 

The  afferent  nerve-fibers,  which 
constitute  some  of  the  cranial  ner^^es 
and  all  the  dorsal  roots  of  the  spinal 
ners'es,  develop  outside  of  the  central 
nerve  system  and  only  subsequently 
become  connected  with  it.  (See  Fig. 
48.)  At  the  tim^e  of  the  closure  of 
the  medullary  tube  a  band  or  ridge  of 
epithelial  tissue  develops  near  the  dor- 
sal surface,  which,  becoming  seg- 
mented, moves  outward  and  forms 
the  rudimentarv  spinal  ganglia.     The 

cells    in    this   situation   develop   two       ^^°- ^t  "^.'Sf  ilSTJ^^vn^  nol^'.A^ 

/^  OF  Origin  of  the  Ventral  and  Dorsal 

axons,  one  from  each  end  of  the  cell,  -roots.— {Edinger,  after  His.) 

which  pass  in  opposite  directions,  one 

toward    the    spinal    cord,   the  other  toward  the  periphery.     In  the  adult 

condition  the  two  axons  shift  their  position,  unite,  and  form  a  T  shaped 

process,  after  which   a  division  into  two  branches  again  takes  place.     In 

the   ganglia   of  all  the  sensori-cranial  and  sensori-spinal  nerves  the  cells 

have  this  histologic  peculiarity. 

The  efferent  fibers  are  therefore  to  be  regarded  as  outgrowths  from  the 
nerve-cells  in  the  ventral  horns  of  the  gray  matter,  and  serve  to  bring  the 
cells  into  anatomic  and  physiologic  relationship  directly  with  the  skeletal 
muscles  and  indirectly,  through  the  intermediation  of  ganglia  (see  sym-, 
pathetic  nerve  system),  with  visceral  muscles,  blood-vessels,  and  glands. 

The  afferent  fibers  are  to  be  regarded  as  outgrowths  from  the  cells  of  the 
dorsal  nerve  ganglia,  and  serve  to  bring  the  skin,  mucous  membrane,  and 
certain  visceral  structures  into  relation  with  specialized  centers  in  the  central 
nerve  system. 

Nerve  Degeneration. — If  any  one  of  the  cranial  or  spinal  nerves  be  di- 
vided in  any  portion  of  its  course,  the  part  in  connection  with  the  periphery  in 
a  short  time  exhibits  certain  structural  changes,  to  which  the  term  degenera- 
tion is  applied.     The  portion  in  connection  with  the  brain  or  cord  retains  its 


ntefu/r 
£006 


96 


TEXT-BOOK  OF  PHYSIOLOGY. 


normal  condition  with  the  exception  of  a  few  millimeters  at  its  peripheral 
end.  The  degenerative  process  begins  simultaneously  throughout  the  entire 
course  of  the  nerve,  and  consists  in  a  disintegration  and  reduction  of  the 
myelin  and  axis-cylinder  into  nuclei,  drops  of  myelin,  and  fat,  which  in  time 
disappear  through  absorption,  leaving  the  neurilemma  intact.  Coincident 
with  these  structural  changes  there  is  a  progressive  alteration  and  diminution 
in  the  excitability  of  the  nerve.  Inasmuch  as  the  central  portion  of  the  nerve, 
which  retains  its  connection  with  the  nerve-cell,  remains  histologically 
normal,  it  has  been  assumed  that  the  nerve-cells  exert  over  the  entire  course 
of  the  nerve-fibers  a  nutritive  or  a  trophic  influence.  This  idea  has  been 
greatly  strengthened  since  the  discovery  that  the  axis-cylinder,  or  the  axon, 
has  its  origin  in  and  is  a  direct  outgrowth  of  the  cell.  When  separated  from 
the  parent  cell,  the  fiber  appears  to  be  incapable  in  itself  of  maintaining  its 
nutrition. 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to  their 
nutrition,  is  demonstrated  by  the  results  which  follow  section  of  the  ventral 
and  dorsal  roots  of  the  spinal  nerves.  If  the  ventral  root  alone  be  divided 
the  degenerative  process  is  confined  to  the  peripheral  portion,  the  central 


Fig.  49. — Degeneration  of   Spinal  Nerves  and   Nerve-roots  after  Section     A, 
Section  of  nerve-trunk  beyond  the  ganglion      B,Sectonof  venral  root      C,  Section  of  dorsal 
D.  Excision  of  ganglion,     a.  Ventral  root.     p.  Dorsal,     g.  Ganglion. — {Dalton.) 

portion  remaining  normal.  If  the  dorsal  root  be  divided  on  the  peripheral 
side  of  the  ganglion,  degeneration  takes  place  only  in  the  peripheral  portion 
of  the  nerve.  (See  Fig.  49.)  If  the  root  be  divided  between  the  ganglion 
and  the  cord,  degeneration  takes  place  only  in  the  central  portion  of  the  root. 
From  these  facts  it  is  evident  that  the  trophic  centers  for  the  ventral  and 
dorsal  roots  He  in  the  spinal  cord  and  spinal  nerve  ganglia,  respectively,  or, 
in  other  words,  in  the  cells  of  which  they  are  an  integral  part.  The  structural 
changes  which  nerves  undergo  after  separation  from  their  centers  are  degen- 
erative in  character,  and  the  process  is  usually  spoken  of,  after  its  discoverer, 
as  the  Waller ian  degeneration. 
^j  When  the  nerve-cells  from  which  the  nerve-fibers  arise,  whether  efferent 
or  afferent,  undergo  degeneration  from  any  cause  whatever,  the  nerve-fiber 
becomes  involved  in  the  degenerative  process  and  when  it  is  completed  the 
structures  to  which  they  are  distributed,  especially  the  muscles,  undergo  an 
atrophic  or  fatty  degeneration,  with  a  change  or  loss  of  their  irritability. 
This  is,  apparently,  not  to  be  attributed  merely  to  inactivity,  but  rather  to  a 
loss  of  nerve  influences,  inasmuch  as  inactivity  merely  leads  to  atrophy  and 
not  to  degeneration. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  97 

|Reunion  and  Regeneration. — When  a  nerve-trunk  is  divided  there  is 
a  loss  of  function  of  the  parts  to  which  it  is  distributed,  and  usually  involves 
both  motion  and  sensation.  This,  however,  is  not  necessarily  permanent, 
for  after  a  variable  period  of  time  it  not  infrequently  happens  that  the  func- 
tions are  restored  because  of  a  reunion  of  the  separated  ends  and  a  regenera- 
tion of  the  peripheral  portion.  A  histologic  study  of  the  nerve-fibers  after 
separation  from  the  nerve-cells  shows  that  conincidently  with  the  degenerative 
process  there  occurs  a  regenerative  process,  consisting  in  a  multiplication  of 
the  nuclei  lying  just  beneath  the  neurilemma  and  an  accumulation  around 
them  of  a  granular  protoplasm  which  in  due  time  completely  fill  the  neuril- 
emma. At  this  stage  the  fiber  is  known  as  a  band-fiber.  If  now  the  physical 
conditions  are  such  as  to  permit  of  a  reunion  of  the  nerve,  this  takes  place, 
and  under  the  nutritive  influence  of  the  cell  the  axis-cylinder  grows  into  the 
band-fiber  and  the  protoplasm  becomes  transformed  into  myehn  as  in  the 
original  fiber.  The  axis-cylinder  continues  to  grow  and  extend  itself 
forward  until  it  reaches  its  ultimate  termination. 

CLASSIFICATION  OF  NERVES. 

The  efferent  nerves  may  be  classified,  in  accordance  with  the  character- 
istic forms  of  activity  to  which  they  give  rise,  into  several  groups,  as  follows: 

1.  Skeletal-muscle  or  motor  nerves,  those  which  convey  nerve  energy  or  nerve 

impulses  directly  to  skeletal-muscles  and  excite  them  to  activity. 

2.  Gland  or  secretor  nerves,  those  which  convey  nerve  impulses  to  glands  by 

way  of  ganglia  and  cause  the  formation  and  discharge  of  the  secretion 
peculiar  to  the  gland. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerve  impulses  to  the 

muscle-fibers  of  the  blood-vessels  and  change  in  one  direction  or  the 
other  the  degree  of  their  natural  contraction.  Those  which  increase  the 
contraction  are  known  as  vaso-constrictors  or  vaso-augmentors;  those 
which  decrease  the  contraction  are  known  as  vaso-dilatators  or  vaso- 
inhibitors.  The  nerves  which  pass  to  that  specialized  part  of  the 
vas"ular  apparatus,  the  heart,  transmit  nerve  impulses  which  on  the 
one  hand  accelerate  its  rate  or  augment  its  force,  and  on  the  other  hand 
inhibit  or  retard  its  rate  and  diminish  its  force.  For  this  reason  they 
are  termed  cardiac  nerves,  one  set  of  which  is  known  as  cardio-accelera- 
,    tor  and  cardio-augmentor,  the  other  as  cardio-inhibitor  nerves. 

4.  Visceral  or  visccro-motor  nerves,  those  which  transmit  nerv^e  impulses  to 

the  muscle. walls  of  the  viscera  and  change  in  one  direction  or  another 
the  degree  of  their  contraction.  Those  which  increase  or  augment  the 
contraction  are  known  as  viscero-augmentor,  while  those  which  decrease 
or  inhibit  the  contraction,  are  known  as  viscero-inhibitor  nerv^es. 

5.  Hair  bulb  or  pilo-motor  nerves,  those  which  transmit  nerve  impulses  to  the 

muscle-fibers  which  cause  an  erection  of  the  hairs. 
Of  the  foregoing  nerves  the  skeletal-muscle  or  motor  nerves  alone  pass 
directly  to  the  muscle.  The  gland,  the  vascular  and  the  visceral  nerves,  all 
terminate  at  a  variable  distance  from  the  peripheral  organ  around  a  local 
sympathetic  ganglion,  which  in  turn  is  connected  with  the  peripheral  organ. 
The  former  are  termed  pre-ganglionic.  The  latter  post-ganglionic  fibers. 
(See  Fig.  13.) 

7 


98  TEXT-BOOK  OF  PHYSIOLOGY, 

The  afferent  nerves  may  also  be  classified,  in  accordance  with  their 
distribution  and  the  character  of  the  sensations  or  other  modes  of  nerve 
activity  to  which  they  give  rise,  into  several  groups,  as  follows: 

1.  Tegumentary  nerves,  comprising  those  distributed  to  skin,  mucous  mem- 

branes and  sense  organs  and  which  transmit  nerve  impulses  from  the 
periphery  to  the  nerve  centers.  They  may  be  divided  into  refiex  and 
sensorifacient  nerves. 

A.  Reflex  ner\-es,  those  which  transmit  ner\-e  impulses  to  the  spinal 
cord  and  medulla  oblongata,  where  they  give  rise  to  different 
modes  of  nerve  activity.     They  may  be  divided  into: 

1.  Reflex  excitator  ner^'es,  which  transmit  ner\'e  impulses  which 
cause  an  excitation  of  nerve  centers  and  in  consequence  in- 
creased activity  of  peripheral  organs,  e.g.,  skeletal  muscles, 
glands,  blood-vessels  and  viscera. 

2.  Reflex  inhibitor  nerv^es,  which  transmit  nerve  impulses  which 
cause  an  inhibition  of  nerve  centers  and  in  consequence, 
decreased  activity  of  the  peripheral  organs.  It  is  quite  prob- 
able that  one  and  the  same  ner\'e  may  subserve  both  sensation 
and  reflex  action,  owing  to  the  collateral  branches  which  are 
given  off  from  the  afferent  roots  as  they  ascend  the  posterior 
column  of  the  cord. 

B.  Sensorifacient  nerves,  those  which  transmit  nerve  impulses  to  the 
brain  where  they  give  rise  to  conscious  sensations.  They  may  be 
subdivided  into: 

1.  Xerves  of  special  sense — e.g.,  olfactory,  optic,  auditory, 
gustatory,  tactile,  thermal,  pain,  pressure — which  give  rise  to 
correspondingly  named  sensations. 

2.  Xers-es  of  general  sense — e.g.,  the  visceral  afferent  nerves — 
those  which  give  rise  normally  to  vague  and  scarcely  perceptible 
sensations,  such  as  the  general  sensations  of  well-being  or  dis- 
comfort, hunger,  thirst,  fatigue,  sex,  want  of  air,  etc. 

2.  Muscle  nerves,  comprising  those  distributed  to  muscles  and  tendons  and 

w^hich  transmit  nerve  impulses  from  muscles  and  tendons  to  the  brain 
where  they  give  rise  to  the  so-called  muscle  sensations,  e.g.,  the  direction 
and  the  duration  of  a  movement,  the  resistance  offered  and  the  posture 
of  the  body  or  of  its  individual  parts. 

PHYSIOLOGIC  PROPERTIES  OF  NERVES. 

Nerve  Irritability  or  Excitability  and  Conductivity. — These  terms 
are  employed  to  express  that  condition  of  a  nerv^e  which  enables  it  to  develop 
and  to  conduct  nerve  impulses  from  the  center  to  the  periphery,  or  from 
the  periphery  to  the  center,  in  response  to  the  action  of  stimuli.  A  nerve 
is  said  to  be  excitable  or  irritable  so  long  as  it  possesses  these  capabilities  or 
properties.  For  the  manifestation  of  these  properties  the  nerve  must 
retain  a  state  of  physical  and  chemic  integrity;  it  must  undergo  no  change 
in  structure  or  chemic  composition.  The  irritability  of  an  efferent  nerve  is 
demonstrated  by  the  contraction  of  a  muscle,  by  the  secretion  of  a  gland,  or 
by  a  change  in  the  caliber  of  a  blood-vessel,  whenever  a  corresponding  nerve 
is  stimulated.     The  irritability  of  an  afferent  nerve  is  demonstrated    by  the. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  99 

production  of  a  sensation  or  a  reflex  action  whenever  it  is  stimulated.  The 
irritability  of  nerves  continues  for  a  certain  period  of  time  after  separation 
from  the  ner\-e-centers  and  even  after  the  death  of  the  animal,  the  time 
varying  in  different  classes  of  animals.  In  the  warm-blooded  animals,  in 
which  the  nutritive  changes  take  place  with  great  rapidity,  the  irritability 
soon  disappears — a  result  due  to  disintegrative  changes  in  the  nerve,  caused 
by  the  withdrawal  of  the  blood-supply  and  other  non-physiologic  conditions. 
In  cold-blooded  animals,  on  the  contrary,  in  which  the  nutritive  changes 
take  place  relatively  slowdy,  the  irritability  lasts,  under  favorable  conditions, 
for  a  considerable  time.  Other  tissues  besides  nerves  possess  irritability, 
that  is,  the  property  of  responding  to  the  action  of  stimuli — e.g.,  glands  and 
muscles,  which  respond  by  the  production  of  a  secretion  or  a  contraction. 

Independence  of  Tissue  Irritability. — The  irritability  of  ner%'es  is 
distinct  and  independent  of  the  irritability  of  muscles  and  glands,  as  shown 
by  the  fact  that  it  persists  in  each  a  variable  length  of  time  after  their  histo- 
logic connections  have  been  impaired  or  destroyed  by  the  introduction  of 
various  chemic  agents  into  the  circulation.  Curara,  for  example,  induces 
a  state  of  complete  paralysis  by  modifying  or  depressing  the  conductivity  of 
the  end-organs  of  the  nerves  just  where  they  come  in  contact  with  the  mus- 
cles, without  impairing  the  irritability  of  either  nen^-trunks  or  muscles. 
Atropin  induces  complete  suspension  of  gland  activity  by  impairing  the 
terminal  organs  of  the  secretor  nerv-es  just  where  they  come  into  relation  with 
the  gland-cells,  without  destroying  the  irritability  of  either  gland-cell  or  nerve. 

Nerve  Stimuli. — Nerves  do  not  possess  the  power  of  spontaneously 
generating  and  propagating  nerve  impulses;  they  can  be  aroused  to  activity 
only  by  the  action  of  an  external  stimulus.  In  the  physiologic  condition  the 
stimuli  capable  of  throwing  the  nerve  into  an  active  condition  act  for  the 
most  part  on  either  the  central  or  peripheral  end  of  the  nerve.  In  the  case 
of  motor  nerves  the  stimulus  to  the  excitation,  originating  in  some  molecular 
disturbance  in  the  nerve-cells,  acts  upon  the  nerve-fibers  in  connection  with 
them.  In  the  case  of  sensor  or  afferent  nerves  the  stimuli  act  upon  the  pecul- 
iar end-organs  with  which  the  sensor  nerves  are  in  connection,  which  in 
turn  excite  the  nerve-fibers.  Experimentally,  it  can  be  demonstrated  that 
nerves  can  be  excited  by  a  sufficiently  powerful  stimulus  applied  in  any 
part  of  their  extent. 

Nerves  respond  to  stimulation  according  to  their  habitual  function; 
thus,  stimulation  of  a  sensor  nerve,  if  sufficiently  strong,  results  in  the  sensa- 
tion of  pain;  of  the  optic  nerv^e,  in  the  sensation  of  light;  of  a  motor  nerve, 
in  contraction  of  the  muscle  to  which  it  is  distributed;  of  a  secretor  nerve, 
in  the  activity  of  the  related  gland,  etc.  It  is,  therefore,  evident  that  pecul- 
iarity of  ner\'e  function  depends  neither  upon  any  special  construction  or 
activity  of  the  nerve  itself  nor  upon  the  nature  of  the  stimulus,  but  entirely 
upon  the  peculiarities  of  its  central  and  peripheral  end-organs. 

Nerve  stimuli  may  be  divided  into — 

1.  General  stimuli,  comprising  those  agents  which  are  capable  of  exciting 

a  nerve  in  any  part  of  its  course. 

2.  Special  stimuli,  comprising   those   agents  which   act   upon   nerves   only 

through  the  intermediation  of  the  end-organs. 
The  end-organs  are  specialized  highly  irritable  structures  placed  between 


loo  TEXT-BOOK  OF  PHYSIOLOGY. 

the   nerve-fibers  and   the   surface.     They  are   especially   adapted   for  the 
reception  of  special  stimuli  and  for  the  liberation  of  energy,  which  in  turn 
excites  the  nerve-fiber  to  activity. 
General  stimuli: 

1.  Mechanic:  Sharp  taps,  sudden  pressure,  cutting,  etc. 

2.  Thermic:  Sudden  application  of  heated  object. 

3.  Chemic:  Contact  of  various  substances  w^hich  alter  their  chemic  composi- 

tion quickly,  e.g.,  strong  acids  or  alkalies,  sol.  sodium  chlorid  15  per 
cent.,  sugar,  urea,  etc. 

4.  Electric:  Either  the  constant  or  induced  current. 
Special  stimuli: 

For  afferent  nerves— 

1.  Light  or  ethereal  vibrations  acting  upon  the  end-organs  of  the  optic 

nerve  in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end-organs  of  the 

auditory  nerve. 

3.  Heat  or  vibrations  of  the  air  acting  upon  the  end-organs  in  the  skin. 

4.  Chemic  agencies  acting  upon  the  end-organs  of  the  olfactory  and  gusta- 

tory nerves. 
For  efferent  nerves — 

A  molecular  disturbance  in  the  central  nerve-cells  from  which  they  arise, 
the  nature  of  which  is  unknown. 

Nature  of  the  Nerve  Impulse. — -As  to  the  nature  of  the  nerve  impulse 
generated  by  any  of  the  foregoing  stimuli,  either  general  or  special,  but  little 
is  known.  It  has  been  supposed  to  partake  of  the  nature  of  a  molecular 
disturbance,  a  combination  of  physical  and  chemic  processes  attended  by 
the  liberation  of  energy,  which  propagates  itself  from  molecule  to  molecule. 
The  passage  of  the  nerve  impulse  is  accompanied  by  changes  of  electric 
tension,  the  extent  of  which  is  an  indication  of  the  intensity  of  the  molecular 
disturbance.  Judging  from  the  deflections  of  the  galvanometer  needle  it  is 
probable  that  when  the  nerve  impulse  makes  its  appearance  at  any  given 
point  it  is  at  first  feeble,  but  soon  reaches  a  maximum  development,  after 
which  it  speedily  declines  and  disappears.  It  may,  therefore,  be  graphically 
represented  as  a  wave-like  movement  with  a  definite  length  and  time  dura- 
tion. (See  page  104.)  Under  strictly  physiologic  conditions  the  nerve 
impulse  passes  in  one  direction  only;  in  efferent  nerves  from  the  center  to  the 
periphery,  in  afferent  nerves  from  the  periphery  to  the  center.  Experimen- 
tally, however,  it  can  be  demonstrated  that  when  a  nerve  impulse  is  aroused 
in  the  course  of  a  nerve  by  an  adequate  stimulus  it  travels  equally  well  in 
both  directions  from  the  point  of  stimulation.  When  once  started,  the 
impulse  is  confined  to  the  single  fiber  and  does  not  diffuse  itself  to  fibers 
adjacent  to  it  in  the  same  nerve-trunk. 

Rapidity  of  Conduction  of  the  Nerve  Impulse. — The  passage  of  a 
nerve  impulse,  either  from  the  brain  to  the  periphery  or  in  the  reverse  direc- 
tion, requires  an  appreciable  period  of  time.  The  velocity  with  which  the 
impulse  travels  in  human  sensory  nerves  has  been  estimated  at  about  50 
meters  a  second,  and  for  motor  nerves  at  from  28  to  t,t,  meters  a  second.  The 
rate  of  movement  is,  however,  somewhat  modified  by  temperature,  cold 
lessening  and  heat  increasing  the  rapidity;  it  is  also  modified  by  electric 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


conditions,  by  the  action  of  drugs,  the  strength  of  the  stimulus,  etc.  The 
rate  of  transmission  through  the  spinal  cord  is  considerably  slower  than  in 
nerves,  the  average  velocity  for  voluntary  motor  impulses  being  only  ii 
meters  a  second,  for  sensory  impulses  12  meters,  and  for  tactile  impulses  40 
meters  a  second. 

Nerve  Fatigue.— Inasmuch  as  nerves  are  parts  of  living  cells,  the  seat 
of  nutritive  changes,  it  might  be  supposed  that  the  passage  of  nen^e  impulses 
would  be  attended  by  the  disruption  of  energy-holding  compounds,  the  pro- 
duction of  waste  products,  the  liberation  of  heat,  and  in  time  by  the  phenom- 
ena of  fatigue.  Though  it  is  probable  that  changes  of  this  character  occur, 
yet  no  reliable  experimental  data  have  been  obtained  which  afford  a  clue  as 
to  the  nature  or  extent  of  any  such  changes.  Stimulation  of  motor  nerves 
with  the  induced  electric  current  for  four  hours  appears  to  be  without  influ- 
ence either  on  the  intensitv  of  the  nerve  impulse  or  the  rate  of  its  conduction. 

Identity  of  Efferent  and  Afferent  Nerves  and  Nerve  Impulses.— 
Notwithstanding  the  classification  of  nerve-fibers  based  on  difi"erences  of 
physiologic  actions,  there  are  no  characters,  either  histologic  or  chemic, 
which  serve  to  distinguish  them  from  one  another.  Moreover,  as  the  nerve 
impulse  is  conducted  through  a  nerve-fiber  equally  well  in  both  directions, 
as  determined  by  experiments,  it  is  probable  that  it  does  not  differ  in  char- 
acter in  the  two'  classes  of  nen-es.  That  the  efferent  fibers  conduct  the 
nerv-e  impulses  from  the  nerve-centers  to  the  periphery,  and  the  aft'erent 
nen^es  from  the  periphery  to  the  centers,  is  because  of  the  fact  that  they 
receive  their  stimulus  physiologically  only  in  the  centers  or  at  the  periphery. 
The  fundamental  reason  for  diiTerence  of  effects  pro- 
duced by  stimulation  of  different  nerves  is  the  character 
of  the  organ  to  which  the  nerA'c  impulse  is  conducted. 
A  nerve  is  merely  the  transmitter  of  the  nerve  impulse, 
which  if  conducted  to  a  muscle  excites  contraction;  to  a 
gland,  secretion;  to  a  blood-vessel,  variation  in  caliber; 
to  special  areas  in  the  brain,  sensations  of  light,  sound, 
pain,  etc. 

Electric  Excitation  of  Nerves. — For  the  purpose  of 
studying  the  physiologic  activities  of  nerves  it  has  been 
found  convenient  to  employ  the  nerve-muscle  prepara- 
tion (the  gastrocnemius  muscle  and  sciatic  nerve)  and  to 
use  as  a  stimulus  the  induced  electric  current.  (See 
Fig.  50.)  When  kept  moist,  this  preparation  is  ex- 
tremely sensitive  to  either  the  galvanic  or  the  induced 
current. 

Though  the  development  and  conduction  of  a  nerve 
impulse  may  be  demonstrated  by  the  deflection  of  the 
galvanometer  needle  or  the  movement  of  the  mercury  in 
the  capillary  electrometer,  it  is  more  conveniently  demonstrated  by  the  con- 
traction of  a  muscle,  the  vigor  of  which,  within  limits,  may  be  taken  as  a 
measure  of  the  intensity  of  the  impulse.  The  preparation  should  be  en- 
closed in  a  moist  chamber  and  the  nerve  connected  with  the  inductorium 
through  the  intervention  of  non-polarizable  electrodes.  The  muscle  may 
be  attached  to  the  muscle-lever  and  its  contractions  recorded. 


Fig.  50. — Nerve. 
MUSCLE  Prepara- 
tion OF  A  Frog.  F. 
Femur.  S.  Sciatic 
nerve.  I.  Ten  do 
Achillis.  —  (Landois 
and    Stirling.) 


I02  TEXT-BOOK  OF  PHYSIOLOGY. 

A  single  shock  of  an  induced  current  develops,  it  is  believed,  a  single 
nerve  impulse  followed  by  a  single  muscle  contraction.  A  minimal  con- 
traction following  a  minimal  electric  stimulus  presupposes  the  development 
of  a  nerve  impulse  of  low  intensity.  Within  certain  limits  a  maximal  con- 
traction following  a  maximal  electric  stimulus  presupposes  the  development 
of  a  nerve  impulse  of  high  intensity.  Intermediate  contractions  indicate 
nerve  impulses  of  corresponding  intensity. 

Tetanization  of  a  muscle  indicates  that  the  nerve  impulses  arrive  at  the 
muscle  with  a  frequency  so  great  that  the  muscle  does  not  succeed  in  relaxing 
from  the  effect  of  one  stimulus  before  the  next  arrives.  Complete  as  well  as 
incomplete  tetanus  may  be  developed  by  gradually  increasing  the  frecjuency 
of  the  stimulus.  The  character  of  the  contraction  caused  by  indirect 
stimulation — i.e.,  through  the  nerve — does  not  differ  in  any  essential  respect 
from  that  due  to  direct  stimulation. 


ELECTRIC  PHENOMENA  OF  NERVES. 

Electric  Currents  from  Injured  Nerves. — It  was  discovered  by  dn 

Bois-Reymond  that  electric  currents  can  be 
obtained  from  nerves  as  well  as  from  muscles, 
and  that  the  electric  properties  of  the  former 
correspond  in  most  respects  to  those  of  the 
latter.  The  laws  governing  the  development 
and  mode  of  action  of  the  currents  derived 
from  muscles  are  equally  applicable  to  the 
currents  derived  from  nerves. 

A  nerve-cylinder  obtained  by  making  two 
transverse  sections  of  any  given  nerve  presents, 
as  in  the  case  of  muscles,  a  natural  and  two 
artificial  transverse  surfaces.  A  line  drawn 
around  the  cylinder  at  a  point  lying  midway 
between  the  two  end  surfaces  constitutes  the 
equator.  From  such  a  cylinder  strong  cur- 
rents are  obtained  when  the  natural  longitud- 
inal surface  and  the  transverse  surface  are 
connected  with  the  electrodes  of  the  galvano- 
meter circuit.  The  strength  of  the  current 
thus  obtained  will  diminish  or  increase  accord- 
ing as  the  electrode  on  the  longitudinal  sur- 
face is  removed  from  or  brought  near  to  the 
equator.  If  two  symmetric  points  on  the 
longitudinal  surface  equidistant  from  the 
equator  are  united,  no  current  is  obtainable. 
When  asymmetric  points  on  the  longitudinal 
surface  are  connected,  weak  currents  are  ob- 
tained, in  which  case  the  point  lying  nearer  the  equator  becomes  positive 
to  the  point  more  distant,  which  becomes  negative.  From  these  facts  it  is 
evident  that  all  points  on  the  longitudinal  surface  are  electrically  positive  to 
the  transverse  surface  and  that  the  point  of  greatest  positive  tension  is  sit- 
uated near  the  equator  (Fig.  51). 


Fig.  51. — Diagram  to  Illus- 
trate THE  Currents  in  Nerves. 
The  arrowheads  indicate  the  direc- 
tion; the  thickness  of  the  lines  in- 
dicates the  strength  of  the  currents. 
— {Landois  and  Stirling.) 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  103 

The  electromotive  force  of  the  nerve  current  varies  in  strength  with  the 
length  and  thickness  of  the  nerve.  The  strongest  current  obtained  from 
the  nerve  of  the  frog  is  equal  to  the  0.002  of  a  Daniell  cell;  that  obtained 
from  the  nerve  of  the  rabbit,  0.026  of  a  Daniell.  The  existence  of  the  nerve 
current,  its  strength,  duration,  etc.,  depend  largely  on  the  maintenance  of 
physiologic  conditions.  All  influences  which  impair  the  nutrition  of  the 
nerv^e  diminish  the  current.  With  the  death  of  the  nerve  all  electric  phenom- 
ena disappear. 

Negative  Variation  of  the  Nerve  Current. — During  the  passage  of 
the  nerve  impulse  the  resting  nerve  current,  or  the  demarcation  current, 
diminishes  more  or  less  completely  in  intensity,  undergoes  a  negative  varia- 
tion, as  shown  by  the  return  of  the  galvanometer  needle,  due  to  a  change  in 
its  electromotive  condition  or  to  a  diminution  of  the  difference  in  potential 
between  the  positive  longitudinal  and  negative  transverse  sections.  This 
negative  variation  of  the  demarcation  current  is  observed  equally  well  from 
either  the  central  or  peripheral  end  of  the  nerve.  If  the  two  ends  of  the 
nerve  are  connected  with  galvanometers  and  the  nerve  stimulated  in  the 
middle,  the  demarcation  currents  simultaneously  undergo  a  negative  varia- 
tion. This  may  be  taken  as  a  proof  that  the  excitation  process  propagates 
itself  equally  well  in  both  directions.  The  negative  variation  is  intimately 
connected  with  changes  in  the  molecular  condition  of  the  nerve  and  is  not 
due  to  any  extraneous  electric  or  other  influence.  And  du  Bois-Reymond 
was  also  enabled  to  obtain  a  negative  variation  of  the  current  in  the  nerves 
of  a  living  frog  which  were  yet  in  connection  with  the  spinal  cord.  In  this 
experiment  the  sciatic  nerve  was  divided  at  the  knee  and  freed  from  its 
connections  up  to  the  spinal  column;  the  transverse  and  longitudinal  surfaces 
were  then  placed  in  connection  with  the  electrodes  of  the  galvanometer  wires 
and  the  current  permitted  to  influence  the  needle.  The  animal  was  then 
subjected  to  the  action  of  strychnin.  Upon  the  appearance  of  the  muscle 
spasms  the  needle  was  observed  to  swing  backward  toward  the  zero  point  to 
the  extent  of  from  i  to  4  degrees,  and  upon  the  cessation  of  the  spasms  to 
return  to  its  previous  position.  In  an  experiment  of  this  nature  it  is  obvious 
that  the  negative  variation  was  the  result  of  a  physiologic  stimulation  of  the 
nerve  arising  within  the  spinal  cord. 

The  question  also  here  arises  as  to  whether  the  negative  variation  is  due 
to  a  steady,  continuous  decrease  of  the  natural  current,  or  whether  it  is  due 
to  successive  and  rapidly  following  variations  in  its  intensity,  similar  to  that 
observed  in  muscles.  Though  this  cannot  be  demonstrated  with  the  physio- 
logic rheoscope,  as  was  the  case  with  the  muscle,  there  can  be  no  doubt,  both 
from  experimentation  and  analogy,  that  the  latter  supposition  is  the  correct 
one.  It  has  been  shown  that  when  non-polarizable  electrodes  connected 
with  Siemen's  telephone  are  placed  in  connection  with  the  longitudinal  and 
transverse  sections  of  a  nerve,  low,  sonorous  vibrations  are  perceived  during 
tetanic  stimulation — a  proof  that  the  active  state  of  the  nerve  is  connected 
with  the  production  of  discontinuous  electric  currents.  The  oscillations 
of  the  mercurial  column  of  the  capillary  electrometer  also  reveal  similar 
electric  changes.  It  was  also  demonstrated  by  Bernstein  with  a  specially 
devised  apparatus,  the  repeating  rheotome,  that  the  negative  variation  is 
composed    of    a   large   number  of  single   variations  which  succeed   each 


I04  TEXT-BOOK  OF  PHYSIOLOGY. 

other  in  rapid  succession  and  summarize  themselves  in  their  effect  on  the 
needle. 

Electric  Currents  from  Uninjured  Nerves. — The  pre-existence 
of  electric  currents  in  living  and  wholly  uninjured  nerves  while  at  rest  has 
also  been  denied  by  Hermann,  who  regards  all  portions  of  the  nerve  as 
isoelectric,  any  difference  of  potential  being  the  result  of  some  injury  to  its 
surface. 

Action  Currents.— For  reasons  to  be  stated  below,  it  is  very  diffi- 
cult to  determine  the  presence  of  diphasic  action  currents  during  the  pass- 
age of  an  excitatory  impulse  through  the  nerve-fiber.  The  so-called  negative 
variation  of  the  resting  nerve  current — the  demarcation  current — which  is 
occasioned  by  tetanic  stimulation,  Hermann  regards  as  the  expression  of  an 
action  current  which  flows  in  the  nerve  in  a  direction  opposite  to  the  demarca- 
tion current.  The  origin  of  this  action  current  is  to  be  sought  for  in  the 
continuous  negativity  of  that  portion  of  the  longitudinal  surface  of  the  nerve 
in  contact  with  the  diverting  electrode,  while  the  dying  substance  of  the 
transverse  surface  takes  no  part  in  the  excitation.  This  tetanic  action  current, 
or  negative  variation,  was  discovered  by  du  Bois-Reymond,  and  Bernstein 
later  succeeded  in  obtaining  this  action  current  during  the  passage  of  a 
single  excitation  process.  That  the  return  of  the  galvanometer  needle 
toward  the  zero  point  is  not  due  to  an  annulment  of  the  demarcation  current 
itself,  but  to  the  appearance  of  an  action  current,  is  shown  by  the  fact  that 
if  the  former  be  compensated  by  a  battery  current  until  the  needle  rests  on 
the  zero  point  the  appearance  of  the  latter  current  will  cause  the  needle 
to  swing  in  a  direction  the  opposite  of  that  caused  by  the  demarcation  current. 
The  negative  variation  and  action  current  may  therefore  be  regarded  as  one 
and  the  same  thing.  It  is  the  expression  of  the  change  the  nerve  is  under- 
going during  the  passage  of  the  nerve  impulse.  The  rapidity  with  which 
the  negative  variation  or  action  current  travels,  the  variation  in  its  intensity 
from  moment  to  moment,  the  time  required  for  it  to  pass  a  given  point, 
would  express  the  change  in  the  nerve  to  which  the  term  nerve  impulse  is 
given.  From  experiments  made  with  the  differential  rheotome,  Bernstein 
calculated  that  the  speed  of  the  negative  variation  is  about  28  meters  a 
second;  that  it  is  at  first  feeble,  soon  rises  to  a  maximum,  and  then  declines; 
that  is  requires  0.0006  to  0.0008  of  a  second  to  pass  a  given  point.  From  these 
data  it  is  evident  that  the  negative  variation  or  action  current  has  a  space  value 
of  0.0006  of  28  meters  or  about  18  mm.  Transferring  these  statements 
to  the  nerve  impulse,  it  may  be  said  that  it  is  a  molecular  disturbance, 
traveling  at  the  rate  of  about  28  meters  a  second,  is  wave-like  in  character, 
the  wave  being  18  millimeters  in  length,  and  occupying  from  0.0006  to 
0.0008  of  a  second  in  passing  any  given  point. 

Absence  of  Diphasic  Action  Currents. — When  any  two  points  on 
the  longitudinal  surface  which  do  not  exhibit  a  current  are  connected 
with  the  galvanometer  and  a  single  wave  of  excitation  passes  beneath  the 
electrodes,  it  might  be  expected  that,  as  in  the  case  of  the  muscle,  a  diphasic 
action  current  would  be  observed,  from  the  fact  that  the  portions  of  the 
nerv'e  beneath  the  electrodes  become  alternately  negative  with  reference 
to  all  the  rest  of  the  nerve.  This,  however,  is  not  the  case,  the  absence  of 
the  two  opposing  phases  of  the  action  current  being  explained  on  the  suppo- 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  105 

sition  that  the  negativity  of  the  two  led-off  points  is  of  equal  amount,  and  that, 
owing  to  the  great  rapidity  with  which  the  excitation  wave  travels,  the  two 
phases  fall  together  too  closely  in  time  to  alternately  influence  the  galvan- 
ometer needle.  During  stimulation  of  the  nerv^e,  when  two  currentless  or 
isoelectric  points  are  connected,  there  is  also  an  absence  of  the  action 
current,  as  was  observed  first  by  du  Bois-Reymond,  and  which  is  to  be  ex- 
plained on  similar  grounds.  It  is  true  that  an  apparent  action  current 
is  sometimes  seen  when  the  stimulating  current  is  very  powerful  or  the  seat 
of  stimulation  too  near  the  diverting  electrodes.  This,  however,  must  be 
attributed  to  an  electrotonic  state  of  the  nerve. 

The  Effects  of  a  Galvanic  Current  on  a  Nerve. — When  a  constant 
galvanic  current  of  medium  strength  is  made  to  pass  through  a  portion  of  a 
nerve,  several  distinct  effects  are  produced: 

1.  The  development  of  a  nerve  impulse  at  the  moment  the  current  enters 
and  at  the  moment  the  current  leaves  the  nerve,  i.e.,  at  the  moment  the 
circuit  is  made  and  at  the  moment  it  is  broken.  The  development  of  the 
nerve  impulse  is  made  evident  by  the  contraction  of  the  muscle  if  the  nerve- 
muscle  preparation  be  used.  If  the  current  be  either  very  weak,  or  very 
strong,  the  muscle  contraction  may  not  always  take  place. 

2.  The  development  of  electric  currents  on  each  side  of  the  positive  pole 
or  anode,  and  the  negative  pole  or  kathode  (see  Fig.  52),  which  can  be  led 


GALVANOMETER 


anelectrotonic  katelectroton ic 

currents  currents 

Fig.  52. — Electrotoxic  Currents. 

off  by  means  of  wires  into  a  galvanometer  circuit  from  either  the  artificial 
transverse  and  longitudinal  surfaces,  or  from  any  two  points  on  the  longi- 
tudinal surface  as  shown  by  the  deflection  of  the  galvanometer  needle. 
The  direction  of  these  electric  currents  in  the  nerve  coincides  with  that  of 
the  galvanic  or  "polarizing  current."  The  "natural  nerve  currents,"  the 
currents  of  injury  or  demarcation  currents,  as  they  are  variously  termed, 
are  at  the  same  time  increased  and  decreased  at  opposite  extremities  of  the 
ner\'e  according  to  the  direction  of  the  polarizing  current. 

To  this  changed  condition  of  the  electromotive  forces  in  a  nerve  the 
term  electrotonus  was  given  (du  Bois-Reymond).  The  currents  them- 
selves are  known  as  electrotonic  currents;  from  their  relation  to  the  anode 
and  kathode,  they  are  termed  anelectrotonic  and  katelectrotonic  currents. 
The  condition  of  the  ner\'e  around  the  poles  both  in  the  intra-polar  and 
extra-polar  regions  is  known  as  anelectrotonus  and  katelectrotonus. 

The  electrotonic  currents  vary  considerably  in  strength  and  extent, 
according  to  the  intensity  of  the  polarizing  current,  increasing  steadily 
with  the  intensity  of  the  latter  up  to  the  point  at  which  the  polarizing  current 


io6  TEXT-BOOK  OF  PHYSIOLOGY. 

begins  to  destroy  the  physical  and  chemic  integrity  of  the  nerve.  The 
electrotonic  currents  are  strongest  in  the  immediate  neighborhood  of  the 
electrodes,  but  gradually  diminish  in  strength  as  the  distance  between  the 
polarized  and  led-off  portions  is  increased.  The  distance  to  which  the 
electrotonic  currents  extend  along  the  nerve  will  depend  very  largely  upon 
the  strength  of  the  polarizing  current,  though  it  is  conditioned  by  the  phys- 
ical state  of  the  nerve;  for  if  it  be  ligated  or  injured  beyond  the  polarized 
portion,  the  electrotonic  currents  are  abolished.  The  electrotonic  currents 
have  no  necessary  connection  with  the  natural  nerve  currents,  nor  are  they 
to  be  regarded  as  branchings  of  the  galvanic  current.  They  are  in  all 
probability  of  artificial  origin,  due  to  an  inner  positive  and  negative  polari- 
zation of  the  nerve  which  extends  for  a  variable  distance  on  each  side  of 
the  poles,  and  due  to  the  action  of  the  polarizing  or  the  galvanic  current. 

3.  An  alteration  in  the  excitability  and  conductivity  of  the  nerve  in  the 
neighborhood  of  the  poles,  whereby  the  results  of  nerve  stimulation — that 
is,  muscle  contraction,  sensation,  and  inhibition — are  increased  or  decreased 


Fig.  53. — Scheme  of  the  Electrotoxic  Excitability. — {Landois  and  Stirling.) 

according  to  the  strength  and  direction  of  the  current.  To  this  condition 
the  term  electrotonus  was  also  given  (Pfiiiger).  This  word  has  thus  been 
employed  to  express  two  distinct  series  of  effects  exhibited  by  a  nerve  through 
a  portion  of  which  a  constant  galvanic  current  is  passing.  It  appears  desir- 
able, for  the  sake  of  clearness,  to  limit  the  term  electrotonus  to  the  electric 
or  electrotonic  currents  which  can  be  led  off  from  either  extremity  of  the 
nerve,  and  to  apply  to  the  modifications  of  irritability  which  accompany 
electrotonus  the  expression,  electrotonic  alteration  of  excitability  and  con- 
ductivity. 

During  the  passage  of  the  current  the  excitability  of  the  intra-polar  as 
well  as  the  extra-polar  regions  undergoes  a  change  which,  as  shown  on 
examination,  is  found  to  be  diminished  in  the  neighborhood  of  the  anode  or 
positive  pole  and  increased  in  the  neighborhood  of  the  kathode  or  negative 
pole.  These  alterations  in  the  excitability  are  most  marked  in  the  imme- 
diate vicinity  of  the  electrodes,  though  they  extend  for  some  distance  into 
both  the  extra-polar  and  intra-polar  regions,  though  w4th  gradually  dimin- 
ishing intensity,  until  they  finally  disappear.  Between  the  electrodes 
there  is  a  point  where  the  excitability  is  unchanged  and  known  as  the  neutral 
or  indifferent  point  (Fig.  53).  The  extent  to  which  the  excitability  is  modi- 
fied as  well  as  the  position  of  the  neutral  point  will  depend  largely  on  the 
strength  of  the  polarizing  or  galvanic  current. 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  107 

The  electrotonic  alterations  of  excitability  and  conductivity  can  be 
experimentally  demonstrated  on  the  muscle-nerve  preparation  in  the  fol- 
lowing manner: 

I.      With  a  descending  current  of  medium  strength.     Previous  to  the  clo- 
sure of  the  polarizing  current,  the  nerve  is  stimulated  first  in  the  extra- 


Cm^S 


\ REGION    or 

j  INCREASED  EXCITABILITY 


ANODE 


KATHODE    ii 


J5 
^5 


SECONDARY  COIL 
Fig.  54. — Diagram  Showing  the  Region  of  Increased  Excitability  Caused  by  the 
Passage   of  a   Galvanic   Current,    Stimulation   of   \\-hich   Gives   Rise   to   Increased 
c0ntr.\cti0n. 

polar  anodic  region  and  the  extra-polar  kathodic  region  with  an  induc- 
tion shock  of  medium  intensity  and  the  height  of  the  contraction  re- 
corded. On  repeating  the  stimulation  a}ter  closure  of  the  polarizing 
current  the  contraction  resulting  from  stimulation  of  the  anodic  region 
will  be  enfeebled  or  may  be  entirely  wanting,  while  the  contraction  from 


REGION    OF 
DECREASED    EXCITABILITY 


Fig.  55. — Diagram  Showing  the  Region  of  Decreased  Excitability  Caused  by  the 
Passage   of  a   Galvanic   Cutirent,    Stimulation   of  which  Gives   Rise   to   Decreased 

CONTR.\CriON. 


Stimulation  of  the  kathodic  region  will  be  decidedly  increased.     (See 

Fig.  54-) 

With  an  ascending  current  of  the  same  strength.  After  preliminary 
testing  of  the  excitability  and  the  subsequent  closure  of  the  polarizing 
current,  it  will  be  found  that  stimulation  of  the  extra-polar  anodic 
region  will  provoke  a  much  less  energetic  contraction  or  perhaps  none 


io8 


TEXT-BOOK  OF  PHYSIOLOGY. 


at  all.     Stimulation  ol"  the  extra-kathodic  region,  though  of  increased 

excitability,  as  shown  by  the  previous  experiment,  may  also  fail  to 

provoke  a  contraction,  owing  to  the  diminished  conductivity  of  the 

region  in  the  neighborhood  of  the  anode.     The  impulse  on  reaching 

this  region  is  blocked  in  its  passage.     A  similar  if  not  more  marked 

decrease  in  the  conductivity  may  be  developed  in  the  region  of  the 

kathode  if  the  current  strength  be  very  great.     (See  Fig.  55.) 

The  Law  of  Contraction;  Polar  Stimulation. — It  was  stated  in  a 

previous  paragraph  that  when  a  galvanic  current  of  medium  strength  is 

made  to  enter  a  nerve,  and  when  it  is  withdrawn  from  the  nerve,  there  is  a 

contraction  of  its  related  muscle.     These  are  generally  known  as  the  make 

and  break  effects.     During  the  actual  passage  of  the  current  no  effect  is 

observed  so  long  as  its  strength  remains  uniform.     Any  sudden  variation 

in  the  strength  of  the  current  at  once  arouses  the  nerve  to  activity,  as  shown 

by  a  muscle  contraction. 

The  muscle  response  to  the  make  and  break  of  the  constant  current  is 
more  or  less  variable  unless  the  direction  of  the  current  as  well  as  its  strength 
be  taken  into  consideration.  If  the  current  is  made  to  flow  from  the  central 
toward  the  peripheral  end  of  the  nerve  it  is  termed  a  direct,  descending,  or 
centrifugal  current;  if  it  is  made  to  flow  in  the  reverse  direction,  it  is  termed 
an  indirect,  ascending,  or  centripetal  current.  The  strength  of  the  current  is 
determined  and  regulated  by  means  of  a  rheocord. 

The  make  and  break  of  currents  of  different  but  known  strengths  and 
directions  give  rise  to  contractions  which  occur  with  more  or  less  regularity. 
The  order  in  which  they  occur  under  these  varying  conditions  of  experi- 
mentation has  been  determined  and  tabulated  as  follows  by  Pfluger,  and  is 
termed  the  law  of  contraction: 


Current 

intensity 

Ascendin 

I  current 

Descendin 

g  current 

Make                        Break 

Make 

Break 

Weak 

Medium 

Strong.. 

Contraction. 
Contraction. 
Rest. 

Rest. 

Contraction. 

Contraction. 

Contraction. 
Contraction. 
Contraction. 

Rest. 

Contraction. 
R'est  or  weak  con- 
traction. 

The  results  as  above  tabulated  are  sometimes  complicated  on  the  open- 
ing of  the  circuit  by  a  series  of  irregular  pulsations  of  the  muscle,  an  ap- 
parent tetanus,  and  long  known  as  the  opening  tetanus  of  Ritter,  which  is 
attributed  to  rapid  changes  in  the  irritability  of  the  nerve,  in  the  region  of 
the  anode.  A  similar  tetanic  contraction  of  the  muscle  is  sometimes  ob- 
served on  the  closure  of  the  circuit  due  to  continued  excitation  in  the  region 
of  the  kathode.  This  is  known  as  the  closing  tetanus  of  Wundt  or  of  Pfluger. 
All  the  phenomena  of  the  law  of  contraction  were  explained  by  Pfluger  on 
the  assumption  that  the  current  stimulates  the  nerve  only  at  the  one  electrode, 
at  the  kathode  on  closing,  and  at  the  anode  on  opening;  or,  in  other  words,  by 
the  appearance  of  katelectrotonus  or  by  the  disappearance  of  anelectrotonus, 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE.  109 

both  conditions  being  attended  by  a  rise  of  excitability — not,  however,  by 
the<  opposite  changes.  It  is  further  assumed  that  the  appearance  of  kate- 
lectrotonus  is  more  effective  as  a  stimulus  than  the  disappearance  of  anelec- 
trotonus.  For  these  reasons  the  term  polar  stimulation  is  generally  employed 
in  discussing  the  make  and  break  effects  of  the  galvanic  current.  The  law 
of  contraction  may  then  be  explained  as  follows:  Very  feeble  currents, 
either  ascending  or  descending,  produce  contraction  only  upon  the  closure 
of  the  circuit,  the  sudden  increase  of  the  excitability  in  the  katelectrotonic  area 
being  alone  sufficient  to  generate  an  impulse.  The  contraction  which 
follows  the  closing  of  the  weak  ascending  current  depends  upon  the  fact 
that  the  decrease  of  excitability  and  conductivity  at  the  anode  is  insufficient 
to  inferfere  with  the  conduction  of  the  kathodal  stimulus.  Medium  currents, 
either  ascending  or  descending,  produce  contraction  both  on  closing  and 
opening  the  circuit.  The  appearance  of  katelectrotonus  and  the  disap- 
pearance of  anelectrotonus  are  both  sufficiently  powerful  to  generate  an  im- 
pulse without,  however,  seriously  impairing  the  conductivity  of  the  nerve. 

Very  strong  currents  produce  contraction  only  upon  the  opening  of  the 
ascending  and  closure  of  the  descending  currents,  or  upon  the  passage  of  the 
excitability  in  the  former  from  the  marked  anelectrotonic  decrease  to  the 
normal  condition,  and  in  the  latter  from  the  normal  to  that  of  katelectrotonic 
increase.  The  absence  of  contraction  upon  the  closure  of  the  ascending 
current  is  dependent  upon  the  blocking  of  the  kathodal  stimulus  by  the 
decrease  of  the  excitability  and  conductivity  at  the  anode.  With  the  open- 
ing of  the  descending  current  the  disappearance  of  anelectrotonus  should 
also  be  followed  by  contraction,  which  would  indeed  be  the  case  if  the 
stimulus  so  generated  was  not  blocked  by  the  decrease  of  the  conductivity 
at  the  kathode  in  consequence  of  the  fall  of  a  high  state  of  katelectrotonus 
to  the  normal  condition. 

The  order  in  which  the  contractions  occur  mav  be  tabulated  as  follows: 


With  Ascending  Current. 

With  Descending  Current. 

Weak..  .. 

I.  K.  C.  C 

— 

K.  C.  C.                      — 

Medium  . 

2.  K.  C.  C. 

A.  0.  C- 

K.  C.  C.             A.  0.  C. 

Strong  .  .  . 

3- 

A.  0.  C. 

K.  C.  C.             A.  0.  C.(?) 

Polar  Stimulation  of  Human  Nerves. — The  preceding  statements  as 
to  changes  in  the  excitability  caused  by  the  passage  of  a  constant  current, 
as  well  as  to  the  law  of  contraction,  are  based  entirely  on  experiments  made 
with  the  isolated  nerve  of  the  frog.  It  is  probable,  however,  that  the  same 
phenomena  would  have  been  observed  had  the  nerve  of  a  mammal  been 
used  and  its  excitability  been  maintained. 

If  the  electrodes  connected  with  the  wires  of  a  sufficiently  strong  gal- 
vanic battery  be  applied  to  the  skin  over  the  course  of  a  superficially  lying 
nerve,  e.g.,  the  brachial,  it  will  be  found  that  there  occurs  on  the  closure  of 
the  circuit  an  increase  in  the  excitability  in  the  extra-polar  anelectrotonic 
region  and  a  decrease  in  the  excitability  in  the  extra-polar  katelectrotonic 
region,  as  shown  by  stimulating  the  nerve  in  the  extra-polar  regions  with 
the  induced  current — results  which  are  in  apparent  contradiction  to  those 
obtained  with  the  isolated  nerve.  This  want  of  accordance  in  the  results 
of  the  two  classes  of  experiments  arises  from  a  failure  to  recognize  the  fact 

^K.  C.  C,  kathodal  closing  contraction.  *  A.  O.  C,  anodal  opening  contraction. 


no 


TEXT-BOOK  OF  PHYSIOLOGY. 


that  the  physiologic  anode  and  kathode  do  not  coincide  with  the  physical 
anode  and  kathode. 

It  has  been  experimentally  demonstrated  that  owing  to  the  large  amount 
of  readily  conducting  tissue  by  which  the  nerve  is  surrounded,  the  current 
density,  though  great  immediately  under  the  electrode,  quickly  decreases 
at  a  short  distance  from  it,  so  that  for  the  nerve  it  becomes  almost  nil.  The 
current,  therefore,  shortly  after  entering,  again  leaves  the  nerve  at  various 
points  which  become  physiologic  kathodes.  Stimulation  of  this  physio- 
logic kathode  with  the  induced  current  gives  rise,  therefore,  to  the  phenom- 
enon of  increased  excitability  in  the  region  of  the  anode.  If,  however,  the 
galvanic  and  stimulating  current  be  combined  in  one  circuit  and  both  be 
applied  to  the  same  tract  of  nerve,  results  will  be  obtained  which  harmonize 
with  those  obtained  with  the  frog's  nerve. 

The  changes  in  the  excitability  of  a  nerve  of  a  living  man  and  the  con- 
tractions which  follow  the  closing  and  opening  of  the  constant  current 
have  been  thoroughly  studied  by  Waller  and  de  Watteville.  These  observers 
employed  a  method  similar  to  that  of  Erb,  conjoining  in  one  circuit  the 
testing  and  polarizing  currents.  By  the  graphic  method  they  recorded 
first  the  contraction  produced  by  an  induction  shock  alone;  and,  secondly, 


Fig.  56. — ./^NODE  OF  Battery. 
Polar  region  of  nerve  is  anodic.  Peri- 
polar  region   of   nerve   is   cathodic. 


Fig.  57. — Cathode  of  Battery. 
Polar  region  of  nerve  is  cathodic.  Peri- 
polar region  of  nerve  is  anodic. — (Waller.) 


the  contraction  produced  by  the  same  stimulus  under  the  influence  of  the 
polarizing  current.  As  a  result  of  many  experiments,  they  also  demonstrated 
an  increase  of  the  excitability  in  the  polar  region  when  it  is  cathodic,  and 
a  decrease  when  it  is  anodic.  Following  the  suggestion  of  Helmholtz,  that  the 
current  density  quickly  decreases  with  the  distance  from  the  electrodes, 
they  recognize,  at  the  point  of  entrance  and  exit  "of  the  current  from  the  nerve, 
two  regions — a  polar,  having  the  same  sign  as  the  electrode,  and  a  peripolar, 
having  the  opposite  sign  (Figs.  56  and  57).  The  peripolar  regions  also 
experience  similar  alterations  of  excitability,  though  less  in  degree,  accord- 
ing as  they  are  kathodic  or  anodic. 

As  it  is  impossible  to  confine  the  current  to  the  trunk  of  the  nerve  when 
surrounded  by  living  tissues,  as  is  easily  the  case  when  experimenting  with 
the  frog's  nerves,  it  is  incorrect  to  speak  of  either  ascending  or  descending 
currents.  Waller,^  who  has  thoroughly  studied  the  electrotonic  effects  of 
the  galvanic  current  from  this  point  of  view,  sums  up  his  conclusions  in 
the  following  words:  "We  must  apply  one  electrode  only  to  the  nerve  and 
attend  to  its  effects  alone,  completing  the  circuit  through  a  second  electrode, 
which  is  applied  according  to  convenience  to  some  other  part  of  the  body. 

i"Human  Physiology,"  p.  363,  1891. 


GENERAL  PHYSIOLOGY  OF  XERVE-TLSSUE.  iii 

"  Confining  our  attention  to  the  first  electrode,  let  us  see  what  will 
happen  according  as  it  is  anode  or  kathode  of  a  galvanic  current  (Figs. 
56  and  57).  If  this  electrode  be  the  anode  of  a  current,  the  latter  enters 
the  nerve  by  a  series  of  points  and  leaves  it  by  a  second  series  of  points;  the 
former,  or  proximal  series  of  points,  collectively  constitutes  the  polar  zone 
or  region;  the  latter,  or  distal  series  of  points,  collectively  constitutes  the 
peripolar  zone  or  region.  In  such  case  the  polar  region  is  the  seat  of  entrance 
of  current  into  the  nerve — i.e.,  is  anodic;  the  peripolar  region  is  the  seat 
of  exit  of  current  from  the  nerve — i.e.,  is  kathodic.  If.  on  the  contrary,  the 
electrode  under  observation  be  the  kathode  of  a. current,  the  latter  enters 
the  nerve  by  a  series  of  points  which  collectively  constitute  a  'peripolar' 
region,  and  it  leaves  the  nerve  by  a  series  of  points  which  collectively  con- 
stitute a  'polar'  region.  The  current,  at  its  entrance  into  the  body,  diffuses 
widely,  and  at  its  exit  it  concentrates;  its  'density'  is  greatest  close  to  the 
electrode,  and,  the  greater  the  distance  of  any  point  from  the  electrode,  the 
less  the  current  density  at  that  point;  hence  it  is  obvious  that  the  current 
density  is  greater  in  the  polar  than  in  the  peripolar  region.  These  conditions 
having  been  recognized,  we  may  apply  to  them  the  principles  learned  by 
study  of  frogs'  nerves  under  simpler  conditions. 

"Seeing  that,  with  either  pole  of  the  battery,  whether  anode  or  kath- 
ode, the  nen^e  has  in  each  case  points  of  entrance  (constituting  a  collective 
anode)  and  points  of  exit  to  the  current  (constituting  a  collective  kathode), 
and  admitting  as  proved  that  make  excitation  is  kathodic,  break  excitation 
anodic,  we  may,  with  a  sufficiently  strong  current,  expect  to  obtain  a  con- 
traction at  make  and  at  break  with  either  anode  or  kathode  applied  to  the 
nerve;  and  we  do  so,  in  fact.  When  the  kathode  is  applied,  and  the  current 
is  made  and  broken,  we  obtain  a  kathodic  make  contraction  and  a  kathodic 
break  contraction;  when  the  anode  is  applied,  and  the  current  is  made  and 
broken,  we  obtain  an  anodic  make  contraction  and  an  anodic  break  con- 
traction. These  four  contractions  are,  however,  of  very  different  strengths; 
the  kathodic  make  contraction  is  by  far  the  strongest;  the  kathodic  break 
contraction  is  by  far  the  weakest;  the  kathodic  make  contraction  is  stronger 
than  the  anodic  make  contraction;  the  anodic  break  contraction  is  stronger 
than  the  kathodic  break  contraction.  Or,  otherwise  regarded,  if,  instead 
of  comparing  the  contractions  obtained  with  a  sufficiently  strong  current, 
we  observe  the  order  of  their  appearance  with  currents  gradually  increased 
from  weak  to  strong,  we  shall  find  that  the  kathodic  make  contraction  appears 
first,  that  the  kathodic  break  contraction  appears  last,  and  the  formula 
of  contraction  for  man  reads  as  follows: 

"Weak  current K.  C.  C. 

Medium  current K.  C.  C.  A.  C.  C.  A.  O.  C. 

Strong  current K.  C.  C.  A.  C.  C.  A.  O.  C.  K.  O.  C." 

The  constant  or  the  galvanic  current  is  frequently  used  for  therapeutic 
and  diagnostic  purposes.  In  accordance  with  the  statements  above  quoted, 
one  electrode  should  be  applied  to  the  part  to  be  investigated,  the  other 
to  some  indifferent  region.  The  electrode  conveying  the  current  to  or 
from  this  part  should  be  of  a  size  sufficient  to  localize  the  current  and  to 
increase  its  density.  It  was  discovered  by  Duchenne  that  there  are  certain 
points  all  over  the  body  stimulation  of  which  is  more  quickly  followed  by 


112  TEXT-BOOK  OF  PHYSIOLOGY. 

muscle  contraction  than  others.  It  was  subsequently  discovered  by  Remak 
that  these  points  coincide  with  the  entrance  of  the  nerve  into  the  muscle. 
It  is  to  these  motor  points  that  the  one  electrode  should  be  applied.  The 
position  of  some  of  these  points  on  the  forearm  is  shown  in  Fig.  58. 

Reactions  of  Degeneration. — In  consequence  of  the  degeneration 
and  changes  in  irritability  which  occur  in  nerves  when  separated  from  their 
centers  and  in  muscles  when  separated  from  their  related  nerves,  either 
experimentally  or  as  the  result  of  disease,  the  response  of  these  structures  to 
the  induced,  and  the  make  and  break  of  the  constant  current,  differs  from 
that  observed  in  the  physiologic  condition.  The  facts  observed  under  the 
application  of  these  two  forms  of  electricity  are  of  importance  in  the  diagnosis 


M.  biceps 
M. 


brachii. 
brach.  anticus. 
N.  meiianus. 


M.  pronator  teres. 

M.  flex,  digitor.  commun.  profund. 
M.  flex,  carpi  radialis. 

M.  flex,  digitor.  sublim. 

M.  flex.  dig.  subl.  (dig.  ind.  et  min.) 

^M.  flex.  poll. 

longus. 

iN.med- 

I  ianus. 


N.  ulnaris. 


M.  flexor  carpi  ulnaris. 


N.  ulnaris. 


Fig.  58. — Motor  Points  of  the  Median  and  Ulnar  Nerves,  with  the  Muscles 
Supplied  by  Them. — {Landois  and  Stirling.) 


and  therapeutics  of  the  precedent  lesions.  The  principal  difference  of 
behavior  is  observed  in  the  muscles,  which  exhibit  diminished  or  abolished 
excitability  to  the  induced  current,  while  at  the  same  time  manifesting 
an  increased  excitability  to  the  constant  current;  so  much  so  is  this  the 
case  that  a  closing  contraction  is  just  as  likely  to  occur  at  the  positive  as 
at  the  negative  pole.  This  peculiarity  of  the  muscle  response  is  termed 
the  reaction  of  degeneration.  The  synchronous  diminished  excitability  of  the 
nerves  is  the  same  for  either  current.  The  term  "partial  reaction  of  degen- 
eration" is  used  when  there  is  a  normal  reaction  of  the  nerves,  with  the  de- 
generative reaction  of  the  muscles.  This  condition  is  observed  in  progressive 
muscular  atrophy. 

Reflex  Action. — Inasmuch  as  many  of  the  muscle  movements  of 
the  body,  as  well  as  the  formation  and  discharge  of  secretions  from  glands, 
variations  in  the  caliber  of  blood-vessels,  inhibition  and  acceleration  in 
the  activity  of  various  organs,  are  the  result  of  stimulations  of  the  terminal 
organs  of  afferent  nerves,  they  are  termed,  for  convenience,  reflex  actions, 
and,  as  they  take  place  for  the  most  part  through  the  spinal  cord  and  medulla 


GENERAL  PHYSIOLOGY  OF  NERVE-TISSUE. 


"3 


oblongata  and  independently  of  the  brain  or  of  volitional  influences,  they  are 
also  termed  involuntary  actions.  A  reflex  action  of  skeletal  muscles,  glands, 
or  non-striated  muscles  of  blood-vessels  or  of  viscera,  therefore,  may  be  de- 
fined as  an  action  which  takes  place  independent  of  volition  and  in  response 
to  peripheral  stimulation.  As  many  of  the  processes  to  be  described  in 
succeeding  chapters  are  of  this  character,  requiring  for  their  performance 
the  cooperation  of  several  organs  and  tissues  associated  through  the  inter- 
mediation of  the  nerve  system,  it  seems  advisable  to  consider  briefly,  in 
this  connection,  the  parts  involved  in  a  reflex  action,  as  well  as  their  mode 
of  action.  As  shown  in  Fig.  13,  page  41,  the  necessary  structures  are  as 
follows: 

1.  A  receptive   surface,    skin,   mucous  mem- 

bane,  sense-organs,  etc. 

2.  An  afferent  nerve-fiber  and  cell. 

3.  An  emissive  cell,  from  which  arises — 

4.  An  eft'erent  nerv^e,  distributed  to  a  respon- 

sive organ,  as 

5.  Skeletal  muscle,  gland,  blood-vessel,  etc. 
Such  a  combination  of  structures  consti- 
tutes a  reflex  mechanism  or  arc,  the  nerve 
portion  of  which,  in  the  case  of  skeletal  mus 
cles,  is  composed  of  but  two  neurons — an 
afferent  and  an  efferent.  In  the  case  of  glands 
and  non-striated  muscles,  whether  of  blood- 
vessels or  viscera,  the  efferent  neuron  instead 
of  passing  direct  to  the  responsive  organ, 
arborizes  around  the  nerve-cells  of  a  peri- 
pheral sympathetic  ganglion.  The  reflex  arc 
is  then  continued  by  the  processes  of  the  gang- 
lion cells.  An  arc  of  this  simplicity  would  of 
necessity  subserve  but  a  simple  movement. 
The  majority  of  reflex  activities,  however,  are 
extremely  complex,  and  involve  the  coopera- 
tion and  coordination  of  a  number  of  ners^e 
centers  situated  at  different  levels  of  the  spinal  cord  on  the  same  and  opposite 
side,  and  of  responsive  organs  frequently  situated  at  distances  more  or  less 
remote  from  one  another.  This  implies  that  a  number  of  neurons  are 
associated  in  function.  The  transference  of  nerve  impulses  coming  from 
a  localized  area  of  a  sentient  surface  to  emissive  cells  situated  at  different 
levels  is  accomplished  by  the  intercalation  of  a  third  neuron  situated  in 
the  gray  matter  which  is  in  connection,  on  the  one  hand,  with  the  central 
terminals  of  the  afferent  neuron,  and,  on  the  other  hand,  through  its  colla- 
teral branches  with  the  dendrites  of  the  efferent  neurons  situated  at  different 
levels  of  the  cord.     (Fig.  59.) 

For  the  excitation  of  a  reflex  action  it  is  essential  that  the  stimulus  applied 
to  the  receptive  surface  be  of  an  intensity  sufficient  to  develop  in  the  terminals 
of  the  afferent  nerve  a  series  of  nerve  impulses,  which,  traveling  inward,  will 
be  distributed  to  and  received  by  the  dendrites  of  the  emissive  or  motor  cell. 
With  the  reception  of  these  impulses  there  is  apparently  a  disturbance  of 


Fig.  59. — Diagr.am  Showing 
THE  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  h,  and  to  the  Efferent 
Neurons  c,  c,  c. — {^Ajter  Kblliker.) 


114  TEXT-BOOK  OF  PHYSIOLOGY. 

the  e(|uilibrium  of  its  molecules,  a  liberation  of  energy,  and,  in  consequence, 
a  transmission  outward  of  impulses  through  the  efferent  nerve  to  muscle, 
gland,  or  blood-vessel;  separately  or  collectively,  with  the  production  of 
muscle  contraction,  a  secretion,  vascular  dilatation  or  contraction,  etc. 
The  reflex  actions  take  place,  for  the  most  part,  through  the  spinal  cord  and 
medulla  oblongata,  which,  by  virtue  of  their  contained  centers,  coordinate 
the  various  organs  and  tissues  concerned  in  the  performance  of  the  organic 
functions.  The  movements  of  mastication;  the  secretion  of  saliva;  the 
muscle,  gland,  and  vascular  phenomena  of  gastric  and  intestinal  digestion; 
the  vascular  and  respiratory  movements;  the  mechanism  of  micturition, 
etc.,  are  illustrations  of  reflex  activity. 


CHAPTER  IX. 
FOODS. 

The  functional  activity  of  every  organ  and  tissue  of  the  boby  is  accom- 
panied by  a  more  or  less  active  disintegration  of  the  living  material,  the  bio- 
plasm, of  which  it  is  composed,  as  well  as  of  the  food  materials  circulating 
in  its  interstices.  The  complex  molecules  of  the  living  material  and  of  the 
non-living  food  materials  are  continually  undergoing  disruption  and  falling 
into  less  complex  and  more  stable  compounds;  these,  through  oxidative 
processes,  are  eventually  reduced  through  a  series  of  descending  chemic 
stages  to  a  small  number  of  simpler  compounds  which,  being  of  no  further 
apparent  value  to  the  organism,  are  eliminated  by  the  various  eliminating 
or  excretory  organs,  the  lungs,  skin,  kidneys,  and  liver.  Among  these 
excreted  compounds  derived  from  tissue  and  from  food  metabolism  the 
most  important  are  urea,  uric  acid,  and  carbon  dioxid.  Many  other  com- 
pounds, organic  as  well  as  inorganic,  are  also  eliminated  from  the  body  in 
the  various  excretions,  though  they  are  present  in  but  small  amounts.  Coin- 
cident with  this  metabolic  process  there  is  a  transformation  of  potential 
into  kinetic  energy,  which  manifests  itself  for  the  most  part  as  heat  and 
mechanic  motion. 

In  order  that  the  organs  and  tissues  may  continue  in  the  performance 
of  their  functions,  it  is  essential  that  they  be  supplied  with  nutritive  mate- 
rials similar  to  those  which  enter  into  their  own  composition:  viz.,  proteins, 
fat,  carbohydrates,  water,  and  inorganic  salts.  These  compounds,  though 
originally  derived  from  the  food,  are  immediately  derived  from  the  blood 
as  it  flows  through  the  capillary  blo5d-vessels.  The  blood  is  therefore  to  be 
regarded  as  a  reservoir  of  nutritive  material  in  a  condition  to  be  absorbed 
and  transformed  into  utilizable  and  living  material.  Inasmuch  as  the 
materials  which  are  lost  to  the  body  daily,  through  processes  of  disintegra- 
tion and  oxidation,  are  supplied  by  the  blood,  it  is  evident  that  this  fluid 
would  diminish  rapidly  in  volume,  with  a  corresponding  decline  in  func- 
tional activity,  were  it  not  replenished  by  the  introduction  into  the  body  of 
new  material  in  the  food.  With  the  diminution  of  the  volume  of  the  biood  and 
an  insufficient  supply  to  the  tissues,  there  arise  the  sensations  of  hunger  and 
thirst,  which  lead  to  the  consumption  of  food  and  the  subsequent  restoration 
of  the  physiologic  condition  of  the  tissues.  These  two  sensations  are  also 
partially  dependent  on  the  empty  condition  of  the  stomach  and  the  dryness 
of  the  mucous  membrane  of  the  mouth  and  throat. 

The  foods  which  are  consumed  daily  in  response  to  sensations  of  hunger 
and  thirst  are  complex  in  composition  and  contain,  though  in  vary- 
ing amounts,  proteins,  fats,  carbohydrates,  water,  and  inorganic  salts, 
which,  in  contradistinction  to  foods,  are  termed  food  principles,  or  as  they 
maintain  the  nutrition,  nutritive  principles.  These  compounds  also  contain 
the  potential  energy  necessary  to  maintain  the  energy  equilibrium  of  the 


ii6  TEXT-BOOK  OF  PHYSIOLOGY. 

body  which   becomes    manifest    as    heat    and    mechanic    motion   in    the 
transformations  of  the  niaterial  used  in  the  nutritive  processes. 

It  has  been  stated  in  a  previous  chapter  that  the  animal  body 
may  be  regarded  as  a  machine  capable  of  performing  each  day  a  certain 
amount  of  work  by  the  expenditure  of  a  definite  amount  of  energy.  In 
the  performance  of  its  work,  whether  it  be  the  raising  of  weights  against 
gravity,  or  the  overcoming  of  friction,  cohesion,  or  elasticity,  the  machine 
suffers  disintegration  and  metabolizes  a  portion  of  the  food  materials  and 
loses  a  portion  of  its  available  energy.  UnUke  other  machines,  however, 
it  possesses  the  power,  within  Hmits,  of  self-renewal,  when  supplied  with 
foods  in  proper  quantity  and  (juality. 

QUANTITIES  OF  FOOD  PRINCIPLES  REQUIRED  DAILY. 

In  order  that  the  body  may  continue  in  the  performance  of  its  work  and 
yet  retain  a  given  weight,  it  is  essential  that  the  loss  to  the  body  daily  shall 
be  exactly  compensated  by  the  introduction  and  assimilation  of  a  corre- 
sponding amount  of  food  principles.  If  this  condition  is  realized,  the 
body  neither  gains  nor  loses  in  weight,  but  remains  in  a  condition  of  nutritive 
equilibrium.  The  determination  of  the  extent  of  the  metabolism  is  made 
from  an  examination  of  the  quantity  and  composition  of  the  daily  excretions. 
If  therefore  these  are  collected  and  analyzed,  it  will  become  possible  to  de- 
termine from  their  chief  constituents  the  extent  and  character  of  the  tissue 
and  food  metabolized.  Thus  the  urea  and  other  nitrogen-holding  com- 
pounds contained  in  the  urine  represent  the  proteins  metabolized;  the  carbon 
dioxid  and  water  represent  the  fat  and  carbohydrates  metabolized.  There- 
fore it  becomes  possible  to  determine  from  the  amounts  of  the  urea  and 
carbon  dioxid  eliminated,  the  different  amounts  of  the  food  principles  re- 
quired to  restore  the  nutritive  equilibrium  under  any  given  condition. 
As  the  activity  of  the  nutritive  changes  varies  in  accordance  with  age,  weight, 
climatic  conditions,  work  done,  etc.,  and  as  the  excreted  products  vary  in  the 
same  ratio,  it  is  obvious  that  the  required  amounts  of  food  will  vary  in 
accordance  with  these  varying  conditions,  if  equilibrium  is  to  be  maintained. 

An  experiment  designed  to  collect  the  excretions  for  purposes  of  analysis 
is  termed  a  metabolism  experiment;  its  object  is  to  deduce  from  the  amounts 
of  urea  and  other  nitrogen-holding  compounds,  of  carbon  dioxid  and  water 
discharged,  the  amount  of  the  tissue  and  food  metabolized,  and  hence  from 
them  to  calculate  the  amounts  of  the  food  principles  and  their  ratio  one  to 
another  that  must  be  returned  to  the  body  if  nutritive  equilibrium  is  to  be 
restored.  This  is  accomplished  by  one  of  the  many  forms  of  respiration 
appliances,  which  have  been  devised  for  animals  and  for  man.  The  best 
form  of  apparatus  for  determining  the  metabolism  of  man  is  that  designed 
by  Benedict. 

Many  metabolism  experiments  have  been  performed  by  different  inves- 
tigators under  a  great  variety  of  conditions.  The  results,  though  differing 
in  some  respects,  have  nevertheless  a  general  average  value.  The  following 
table  shows  the  results  of  a  series  of  experiments  made  byVierordt.  On 
the  right  under  the  term  outcome,  are  arranged  the  amounts  of  the  sub- 
stances eliminated;  on  the  left,  under  the  term  income,  the  amounts  of  the 


FOODS. 


117 


food  principles  which  were  calculated  as  necessary  to  replace  the  tissue  and 
food  metabolized. 

COMPARISON  OF  THE  INCOME  AND  OUTCOME. 

Outcome 


Income 


Grams      Ounces 


Grams      Ounces 


Protein 

Fat 

Carbohydrates 

Salts 

Water 

Oxygen 


120 

4-23 

90 

330 

3-17 
11.64 

32 
2818 

756 

I -13 
99-3° 
26.66 

4146 

146 . 13 

Water 

Urea 

Feces,  dry 

Salts 

Carbon  dioxid 

Water  formed  in  bodv . 


40 

38 

32 

922 

296 


99-30 

1 .40 

1 .60 

I -13 

32-37 

10-33 


4146     ,   146.13 


Other  estimates  as  to  the  amounts  of  the  organic  food  principles  recjuired 
daily  based  on  the  amount  of  the  excreted  products  are  as  follows: 

Ranke.  Voit.  Moleschott.  Atwater.  Hultgren. 

Grams.  Grams.  Grams.  Grams.  Grams. 

Protein 100             118  130                      125  134 

Fat 100               56  84                      125  79 

Starch ..   250             500  550                    400  522 

From  the  foregoing  estimates  it  is  assumed  that  for  the  maintenance 
of  nitrogen  equilibrium  an  amount  of  protein,  100  grams  or  more,  or  about 
1.5  to  1.7  grams  for  each  kilogram  of  body  weight  must  be  consumed  each 
day;  and  that  if  the  amount  falls  below  this  minimum  the  tissues  will  be 
called  upon  to  yield  up  a  portion  of  their  protein  and  thereby  undergo 
deterioration  with  a  consequent  loss  of  their  efficiency. 

It  has,  however,  been  established  that  nitrogen  equilibrium  can  be  main- 
tained without  detriment  to  the  body  or  its  activities,  for  a  variable  period 
of  time,  extending  over  months  and  years,  on  a  diet  much  poorer  in  its 
protein  content  than  in  any  of  the  foregoing  diets.  Chittenden  has  demon- 
strated by  a  long  series  of  carefully  conducted  experiments  on  human 
beings,  that  the  protein  intake  can  be  reduced  to  60  grams  or  0.85  grams 
for  each  kilogram  of  body  weight  without  any  impairment  in  the  working 
capacity  of  the  tissues  or  of  the  individual.  Even  this  amount  is  in  actual 
excess  of  the  tissue  needs  as  the  protein  metabolism  according  to  Chittenden's 
experiments  probably  does  not  amount  to  more  than  0.75  grams  for  each 
kilogram  of  body  weight. 

The  daily  observations  of  some  twenty-four  individuals  who  were  placed 
on  a  diet  in  which  the  protein  content  was  low  for  a  period  varying  from 
five  to  eighteen  months  revealed  the  fact  that  they  not  only  maintained  the 
nitrogen  equilibrum  but  that  they  gained  in  weight  and  strength  as  shown 
by  their  capacity  to  meet  successfully  various  endurance  tests.  These 
experiments  would  therefore  indicate  that  the  consumption  of  100  or  more 
grams  of  protein  each  day  is  unnecessary  and  that  any  amount  beyond  that 
actually  needed  for  tissue  repair,  approximately  60  grams  or  even  less  for 
an  individual  weighing  70  kilograms  is  undesirable,  for,  as  will  be  stated  in 
subsequent  pages,  all  protein  when  metabolized  yields  a  series  of  nitrogen- 
holding  bodies  which  must  be  subsequently  eliminated  by  the  kidneys  and 


ii8 


TEXT-BOOK  OF  PHYSIOLOGY. 


perhaps  the  intestinaJ  glands  as  well.  This  necessitates  on  the  part  of  the 
kidneys,  the  chief  eliminating  organs,  the  expenditure  of  a  certain  amount 
of  energy.  The  wear  and  tear  of  these  organs  will  be  proportional  to  the 
amount  of  urea  and  other  materials  which  they  are  called  upon  to  excrete 
and  if  the  kidneys  fail  to  excrete  them,  they  become  deposited  in  the  tissues 
and  give  rise  to  certain  nutritional  disorders.  Any  unnecessary  consump- 
tion of  proteins  should  therefore  be  avoided. 

It  must  be  remembered,  however,  as  protein  yields  energy  when  meta- 
bolized, that  the  heat  value  of  the  excluded  protein  must  be  balanced  by  an 
increase  in  the  amount  of  either  starch  or  fat  or  both,  an  increase  that  will 
yield  on  oxidation  an  equivalent  amount  of  heat. 

In  arranging  tables  showing  the  relation  between  the  income  and  the 
outcome  it  is  generally  customary  to  state  merely  the  amounts  by  weight 
of  the  nitrogen  and  carbon  each  contains.  This  method  furnishes  ac- 
curate information  regarding  the  metabolism  of  the  body,  for  the  reason 
that  the  nitrogen  represents  the  protein,  and  the  carbon,  with  the  exception 
of  that  contained  in  the  protein,  the  fat  and  carbohydrates  which  have  under- 
gone disintegration  or  metabolism. 

The  following  balance  table,  as  given  by  Ranke,  shows  the  relation 
of  the  nitrogen  to  the  carbon  in  the  average  mixed  diet  and  in  the  excretions 
of  a  man  weighing  70  kilograms,  in  a  condition  of  nutritive  equilibrium: 


Income 

Grams 

N. 

C. 

Protein 

100 
100 
250 

15-5 

53 -o 
79.0 

93  0 

Fat 

Carbohydrates 

15-5 

225.0 

Outcome 

Grams 

N. 

C. 

Urea 

3i-5l 
0.5/ 

14.4 
I.I 

Uric  acid 

6.16 

Feces 

10.84 
208  00 

CO, 

15-5 

225.00 

From  the  above  it  will  be  observed  that  the  daily  discharge  for  each 
kilogram  of  body- weight  is  0.22  gram  nitrogen  and  3.21  grams  of  carbon; 
the  relation  of  the  two  being  ^=1.46.  On  a  diet  in  which  there  is  an 
excess  of  either  protein  or  carbohydrates  this  ratio  necessarily  changes. 

CLASSIFICATION  OF  FOOD  PRINCIPLES. 


Though  the  food  principles  are  grouped  as  proteins,  fats,  carbohydrates, 
etc.,  the  members  of  each  group  differ  somewhat  in  chemic  composition, 
digestibility,  and  nutritive  value.     These  groups  are  as  follows: 


FOODS.  119 


I.  Proteins. 


Principle.  Where  found. 

Myosin Flesh  of  animals. 

Al'buniin,  vitelUn White  of  egg,  yolk  of  egg. 

Caseinogen Milk . 

Serum  albumin,  fibrin Blood  contained  in  meat. 

Gliadin  and  (^lutinin Grain  of  wheat  and  some  other  cereals. 

Vegetable  albumin . .  Soft-growing  vegetables. 

Le^umin Peas,  beans,  lentils,  etc. 

2.  Fats. 

Animal  fats In  adipose  tissue  of  animals. 

Vecretable  oils In  seeds,   grains,   nuts,  fruits,   and  other 

vegetable  tissues. 

3.  Carbohydrates. 

De.xtrose  or  grape-sugar \  j^  huits. 

Levulose  or  fruit-sugar j 

Lactose  or  milk-sugar Milk. 

Saccharose  or  cane-sugar.  .Sugar-cane,  beet  roots. 

Maltose .Malt  and  malted  foods. 

'  Cereals,  tuberous  roots,  and  leguminous 
Starch ]       plants. 

Glycogen • •  Liver,  muscles. 

4.  Inorg.axic. 

Water 1 

Sodium  and  potassium  chlorid i 

Sodium,  potassium,  and  calcium  phosphates  \  In  nearly  all  animal  and  vegetable  foods. 

and  carbonates ! 

Iron J 

5.  Veget.\ble  Acids. 

Citric,  tartaric,  acetic,  maUc In  fruits  and  vegetables. 

6.  Accessory  Foods. 

Coffee,  Tea,  Cocoa,  Condiments,  Spices,  Alcohol. 

Disposition  of  Food. — The  protein  principles  of  the  food  while  in  the 
alimentary  canal  undergo  a  series  of  disintegrative  changes  by  virtue  of 
which  they  are  reduced  in  part  to  simple  nitrogen-holding  bodies,  amino- 
and  diamino-acids  and  ammonia,  and  in  part  to  their  immediate  antecedents 
peptids  and  polypeptids.  Under  these  forms  the  nitrogen-holding  con- 
stituents of  the  food  are  absorbed  from  the  intestine.  During  the  act  of 
absorption  they  are  for  the  most  part  transformed  into  the  forms  of  protein 
characteristic  of  blood  and  more  particularly  that  form  known  as  plasma 
or  serum  albumin.  After  being  distributed  by  the  blood-stream  to^  the 
tissues,  they  are  brought  into  relation  with  the  living  cells.  The  disposition 
made  of  the  protein  material  by  the  bioplasm  of  the  cell  has  not  been  definitely 
determined.  According  to  Voit,  of  the  protein  thus  brought  into  con- 
tact with  the  living  tissues,  only  a  small  percentage  is  utilized  and  assim- 
ilated for  tissue  repair.  This  he  terms  tissue  or  organ  protein.  The  re- 
maining large  percentage  circulating  in  the  interstices  of  the  tissues,  though 
not  forming  an  integral  part  of  them,  is  acted  on  directly  by  them,  merely 
by  virtue  of  contact— split,  oxidized,  and  reduced  to  simpler  compounds. 
This  he  terms  circulating  protein. 


I20  TEXT-BOOK  OF  PHYSIOLOGY. 

According  to  Pfliigcr  and  others,  this  view  is  not  tenable.  Pfliiger 
asserts  that,  as  material  changes  or  metabolism  can  take  place  only  within 
living  cells,  all  the  protein  must  first  be  assimilated  and  organized  by  the 
cells  before  it  can  undergo  metabolic  changes.  Metabolism  by  contact 
action  is  denied,  and  the  division  of  proteins  into  organ  and  circulating 
protein  is  not  justifiable. 

In  the  process  of  metabohsm  the  protein  suffers  disintegration,  giving 
rise  through  oxidation  to  some  carbon-holding  compound,  possibly  fat, 
possibly  sugar  and  to  some  nitrogen-holding  compounds,  which  eventually 
give  rise  to  urea.  The  intermediate  stages,  however,  are  not  definitely 
known;  the  immediate  antecedents  of  urea  are  probably  carbamate  and 
carbonate  of  ammonia.  The  disintegration  of  the  proteins  is  attended  by 
the  liberation  of  heat,  thus  contributing  to  the  general  store  of  the 
energy  of  the  body. 

The  amino-acids  that  are  not  utilized  in  the  synthesis  of  the  necessary 
blood  proteins  are  absorbed  by  the  intestinal  epithelium  and  deprived  of 
their  amidogenic  nitrogen  (NHj).  The  latter  is  then  converted  into  am- 
monia, carried  to  the  liver  and  converted  into  urea.  The  remainder  of  the 
amino-acid  is  carried  into  the  circulation,  and  is  eventually  oxidized,  thus 
giving  rise  to  heat.  It  is  also  possible  that  some  of  the  amino-acids  are 
carried  to  the  tissues  and  there  directly  used  in  tissue  formation. 

The  fat  principles  while  in  the  alimentary  canal  also  undergo  a  series  of 
^  changes  whereby  they  are  reduced  to  soap  and  glycerin,  under  which  forms 
they  are  absorbed.  During  the  act  of  absorption  the  soap  and  glycerin 
are  synthesized  to  human  fat.  The  fine  particles  thus  formed  in  the 
intestinal  wall  are  carried  by  the  lymph  vessels  to  the  thoracic  duct,  and 
thence  into  the  blood  stream,  from  which  they  rapidly  disappear.  Though 
it  is  possible  that  a  portion  of  the  fat  enters  directly  into  the  formation  of 
the  living  material,  it  is  generally  believed  that  it  is  at  once  oxidized  and 
reduced  to  carbon  dioxid  and  water  with  the  liberation  of  energy.  The 
natural  supposition  that  a  portion  of  the  ingested  fat  is  directly  stored 
up  in  the  cells  of  the  areolar  connective  tissue,  thus  giving  rise  to  adipose  tissue, 
has  been  a  subject  of  much  controversy,  though  modern  experimentation 
renders  this  very  probable.  The  body-fat,  under  physiologic  conditions, 
is  also  a  product  of  the  metabolic  activity  of  connective-tissue  cells  and  is  a 
derivative  of  both  proteins  and  carbohydrates. 

The  carbohydrate  principles  are  reduced  during  digestion  to  simple  forms 
of  sugar,  chiefly  dextrose  and  levulose.  Under  these  forms  they  are  absorbed 
into  the  blood.  These  compounds  are  then  carried  to  the  liver  and  to  the 
muscles  where  they  are  dehydrated  and  stored  under  the  form  of  starch, 
termed  animal  starch  or  glycogen.  Subsequently  glycogen  is  transformed 
by  hydration  to  sugar,  after  which  it  is  oxidized  to  carbon  dioxid  and  water. 
The  intermediate  stages  through  which  sugar  passes  before  it  is  reduced 
to  carbon  dioxid  and  water  are  only  imperfectly  known.  Though  a  large 
part  of  the  carbohydrate  material  is  at  once  oxidized,  it  is  now  well  estab- 
lished that  another  portion  contributes  to  the  formation  of,  if  it  is  not  directly 
converted  into,  fat.  As  the  carbohydrates  form  a  large  portion  of  the  food, 
they  contribute  materially  to  the  production  of  energy. 

The   inorganic   principles,    though    apparently    not   playing   as    active 


FOODS.  121 

a  part  in  the  metabolism  of  the  body  as  the  organic,  are  nevertheless  essential 
to  its  physiologic  activity.  ^ 

Water  is  promptly  absorbed  after  ingestion  and  becomes  a  part  of  the 
circulating  fluids — blood  and  lymph.  In  the  digestive  apparatus  it  favors 
the  occurrence  of  those  chemic  changes  in  the  food  necessary  for  their 
absorption,  it  promotes  absorption  of  the  food,  holds  various  constituents 
of  the  blood  and  other  fluids  in  solution,  hastens  the  general  metabolism 
of  the  body,  holds  in  solution  various  products  of  metabolic  activity,  and, 
leaving  the  body  through  the  excretory  organs,  promotes  their  elimination. 

Sodium  chlorid  is  absorbed  into  the  blood  and,  unless  taken  in  excess, 
is  utilized  in  replacing  that  which  is  lost  to  the  organism  daily.  The  exact 
role  which  sodium  chlorid  plays  in  the  nutritive  process  is  unknown;  but, 
as  it  is  present  as  a  necessary  constituent  in  all  the  fluids  and  solids  of  the 
body,  and  as  it  is  instinctively  employed  as  a  condiment,  it  may  be  assumed 
to  have  a  more  or  less  important  function. 

When  taken  as  a  condiment,  it  imparts  sapidity  to  the  food  and  excites 
the  flow  of  the  digestive  fluids;  it  ultimately  furnishes  the  chlorin  for  the 
hydrochloric  acid  of  the  gastric  juice.  Judging  from  the  impairment  of  the 
nutrition  as  observed  in  animals  after  deprivation  of  salt  for  a  long  period 
of  time,  it  favorably  influences  the  growth  and  functional  activity  of  all 
tissues. 

It  is  well  known  that  herbivorous  animals,  races  of  men  as  well  as 
individuals  who  live  largely  on  vegetable  foods,  require  a  larger  addi- 
tional amount  of  sodium  chlorid  than  carnivorous  animals  or  human 
beings  who  live  largely  on  animal  foods,  even  though  the  two  classes  of 
foods  contain  relatively  the  same  amounts.  The  explanation  is  that  the 
vegetable  foods  contain  potassium  salts  which,  meeting  in  the  blood  with 
sodium  chlorid,  undergo  decomposition  into  potassium  chlorid  and  sodium 
carbonate  or  phosphate,  all  of  which,  when  in  excess,  are  at  once  eliminated 
by  the  kidneys.  The  blood,  therefore,  becomes  poorer  in  sodium  chlorid,  one 
of  its  necessary  constituents. 

Potassium  phosphate  and  carbonate  are  also  essential  to  the  normal 
composition  of  the  solids  and  fluids.  They  impart  a  certain  degree  of 
alkalinity  to  the  blood  and  lymph,  one  of  the  conditions  necessary  to  the 
life  and  activity  of  the  tissue-cells  bathed  by  them.  When  administered 
in  small  doses,  they  increase  the  force  of  the  heart,  raise  the  arterial  pressure 
and  increase  the  activity  of  the  circulation. 

Calcium  phosphate  and  carbonate  are  partly  utilized  in  maintaining 
the  solidity  of  the  bones  and  teeth,  replacing  the  amount  metabolized 
daily.  Inasmuch  as  the  metabolism  of  these  two  tissues  is  slight,  there  is 
not  much  need  in  the  adult  for  lime  as  an  article  of  food.  In  young  animals 
lime  is  essential  to  the  solidification  and  development  of  bone.  WTien 
deprived  of  it,  the  skeleton  undergoes  a  defective  development  similar 
to  the  pathologic  condition  known  as  rickets.  Lime  is  present  in  milk  to 
the  extent  of  0.15  per  cent.,  as  well  as  in  eggs  and  peas  in  relatively  large 
quantities. 

Iron  is  contained  in  both  animal  and  vegetable  foods,  not,  however, 
in  the  form  of  inorganic  iron,  nor  in  the  form  of  an  organic  salt,  but  as  a 
compound  with  nuclein,  thus  forming  an  integral  part  of  the  proteid  molecule. 


122  TEXT-BOOK  OF  PHYSIOLOGY. 

After  absorption  the  iron  is  utilized  in  the  formation  of  the  coloring-matter 
of  the  blood-corpuscles — hemoglobin.  The  organic  compounds  of  iron 
and  the  nucleins  have  been  termed  hematogens.  The  amount  of  iron  ingested 
has  been  estimated  at  lo  to  90  milligrams  daily,  the  larger  part  of  which  is 
eliminated  in  the  feces.  The  relatively  small  part  eliminated  by  the  kidneys 
and  liver  is  usually  taken  as  the  amount  metabolized,  though  it  is  probable 
that  this  is  not  wholly  true,  as  there  is  evidence  that  iron  can  be  retained  in 
the  body  and  utilized  again  in  the  formation  of  new  hemoglobin.  Contrary 
to  what  might  be  expected,  milk  contains  but  a  very  small  quantity  of  iron 
not  more  than  3  or  4  milligrams  in  1000  grams  (human  milk) — an  amount 
insufficient  for  the  development  of  the  necessary  hemoglobin.  This  is 
compensated  for,  however,  by  the  accumulation  of  iron  in  the  liver  during 
intrauterine  life.  According  to  Bunge,  the  liver  of  a  newly  born  rabbit 
contains  as  much  as  18.2  milligrams  per  100  grams  of  body-weight,  while  at 
the  end  of  twenty-four  days  it  contains  only  3.2  milligrams  per  100  grams  of 
body-weight. 

Vegetable  acids  increase  the  secretions  of  the  alimentary  canal,  and 
are  apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea.  After 
entering  into  combination  with  bases  to  form  salts,  they  stimulate  the 
action  of  the  kidneys  and  promote  a  greater  elimination  of  all  the  urinary 
constituents.  In  some  unknown  way  they  influence  nutrition;  when  deprived 
of  these  acids,  the  individual  becomes  scorbutic. 

The  accessory  foods — coffee,  tea,  and  cocoa— when  taken  in  modera- 
tion have  a  stimulating  influence  on  the  nervous  system,  as  shown  by  the 
removal  of  both  mental  and  physical  fatigue,  by  an  increased  capacity 
for  sustained  mental  work,  and  by  the  persistent  wakefulness  among  those 
unaccustomed  to  their  use.  Coffee  more  especially  increases  the  frequency 
and  force  of  the  heart-beat,  raises  the  arterial  pressure,  and  hastens  the 
general  blood-flow.  It  has  no  influence  either  in  the  way  of  increasing  or 
decreasing  protein  metabolism. 

Tea  frequently  acts  as  an  astringent  on  the  alimentary  canal  on  account 
of  the  tannin  which  passes  into  the  water  when  the  infusion  is  made.  In- 
asmuch as  tannin  also  coagulates  peptones,  the  excessive  use  of  tea  as  a 
beverage  is  apt  to  derange  the  digestive  organs  and  the  general  process  of 
digestion. 

Cocoa  is  more  nutritive  than  either  coffee  or  tea,  on  account  of  the 
large  amount  of  fat  and  protein  it  contains.     It  is,  however,  less  stimulating. 

The  active  principles  in  coffee,  tea,  and  cocoa,  and  to  which  their  effects 
are  to  be  attributed,  are  caffein,  thein,  and  theohromin  respectively.  These 
alkaloids  are  chemically  closely  related  one  to  the  other  and  to  the  compound 
xanthin.  They  are  present  in  the  coffee  seeds,  the  tea  leaves,  and  the  cocoa 
bean  to  the  extent  of  1.7  per  cent.,  1.4  per  cent.,  and  1.6  per  cent,  respectively. 
When  prepared  as  a  beverage,  however,  there  is  three  times  as  much  caffein 
in  coffee  as  thein  in  tea. 

Alcohol  when  taken  in  small  ciuantities  stimulates  the  digestive  glands 
to  increased  activity  and  thus  promotes  digestive  power.  Its  absorption  into 
the  blood  is  followed  by  increased  action  of  the  heart,  dilatation  of  the  cutane- 
ous blood-vessels,  a  sensation  of  warmth,  and  an  excitation  of  the  brain. 
In  large  quantities  it  acts  as  a  paralyzant,  depressing  more  especially  the 


FOODS.  123 

vaso-constrictor  nerve-centers  and  certain  areas  of  the  brain,  as  shown  by 
an  impairment  in  the  power  of  sustained  attention,  clearness  of  judgment, 
and  muscle  coordination. 

Alcohol  is  undoubtedly  oxidized  in  the  body,  as  only  about  2  per  cent, 
can  be  obtained  from  the  urine  and  expired  air.  It  thus  contributes  to  the 
store  of  the  body-energy.  Whether  for  this  reason  it  can  be  regarded 
as  a  food — that  is,  whether  it  can  be  substituted  in  part  at  least  for  fat  or 
carbohydrate  material  without  impairing  the  protein  metabolism — is  at 
present  a  subject  of  experimentation  and  discussion.  According  to  some 
investigators,  alcohol  does  not  retard  protein  metabolism,  for  when  it  is 
introduced  into  the  body  in  amounts  equivalent  to  the  carbohydrates  with- 
drawn from  the  food  there  is  at  once  a  rise  in  the  amount  of  nitrogen  excreted. 
Hence  it  cannot  be  regarded  as  a  food.  According  to  other  investigators, 
alcohol  retards  or  protects  protein  metabolism  just  as  effectually  as  an 
equivalent  amount  of  starch  or  sugar.  Many  more  experiments  are  required 
to  decide  this  question.  When  taken  habitually  in  large  c[uantities,  alcohol 
deranges  the  activities  of  the  digestive  organs,  lowers  the  body  temperature, 
impairs  muscle  power,  lessens  the  resistance  to  depressing  external  con- 
ditions, diminishes  the  capacity  for  sustained  mental  work,  and  leads  to  the 
development  of  structural  changes  in  the  connective  tissues  of  the  brain, 
spinal  cord,  and  other  organs.  In  infectious  diseases  and  in  cases  of  depres- 
sion of  the  vital  powers  it  is  most  useful  as  a  restorative  agent. 

THE  ENERGY  OR  HEAT  VALUE  OF  THE  FOOD  PRINCIPLES. 

The  food  consumed  not  only  restores  the  material  metabolized  and 
discharged  from  the  body,  but  also  the  energy  which  has  been  expended  as 
heat  and  mechanic  motion.  The  food  principles  are  products  of  the  con- 
structive processes  taking  place  in  the  vegetable  world  during  the  period  of 
growth  and  activity.  At  the  time  of  their  formation  there  is  an  absorption 
and  storing  of  the  sun's  energy  which  then  exists  in  a  potential  condition. 
During  the  metabolism  of  the  animal  body  these  compounds  are  reduced 
through  oxidation  to  relatively  simple  bodies,  such  as  carbon  dioxid,  water, 
urea,  etc.,  with  the  liberation  of  their  contained  energy.  All  of  the  energy 
of  the  body,  whatever  its  manifestations  may  be,  can  be  traced  to  chemic 
changes  going  on  in  the  tissues,  and  more  particularly  to  those  changes 
involved  in  the  oxidation  of  the  food  principles. 

It  becomes,  therefore,  a  matter  of  interest  to  determine  the  heat  loss 
from  the  body  in  twenty-four  hours  for  the  purpose  of  subsequently  deter- 
mining if  the  energy  contained  in  the  foods,  expressed  in  terms  of  heat,  is 
present  in  amounts  sufiEicient  to  compensate  for  the  loss.  The  total  quan- 
tity of  heat  liberated  in  the  body  and  dissipated  from  it  in  twenty-four  hours 
is  determined  by  placing  the  subject  in  a  respiration  chamber  provided  with 
appliances  containing  water,  by  means  of  which  the  heat  can  be  absorbed 
and  measured.  (See  chapter  on  Animal  Heat.)  The  unit  of  heat  measure- 
ment is  the  Calorie,  which  is  defined  as  the  amount  of  heat  necessary  to  raise 
the  temperature  of  one  kilogram  of  water  1°  C.  If  therefore  the  volume 
of  the  water  employed  in  the  experiment  expressed  in  kilograms  be  multi- 
plied by  the  number  of  degrees  of  temperature  through  which  it  has  been 


124  TEXT-BOOK  OF  PHYSIOLOGY. 

raised,  the  number  of  Calories  will  be  known.  The  average  number  of 
Calories  dissipated  in  various  ways,  e.g.,  radiation,  vaporization  of  water 
from  lungs  and  skin,  warming  of  foods,  air,  etc.,  has  been  estimated  at  from 
2500  to  3000  each  day.  The  question  then  to  be  determined  is,  whether 
any  given  diet  scale  contains  this  amount  of  heat  and  whether  it  can  be 
liberated  on  oxidation  in  the  body. 

The  amount  of  heat  or  energy  which  any  given  food  principle  will  yield 
can  be  determined  by  burning  a  definite  amount  (e.g.,  i  gram)  to  carbon 
dioxid  and  water  and  ascertaining  the  extent  to  which  the  heat  thus  liberated 
will  raise  the  temperature  of  a  given  amount  of  water  {e.g.,  1  kilogram). 
The  amount  of  heat  may  be  expressed  in  gram  or  kilogram  degrees  or 
calories,  a  gram  calorie  or  kilogram  Calorie  being  the  amount  of  heat  required 
to  raise  the  temperature  of  a  gram  or  a  kilogram  (1000  grams)  of  water  1°  C. 
The  apparatus  employed  for  this  purpose  is  termed  a  calorimeter,  and  con- 
sists essentially  of  a  closed  chamber  in  which  the  oxidation  takes  place, 
surrounded  by  a  water  jacket,  the  rise  in  temperature  of  the  water  indicating 
the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calorimeters 
and  different  food  principles  of  the  same  group  vary,  though  within  certain 
limits:  e.g.,  1  gram  of  casein  yields  5.867  kilogram  Calories;  i  gram  of  lean 
beef,  5.656  Calories;  i  gram  of  fat  yields  9.353,  9.423,  9.686  Calories;  i  gram 
of  carbohydrate,  4.182,  4.479,  etc.,  Calories.  These  numbers  represent 
the  physical  calorimetric  heat  values  of  these  food  principles. 

In  the  human  body  as  determined  by  calorimetric  methods  the  oxidation 
of  the  food  principles  yields  practically  the  same  amount  of  heat  they  yield 
when  oxidized  outside  the  body,  with  the  exception  of  the  protein,  which 
is  oxidized  only  to  the  stage  of  urea.  As  this  compound  is  capable  of 
further  reduction  in  the  calorimeter  to  carbon  dioxid  and  water  with  the 
liberation  of  heat,  the  quantity  of  heat  it  contains  must  therefore  be  deducted 
from  the  calorimetric  heat  value  of  the  protein.  According  to  Rubner, 
I  gram  of  urea  will  yield  2.523  kilogram  Calories.  As  the  urea  which  results 
from  the  oxidation  of  i  gram  of  protein  is  about  -J  of  a  gram,  the  amount  of 
heat  to  be  deducted  from  the  heat  value  of  the  protein  is  ^  of  2.523,  or 
0.841  Calories.  It  has  also  been  shown  that  some  of  the  ingested  protein 
escapes  in  the  feces,  the  heat  value  of  which  must  also  be  determined  and 
deducted.  This  having  been  done,  the  physiologic  heat  value  becomes 
4.124  Calories. 

The  following  estimates  give  approximately  the  number  of  kilogram 
Calories  produced  when  the  food  is  burned  to  carbon  dioxid,  water,  and 
urea  in  the  body: 

I  gram  of  protein  yields 4  •  124  Calories. 

I  gram  of  fat  yields 9-353  Calories. 

I  gram  of  carbohydrate  yields 4. 116  Calories. 

The  total  number  of  kilogram  Calories  or  kilogram  degrees  of  heat 
yielded  by  any  of  the  previously  given  diet  scales  can  be  readily  determined 
by  multiplying  the  quantities  of  food  principles  consumed  by  the  above- 
mentioned  factors.  The  diet  scale  of  Vierordt,  for  example,  yields  the 
following: 


FOODS.  125 

120  grams  of  protein  yields ■ 494  •  88  Calories. 

90  grams  of  fat  yields 841 .  77  Calories. 

330  grams  of  starch  yields 1358-28  Calories. 

2694.93  Calories 

The  total  Calories  obtained  from  other  diet  scales  would  be  as  follows: 
Ranke,,  2335;  Voit,  3387;  Moleschott,  2984;  Atwater,  3331;  Hultgren, 
3436. 

Starvation. — The  relation  of  the  different  food  principles  to  the  general 
nutritive  process  becomes  more  apparent  from  an  examination  of  the 
excretions  from  the  body  during  the  process  of  starvation  combined  with  an 
examination  of  the  organs  and  tissues  after  death.  If  an  animal  be  deprived 
entirely  of  food,  a  decline  in  body-weight  at  once  sets  in,  which  continues 
until  about  40  per  cent,  of  the  weight  has  been  lost,  when  death  generally 
ensues.  This  results  from  the  fact  that  the  active  tissue  cells  consume, 
for  the  purpose  of  maintaining  the  normal  temperature  of  the  body,  not 
only  their  own  reserve  food  material,  but  that  of  the  less  active  or  storage 
tissues  as  well;  and,  in  consequence,  there  is  a  progressive  diminution  in 
weight. 

The  phenomena  which  characterize  this  non-physiologic  condition  are 
as  follows:  hunger,  intense  thirst,  gastric  and  intestinal  uneasiness  and 
pain,  diminished  pulse-rate  and  respiration,  muscular  weakness  and  emacia- 
tion, a  lessening  in  the  amount  of  urine  and  its  constituents,  diminished 
expiration  of  carbon  dioxid,  an  exhalation  of  a  fetid  odor  from  the  body, 
vertigo,  stupor,  delirium,  at  times  convulsions,  a  sudden  fall  in  body  tem- 
perature, and  finally  death.  The  duration  of  life  after  complete  depriva- 
tion of  food  varies  from  eight  to  thirteen  days  or  more,  though  this  period 
can  be  prolonged  if  the  animal  be  supplied  with  water,  this  being  more 
essential  under  the  circumstances  than  the  organic  materials  which  can  be 
supplied  by  the  organism  itself.  The  duration  of  the  starvation  period 
will  naturally  vary  in  accordance  with  the  previous  condition  of  the  animal 
and  the  amount  of  reserve  food  the  body  contains. 

The  extent  and  the  character  of  the  metabolism  that  the  body  undergoes 
in  starvation  can  be  determined  from  an  examination  of  the  excretions. 
Thus  the  excretion  of  urea  declines  very  rapidly  during  the  first  few  days — 
a  fact  which  has  been  attributed  to  the  consumption  of  the  surplus  protein 
food.  After  this  period,  when  the  tissues  begin  to  metabolize  their  own 
protein,  the  excretion  remains  fairly  constant,  about  13  grams,  until 
toward  the  close  of  the  starvation  period,  when  the  amount  eliminated 
falls  very  rapidly.  As  proteins  contain  about  16  per  cent,  of  nitrogen, 
I  part  of  nitrogen  equals  6.25  parts  of  protein.  Hence,  for  every  i  gram  of 
nitrogen  or  2.14  grams  urea  excreted,  it  may  be  assumed  that  6.25  grams  of 
protein  or,  according  to  Voit,  30  grams  of  flesh  have  been  metabolized. 
The  daily  excretion  of  urea,  after  the  first  five  days  therefore,  indicates  fairly 
accurately  the  extent  of  the  metabolism  of  the  tissue  protein. 

It  has  been  observed  also  that  there  is  a  steady  diminution  in  the  excre- 
tion of  carbon  dioxid,  though  this  is  greatest  in  the  last  few  days.  As  fat 
contains  about  76  per  cent,  of  carbon,  i  part  of  carbon  equals  1.31  parts  of 
fat.  Hence,  for  every  i  gram  of  carbon  or  3.66  grams  carbon  dioxid  ex- 
creted it  may  be  assumed  that  1.3 1  grams  of  fat  have  been  metabolized. 


126 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  daily  excretion  of  carbon,  therefore,  indicates  the  extent  of  fat  metab- 
olism. The  carbohydrates  are  here  left  out  of  consideration,  as  they  con- 
stitute only  about  i  per  cent,  of  the  body-weight.  It  must  be  borne  in 
mind,  however,  that  in  the  metabolism  of  protein  a  certain  C|uantity  of  fat 
or  sugar  is  produced,  which  also  undergoes  oxidation.  The  amount  of  the 
carbon  or  the  fat  that  the  protein  would  give  rise  to,  as  previously  deter- 
mined, must  therefore  be  subtracted  from  that  eliminated  by  the  lungs,  etc., 
in  order  to  determine  the  amount  of  body-fat  metabolized.  Observations 
of  human  beings  in  the  fasting  condition  show  that  for  a  period  of  ten  days 
there  is  a  daily  excretion  of  about  24  grams  of  urea-,  equivalent  to  about  72 
grams  of  protein.  This  amount,  however,  may  be  reduced  to  from  40  to  50 
per  cent,  if  the  individual  has  a  surplus  of  body-fat.  Human  beings  under 
similar  circumstances  may  lose  during  the  first  few  days  from  180  to  200 
grams  of  fat. 

The  following  table  shows  the  excretion  of  nitrogen  and  carbon  and 
the  calculated  amounts  of  protein  and  fat  metabolized  from  an  experiment 
made  by  Ranke  on  himself  during  a  fast  of  twenty  hours,  beginning  twenty- 
four  hours  after  the  last  meal: 


Disintegration  of  Tissue 
(Calculated) 

Expenditure 

Nitrogen 

Carbon 

Urea,  17  gm \ 

Uric  acid,  0.2  gm / 

Nitrogen 

Carbon 

Protein,  k,o  gm 

.     7.8 

26.'; 

7.2 
0.0 

3-4 

Fat,  199.6  gm 

0.0 

157-5 

Carbon  dioxid . 

180.6 

7.8 

184.0 

7.2 

184.0 

Coincidentiy  with  these  losses  to  the  body  there  is  also  a  gradual  loss  of 
inorganic  salts,  and  toward  the  termination  of  the  period  a  sudden  fall  in 
temperature  of  several  degrees  centigrade,  in  consequence  of  the  final  con- 
sumption of  all  available  foods,  when  death  ensues,  in  all  probability,  from 
a  cessation  in  the  action  of  the  heart. 

Post-mortem  Appearances. — It  has  been  experimentally  determined  that 
animals  die  when  the  body-weight  has  declined  to  about  40  per  cent.  Post- 
mortem examination  shows  that  the  loss  of  material,  though  very  generally 
distributed  throughout  the  body,  is  greatest  in  organs  and  tissues  least 
essential  to  life. 

The  results  of  an  analysis  of  the  organs  and  tissues  of  a  cat  after  a  thirteen- 
day  period  of  starvation,  during  which  the  animal  lost  1017  grams  in  weight, 
are  given  in  the  following  table,  based  on  data  furnished  by  Voit: 

It  will  be  observed  from  this  table  that  the  adipose  tissue  suffers  the 
greatest  loss,  the  entire  amount  disappearing  with  the  exception  of  a  small 
portion  in  the  posterior  part  of  the  orbital  cavity  and  around  the  kidneys. 
The  muscles,  though  losing  only  31  per  cent,  of  their  weight,  yet  furnish 
429  grams  of  presumably  protein  material,  for  nutritive  purposes.  The 
heart  and  nervous  system  experience  but  slight  loss. 


Percentage 

Actual  Loss 

Loss  of  weight 

of  Tissue 

Grams 

97 

267 

67 

6 

54 

49 

40 

I 

31 

429 

27 

37 

26 

7 

21 

89 

18 

3 

18 

21 

FOODS.  127 

Organ 


Adipose  tissue 

Spleen 

Liver 

Testes 

Muscles 

Blood 

Kidneys 

Skin  and  hair 

Lungs 

Intestines 

Pancreas 17  ^ 

Bones 14  55 

Heart 3  ° 

Nervous  system 3  1  i 

Mixed  Diet. — The  chemic  composition  of  the  tissues,  taken  in  con- 
nection with  their  metaboHsm  during  starvation,  implies  that  no  one  article 
of  food  is  sufficient  for  tissue  repair  and  heat  production;  but  that  all  classes 
of  foods — in  other  words,  a  mixed  diet — are  essential  to  the  maintenance  of 
a  normal  nutrition.  Experimental  investigation  has  also  conclusively 
established  this  fact.  Moreover,  the  amounts  of  nitrogen  and  carbon  elimi- 
nated daily,  and  the  ratio  existing  between  them,  indicate  the  amounts 
of  protein,  fat,  and  carbohydrate  which  are  required  to  cover  the  loss. 

Metabolism  on  a  Purely  Protein  Diet. — Notwithstanding  the  chemic 
composition  of  the  proteins  and  the  possibility  of  their  giving  rise  to  either 
fat  or  a  carbohvdrate  during  their  metabolism  it  has  been  found  extremely 
difficult  to  maintain  the  normal  nutrition  for  any  length  of  time  on  a  pure 
protein  or  fat-free  flesh  diet.  This,  however,  has  been  accomplished  wdth 
dogs.  It  was  found,  however,  that,  in  order  to  maintain  the  equilibrium, 
it  was  necessary  to  increase  the  proteins  from  two  to  three  times  the  usual 
amount.  Thus,  a  dog  weighing  30  to  35  kilograms  required  from  1500  to 
1800  grams  of  flesh  daily  in  order  to  get  the  requisite  amount  of  carbon  to 
prevent  consumption  of  its  own  adipose  tissue.  Under  similar  circumtances, 
a  human  being  weighing  70  kilograms  would  require  more  than  2000  grams 
of  lean  beef — an  amount  which,  from  the  nature  of  the  digestive  apparatus, 
it  would  be  practically  impossible  to  digest  and  assimilate  for  any  length 
of  time.  Even  the  slight  habitual  excess  beyond  the  amount  normally 
required  is  imperfectly  assimilated  and  gives  rise  to  the  production  of 
nitrogen-holding  compounds  w^hich,  on  account  of  the  difficulty  with  which 
they  are  eliminated  by  the  kidneys,  accumulate  within  the  body  and  develop 
the  gouty  diathesis,  with  all  its  protean  manifestations. 

Metabolism  on  a  Fat  and  Carbohydrate  Diet. — As  nitrogen  is  an 
indispensable  constituent  of  the  tissues,  it  is  evident  that  neither  fat  nor 
carbohydrates  can  maintain  nutritive  equilibrium  except  for  very  short 
periods.  On  such  a  diet  the  tissues  consume  their  own  proteins,  as  shown  by 
the  continuous  excretion  of  urea,  though  the  amount  is  less  than  during 
starvation.  An  excess  of  fat  retards  the  metabolism  of  proteins.  The  same 
holds  true  for  the  carbohydrates. 


128  TEXT-BOOK  OF  PHYSIOLOGY. 

Thus,  in  any  well-arranged  dietary  there  should  be  a  combination 
of  proteins,  fats,  and  carbohydrates  in  amounts  sufficient  to  maintain  nutritive 
equilibrium;  in  other  words,  to  repair  the  loss  of  tissue  and  to  furnish  the 
requisite  amount  of  heat  in  accordance  with  work  done,  as  well  as  with 
climatic  and  seasonal  variations. 

COMPOSITION  OF  FOODS. 

The  food  principles  essential  to  the  maintenance  of  the  nutrition  of 
the  body  are  contained  in  varying  proportions  in  compound  substances 
termed  foods;  e.g.,  meat,  milk,  wheat,  potatoes,  etc.  Their  nutritive  value 
depends  partly  on  the  amounts  of  their  contained  food  principles  and 
partly  on  their  digestibility.  The  dietary  of  civilized  man  embraces  foods 
derived  from  both  the  animal  and  vegetable  w^orlds. 

The  following  tables  show  the  percentage  composition  of  the  edible 
portions  of  foods  as  well  as  the  amount  of  heat  liberated  per  pound  when 
oxidized  in  the  body,  according  to  Atwater  and  Bryant. 

Composition  of  Animal  Foods. — The  following  table  shows  the  average 
percentage  composition  of  various  kinds  of  meats,  cow's  milk,  and  eggs: 


Kind  of  Food 
Materials 


Water 


Unavail- 
able 
Nutrients 


Proteins       Fat 


Carbo- 
hydrates 


Ash 


Fuel  Value 

Per  Lb. 
453.6  Grams 


Per 

cent. 
Beef 

Loin,  lean 67  .0 

Loin,  fat. . . , 

Round,  lean 

Round,  fat bo. 4 

Veal: 

Cutlets  (round) 

Liver 

Mutton: 

Leg 

Loin 

Pork: 

Loin  chops 52  .0 

Ham 53.9 

Fowl:  63.7 

Turkey 55.5 

Mackerel '     73-4 

Halibut 75-4 

Milk 87.0 

Eggs,  boiled 73  .2 


54-7 
70  .0 


70,7 
73  o 


62.8 
50.2 


Per 

cent. 

1 .2 
1.9 
1 .0 
1.6 


Per  Per 

cent.         cent. 


Per       I    Per 
cent.      I  cent. 


2  .1 
1.6 
1.9 
1-3 


19. 1 
17.0 
20.7 
18.9 

19.7 
9-7 

17.9 

15-5 

16. 1 
14.8 
18.7 
20.5 
18. 1 
18.0 

3  - 
12.8 


12  .1 
26.2 

7-5 
18.5 

7-3 
50 


31-4 

28.6 

27-5 

15-5 

21.8 

6.7 

4-9 

3-8 

II. 4 


1 .0 
0.9 

1 .1 
i.o 

0.8 


0.8 
0.6 

0.8 
0.6 
0.8 
0.8 
0.9 
0.8 

0-5 
0.6 


Calories 


900 

1470 

735 

1175 


410 

1095 
1660 

1555 
1480 
1040 

853 
650 

570 
310 

755 


Meats. — It  will  be  observed  from  these  analyses  that  the  meats  contain 
from  18  to  20  per  cent,  of  protein  material.  The  proteins  are  two  in  number 
and  are  known  as  paramyosinogen  or  myosin  and  myosinogen  or  myogen, 
both  of  which  are  in  a  semi-fluid  condition.  The  latter  is  four  or  five  times 
as  abundant  as  the  former.  After  death  these  substances  undergo  coagu- 
lation and  give  rise  to  two  solid  substances  known  as  myosin-fibrin  and 
myogen-fibrin.     After  being  subjected  to  the  cooking  process,  meats  contain 


FOODS.  129 

the  albuminoid  body  gelatin,  a  product  of  the  transformation  of  the  proteins 
of  the  connective  tissue. 

The  percentage  of  fat  contained  within  the  meat  substance  is  very 
small  except  in  mutton  and  pork,  where  it  rises  to  5.4  per  cent,  and  5.8 
per  cent,  respectively.  The  fat-globules  in  these  meats  are  packed  closely 
between  the  muscle-fibers,  and  prevent  the  easy  entrance  of  the  digestive 
fluids,  and  hence  they  are  more  difficult  of  digestion  than  beef.  The  large 
percentage  of  fat  represented  in  the  foregoing  table  is  due  to  the  presence 
in  the  food  as  eaten  of  adipose  tissue  which  is  an  addition  not  a  constituent 
of  meat. 

The  carbohydrates  vary  from  0.5  to  i  per  cent.,  and  are  represented 
by  glycogen.  The  principal  inorganic  salts  are  potassium  phosphate  and 
sodium  chlorid. 

The  composition  of  meat  will  vary  in  composition  in  nutritive  value  and 
in  energy- lib  crating  capacity  in  accordance  with  the  region  of  the  body 
from  which  the  specimen  is  taken. 

Cooking,  when  properly  done,  not  only  makes  the  meat  more  palatable 
and  appetizing  from  the  development  of  agreeable  flavors,  but  converts 
the  connective  tissue,  which,  in  old  animals  especially,  is  tough  and  resisting, 
into  gelatin,  thus  rendering  it  more  easy  of  mastication  and  digestion.  At 
the  same  time  parasitic  organisms,  such  as  the  embryonic  forms  of  tenia  or 
tapeworm,  and  Trichina  spiralis,  as  well  as  bacterial  growths,  which  fre- 
quently infest  the  bodies  of  animals,  are  destroyed  and  made  harmless. 

Milk  is  the  natural  food  of  the  young  of  all  mammals,  and  is  usually 
regarded  as  typical  on  account  of  the  ratio  existing  among  its  nutritive 
principles.  The  analysis  given  above  is  that  of  cow's  milk.  Examined 
microscopically,  milk  is  seen  to  consist  of  a  clear  fluid,  the  milk  plasma, 
holding  in  suspension  an  enormous  number  of  small,  highly  refractive  oil- 
globules,  which  measure  on  the  average  about  m^oQ  of  an  inch  in  diameter. 
Each  globule  is  supposed  by  some  observers  to  be  surrounded  by  a  thin 
albuminous  envelope,  which  enables  it  to  maintain  the  discrete  form. 
Others  deny  the  existence  of  such  a  membrane.  The  chief  protein  con- 
stituent of  milk,  caseinogen,  is  held  in  solution  by  the  presence  of  phosphate 
of  lime.  On  the  addition  of  acetic  acid  or  sodium  chlorid  up  to  the  point  of 
saturation  the  caseinogen  is  precipitated  as  such  and  may  be  collected  by 
appropriate  chemic  methods.  When  taken  into  the  stomach,  caseinogen  is 
coagulated;  that  is,  it  is  changed  into  casein  or  tyrein.  This  change  is 
brought  about  by  the  presence  in  the  gastric  juice  of  a  special  ferment 
termed  rennin  or  pexin. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures.  It 
is  a  combination  of  olein,  palmitin,  and  stearin,  with  a  small  quantity 
of  butyrin  and  caproin.  When  milk  is  allowed  to  stand  for  some  time, 
the  fat-globules  rise  to  the  surface  and  form  a  thick  layer  known  as  cream. 
When  subjected  to  the  churning  process,  fat-globules  run  together  and 
form  a  coherent  mass — butter. 

Lactose  is  the  particular  form  of  sugar  found  in  milk.     In  the  presence 

of  Bacillus  acidi  lactici,  the   lactose  is  decomposed   into  lactic  acid  and 

carbon  dioxid,  the  former  of  which  not  only  imparts  a  sour  taste  to  the  milk, 

but  causes  a  precipitation  of  the  caseinogen.     The  chief  salt  found  in  milk 

9 


I30 


TEXT-BOOK  OF  PHYSIOLOGY. 


is  phosphate  of  lime,  and  this  is  the  chief  source  of  this  agent  in  the  formation 
of  bones.     Sodium  and  potassium  chlorids  are  also  present. 

Eggs  are  also  to  be  regarded  as  complete  natural  foods,  inasmuch 
as  they  contain  all  the  necessary  food  principles.  The  analysis  given  in 
the  foregoing  table  represents  the  composition  of  the  entire  egg.  The 
white  of  the  egg  contains  12  per  cent,  of  protein  and  2  per  cent,  of  fat. 
The  yolk,  however,  contains  15  per  cent,  of  protein  and  30  per  cent,  of  fat. 

Composition  of  Cereal  Foods. — The  average  composition  of  the 
principal  cereals  is  shown  in  the  following  table: 


Kind  of  Food 
Material 


Water 


Unavail- 
able 
Nutrients 


Proteins       Fat 


Carbo- 
hydrates 


Ash 


Fuel  \'^alue 

Per  Pound 

453.6  Grams 


Per 


Per 


Entire  wheat  flour 

Rye  flour 

Rice 

Barley,  pearled ii 

Buckwheat  flour 

Corn  meal 

Oat  meal 

Whole  wheat  bread. . 

White  bread 35 

Graham  crackers 


cent. 

cent. 

II. 4 

4-5 

12 

9 

3-6 

12 

3 

3  -7 

II 

.1 

4.0 

13 

b 

3-5 

12 

5 

4.0 

7 

8 

5-6 

3« 

4 

3-2 

35 

3 

3-3 

-\ 

4 

4.8 

Per 

cent. 

10.7 
5-3 
6.5 
6.6 

5-2 
7-5 
13-4 
7-5 
71 
7-7 


Per 
cent. 


1-7 
0.8 

03 


1 .1 

1-7 
6.6 
0.8 

1 .2 


Per 

cent. 


70 
76 
76 
76 
75 
73 
65 
49 
52 
8.5    72 


Per 

cent. 
0.8 
0-5 
0-3 
0.8 
0.7 
0.8 
1.4 
1 .0 
0.8 
I.I 


Calories 

1645 
1610 
1610 
1630 
1600 
162; 

1795 
1125 

"95 
1900 


That  the  cereals  are  most  important  and  useful  articles  of  diet  is  evident 
from  their  composition,  consisting,  as  they  do,  of  proteins  and  carbohydrates 
in  large  proportion.  Owing  to  the  cellulose  or  woody  fiber  which  envelops 
and  penetrates  the  grain,  they  are  somewhat  difficult  of  digestion.  A  section 
of  a  grain  of  wheat  shows  the  external  cellulose  envelope,  the  husk,  beneath 
which  is  a  layer  of  large  cells  containing  the  chief  protein — the  gluten. 
The  interior  of  the  grain  consists  of  small  cavities,  the  walls  of  which  are 
formed  of  cellulose  and  which  contain  the  granules  of  starch,  fat,  small 
quantities  of  protein,  and  inorganic  salts.  All  other  cereals  have  a  similar 
structure. 

In  the  preparation  of  white  flour  from  wheat  it  is  customary  to  remove 
the  husk,  a  process  which  involves  the  removal  also  of  a  portion,  if  not  all, 
of  the  gluten  cells,  so  that  such  flour  contains  less  nitrogenized  material 
than  the  original  grain.  It  is  possible,  however,  in  the  milling  of  wheat, 
to  remove  only  the  husk  and  retain  the  gluten  in  the  flour,  as  in  the  prepa- 
ration of  whole  wheat  flour. 

Bread  is  an  artificially  prepared  food  made  either  of  wheat  or  rye. 
Owing  to  the  fact  that  the  proteins  of  the  other  cereals  do  not  possess  the 
same  adhesive  properties  when  kneaded  with  water,  they  cannot  be  used 
for  bread-making  purposes.  In  the  making  of  bread,  the  flour  is  kneaded 
with  water  until  a  glutinous  mass — dough — is  formed.  During  this  process, 
salt,  sugar,  and  yeast  are  added.  It  is  then  kept  in  a  temperature  of  about 
100°  F.  In  the  presence  of  heat  and  moisture  the  natural  ferment  of  the 
flour — diastase — converts  a  portion  of  the  starch  into  sugar,  which  in  turn 


FOODS. 


131 


is  split  up  into  carbon  dioxid  and  alcohol  by  the  yeast  plant.  The  sugar  that 
is  added  undergoes  a  similar  change  and  hastens  the  process.  The  bubbles 
of  carbon  dioxid,  becoming  entangled  in  the  dough,  cause  it  to  swell  or  rise 
and  subsequently  give  the  porous  or  spongy  character  to  the  bread.  When 
baked  at  a  temperature  of  400°  F.,  the  alcohol  is  largely  driven  off;  yeast  cells 
and  other  organisms  are  destroyed;  the  starch,  particularly  that  on  the 
surface,  is  dextrinized.  The  principal  salts  contained  in  wheat  flour  are 
potassium  and  magnesium  phosphate. 

Composition    of    Vegetable    Foods. — The    average    composition    of 
some  of  the  principal  vegetables  is  shown  in  the  following  table: 


Kind  of  Food 
Material 


Water 


Unavail- 
able 
Nutrients 


Proteins       Fat 


Carbo- 
hydrates 


I   Ash 


Fuel  Value 

Per  Pound 

453.6  Grams 


Beans,  lima,  dried. . . . 
Beans,  lima,  green.. . . 
Beans,  white,  dried . .  . 
Beans,  string,  cooked' 

Peas,  dried 

Peas,  green,  cooked'. . 
Potatoes,    boiled, 
cooked '. 

Potatoes,  sweet 

Beets,  cooked ' 

Cabbage . . 
Tomatoes .    . 

Turnips 

Egg-plant 

Spinach,  fresh 

Asparagus,  cooked.  .  . 


Per 

cent. 
10 
68 


Per 

cent. 
6.7 
2.7 

7-5 
0-5 
7.6 


30 

1 .2 

0-7 
0.4 
0.8 
0.6 
1 .0 
1 .0 


Per 

cent. 
12.8 

5-3 
15.8 

C.6 
17-3 

5-1 

1.9 

2  .2 

1-7 

1 .2 
0.7 
1 .0 

O.Q 

1.6 
1-7 


Per 
cent. 
1.4 
0.6 
1.6 
1 .0 
0.9 
31 


1.9 


0-3 
0.4 
0.2 
0-3 


Per 

Per 

cent. 

cent. 

65.6 

31 

21 

6 

1-3 

59 

9 

2.6 

I 

9 

0.7 

62 

■"J 

2  .2 

14 

4 

1 .1 

20 

0 

0.8  1 

40 

3 

0-7 

7 

2 

1 .2 

5 

.S 

0.8 

3 

8 

0.4 

7 

8 

0.6 

4 

9 

0.4 

3 

2 

1.6 

2 

I 

0.6 

Calories 

1565 
525 

1530 
90 

1508 
490 
41S 

88s 
170 
140 
100 

175 
120 
100 
19s 


*  With  butter  etc.,  added. 

The  vegetable  foods,  as  a  class,  vary  considerably  in  nutritive  value  and 
digestibility,  the  latter  depending  on  the  amount  of  cellulose  they  contain. 
A  section  of  a  vegetable  shows  not  only  the  presence  of  an  external  cellulose 
envelope,  but  also  an  inner  framework  which  penetrates  its  substance  in  all 
directions.  The  nutritive  principles  are  contained  in  small  cavities,  the 
walls  of  which  are  formed  by  the  framework.  Nearly  all  vegetables  require 
cooking  before  being  eaten.  When  subjected  to  heat  and  moisture,  not 
only  is  the  texture  of  the  vegetable  softened  and  disintegrated,  but  the 
starch  grains  are  hydrated  and  partially  prepared  for  conversion  into  dextrin 
and  sugar.  At  the  same  time  various  savor)-  substances  are  set  free,  which 
make  the  food  more  palatable. 

Beans  and  peas  contain  large  quantities  of  a  protein,  legumin,  anri 
starch,  and  hence  are  especially  valuable  as  nutritive  foods.  The  presence 
of  the  cellulose  envelope,  especially  in  ripe  beans  and  peas,  combined  with 
rather  a  dense  texture,  renders  them  somewhat  difficult  of  digestion.  Pota- 
toes, though  largely  employed  as  food,  are  extremely  poor  in  protein,  2  per 
cent.,  and  carbohydrates,  20  per  cent.  When  sufficiently  cooked  they  are 
easily  digested,  owing  to  the  small  amount  of  cellulose  they  contain. 


132 


TEXT-BOOK  OF  PHYSIOLOGY. 


Green  vegetables, — e.g.,  lettuce,  spinach,  tomatoes,  asparagus,  onions, 
etc.,  though  containing  food  principles  in  small  amounts,  are,  nevertheless, 
valuable  adjuncts  to  the  dietary,  for  the  reason  that  they  contain  inorganic 
as  well  as  organic  salts,  which  appear  to  be  necessary  to  the  maintenance 
of  the  normal  nutrition.  The  want  of  green  vegetables  has  been  supposed 
to  be  the  cause  of  scurv^y. 

Ripe  fruits,  grapes,  cherries,  apples,  pears,  peaches,  strawberries,  lemons, 
oranges,  etc.,  though  consumed  largely,  possess  but  little  nutritive  value. 
They  consist  largely  of  water,  75  to  85  per  cent.,  proteins  a  trace,  sugar 
from  5  to  13  per  cent.,  organic  acids  (citric,  malic,  tartaric),  pectose,  and 
various  inorganic  salts. 

Relative  Value  of  Animal  and  Vegetable  Foods. — Though  both 
animal  and  vegetable  foods  contain  the  different  classes  of  food  principles, 
it  is  not  a  matter  of  entire  indifference  as  to  which  are  consumed.  It  has 
been  found  by  experiment  that  animal  proteins  are  more  easily  and  com- 
pletely digested  and  absorbed  than  vegetable  proteins;  that  cellulose  is  not 
only  highly  indigestible,  but  by  its  presence  in  large  quantities  retards  the 
digestive  process  and  impairs  the  activity  of  the  entire  digestive  mechanism, 
though  in  moderate  quantity  it  undoubtedly  aids  digestion  indirectly  by 
mechanically  promoting  peristalsis.  The  following  table  shows  the  relative 
digestibility  of  the  two  classes  of  foods: 


Weight  of  Food 


Vegetable 


Animal 


Digested         Undigested  j     Digested         Undigested 


Of  100  parts  of  solids 75.5  24.5  89.9  10. i 

Of  100  parts  of  protein 46.6  53-4  81.2  18.8 

Of  100  parts  of  fats  or  carbohydrates.  90.3  9.7  96.9  3.1 


CHAPTER  X. 
DIGESTION. 

Digestion  is  a  process  partly  physical,  partly  chemic,  by  which  the 
nutritive  principles  of  the  foods  are  prepared  for  absorption.  The  reason 
for  these  changes  lies  in  the  fact  that  the  foods  as  consumed  are  hetero- 
geneous compounds  consisting  of  organic  and  inorganic  nutritive  principles 
associated  with  a  varying  amount  of  non-nutritive  material,  such  as  the 
dense  parts  of  the  connective  tissue  of  the  animal  foods  and  the  woody 
fiber  or  cellulose  of  the  vegetable  foods,  from  which  the  nutritive  prin- 
ciples must  be  freed  before  they  can  be  utilized;  and  in  the  further  fact,  that 
even  when  consumed  in  the  free  state,  the  food  principles  are  seldom  in  a 
condition  to  be  absorbed  into  the  blood  and  assimilated  by  the  tissues. 
When  foods  are  consumed  in  their  natural  state  or  after  they  have  been 
subjected  to  the  cooking  process,  they  are  subjected  while  in  the  food  canal 
to  the  solvent  action  of  various  fluids  by  which  they  are  disintegrated  and 
reduced  to  the  liquid  condition.  The  nutritive  principles  freed  from  their 
combinations  are  changed  in  chemic  composition  and  transformed  into 
substances  capable  of  absorption.  To  all  the  physical  and  chemic  changes 
which  foods  undergo  in  the  food  canal  the  term  digestion  has  been  given. 

The  digestive  apparatus  comprises  the  entire  alimentary  or  food  canal 
and  its  various  appendages:  the  lips,  the  teeth,  the  tongue,  the  salivary 
glands,  the  gastric  and  intestinal  glands,  the  pancreas,  and  the  liver  (Fig.  60). 

The  canal  itself  is  a  musculo-membranous  tube  about  thirty-two  feet 
in  length,  and  extends  from  the  mouth  to  the  anus.  It  may  be  subdivided 
into  several  distinct  portions,  as  mouth,  pharynx,  esophagus,  stomach, 
small  and  large  intestines.  The  mouth  is  pro\'ided  (i)  with  teeth,  by  which 
the  food  is  divided,  (2)  with  the  tongue,  and  (3)  with  glands,  by  which  a 
solvent  fluid,  the  saliva,  is  secreted.  The  glands,  though  situated  for  the 
most  part  outside  the  mouth,  are  connected  with  it  by  means  of  ducts. 
Posteriorly  the  mouth  opens  into  the  pharynx  or  throat,  a  somewhat  py- 
ramidal-shaped structure  about  five  inches  in  length,  which  in  turn  is 
followed  by  the  esophagus  or  gullet,  a  tube  about  nine  inches  in  length.  As 
the  esophagus  passes  through  the  diaphragm  it  expands  into  the  stomach, 
a  curved  pyriform  organ,  which  serves  as  a  reservoir  for  the  reception  and 
retention  of  the  food  for  a  varying  length  of  time.  The  small  intestine  is 
that  portion  of  the  alimentary  canal  extending  from  the  end  of  the  stomach 
to  the  beginning  of  the  large  intestine  in  the  right  iliac  fossa;  owing  to  its 
length,  about  twenty-two  feet,  it  presents  a  very  convoluted  appearance  in  the 
abdominal  cavity.  Embedded  in  its  walls  are  the  intestinal  glands  which 
open  on  its  surface  and  secrete  the  intestinal  fluid.  In  the  upper  portion  of 
the  small  intestine,  within  five  inches  of  the  stomach,  there  is  an  orifice,  the 
outlet  of  a  small  pouch,  the  Ampulla  of  Vater,  into  which  open  the  termina- 
tions of  the  ducts  of  the  liver  and  pancreas,  organs  which  secrete  the  bile  and 

133 


134 


TEXT-BOOK  OF  PHYSIOLOGY. 


pancreatic  juice  respectively.  The  large  intestine  is  from  five  to  six  feet  in 
length  and  extends  from  the  end  of  the  small  intestine  to  the  anus.  Its  walls 
contain  a  large  number  of  glands. 

The  general  process  of  digestion  is  largely  accomplished  by  the  chemic 
action  of  the  digestive  fluids  secreted  by  glands,  some  of  which  are  imbedded 
in  the  walls  of  the  canal  while  others  are  situated  outside  of  it  and  com- 
municate with  it  onlv  bv  means  of  ducts.     These  fluids  are  the  saliva,  the 


Salivary  O/and" 


Large' 
intestine 


Vermiform  Appendix  ■- 


Fig.  6o. — Diagram  of  the  Alimentary  Canal. — {Modified  from  Landois.) 

gastric,  intestinal,  and  pancreatic  juices,  and  the  bile.  Though  taking 
place  throughout  a  large  portion  of  the  food  canal,  the  process  may  be  sub- 
divided into  several  stages:  viz.,  prehension,  mouth  digestion,  deglutition, 
gastric  digestion,  and  intestinal  digestion. 

As  a  result  of  the  action  of  these  fluids  the  nutritive  principles  are  pre- 
pared for  absorption  into  the  blood;  the  non-nutritive  principles,  along 
with  certain  waste  products,  pass  into  the  large  intestine  to  be  finally  ex- 
truded from  the  bodv. 


DIGESTION.  135 

FERMENTS;  ENZYMES. 

In  a  preceding  chapter  it  was  stated  that  under  favorable  conditions 
the  carbohydrates,  fats,  and  proteins  undergo  reduction  to  simpler  com- 
pounds as  a  result  of  the  action  of  agents  such  as  the  yeast  plant  and  various 
forms  of  bacteria.  To  this  process  of  reduction  the  term  fermentation,  and 
to  the  agent  which  causes  the  fermentation  the  term  ferment,  or  enzyme  has 
been  given.  As  these  compounds  undergo  reduction  to  simpler  substances 
somewhat  different  in  character  in  the  alimentary  canal  during  the  period  of 
digestion  as  a  result  of  the  action  of  ferments,  it  will  be  conducive  to  clearness 
of  ideas  regarding  the  nature  of  the  digestive  process  if  the  nature  and  prop- 
erties of  ferments  in  general  is  briefly  considered  at  this  time. 

A  ferment  or  an  enzyme  may  be  defined  as  an  agent  that  induces  a  change 
of  state,  or  a  change  in  composition  of  an  organic  compound  without  itself 
being  utilized  in  the  process  or  appearing  in  the  end-results  of  the  process. 

Ferments  have  been  divided  for  a  long  time  into  two  classes,  viz.,  organ- 
ized and  unorganized.  Among  the  organized  ferments  may  be  mentioned 
the  yeast  plant  (Saccharomycetes)  and  various  forms  of  bacteria;  among 
the  unorganized  ferments  may  be  mentioned  the  diastase  that  transforms 
the  starch  of  barley,  wheat,  or  other  cereals  into  sugar,  as  well  as  ptyalin, 
pepsin,  steapsin  and  other  ferments  contained  in  the  digestive  fluids  that 
transform  or  reduce  the  food  principles  to  simpler  compounds. 

It  will  be  recalled  that  if  the  yeast  plant  is  added  to  a  sugar  solution 
containing  in  addition  some  protein  and  various  inorganic  salts  such  as 
phosphates  and  the  solution  kept  at  a  favorable  temperature  the  yeast  cells 
soon  begin  to  grow  and  multiply.  Coincidently  the  sugar  is  reduced  for 
the  most  part  to  carbon  dioxid  and  alcohol.  The  carbon  dioxid  bubbling 
through  the  solution  as  steam  bubbles  through  water  that  is  boiling,  gave  rise 
to  the  expression  fermentation  (from  fervere,  to  boil),  and  as  this  was 
attributed  to  the  life  activities  of  the  yeast  plant  it  was  called  a  ferment. 

Again,  if  dead  protein  matter  is  exposed  to  air  and  moisture  at  a  suitable 
temperature  it  will  be  invaded  by  various  species  of  bacteria,  which  in  a  short 
time  will  begin  to  grow  and  multiply.  Coincidently  the  protein  molecules  are 
reduced  to  simpler  compounds,  such  as  hydrogen  sulphid,  ammonia, 
carbon  dioxid  and  a  number  of  other  compounds,  the  nature  of  which  will 
vary  with  the  character  of  the  protein.  As  this  reduction  is  accompanied  by 
the  bubbling  of  gases  through  the  surrounding  liquid,  it  too  has  received  the 
name  of  fermentation,  and  as  the  reduction  is  attributed  to  the  life  activities 
of  the  bacteria  they  too  have  been  called  ferments.  In  both  instances  the 
ferment  is  a  unicellular  plant  possessing  a  distinct  organization.  For  this 
reason  they  have  been  termed  organized  ferments. 

When  grains  of  barley  or  other  cereals  containing  starch  are  exposed  to 
moisture  and  a  suitable  temperature,  the  starch  is  gradually  changed  to 
sugar,  a  transformation  attributed  to  the  action  of  a  ferment.  When  the 
starches,  fats,  proteins,  and  compound  sugars  are  introduced  into  the 
alimentary  canal  they  are  also  reduced  to  simpler  compounds,  a  reduction 
attributed  to  the  action  of  a  series  of  distinct  and  specific  ferments.  In 
addition  to  the  changes  that  the  food  principles  undergo  in  the  alimentary 
canal,  the  corresponding  principles  as  well  as  many  other  compounds  under- 


136  TEXT-BOOK  OF  PHYSIOLOGY. 

go  similar  changes  in  the  body  tissues  as  the  result  of  the  action  of  ferments, 
changes  that  underlie  and  condition  many  if  not  all  the  phenomena  of  the 
nutritive  process.  In  none  of  these  instances  however,  has  the  ferment  been 
satisfactorily  isolated  or  its  chemic  or  physical  features  determined.  For 
this  reason  these  ferments  have  been  termed  unorganized  ferments. 
Investigations  have  demonstrated,  however,  that  they  are  products  of  the 
metabolism  of  the  cells  of  plant  and  animal  tissues. 

In  recent  years  the  distinction  between  organized  and  unorganized 
ferments  has  become  untenable  owing  to  the  fact  that  chemists  have  succeeded 
in  extracting  from  yeast  cells  as  well  as  from  bacterial  cells,  enzymes  or 
ferments  that  produce  in  sugar  and  protein  the  same  reduction  effects  under 
the  same  conditions  as  in  the  case  of  yeast  cells  and  bacteria  themselves. 
It  is  therefore  probable  that  these  organized  cells  act  not  directly  by  virtue  of 
their  own  activities,  but  indirectly,  by  virtue  of  an  unorganized  ferment 
which  they  secrete  and  discharge  into  the  surrounding  medium.  All  enzymes 
that  produce  their  effects  after  being  discharged  from  cells  are  termed 
extra-cellular  enzymes,  while  those  that  produce  their  effects  in  the  interior 
of  cells  are  termed  intra-cellular  enzymes. 

The  Nature  of  Enzymes. — An  enzyme  is  in  all  probability  organic  in 
character,  though  neither  its  chemic  nature  nor  composition  has  been 
determined.  Some  of  them  exhibit  protein,  others  carbohydrate  reactions  but 
by  reason  of  the  difficulty  in  isolating  enzymes  and  of  freeing  them  absolutely 
from  all  traces  of  protein  and  carbohydrates  it  is  not  possible  to  state 
positively  whether  the  reactions  observed  are  due  to  the  enzyme  or  its 
associated  organic  matter.  The  purer  the  preparation,  however,  the  less  of 
any  chemic  reaction  is  exhibited. 

From  what  is  known  of  their  action,  of  the  effects  produced  and  of  the  condi- 
tions under  which  they  act,  ferments  have  a  resemblance  to  various  inorganic  sub- 
stances or  agents  that  produces  changes  of  composition  and  decomposition 
apparently  by  their  presence  alone,  for,  as  far  as  the  evidence  goes,  they  neither 
enter  into  the  end-products  of  the  reaction  nor  are  they  destroyed.  A  chemic 
change  thus  produced  is  termed  catalysis  and  the  agent  causing  it  is  termed  a 
catalyzer  or  catalyst.  The  substance  on  which  the  catalyst  acts  is  termed  the 
substrate.  In  most,  if  not  in  all  instances  a  catalyst  acts  not  as  an  initiator,  but 
as  an  accelerator  of  a  change  that  would  spontaneously  take  place  with  extreme 
slowness  and  in  some  instances  with  results  so  slight  as  to  be  inappreciable. 
Oxygen  and  hydrogen,  for  example,  spontaneously  combine,  there  are  reasons  for 
believing,  at  room  temperatures  though  at  such  a  slow  rate  that  the  formation  of 
water  cannot  be  detected,  but  if  a  small  quantity  of  finely  divided  platinum  be 
added  the  combination  takes  place  almost  immediately;  carburetted  hydrogen  and 
oxygen  combine  when  they  pass  over  platinum  with  the  formation  of  carbon 
dioxid  and  water;  saccharose  and  water  in  the  presence  of  hydrochloric  acid  will 
combine  and  be  reduced  to  equal  quantities  of  levulose  and  dextrose;  dilute 
peroxid  of  hydiogen  will  slowly  decompose  spontaneously  and  yield  up  oxygen, 
but  if  finely  divided  platinum  or  silver  be  added  the  decomposition  is  greatly  ac- 
celerated. In  all  these  instances,  to  which  many  more  might  be  added,  the 
catalyst,  simply  by  its  presence  accelerates  a  change  spontaneously  taking  place 
without  itself  appearing  in  the  end-products  of  the  reaction. 

It  has  been  experimentally  demonstrated  that  the  finer  the  catalyst  is  divided 
or  the  greater  the  surface  it  presents  the  more  energetically  it  acts.  Thus,  if  platinum, 


DIGESTION.  137 

silver,  or  gold  be  changed  to  the  colloidal  state,'  a  state  in  which  the  particles  of  ultra- 
microscopic  size  are  held  in  solution  or  perhaps  suspension  they  become  extremely 
active  catalyzers  even  in  exceedingly  small  quantities. 

From  the  foregoing  facts  and  from  many  others  it  may  be  assumed 
that  the  unorganized  enzymes  exist  in  the  colloidal  state. 

The  Rate  and  Completeness  of  Enzymic  Action.— The  rate  and 
completeness  of  enzymic  action  are  influenced  by  a  variety  of  conditions, 
among  the  more  important  of  which  are  temperature  and  the  rapidity  of 
the  removal  of  the  products  of  their  action. 

Temperature. — All  enzymes  are  sensitive  to  changes  in  temperature. 
At  0°  C.  they  appear  to  be  incapable  of  inducing  changes  in  organic  matter. 
As  the  temperature  is  raised  their  reaction  properties  develop  and  increase 
in  velocity,  until  a  temperature  of  40°  C.  to  50°  C.  is  reached,  when  they  are 
at  their  maximum.  For  this  reason  this  degree  of  temperature  is  spoken  of  as 
the  optimum  temperature.  Beyond  50°  C.  the  velocity  of  their  action  begins 
to  decrease  and  at  60°  C.  it  comes  to  an  end  for  the  majority  of  enzymes. 
At  100°  C.  all  reaction  ceases  for  the  reason  that  the  enzymes  are  destroyed 
especially  if  they  are  moist. 

The  Removal  of  the  Products  of  Enzymic  Action.— T\\q  completeness 
of  enzymic  action  will  depend  on  the  rapidity  with  which  the  products  of 
enzyme  activity  are  removed.  This  is  illustrated  in  the  following:  If  a 
substrate  such  as  fat  and  the  enzyme  lipase  be  mixed  with  water  in  a  dialyzing 
test-tube,  the  fat  will  combine  with  water,  after  which  the  fat  will  undergo 
a  cleavage  into  a  fat  acid  and  glycerin.  If  the  products  of  the  reaction  are 
removed  practically  as  rapidly  as  they  are  formed  the  reaction  will  continue 
until  all  the  fat  is  so  transformed.  Under  such  circumstances  the  action 
will  be  complete.  If,  however,  the  reaction  takes  place  in  a  receptacle  the 
character  of  which  prevents  the  removal  of  the  fat  acid  and  glycerin,  the 
reaction  will  in  time  come  to  an  end,  leaving  apparently  a  percentage  of 
fat  unchanged.  The  explanation  at  one  time  given  for  the  cessation  of  the 
reaction  was  that  the  accumulation  of  the  products  interfered  with  the 
further  action  of  the  enzyme.  It  is,  however,  now  generally  admitted  that 
under  the  circumstances  the  ferment,  shortly  after  the  appearance  of  the 
cleavage  products,  initiates  a  reverse  action,  i.e.,  recombines  the  fat  acid  and 
glycerin  with  the  re- formation  of  the  fat  until  a  condition  of  chemic 
equilibrium  is  established  between  the  opposing  tendencies.  The  dis- 
covery that  many  ferments  are  thus  capable  of  secondarily  reversing  their 
primary  action  has  assisted  in  the  interpretation  of  a  number  of  obscure 
physiologic  processes.  It  must  not  be  overlooked  that  in  this  instance  the 
enzyme  does  not  initiate  the  reverse  action,  but  merely  hastens  what  would 
take  place  by  reason  of  a  want  of  chemic  equilibrium  between  the  substances 
present.^ 

'  The  colloidal  state  may  be  developed  by  passing  an  electric  current  through  electrodes  of 
these  metals  placed  in  distilled  water.  With  the  passage  of  the  current  particles  of  the  metals 
are  discharged  from  one  of  the  electrodes  into  the  water  in  the  form  of  a  cloud. 

-  Reversibihty  of  a  chemic  reaction  may  be  defined  as  a  recombination  of  the  products  of  the 
reaction  of  the  original  compound  until  a  condition  of  equilibrium  is  established  between  the 
analytic  and  the  synthetic  tendencies.  A  classic  illustration  of  the  two  phases  of  a  chemic  reaction 
is  the  following:  If  chemically  equivalent  amounts  of  acetic  acid  and  ethyl  alcohol  are  mixed  at  a 
definite  temperature  a  reaction  occurs  which  eventuates  in  the  formation  of  ethyl  acetate  and 
water.     In  this  instance  after  a  certain  percentage  of  these  substances  has  thus  united  the  prod- 


138  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Specific  Action  of  Enzymes. — The  number  of  enzymes  in  the 
digestive  fluids  and  in  the  various  tissues  of  the  body  has. given  rise  to  the 
idea,  which  has  been  confirmed  by  experiment,  that  an  enzyme  exerts  its 
action  on  but  one  substrate,  in  other  words,  that  its  action  is  specific.  Thus 
an  enzyme  that  would  transform  starch  into  sugar  would  not  be  capable  of 
causing  a  cleavage  of  fat  into  a  fat  acid  and  glycerin;  an  enzyme  that  would 
cause  a  cleavage  of  saccharose  would  not  be  capable  of  causing  a  cleavage  of 
lactose.  So  with  all  other  enzymes.  Each  seems  to  be  specially  adapted  to 
catalyze  but  one  substrate  under  given  conditions.  Various  other  features 
of  enzymes,  their  mode  of  action,  their  origin  from  preexisting  substances, 
the  methods  by  which  they  are  made  active,  the  conditions  under  which 
they  are  most  active,  etc.,  will  be  mentioned  in  connection  with  a  considera- 
tion of  the  fluids  and  tissues  in  which  they  are  present. 

MOUTH  DIGESTION. 

The  digestion  of  the  food  as  it  takes  place  in  the  mouth  comprises 
a  series  of  physical  and  chemic  changes,  the  result  of  the  action  of  the 
teeth,  the  tongue,  and  the  saliva.  The  mechanic  division  of  the  food 
and  the  incorporation  of  the  saliva  with  it  are  termed  respectively  mastication 
and  insalivation. 

MASTICATION. 

Mastication  is  the  mechanic  division  of  the  food,  and  is  accomplished 
by  the  teeth  and  the  movements  of  the  lower  jaw  under  the  influence  of 
muscle  contractions.  Complete  mechanic  disintegration  of  the  food  is 
important  for  its  subsequent  solution  and  chemic  transformation;  for  when 
finely  divided  it  presents  a  larger  surface  to  the  action  of  the  digestive 
fluids  and  thus  enables  them  to  exert  their  respective  actions  more  effectively 
and  in  a  shorter  period  of  time. 

The  Teeth. — In  man  passing  from  childhood  to  adult  life  two  sets 
of  teeth  make  their  appearance.  The  first  set  constitute  the  temporary, 
deciduous,  or  milk  teeth;  the  second  set  constitute  the  permanent  teeth, 
which  should  last  with  proper  care  through  life  or  to  an  advanced  age. 

The  temporary  teeth,  twenty  in  number,  ten  in  each  jaw,  though  smaller 
than  the  permanent  teeth,  have  the  same  general  conformation.  They 
are  divided  into  four  incisors,  two  cuspids  or  canines,  and  four  molars  for 
each  jaw. 

The  permanent  teeth,  thirty-two  in  number,  sixteen  in  each  jaw,  are 
divided  into  four  incisors,  two  cuspids  or  canines,  four  bicuspids  or  pre- 
molars, and  six  molars  for  each  jaw. 

Each  tooth  may  be  said  to  consist  of  three  portions:  (i)  the  crown, 
or  that  portion  which  projects  above  the  gums;  (2)  the  root  or  fang,  that 

ucts  of  the  reaction  begin  to  recombine  with  the  formation  of  the  original  compounds  until  the 
opposing  tendencies  are  in  equilibrium,  a  state  in  which  they  remain  so  long  as  the  conditions 
remain  unchanged.  Again,  if  maltose  and  water  be  mixed,  a  reaction  occurs  which  eventuates  in 
the  formation  of  dextrose.  In  time  the  dextrose  molecules  combine  to  form  maltose  and  water 
until  a  condition  of  equilibrium  is  established.  If  the  yeast  enzyme  be  added  the  reactions  both 
analytic  and  synthetic  are  increased  in  velocity.  The  enzyme,  however,  does  not  initiate,  but 
merely  hastens  a  reaction  aheady  taking  place. 


DIGESTION. 


139 


portion  embedded  in  the  alveolar  socket;  (3)  the  constricted  portion  or  neck, 
which  is  surrounded  by  the  free  margin  of  the  gum.  The  teeth  are  firmly 
secured  in  their  sockets  by  a  fibrous  membrane,  the  peridental  membrane, 
which  is  attached,  on  the  one  hand,  to  the  alveolar  process,  and,  on  the  other, 
to  the  cementum. 

A  vertical  section  of  a  tooth  shows  that  it  consists  of  three  distinct 
solid  structures,  the  enamel,  the  dentine,  and  the  cementum,  which  have 
the  anatomic  relationship  represented  in  Fig.  61.  In  the  center  of  j^ the 
dentine  there  is  a  cavity  the  general  shape 
of  which  varies  in  different  teeth,  and 
which  is  occupied  during  the  living  condi- 
tion by  the  tooth  pulp. 

Microscopic  examination  of' the  tooth 
reveals  the  presence  of  irregular  stellate 
spaces,  the  interglobular  spaces,  between 
the  dentine  and  the  cementum,  which  are 
occupied  by  connective-tissue  cells.  Clefts 
of  varying  size  are  also  observed  at  the 
junction  of  the  dentine  and  the  enamel, 
and  extending  for  some  distance  into  the 
latter. 

The  enamel  is  composed  of  dense  hard 
cylinders  which,  on  account  of  their  small 
size  and  close  relationship,  appear  to  be 
hexagonal  in  shape.  These  cylinders  are 
held  together  by  cement  substance.  The 
free  border  of  the  enamel  is  covered  by  a 
thin  membrane  known  as  the  cuticle  or 
membrane  of  Nasmyth. 

The  dentine  is  somewhat  less  dense  than 
the  enamel.  It  is  composed  of  connective- 
tissue  fibers  embedded  in  a  ground-sub- 
stance, both  of  which  have  undergone 
calcification  in  the  course  of  development. 
The  dentine  is  penetrated  by  a  series  of  fine 
canals,  the  dentine  canals  or  tubules,  which 
begin  by  open  mouths  on  the  pulp  side.  From  this  point  the  tubules  pass 
outward  to  the  cementum  and  enamel,  where  their  terminal  branches  com- 
municate with  and  terminate  in  the  interglobular  spaces  and  clefts.  In 
their  course  the  tubules  give  off  a  series  of  branches  which  communicate 
freely  with  one  another.  The  dentine  bordering  the  tubule  is  somewhat 
more  dense  than  the  intertubular  portion  and  constitutes  what  is  known  as 
the  dentinal  sheath  or  Neumann's  sheath. 

The  cementum  resembles  bone  because  it  contains  both  lacunae  and 
canaliculi,  though  it  is,  as  a  rule,  devoid  of  Haversian  canals. 

The  pulp  consists  of  a  framework  of  connective  tissue  which  affords 
support  for  blood-vessels  and  nerves,  both  of  which  enter  the  pulp  chamber 
through  a  small  foramen  at  the  apex  of  the  root.  The  outer  surface  of 
the  pulp  is  covered  with  a  layer  of  large  spheric  cells,  the  odontoblasts. 


Fi(  61, — \naic\i  SrcTiON  01 
Tool  11  IN  Jaw.  L  Lnamcl.  D. 
Dentine.  P.  M.  Periodontal  mem- 
brane. P.C.  Pulp  cavity.  C.  Ce- 
ment. B.  Bone  of  lower  jaw.  V. 
Vein.  a.  Arter>'.  N.  Nerve. — {Stir- 
ling.) 


I40  TEXT-BOOK  OF  PHYSIOLOGY. 

Each  cell  presents  on  its  inner  surface  short  processes  which  pass  into  the 
pulp;  on  its  outer  surface  it  presents  a  long  process  which  enters  a  dentine 
tubule  and  extends  as  far  as  its  ultimate  terminations.  Collectively  these 
processes  are  known  as  the  dentine  fibers.  Inasmuch  as  the  fibers  do  not 
completely  occupy  the  lumen  of  the  tubule,  it  is  probable  that  there  is  a 
free  circulation  of  lymph  from  the  pulp  chamber  through  the  dentine  tubules 
into  the  enamel  clefts,  into  the  interglobular  spaces,  and  possibly  into  the 
lacunae  of  the  cementum. 

The  peridental  membrane  is  composed  of  connective-tissue  fibers  abun- 
dantly supplied  with  blood-vessels  and  nerves. 

Movements  of  the  Lower  Jaw. — The  lower  jaw  is  capable  of  a  down- 
ward and  upward,  an  antero-posterior,  and  a  lateral  movement,  all  depend- 
ent on  the  peculiar  construction  of  the  joint. 

Temporo-maxillary  Articulation. — -This  articulation  is  formed  by  the 
anterior  portion  of  the  glenoid  cavity,  the  eminentia  articularis,  and  the 
condyle  of  the  inferior  maxilla,  all  of  which  are  united  by  means  of  liga- 
ments. Situated  between  the  glenoid  cavity  and  the  condyle  is  a  plate  of 
fibro-cartilage  oval  in  shape  and  biconcave.  This  cartilage  divides  the 
joint  into  two  cavities — one  above,  the  other  below^ — each  of  which  is  pro- 
vided with  a  synovial  membrane.  The  function  of  the  cartilage  is  to  present 
constantly  an  articulating  surface  to  the  condyle  in  the  various  movements 
of  the  lower  jaw,  which  it  is  enabled  to  do  by  virtue  of  its  mobility. 

In  the  downward  movement  of  the  lower  jaw  each  condyle  glides  for- 
ward, carrying  with  it  the  interarticular  fibro-cartilage,  the  upper  concave 
surface  of  which  is  applied  to  the  convex  surface  of  the  eminentia  articularis. 
In  the  upward  movement  of  the  jaw  both  the  condyles  and  the  cartilages  pass 
backward  and  resume  their  normal  position.  The  movements  of  depres- 
sion and  elevation  are  made  possible  by  the  slightly  oblique  direction  of  the 
condyle.  In  the  carnivorous  animals,  whose  food  requires  considerable 
cutting,  these  movements  are  especially  well  developed.  In  these  animals 
the  condyles  are  transversely  arranged  and  at  right  angles  to  the  long  axis 
of  the  jaw.  In  the  antero-posterior  movement  the  jaw  moves  in  a  hori- 
zontal direction  and  the  condyles  and  the  articular  cartilages  glide  forward 
and  backward  in  the  glenoid  fossae.  In  the  rodent  animals  the  long  axis 
of  the  condyle  runs  in  the  antero-posterior  direction,  which  allows  of  a 
considerable  gliding  movement.  When  the  jaw  performs  a  lateral  movement, 
the  condyle  and  cartilage  of  one  side  may  remain  in  their  natural  position 
while  the  opposite  condyle  and  cartilage  glide  forward  in  the  glenoid  fossa, 
directing  the  symphysis  of  the  jaw  to  the  opposite  side  of  the  median  line. 
The  lateral  movements  are  well  exhibited  by  the  herbivorous  animals,  in 
which  they  are  quite  extensive,  and  made  possible  by  the  small  size  of  the 
condyle  and  the  large  extent  of  articulating  surface.  In  man  the  structure 
of  the  joint  is  such  as  to  admit  of  all  these  possibilities,  and  the  lower  jaw 
acquires  in  consequence  great  freedom  of  movement. 

The  Functions  of  the  Muscles  of  Mastication. — The  movements  of 
the  lower  jaw  are  caused  by  the  action  of  numerous  muscles,  which,  having 
fixed  points  of  origin,  are  attached  to  various  points  on  its  surface.  The 
muscles  concerned  in  the  movements  of  mastication  are  presented  in  the 
following  table: 


DIGESTION. 


141 


Anterior  belly  of  digastric 

Mylohyoid 

Geniohyoid 

Temporal 

Internal  portion  of  masseter 

Internal  pterygoids 

External  pterygoids 

External  portion  of  masseter 

Anterior  fibers  of  temporal 

Posterior  fibers  of  temporal 

Internal  portion  of  masseter 

Digastric,  mylohyoid,  and  geniohyoid 

Internal  pterj-goids 

External  pterygoids 

Pterygoids,  external  and  internal 

Temporal 

Masseter 


Depress  the  lower  jaw  and  open  the 
mouth. 

Elevate  the  lower  jaw  and  close  the 
mouth. 

Draw  the  lower  jaw  forward  and  cause 
the  lower  teeth  to  project  beyond 
the  upper. 

Draw  the  lower  jaw  back  to  its  normal 
position. 

\  Contracting  alternately,  draw  the  jaw 
/       to  the  opposite  side. 

Produce  grinding  movements  of  the 
lower  jaw. 


The  action  of  the  depressor  muscles  becomes  apparent  when  their  points 
of  origin  and  insertion  are  considered.  The  anterior  belly  of  the  digastric, 
the  mylohyoid,  and  the  geniohyoid  muscles,  agree  in  having  a  similarity  of 
origin — the  hyoid  bone — and  a  common  area  of  insertion,  the  anterior 
portion  of  the  lower  jaw.  Their  anatomic  relation  is  such  that  their 
combined  action  will  depress  the  lower  jaw  and  open  the  mouth. 

The  action  of  the  elevator  muscles  becomes  apparent  when  their  points 
of  origin  and  insertion  are  considered.  The  elevator  muscles  arise  from 
various  points  on  the  side  of  the  head,  and  are  inserted  into  the  coronoid 
process,  ramus,  and  internal  surface  of  the  angle  of  the  lower  jaw.  After  the 
mouth  has  been  opened,  the  simultaneous  contraction  of  these  muscles  will 
elevate  the  jaw  and  closes  the  mouth  with  considerable  force.  The  power 
of  these  muscles,  which  is  very  great,  depends  on  the  shortness  and 
thickness  of  the  muscle-bundles. 

The  action  of  the  rotator  muscles,  the  external  and  internal  pterygoids, 
those  which  give  rise  to  the  lateral  movements  of  the  jaw,  depends  in  like 
manner  on  their  origin  and  insertion.  The  first  arises  from  the  outer 
surface  of  the  external  pterygoid  plate  and  the  great  wing  of  the  sphenoid 
bone;  the  second  arises  mainly  from  the  inner  surface  of  the  external  ptery- 
goid plate;  they  are  inserted  into  the  neck  of  the  condyle  and  angle  of  the 
lower  jaw  respectively.  WTien  they  contract,  the  condyle  on  the  correspond- 
ing side  is  drawn  forward,  while  the  opposite  condyle  remains  stationary. 
As  a  result,  the  symphysis  of  the  jaw  is  directed  to  the  opposite  side.  The 
grinding  movements  of  the  jaw  are  produced  by  the  coordinated  action  of 
all  the  groups  of  muscles  acting  more  or  less  successively. 

For  the  proper  mastication  of  the  food  it  is  essential  that  it  be  kept 
between  the  opposing  surfaces  of  the  teeth.  This  is  accomplished  by  the 
contraction  of  the  orbicularis  oris  and  buccinator  muscles  from  without  and 
the  tongue  muscles  from  within. 

The  Nerve  Mechanism^  of  Mastication. — Mastication  is  a  complex 
act  and  involves  the  cooperation  of  a  number  of  muscles,  afferent  and  efferent 
nerves,  and  a  central  mechanism  by  which  they  are  excited  to,  and  coordi- 
nated in  their  activity.  The  central  mechanism  is  located  in  the  medulla 
oblongata  in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle. 


^  By  this  term  is  meant  a  combination  of  nerves,  afferent  and  efferent,  and  nerve  centers 
which  when  excited  to  action  coordinates  the  actions  of  the  organs  with  which  it  is  associated. 


142  TEXT-BOOK  OF  PHYSIOLOGY. 

During  the  intervals  of  mouth  digestion  the  mouth  is  closed  by  the  con- 
traction of  the  elevator  muscles  of  the  lower  jaw.  When  the  occasion  arises 
for  the  introduction  of  food,  the  mouth  is  opened  by  the  depressor  muscles; 
after  the  food  is  introduced  into  the  mouth  it  is  again  closed  and  that 
combination  of  muscle  contractions  initiated  which  when  continued  results 
in  the  mechanical  division  of  the  food. 

The  nerves  and  nerve  centers  constituting  the  nerve  mechanism  for 
mastication  are  shown  in  the  following  table: 

Afferent  Nerves.  Nerve-center.  Efferent  Nerves. 

1.  Lingual  and  buccal  branches  of  Medulla  oblongata.  i.  Small   root  of   the   trigeminal 
the  trigeminal  nerve.  nerve. 

2.  Glosso-pharyngeal.  2.  Hypoglossal. 

3.   Facial  or  portio  dura. 

The  Efferent  Nerves. — The  efferent  nerves  that  transmit  nerve  im- 
pulses to  the  various  muscles  of  mastication  are  the  small  root  of  the  tri- 
geminal, the  hypoglossal,  and  the  facial. 

The  small  root  of  the  trigeminal  nerve  after  emerging  from  the  cavity 
•of  the  cranium  through  the  foramen  ovale  joins  the  inferior  maxillary  divi- 
sion of  the  large  sensor  root.  After  a  short  course  the  efferent  fibers  sepa- 
rate into  two  groups,  an  upper  and  a  lower;  the  upper  group  is  distributed  to 
the  masseter,  temporal,  internal  and  external  pterygoid  muscles,  the  lower 
group  is  distributed  to  the  mylohyoid  and  anterior  belly  of  the  digastric 
muscles.  The  hypoglossal  nerve,  after  emerging  from  the  cranium  through 
the  anterior  condyloid  foramen,  passes  downward  and  forward  to  be  dis- 
tributed to  the  intrinsic  and  extrinsic  muscles  of  the  tongue.  The  facial 
or  portio  dura  after  emerging  from  the  stylo-mastoid  foramen  is  distributed 
to  the  superficial  muscles  of  the  face. 

Stimulation  of  any  one  of  these  nerves  with  induced  electric  currents 
gives  rise  to  convulsive  movements  in  the  muscles  to  which  it  is  distributed 
while  its  division  is  followed  by  paralysis  of  the  muscles. 

The  Central  Mechanism. — The  central  mechanism  that  excites  and 
coordinates  the  action  of  the  nerve-cells  from  which  these  nerves  emerge,  may 
be  excited  to  activity  (i)  by  nerve  impulses  descending  from  the  cerebrum  as 
a  result  of  volitional  efforts;  and  (2)  by  nerve  impulses  transmitted  through 
afferent  nerves  from  the  mouth.  Though  movements  of  mastication  are  pri- 
marily volitional  and  may  so  continue,  nevertheless  when  once  initiated  they 
continue  for  an  indefinite  period,  so  long  in  fact  as  the  nerve  impulses  which 
the  food  develops  in  afferent  ner\^es  are  received  by  the  central  mechanism, 
thus  falling  into  the  category  of  secondary  or  acquired  reflex  acts.  That 
the  masticatory  movements  are  of  this  reflex  character  is  indicated  by  the 
fact  that  they  will  be  maintained,  even  though  the  volitional  effort  that 
called  them  forth  has  subsided  and  the  attention  has  been  directed  to  some 
entirely  different  subject.  It  would  appear  that  all  that  is  necessary  under 
such  circumstances  is  the  stimulating  action  of  the  food  upon  the  peripheral 
terminations  of  the  afferent  nerves  distributed  to  the  mucous  membrane 
of  the  tongue  and  mouth. 

The  Afferent  Nerves. — The  afferent  nerves,  stimulation  of  which 
excites  the  central  mechanism,  are  the  lingual  and  buccal  branches  of  the 
superior  and  inferior  maxillary  divisions  of  the  trigeminal  nerve,  the  lingual 


DIGESTION. 


143 


branches  of  the  glosso-pharyngeal,  and  in  all  probability  the  gustatory  fibers 
of  the  chorda  tympani.  The  introduction  of  food  into  the  mouth  develops 
in  the  peripheral  terminations  of  these  nerves,  by  reason  of  its  physical  and 
chemic  properties,  nerve  impulses  which  are  then  transmitted  to  the  central 
mechanism.  If  these  nerves  are  divided,  mastication  is  seriously  impaired. 
When  divided  and  their  central  ends  stimulated  with  induced  electric 
currents,  the  muscle  will  reflexly  be  thrown  into  contraction. 


^ 


Intercalated 
pieces. 


INSALIVATION. 

Insalivation  is  the  incorporation  of  the  saliva  with  the  food,  and  takes 
place  for  the  most  part  during  mastication.  The  saliva  ordinarily  present 
in  the  mouth  is  a  complex  fluid  composed 
of  the  various  secretions  of  the  parotid, 
submaxillary,  and  sublingual  glands  and 
the  muciparous  follicles  of  the  mouth,  which 
collectively  constitute  the  salivary  ap- 
paratus. 

The  parotid  gland  is  situated  in  front 
of  and  partly  below  the  external  ear,  where 
it  is  held  in  position  by  the  fascia  and  skin. 
From  the  anterior  border  of  the  gland  there 
emerges  a  duct  (Stenson's),  which,  after 
crossing  the  masseter  muscle  to  its  anterior 
border,  turns  inward,  pierces  the  buccin- 
ator muscle  and  opens  on  the  surface  of 
the  cheek  opposite  the  second  upper  molar 
tooth. 

The    submaxillary  gland  is  situated 
below  the  jaw  in  the  anterior  part  of  the 
submaxillary    triangle.     From    the    gland 
there  emerges  a  duct  (Wharton's)  which  opens  into  the  mouth  by  a  minute 
orifice  on  the  surface  of  a  papilla  by  the  side  of  the  tongue. 

The  sublingual  gland  is  situated  just  beneath  the  mucous  membrane 
in  the  anterior  part  of  the  mouth,  where  it  forms  a  projection  between  the 
gums  and  tongue.  The  posterior  part  of  the  gland  gives  origin  to  a  duct 
(the  duct  of  Rivinus,  described  also  by  Bartholin)  which  opens  into  the 
mouth  with  or  very  near  to  the  duct  of  Wharton.  The  anterior  part  of  the 
gland  gives  origin  to  a  varying  number  of  ducts  (the  ducts  of  Walther) 
which  open  separately  along  the  edge  of  the  sublingual  plica  of  the  mucous 
membrane. 

Histologic  Structure  of  the  Salivary  Glands. — In  their  ultimate 
structure  the  salivary  glands  bear  a  close  resemblance  to  one  another. 
They  are  compound  tubulo-alveolar  glands  composed  of  small  irregularly 
shaped  lobules  united  by  areolar  tissue,  and  connected  by  branches  of  the 
salivary  ducts.  Each  lobule  is  made  up  of  a  number  of  small  alveoli  or 
acini  more  or  less  tubular  in  shape  which  are  the  terminal  expansions  of  the 
smallest  ducts.  (See  Fig.  62).  The  wall  of  the  acinus  is  formed  by  a 
reticulated  basement  membrane,  surrounded  externally  by  blood-vessels, 


Fig.  62. — Scheme  of  the  Human 
Submaxillary  Gland. — {Stdhr.) 


144 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  spaces  between  which  constitute  lymph-spaces  or  channels.  The  inner 
surface  of  the  acinus  membrane  supports  a  single  layer  of  irregular  spheric 
or  polygonal  epithelial  cells.  The  cells  do  not  entirely  fill  up  the  cavity 
of  the  acinus,  but  leave  an  intercellular  space,  the  lumen,  which  constitutes 
the  beginning  of  the  duct  for  the  transmission  of  the  secretion  to  the  mouth. 
From  each  acinus  there  passes  a  narrow  intercalary  duct  lined  by  a  layer  of 
flattened  cells.  The  common  excretory  duct — formed  by  the  union  of  the 
intralobular  and  interlobular  ducts — consists  also  of  a  basement  membrane, 
lined,  however,  by  tall  columnar  epithelial  cells.  The  salivary  glands  are 
abundantly  supplied  with  blood-vessels  and  nerves  which  are  closely  related 
to  their  activity. 

Based  partly  on  the  character  of  their  secretions  and  partly  on  the  micro- 
scopic appearance  of  their  secreting  cells,  the  salivary  glands  have  been 
divided  by  Heidenhain  into  two  classes:  viz.,  serous  or  albuminous,  and 
mucous  glands.     To  the  first  class  belong  the  parotid,  a  portion  of  the  sub- 


3  - 

Z "9y 

1 

3    - 


Fig.  63. — Parotid  Gland  AT  Rest.  1,1, 
Acini;  2,  duct;  3,3,  albuminous  cells  filled 
with  ;fine  granules;  4,4,  nuclei  almost  con- 
cealed.    (Semi-diagrammatic.) 


/ 

/-- 

-f 

3  - 

1 

z- 

3 

t  ■ 

^^'T'^'^^sr:}^ 


4- 


■5 


Fig.  64.^Sxjb  maxillary  Gland  at 
Rest.  1,1,  Acini;  2,  duct;  3,  3,  mucous  cells 
containing  mucin;  4,4,  nuclei,  flattened  and 
dispersed  toward  the  base  of  the  cells;  5,5, 
crescents  of  Giannuzzi. — {After  Vialleton.) 


maxillary,  and  a  portion  of  the  glands  of  the  tongue.  To  the  second  class 
belong  a  portion  of  the  submaxillary  gland,  the  sublingual,  a  portion  of 
the  glands  of  the  tongue,  the  glands  of  the  cheeks,  lips,  palate,  and  pharynx. 
It  is  possible  that  a  single  alveolus  of  any  gland  may  contain  both  albu- 
minous and  mucous  cells. 

In  the  serous  glands  the  cells  are  more  or  less  spheric  in  shape,  nucleated, 
and  almost  completely  filled  with  dark  granular  material  (Fig.  63).  In  the 
mucous  glands  the  cells  are  large,  clear  in  appearance,  and  loaded  with  a 
highly  refracting  material  resembling  mucin  (Fig.  64).  Between  the  base- 
ment membrane  and  the  clear  cells  are  to  be  found  in  the  acini  of  the  sub- 
maxillary glands  small  crescentic  shaped  cells  filled  with  granular  material 
which  stains  deeply  with  various  coloring  matters.  These  are  known  as 
the  demilunes  of  Heidenhain.  At  one  time  it  was  supposed  that  they  were 
young  cells  destined  to  take  the  place  of  the  clear  cells  which  were  believed 
to  be  exhausted  and  to  have  undergone  disintegration.  At  the  present  time 
they  are  regarded  as  albuminous  or  serous  cells  which  exhibit  changes 
similar  to  the  cells  of  the  parotid  gland. 


DIGESTION.  145 

The  glands  embedded  in  the  mucous  membrane  covering  the  tongue, 
lips,  cheek,  palate,  and  pharynx  are  for  the  most  part  lined  with  epithelial 
cells  which  contain  a  highly  refracting  matter  similar  to,  if  not  identical  with, 
that  found  in  the  cells  of  the  submaxillary  and  sublingual  glands. 

Nerve-supply. — Histologic  investigation  has  demonstrated  that  the 
cells  and  blood-vessels  of  the  salivary  glands  are  supplied  with  nerv^e-fibers 
directly  from  ganglion  cells  situated  in  their  immediate  neighborhood. 
Thus  the  cells  and  blood-vessels  of  the  submaxillary  and  sublingual  glands, 
receive  nerve-fibers  from  the  submaxillary,  sublingual  and  superior  cervical 
ganglia,  while  the  cells  and  blood-vessels  of  the  parotid  gland  receive  nerve- 
fibers  from  the  otic  and  the  superior  cervical  ganglia.  From  their  ultimate 
distribution  it  may  be  inferred  that  some  of  the  ganglion  cells  and  fibers 
influence  the  production  of  the  secretions  (secretor  nerves),  while  others 
influence  the  caliber  of  the  blood-vessels  causing  either  constriction  or  dilata- 
tion (vaso-constrictor  and  vaso-dilatator  nerves).  (Fig.  67.)  The  secretor 
fibers  penetrate  the  basement  membrane  enclosing  the  gland  acinus  and 
finally  terminate  between  and  on  the  surface  of  the  secretor  cells.  The 
vaso-motor  fibers  terminate  between  and  on  the  muscle  cells  in  the  walls  of 
the  blood-vessels. 

The  local  ganglion  cells,  howevet,  are  in  anatomic  relation  with  nerve- 
trunks  coming  directly  from  the  medulla  oblongata  and  the  spinal  cord. 
As  they  enter  the  ganglia,  their  terminal  branches  arborize  around  and 
closely  invest  the  cells  of  the  ganglia  and  come  into  intimate  histologic  and 
physiologic  connection  with  them.  The  nerve-fibers  coming  from  the 
central  nerve  system  are  known  as  pre-ganglionic  fibers,  while  those  coming 
from  the  ganglia  are  known  as  post-ganglionic  fibers.  Through  the  inter- 
mediation, therefore,  of  the  ganglion  cells,  the  secretor  cells  of  the  salivary 
glands  and  the  blood-vessels  surrounding  them  are  brought  into  relation 
with  the  central  organs  of  the  nen^e  system  and  become  susceptible  of  being 
influenced  by  them. 

The  Parotid  Saliva. — The  parotid  saliva,  as  it  flows  from  the  orifice  of 
Stenson's  duct,  is  clear,  limpid,  free  from  viscidity,  distinctly  alkaline  in 
reaction,  with  a  specific  gravity  of  1.003.  Chemic  analysis  shows  that  it 
consists  of  water,  a  small  quantity  of  protein  matter,  a  trace  of  a  sulpho- 
cyanogen  compound,  and  inorganic  salts.  The  secretion  is  increased  during 
mastication,  and  especially  on  the  side  engaged  in  mastication.  Dry  food 
causes  a  larger  flow  than  moist  food.  The  situation  of  the  orifice  of  the 
parotid  duct  is  such  that  as  the  secretion  is  poured  into  the  mouth  it  is  at 
once  incorporated  with  the  food  by  the  movements  of  the  lower  jaw,  and 
thus  fulfils  the  physical  function  of  softening  and  moistening  it. 

The  Submaxillary  Saliva. — The  submaxillary  saliva  is  clear,  slightly 
viscid,  alkaline  in  reaction,  and  has  a  specific  gravity  of  1.002.  It  consists 
of  water,  protein  matter  (mucin),  and  inorganic  salts. 

The  Sublingual  Saliva. — The  sublingual  saliva  is  clear,  extremely 
viscid,  and  strongly  alkaline  in  reaction.  It  consists  of  water,  protein 
matter  (chiefly  mucin),  and  inorganic  salts. 

The  small  racemose  glands  embedded  in  the  mucous  membrane  on 
the  inner  surface  of  the  cheeks  and  lips,  on  the  hard  and  soft  palate,  and  on 


146  TEXT-BOOK  OF  PHYSIOLOGY. 

the  tongue  and  pharynx,  secrete  a  fluid  which  is  grayish  in  color,  and 
extremely  viscid  and  ropy.     It  contains  a  large  amount  of  mucin. 

Mixed  Saliva. — The  saliva  of  the  mouth  is  a  complex  fluid  composed 
of  the  secretions  of  all  the  salivary  glands.  As  obtained  from  the  mouth 
it  is  frothy,  opalescent,  slightly  turbid,  and  somewhat  viscid.  The  specific 
gravity  is  low.  ranging  from  i.ooo  to  1.006.  The  reaction  is  usually  distinctly 
alkaline.  It  may,  however,  be  neutral  or  even  acid  in  reaction  if  there  is 
any  fermentation  of  food  particles  in  the  mouth  or  in  certain  disorders  of  the 
alimentary  canal.  When  examined  with  the  microscope,  the  saliva  is  seen 
to  contain  epithelial  cells,  salivary  corpuscles  resembling  leukocytes,  particles 
of  food,  and  various  microorganisms,  especiaWy  Leptolhrix  buccalis. 

The  chemic  composition  of  the  saliva  is  shown  in  the  following  table: 

COMPOSITION  OF  HUMAN  SALIVA. 

Water 995 .  16  994 .20 

Epithelium i  .62  2 .20 

Soluble  organic  matter i  .34  i  .40 

Potassium  sulphocyanid o  .06  o  .04 

Inorganic  salts i .  82  2.20 

1000.00  1000.04 

(Jacubowitsch.)     (Hammerbacher.) 

Water  constitutes  the  main  ingredient,  amounting  to  99.5  per  cent, 
the  soluble  organic  matter  is  protein  in  character  and  is  a  mixture  of 
mucin,  globulin,  and  serum-albumin.  The  potassium  sulphocyanid  is 
mainly  derived  from  the  parotid  gland.  Its  presence  can  be  demon- 
strated by  the  addition  of  a  few  minims  of  a  dilute  solution  of  slightly 
acidulated  ferric  chlorid,  when  a  characteristic  red  color  is  developed. 
The  inorganic  constituents  comprise  the  sodium,  calcium,  and  magnesium, 
phosphates,  sodium  carbonate,  and  sodium  and  potassium  chlorids. 

The  relative  amounts  of  the  different  constituents  of  the  saliva  will  depend 
on  the  relative  degree  of  activity  of  the  different  glands,  and  this  in  turn  will 
be  determined  by  the  character  of  the  food.  When  the  food  is  dry,  there 
will  be  an  excess  of  the  parotid  secretion;  when  it  partakes  of  the  consistence 
of  meat,  there  wdll  be  a  larger  secretion  of  the  submaxillary  saliva.  The 
glands  apparently  adapt  their  activity  to  the  character  of  the  food. 

Quantity  of  Saliva. — The  estimation  of  the  total  quantity  of  mixed 
saliva  secreted  in  twenty-four  hours  is  exceedingly  difficult,  and  the  results 
obtained  must  be  only  approximative.  It  is,  of  course,  subject  to  consider- 
able variation,  depending  upon  habit,  the  nature  of  the  food,  etc.  The 
experiments  of  Professor  Dalton  and  the  results  obtained  by  him  are  emi- 
nently trustw^orthy,  and  in  all  probability  represent  as  nearly  as  possible  the 
exact  amount  secreted.  He  found  that  without  any  artificial  stimulus  he 
was  enabled  to  collect  from  the  mouth  about  36  grams  (540  grains)  of  saliva 
per  hour,  but  that  upon  the  introduction  of  any  stimulating  substance  into 
the  mouth  the  quantity  could  be  greatly  increased.  During  mastication  the 
saliva  was  poured  out  in  greater  abundance,  the  amount  depending  upon 
the  relative  dryness  of  the  food.  He  found  that  wheat  bread  absorbed 
55  per  cent,  of  its  weight,  and  fresh  cooked  meat  48  per  cent.  If,  therefore, 
the  average  quantity  of  bread  and  meat  required  daily  by  a  man  of  ordinary 


DIGESTION. 


147 


physical  development  and  activity  be  assumed  to  be  540  grams  (19  oz.)  of 
the  former  and  450  grams  (16  oz.)  of  the  latter,  these  two  substances  would 
absorb  respectively  297  grams  (4573.8  grains)  and  216  grams  (3326.4  grains), 
making  a  total  of  513  grams  (7900  grains).  If,  therefore,  the  amount 
secreted  and  mixed  ^vith  the  food  during  an  estimated  two  hours  of  mastica- 
tion be  added  to  the  amount  secreted  during  the  remaining  twenty-two  hours, 
supposing  that  it  continues  at  the  rate  of  36  grams  per  hour,  we  have  a  total 
amount  of  513+792  grams,  or  1305  grams  (19,780  grains),  or  about  2.8 
pounds. 

Histologic  Changes  in  the  Salivary  Glands  during  Secretion. — 
During  and  after  secretion  very  remarkable  changes  take  place  in  the  cells 
lining  the  acini,  which  are  in  some  way  connected  with  the  production  of  the 
essential  constituents  of  the  salivary  fluids.  In  the  case  of  the  parotid  gland, 
which  may  be  regarded  as  the  type  of  a  serous  or  albuminous  gland,  the 
following  changes  have  been  observed  by  Langley  (Fig.  65).     During  the 


3 /'^  \]y^  ^\\\ 

'#?©  ^/ 

/.. \  t^a(®  W\®1         1 

3 ^^^UJl/'; 

■^ 

Fig.  65.^Parotid  Gland  After  Pro- 
longed Activity.  1,1,  Acini;  2,  duct;  t,,t„ 
albuminous  cells  almost  free  of  granules;  4, 
nuclei  clear  and  well  defined.  (Semi-diagram- 
matic.) 


Fig.  66. — Sl'bmaxillary  Gland  After 
Prolonged  Activity.  1,1,  Acini;  2,  duct; 
3,3,  mucous  cells  free  of  mucin  and  filled 
with  fine  granules;  4,4,  nuclei  rounded  and 
returned  to  the  center  of  the  cell;  5,5,  cells  of 
Guiannuzzi,  large  and  distinct.  {After 
Vialleton.) 


period  of  rest  and  just  previous  to  secretor  activity,  the  epithelial  cells  are 
enlarged  and  swollen,  and  encroach  on  the  lumen  of  the  acinus.  The 
protoplasm  of  the  cells  is  so  completely  filled  with  dark  fine  granules  as  not 
only  to  obscure  the  nucleus,  but  almost  to  obliterate  the  line  of  union  of  the 
cells.  As  soon  as  secretion  becomes  active,  however,  the  granules  begin  to 
disappear  from  the  outer  region  of  the  cell  and  move  toward  the  inner  border 
and  into  the  lumen  of  the  acinus.  From  these  observations  it  might  be 
inferred  that  during  rest  the  protoplasm  of  the  cells  gives  rise  to  granular 
material,  and  that  during  and  after  secretor  activity  there  is  an  absorption 
of  new  material  from  the  lymph  and  a  reconstruction  of  the  granular  material. 
In  the  submaxillary  gland,  a  portion  of  which  may  be  taken  as  a  type  of  a 
mucous  gland,  similar  changes  have  been  observ^ed  (Fig.  66).  During  rest 
the  epithelial  cells  are  large,  clear  in  appearance,  highly  refractive,  and  loaded 
with  small  globules  resembling  mucin.  The  nucleus,  surrounded  by  a  small 
quantity  of  protoplasm,  lies  near  the  margin  of  the  cell.  That  the  granules 
are  not  protoplasmic  in  character  is  shown  by  the  fact  that  they  do  not  stain 


148  TEXT-BOOK  OF  PHYSIOLOGY. 

on  the  addition  of  carmine.  When  treated  with  water  or  dilute  acids,  the 
globules  swell  up,  coalesce,  and  form  a  uniform  mass.  The  chemic  relations 
of  this  substance  indicate  that  it  is  the  precursor  of  mucin — namely,  mucigen. 
During  secretor  activity  the  cells  discharge  these  mucigen  granules  into  the 
lumen  of  the  acinus  where  they  are  transformed  into  mucin.  Though  the 
appearance  of  the  gland-cell  indicates  it,  there  is  no  evidence  for  the  view 
that  the  cell  itself  undergoes  disintegration  in  the  process. 

The  Physiologic  Actions  of  Saliva. — The  constant  presence  of  salivary 
glands  in  the  different  classes  of  animals  and  the  large  amount  of  secretion 
they  pour  daily  into  the  alimentary  canal  point  to  the  conclusion  that  this 
mixed  fluid  plays  an  important  role  in  the  general  digestive  process.  Experi- 
ment has  demonstrated  that  it  has  a  two-fold  action;  viz.,  physical  and 
chemical. 

Physically,  saliva  softens  and  moistens  the  food,  unites  its  particles  into 
a  consistent  mass  by  means  of  its  contained  mucin,  and  thus  facilitates 
swallowing. 

Chemically  it  converts  starch  into  sugar.  This  action  is  more  marked 
with  boiled  than  with  raw  starch,  a  fact  which  depends  on  the  physical 
structure  of  the  starch  grain.  In  the  natural  condition  each  starch  grain  con- 
sists of  a  cellulose  envelope  or  stroma  in  the  meshes  of  which  is  contained  the 
true  starch  material,  the  granulose.  When  boiled  for  some  minutes,  the 
starch  grain  absorbs  water,  the  granulose  swells  and  ruptures  the  cellulose 
envelope,  after  which  it  passes  into  an  imperfect  opalescent  solution  more  or 
less  viscid,  depending  on  the  relative  amounts  of  water  and  starch.  This  is 
the  change  largely  brought  about  by  the  process  of  cooking.  If  a  portion  of 
this  hydrated  starch  be  kept  in  the  mouth  for  a  few  minutes  it  will  be  con- 
verted into  sugar,  a  fact  made  evident  by  the  sense  of  taste. 

The  chemic  action  of  saliva  in  converting  starch  into  sugar,  as  well  as  the 
intermediate  stages,  can  be  experimentally  shown  in  the  following  manner: 
To  5  volumes  of  a  thin  starch  solution  in  a  test-tube  add  two  volumes  of 
j&ltered  saliva.  Place  the  mixture  in  a  water-bath  at  a  temperature  of  35°  C. 
In  a  few  minutes  the  starch  passes  into  a  soluble  condition  and  the  fluid 
becomes  clear  and  transparent.  On  testing  the  solution  from  time  to  time 
with  iodin  the  characteristic  blue  reaction  will  be  found  to  disappear,  gradu- 
ally, the  color  passing  from  blue  to  violet,  to  red,  to  yellow.  If  now  the 
solution  be  boiled  with  a  solution  of  cupric  hydroxid  (Fehling's  solution)  a 
copious  red  or  yellow  precipitate  of  cuprous  oxid  is  formed,  which  indicates 
the  presence  of  sugar.  The  polariscope  shows  that  this  sugar  is  maltose, 
CjjHo^Ojj.  During  the  conversion  of  the  starch  intermediate  substances 
are  formed  to  which  the  term  dextrin  is  applied.  After  the  starch  has  been 
rendered  soluble  it  undergoes  a  cleavage  into  maltose  and  a  dextrin,  which, 
as  it  gives  rise  to  a  red  color  with  iodin,  is  termed  erythrodextrin.  At  a 
later  stage  this  erythrodextrin  also  undergoes  a  cleavage  into  maltose 
and  a  second  variety  of  dextrin,  which,  as  it  does  not  give  rise  to  any 
color  with  iodin,  is  termed  achroodextrin.  It  is  claimed  by  some  investi- 
gators that  this  form  can  also  in  time  be  transformed  into  sugar.  It  is 
possible  that  a  small  quantity  of  dextrose  is  also  formed. 

The  successive  stages  of  the  conversion  of  starch  into  sugar  may  be 
represented  by  the  following  schema: 


DIGESTION.  149 

r  T7     *u     J     *_•        f  Achroodextrin. 
fErythrodextrin=     ,r.ltn=<. 


Starch  =  Soluble  Starch-  |  ^'^l'-^'^^;"""""'      \  Maltose. 

This  change  consists  in  the  assumption  by  the  starch  of  a  molecule  of  water, 
and  for  this  reason  the  process  is  termed  hydrolysis.  The  nature  of  the 
chemic  change  is  shown  in  the  following  formula: 

3(C6H,oOJ  +  H,0  =  C,jHj,Oi,  +  CeH,oO, 

Starch  +  Water  =  Maltose  +  Dextrin. 

The  amylolyticS  amyloclastic,  or  starch-changing  action  of  saliva  depends 
on  the  presence  of  an  unorganized  ferment  or  enzyme  known  as  ptyalin  or 
amylase.  This  enzyme  is  present  in  the  secretion  of  each  of  the  salivary 
glands.  The  chemic  character  of  ptyaHn  is  unknown,  though  there  are 
reasons  for  belfeving  that  it  partakes  of  the  nature  of  a  protein.  It  is  a  prod- 
uct in  all  probabiHty  of  the  katabolic  activity  of  the  secretor  cells.  According 
to  Latimer  and  Warren,  ptyalin  is  a  derivative  of  the  zymogen,  ptyalogen. 
This  latter  compound  has  been  shown  to  be  present  in  the  glands  of  the 
dog,  cat,  and  sheep.  Ptyalin  effects  the  transformation  of  starch  merely 
by  its  presence,  and  undergoes  no  perceptible  consumption  in  the  process. 
The  activity  of  this  enzyme  is  very  great,  and  unless  interfered  with  by  an 
excess  of  sugar  and  dextrin,  it  acts  practically  indefinitely. 

The  activity  of  ptyalin  is  modified  by  various  external  conditions,  among 
which  may  be  mentioned  the  chemic  reaction  of  the  medium  in  which  it  is 
placed.  It  is  most  active  when  the  medium  is  moderately  alkaline.  Its 
activity  is  arrested  by  strong  alkalies  or  acids,  though  the  presence  of  a 
small  percentage  of  an  acid  does  not  appear  to  have  any  effect  in  either 
hastening  or  retarding  the  process.  This  fact  has  a  bearing  upon  the  ques- 
tion as  to  whether  the  action  of  the  saliva  is  interfered  with  in  the  stomach 
by  the  presence  of  the  gastric  juice.  At  present  it  is  a  disputed  matter,  but 
the  weight  of  authority  is  in  favor  of  the  view  that  the  transforming  action 
may  continue  for  almost  half  an  hour  during  the  early  stages  of  gastric 
digestion.  The  temperature  also  influences  the  rapidity  with  which  the 
transformation  of  the  starch  is  effected.  At  a  temperature  of  95°  to  106°  F. 
the  ptyalin  acts  most  energetically,  while  its  activity  is  entirely  arrested  by 
reducing  the  temperature  to  the  freezing-point  or  raising  it  to  the  boiling 
point. 

The  Nerve  Mechanism  of  the  Secretion  of  Saliva. — The  secretion  of 
saliva  is  a  complex  act  and  involves  the  cooperation  of  gland  cells,  blood- 
vessels, efferent  and  aft'erent  nerves  contained  in  different  cranial  nerves, 
and  a  central  mechanism  by  which  they  are  excited  to  and  coordinated  in 
activity.  The  central  mechanism  is  located  in  the  medulla  oblongata  in 
the  gray  m.atter  beneath  the  floor  of  the  fourth  ventricle. 

During  the  interv^als  of  mouth  digestion  the  glands  are  practically  at  rest 
as  far  as  the  discharge  of  saliva  is  concerned.  The  cells,  however,  are 
actively  engaged  in  absorbing  from  the  surrounding  lymph-spaces  materials 
derived  from  the  blood,  out  of  which  they  construct  their  characteristic  con- 

'  The  term  amylolytic  has  been  objected  to  on  the  ground  that  it  does  not  correctly  express 
the  fact,  but  is  analogous  with  electrolytic  and  means  a  transformation  by  means  of  starch. 
Fergussen  has  suggested  the  use  of  the  term  amyloclastic  as  well  as  proteoclastic  and  hpoclastic 
for  the  terms  now  generally  employed. 


ISO 


TEXT-BOOK  OF  PHYSIOLOGY. 


tents.  The  blood-vessels  possess  that  degree  of  dilatation  necessary  for 
nutritive  purposes. 

With  the  introduction  of  food  into  the  mouth  and  the  onset  of  mastication 
the  blood-vessels  suddenly  dilate,  the  blood-supply  is  increased,  and  the 
gland-cells  begin  to  discharge  water,  inorganic  salts,  and  their  organic 
constituents  into  the  lumen  of  the  acinus,  materials  that  collectively  consti- 
tute the  saliva  characteristic  of  any  one  of  the  glands.  This  continues  until 
mastication  ceases,  when  all  the  structures  return  to  their  former  condition 
of  relative  inactivity. 

The  nerves  and  nerve  centers  that  constitute  the  nerve  mechanism  for 
the  secretion  of  saliva,  as  determined  by  experimental  investigations  are 
shown  in  the  following  table: 


Afferent  Nerves. 


Lingual  and  buccal  branches  of 
the  trigeminal  nerve. 


Nerve-centers. 
Medulla  oblongata. 


Taste    fibers 
tympani. 


in     the    chorda 


Efferent  Nerves. 

The  chorda  tympani  and  its  post- 
ganglionic continuations  for  the  sub- 
maxillary and  sublingual  glands; 
the  glosso-pharyngeal  nerve  and  its 
postganglionic  continuations  con- 
tained in  the  auriculo-temporal 
branch  of  the  trigeminal  nerve,  for 
the  parotid  gland. 

The  sympathetic  nerve  for  all  the 
glands. 


3.  Taste    fibers    in     the    glosso- 
pharyngeal. 

The  Efferent  Nerves. — The  efferent  nerve-fibers,  as  stated  in  the  fore- 
going paragraph,  that  transmit  nerve  impulses  to  the  submaxillary,  sub- 
lingual, and  parotid  glands,  as  well  as  to  their  associated  blood-vessels,  are 
contained  respectively  in  the  chorda  tympani  and  its  postganglionic  con- 
tinuations, in  the  glosso-pharyngeal  and  its  postganglionic  continuations 
contained  in  the  auriculo-temporal  branch  of  the  fifth  nerve,  and  in  the 
branches  of  the  sympathetic  nerve  derived  from  the  superior  cervical  ganglion. 
That  these  nerves  transmit  the  nerve  impulses  to  the  salivar\^  apparatus  is 
shown  by  the  effects  that  follow  their  division  and  stimulation. 

The  Chorda  Tympani. — The  chorda  tympani  nerve  is  apparently  a 
branch  of  the  facial,  the  trunk  of  which  it  leaves  in  the  aqueduct  of  Fallopius. 
It  then  crosses  the  tympanic  cavity,  emerges  through  the  Glaserian  fissure, 
and  joins  the  lingual  branch  of  the  inferior  maxillary  division  of  the  fifth 
nerve.  After  passing  forward  as  far  as  the  sublingual  gland,  nearly  all  of 
the  fibers  leave  the  lingual  nerve  by  four  or  five  strands  to  become  connected 
by  terminal  branches  with  nerve  ganglion  cells  in  relation  with  the  sub- 
maxillary and  sublingual  glands.     (See  Fig.  67.) 

The  effects  on  the  secretion  and  flow  of  saliva  from  the  submaxillary 
gland  which  follow  division  and  stimulation  of  the  chorda  tympani  nerve 
are  shown  in  the  following  way:  A  cannula  is  inserted  into  Wharton's  duct 
and  the  rate  of  flow  estimated;  the  nerve  is  then  divided,  after  which  the 
flow  ceases.  The  peripheral  end  of  the  nerve  is  then  stimulated  with 
induced  electric  currents  when  a  copious  secretion  of  a  thin  saliva  takes 
place,  accompanied  by  a  marked  dilatation  of  the  blood-vessels  of  the  gland. 
The  quantity  of  blood  passing  through  the  vessels  is  so  great  as  to  give  to 
the  venous  blood  an  arterial  hue  and  to  the  small  veins  a  distinct  pulsation. 
It  would  appear  from  these  effects  that  the  chorda  contains  two  sets  of  fibers, 


DIGESTION. 


151 


one  of  which  inhibits  the  action  of  a  local  vaso-motor  mechanism  permitting 
the  blood-vessels  to  dilate  (vaso-dilatator  fibers),  the  other  of  which  stimu- 
lates the  secretor  cells  to  activity,  through  the  intermediation  of  local  ganglia. 
That  local  ganglia  are  involved  is  shown  by  the  effects  which  follow  the 
injection  of  nicotin  into  the  circulation.  After  a  sufficient  dose — 10  milli- 
grams for  the  cat — stimulation  of  the  chorda  has  no  effect.  Stimulation  of 
the  nerve-plexus  beyond  the  ganglion,  the  postganglionic  fibers,  however,  is 
at  once  followed  by  vascular  dilatation  and  secretion,  a  fact  that  would 
indicate  that  the  ganglia  are  not  only  stimulated  by  the  chorda  tympani  but 
that  the  conductivity  of  the  nerve  endings  of  the  chorda  around  the  ganglia 
is  impaired. 

Glosso-Pharyngeal 
Otic  Oan(jJ.ion 


rarottd  Gland ^ 


•Jaco6sens  JVerve 


Suh  Maxilla  ri/  (//an^^^^^lf   " 

uAorda.Tt/mpaniMerce, 


bup. Cervical  6 any  lion. 
Sympathetic  Jveroes 


Fig.    67.— rSCHEME    OF   THE   NeRVES    INVOLVED    IN    THE   SECRETION    OF   vSaLIVA. 

It  might  be  inferred  that  the  increase  in  the  flow  of  saliva  is  due  to  filtra- 
tion, the  result  of  the  increased  blood-supply  to  the  gland,  and  not  to  the 
influence  of  any  true  secretor  fibers  stimulating  the  activities  of  the  secretor 
cells.  That  this  is  not  the  case,  however,  can  be  demonstrated  in  several 
ways:  first,  the  pressure  in  the  duct  of  the  submaxillary  gland,  as  shown 
by  the  mercurial  manometer,  rises,  when  the  gland  is  secreting,  considerably 
above  the  pressure  in  the  carotid  artery,  which  could  not  be  the  case  if  it 
were  due  to  a  mere  filtration;  for  if  pressure  alone  were  the  cause,  the  flow 
of  saliva  would  cease  as  soon  as  the  pressure  in  the  tube  equaled  the  pressure 
in  the  blood-vessels.  Second,  even  in  the  absence  of  blood  the  gland  can  be 
made  to  yield  a  secretion,  as  shown  by  stimulating  the  nerv^e  in  a  recently 
killed  animal.  Third,  after  the  injection  of  atropin  into  the  circulation  the 
secretion  is  abolished,  but  the  local  vaso-motor  mechanism  is  unimpaired, 
for  stimulation  of  the  nerve,  as  in  the  previous  instance,  gives  rise  to  a  dilatation 


152  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  vessels  and  an  increased  blood-supply.  There  is  thus  abundant 
proof  that  the  chorda  tympani  contains  two  sets  of  fibers — one  regulating 
the  blood-supply  to  the  gland,  the  other  stimulating  the  secretor  cells. 

The  efferent  fibers,  vaso-motor  and  secretor,  which  constitute  in  part 
the  chorda  tympani  nerve  have  their  origin  in  cells,  the  nucleus  salivalorius, 
located  beneath  the  floor  of  the  fourth  ventricle,  from  which  they  emerge  in 
the  nerve  of  Wrisberg  or  pars  intermedia,  and  enter  the  trunk  of  the  facial 
nerve  at  the  bottom  of  the  internal  auditory  canal  after  which  they  pursue 
the  course  stated  above. 

The  Glosso-pharyngeal  Nerve. — The  nerve-fibers  that  conduct  nerve 
impulses  outward  from  the  medulla  to  the  parotid  gland  are  believed  to  pass 
through  the  glosso-pharyngeal  nerve,  through  the  tympanic  branch  or  nerve 
of  Jacobson,  to  the  otic  ganglion,  with  which  they  become  connected.  From 
this  ganglion  new  nerve-fibers  arise  which  pass  into  the  fifth  nerve  and 
reach  the  secretor  cells  of  the  parotid  gland  through  the  auriculotemporal 
nerve.  The  trunk  of  this  latter  nerve  contains  therefore  postganglionic 
fibers  that  bear  the  same  relation  to  the  parotid  gland  and  blood-vessels 
that  the  postganglionic  fibers  from  the  submaxillary  ganghon  bear  to  the 
submaxillary  gland  and  blood-vessels. 

The  influence  of  the  efferent  fibers  in  the  trunk  of  the  glosso-pharyngeal 
on  the  parotid  gland  is  similar  to  the  influence  of  the  chorda  tympani  on  the 
submaxillary  gland ;  for  if  the  glosso-pharyngeal  nerve  or  its  post  ganglionic 
continuations  in  the  auriculo-temporal  nerve  be  stimulated  in  any  part  of  its 
course  with  induced  electric  currents  there  follows  a  dilatation  of  the  blood- 
vessels and  an  abundant  discharge  of  a  thin  saliva  rich  in  water  and  salts 
but  poor  in  the  amount  of  organic  matter.  Division  of  the  glosso-pharyngeal 
nerve,  extirpation  of  the  otic  ganglion  or  division  of  the  auriculo-temporal 
nerv'e  is  followed  by  a  loss  of  reflex  secretion.  Stimulation  of  the  branch 
connecting  the  glosso-pharyngeal  with  the  otic  ganglion  (Jacobson's  nerve) 
gives  rise  to  the  secretion  as  shown  by  Heidenhain.  Division  of  the  nerve 
is  also  followed  by  a  loss  of  reflex  secretion. 

The  Sympathetic  Nerves. — The  sympathetic  fibers  which  influence 
the  salivary  secretion  emerge  from  the  spinal  cord  mainly  through  the  second, 
third,  and  fourth  thoracic  nerves.  After  passing  into  the  sympathetic  chain 
they  ascend  to  the  superior  cervical  ganglion,  with  the  cells  of  which  they 
become  connected  through  the  intermediation  of  fine  terminal  branches. 
From  this  point  non-medullated  nerve-fibers  follow  the  branches  of  the 
external  carotid  artery  to  the  different  glands.  There  is  no  evidence  that 
these  fibers  have  any  connection,  anatomic  or  physiologic,  with  ocal  ganglia 
at  or  near  the  submaxillary,  sublingual,  or  parotid  glands.  If  the  sympa- 
thetic nerve  in  the  neck,  especially  in  the  dog,  be  divided  and  the  peripheral 
end  stimulated  with  induced  electric  currents,  there  is  at  once  a  contrac- 
tion of  the  smaller  blood-vessels  of  the  submaxillary  and  sublingual  glands 
and  a  diminution  of  the  blood-supply,  a  result  showing  the  presence  of  vaso- 
constrictor fibers.  Nevertheless  both  the  submaxillary  and  sublingual 
glands  pour  out  a  saliva  which  is  different  from  that  poured  out  when  the 
chorda  tympani  is  stimulated.  The  quantity  is  less,  it  is  more  viscid,  richer 
in  organic  matter,  of  a  higher  specific  gravity,  and  more  active  in  the  trans- 
formation of  starch  into  sugar. 


DIGESTION.  153 

Stimulation  of  the  sympathetic  fibers  passing  to  the  parotid  gland  is 
followed  by  contraction  of  the  vessels  and  an  abolition  of  the  secretion;  but 
at  the  same  time  there  is  an  increased  activity  of  the  secretor  cells,  for  sub- 
sequent stimulation  of  the  auriculo-temporal  nerve  not  only  causes  an 
increase  in  the  amount  of  water  and  inorganic  salts,  but  an  increase  also  in 
the  amount  of  organic  matter  far  beyond  that  produced  when  the  auriculo- 
temporal alone  has  been  stimulated.  Histologic  examination  shows  that 
the  small  ducts  of  the  gland  are  filled  with  thick  organic  matter  after  stimula- 
tion of  the  cervical  sympathetic. 

The  foregoing  facts  led  Heidenhain  to  the  conclusion  that  there  are  two 
physiologically  distinct  efferent  nerve-fibers  distributed  to  the  glands,  viz., 
trophic  nerves,  derived  from  the  sympathetic  which  stimulate  the  cells  to  the 
production  of  organic  constituents;  and  secretor  nerves,  derived  from  the 
cranial  nerves,  chorda  tympani  and  glosso-pharyngeal,  which  stimulate 
the  cells  to  the  production  of  water  and  inorganic  salts.  This  view  has 
however,  been  controverted  by  Langley,  who  regards  the  secretor  fibers  to  the 
glands  as  essentially  the  same,  and  considers  the  differences  in  the  character 
of  the  secretion  to  be  dependent  on  differences  in  the  quantity  of  the  blood- 
supply  induced  by  the  simultaneous  stimulation  of  the  vaso-motor  nerves. 

The  Central  Mechanism. — The  central  mechanism  that  excites  the 
glands  and  blood  vessels  to  activity  through  efferent  nerv-es  originating  in  its 
cells  maybe  aroused  to  action  (i)  by  nerve  impulses  descending  from  the  cere- 
brum in  consequence  of  psychic  states  induced  by  the  sight  or  the  odor  of 
foods  especially  after  long  abstinence;  (2)  by  nerve  impulses  transmitted 
through  afferent  nerves  from  the  mouth,  developed  by  the  contact  of  the  food 
with  the  peripheral  terminations  of  the  gustatory  or  general  sensor  nerves. 

That  psychic  states,  ideas  and  feelings  aroused  by  the  sight,  odor,  and 
■contemplation  of  food  can  give  rise  to  a  stimulation  of  the  cells  of  the  central 
mechanism  in  the  manner  just  stated  is  shown  by  the  flow  of  saliva  which  is 
familiarly  known  as  watering  of  the  mouth.  This  fact  has  been  experiment- 
ally demonstrated  by  Pawlow  on  dogs.  This  investigator  cause  the  ducts 
of  the  glands  to  be  brought  to  the  surface  in  such  a  manner  that  they  healed 
into  the  edges  of  the  skin  wounds.  By  means  of  suitable  receivers  applied 
over  the  orifices  of  the  ducts  the  saliva  could  be  readily  collected.  When  the 
dog  under  such  circumstances  was  tempted  by  the  sight  of  foods  there  was 
at  once  a  free  discharge  of  saliva,  the  quantity  and  quality  of  which  varied 
with  the  character  of  the  foods. 

That  the  central  mechanism  can  be  excited  to  activity  by  nerve  impulses 
reflected  from  the  periphery  can  be  demonstrated  by  the  introduction  of  food 
into  the  mouth,  as  well  as  by  stimulation  of  the  branches  of  the  afferent 
nerves  distributed  to  the  mouth  which  constitute  the  afferent  part  of  this 
mechanism. 

The  Afferent  Nerves. — ^The  afferent  nerves  that  transmit  nerve  impulses 
from  the  mouth  to  the  central  mechanism,  are  the  taste  fibers  in  the  chorda 
tympani,  the  taste  and  sensor  fibers  of  the  glosso-pharyngeal,  and  the  sensor 
fibers  of  the  lingual  and  buccal  branches  of  the  trigeminal  nerve.  This  of 
show^n  by  the  fact  that  if  they  are  transversely  divided  there  is  a  cessation  of 
the  discharge  of  saliva  when  the  peripheral  nerve  endings  in  the  mouth  are 
stimulated  by  the  presence  of  food.     With  these  nerves  intact  the  introduc- 


154  TEXT-BOOK  OF  PHYSIOLOGY. 

tion  of  food  into  the  mouth  will  invariably  be  followed  by  a  flow  of  saliva. 
Pawlow  has  apparently  demonstrated  that  this  general  fact  must  be  supple- 
mented by  the  further  fact,  that  there  is  a  special  adaptation  between  the 
character  of  food  and  the  different  glands.  Thus,  solid  dry  foods,  cause  a 
large  flow  of  a  thin  saliva  from  the  parotid  glands,  but  a  slight  flow  from  the 
submaxillary;  moist  foods  and  especially  meat  causes  a  large  flow  from  the 
submaxillary  gland,  but  a  slight  flow  from  the  parotid.  It  is  also  probable 
that  the  glands  respond  by  discharging  a  secretion  of  special  quality  in 
accordance  with  the  properties  of  the  different  foods. 

Stimulation  of  the  afferent  nerves  with  induced  electric  currents  also  gives 
rise  to  a  discharge  of  saliva.  This  can  be  demonstrated  by  exposing  the 
glands  and  the  afferent  nerves  and  subjecting  them  to  experiment.  Under 
such  circumstances,  if  a  cannula  be  placed  in  the  duct  of  the  submaxillary 
gland,  and  the  lingual  nerve  stimulated  by  induced  electric  currents  of 
moderate  strength,  a  copious  flow  of  saliva  at  once  takes  place.  If  now  the 
glosso-pharyngeal  nerve  or  the  central  end  of  the  divided  chorda  tympani 
nerve  be  stimulated  in  a  similar  manner,  the  effect  on  the  secretion  will  be 
the  same.  Division  of  these  nen/es  in  an  animal,  in  such  a  way  as  to  prevent 
the  nerve  impulses  from  reaching  the  medulla  oblongata,  is  followed  by  a 
marked  diminution  in  the  amount  of  saliva  secreted.  The  reflex  centers, 
however,  may  receive  impulses  and  be  excited  to  activity  by  impulses  coming 
through  other  nerves — e.g.,  the  pneumogastric,  when  the  mucous  membrane 
of  the  stomach  is  stimulated;  the  sciatic,  when  after  division,  its  central  end 
is  stimulated. 

Resume  of  the  Factors  involved  in  the  Secretion  of  Saliva. — From 
the  foregoing  statements  it  is  apparent  that  the  secretion  of  saliva  is  a  complex 
act  involving  the  cooperation  of  several  different  factors.  As  the  mechanism 
for  the  elaboration  of  this  secretion  is  typical  of  that  for  many  secretions  it 
will  be  of  advantage  to  summarize  these  factors  and  their  specific  functions. 
These  are  as  follows: 

1.  Epithelial  cells,  the  physiologic  actions  of  which    are    the    production 

of  the  specific  characteristic  constituents  of  the  saliva,  e.g.,  mucin, 
albumin,  the  enzyme  ptyalin,  as  well  as  the  absorption  and  discharge  of 
water  and  inorganic  salts. 

2.  Lymph,  which  contains  the  nutritive  material  necessary  for  the  growth, 

repair,  and  metabolic  activities  of  the  secreting  cells. 

3.  Capillary  blood-vessels,  which  permit  the  passage  of  those  constituents  of 

the  blood  that  collectively  constitute  lymph. 

4.  Vaso-motor  nerves,  some  of  which  at  the  beginning  of  secretor  activity 

dilate  the  blood-vessels  and  thus  increase  the  blood  supply  and  the 
production  of  lymph  (vaso-dilatator  nerves);  others  of  which  at  the 
end  of  secretor  activity  perhaps  actively  contract  the  blood-vessels 
and  thus  decrease  the  blood  supply  to  the  previous  condition  (vaso- 
constrictor nerves). 

5.  Secretor  nerves  which  stimulate  the  epithelial  cells  to  increased  activity 

causing  them  to  discharge  their  specific  metabolic  constituents  along 
with  water  and  inorganic  salt  in  characteristic  proportions  from  the 
orifices  of  the  gland  ducts  (secreto-motor  nerves). 
The  central  mechanism  is  excited  to  coordinate  activity,  primarily,  by 


DIGESTION.  155 

nerve  impulses  descending  from  the  cerebrum  as  a  result  of  psychic  states 
developed  by  the  sight  and  odor  of  food,  and  secondarily,  by  nerve  impulses, 
transmitted  by  the  nerves  of  gustation  and  general  sensibility  and  developed 
by  the  contact  of  food  on  their  peripheral  terminations  during  the  act 
of  mastication. 

Modifications  of  the  Nerve  Mechanism  of  Insalivation  due  to  the 
Physiologic  Action  of  Drugs. — The  functions  of  different  portions  of 
the  nerve  mechanism  of  insalivation  may  be  made  apparent  by  an  analysis  of 
the  effects  that  follow  the  administration  of  physiologic  or  slightly  toxic 
doses  of  the  alkaloids  of  various  drugs.  The  effects  can  be  shown  to  be  due 
to  a  depression  or  stimulation  of  the  normal  activity  of  one  or  more  portions 
of  the  mechanism.  As  a  result  the  secretion  may  be  decreased  or  increased 
in  volume.  The  following  examples  will  illustrate  the  action  of  alkaloids 
in  general. 

Nicotin. — When  nicotin  in  sufficiently  large  doses  is  given  to  an  animal 
hypodermatically,  the  secretion  of  saliva  after  a  variable  period  of  time  ceases 
and  the  mouth  becomes  dry.  If  the  chorda  tympani  nerve,  i.e.,  the  pre- 
ganglionic portion,  be  then  stimulated  with  induced  electric  currents  the 
usual  phenomenon,  viz.,  a  free  flow  of  saliva,  fails  to  occur.  If,  however, 
the  nerve  branches  emerging  from  the  submaxillary  ganglion,  i.e.,  the  post- 
ganglionic portion,  be  stimulated  with  electric  currents,  the  saliva  will  be 
discharged  as  usual.  The  inference  is  that  the  conductivity  of  the  peripheral 
terminations  of  the  preganglionic  chorda  fibers  is  depressed  so  that  the  nerve 
impulses  discharged  by  the  central  mechanism  fail  to  reach,  and  therefore  to 
stimulate,  the  submaxillary  ganglion  cells.  The  inference  as  to  the  seat  of 
action  of  nicotin  is  supported  by  the  fact  that  painting  the  surface  of  the 
superior  cervical  sympathetic  ganglion  wdth  nicotin  will  impair  the  conductiv- 
ity of  the  terminal  branches  of  the  preganglionic  fibers  emerging  from  the 
cord  so  that  stimulation  of  these  fibers  fails  to  produce  beyond  the  ganglion 
the  usual  secretor  effects.  It  is  probable  that  nicotin  has  a  similar  action 
on  the  peripheral  terminations  of  Jacobson's  nerve  which  arborize  around 
the  nerve  cells  of  the  otic  ganglion. 

Atropin. — Atropin  in  doses  of  i  milligram  also  causes  a  complete  cessa- 
tion in  the  flow  of  saliva  and  consequently  an  extreme  dr\mess  of  the  mouth. 
After  the  occurrence  of  this  condition  neither  stimulation  of  the  preganglionic 
chorda  tympani  fibers  nor  of  the  postganglionic  fibers,  will  cause  the  glands  to 
secrete.  But  as  stimulation  of  the  sympathetic  nerv^e  in  the  cervical  region 
will  excite  a  secretion  the  inference  is  that  the  atropin  exerts  a  depressing 
effect  on  the  conductivity  of  the  nerve  endings  in  contact  with  the  gland  cells 
thus  interfering  with  the  transmission  of  nerv^e  impulses,  rather  than  on  the 
gland  cells  themselves.  The  same  holds  true  for  the  nerve  terminations  in  the 
postganglionic  fibers  distributed  to  the  parotid  gland.  The  action  of  atropin 
is  not  limited,  however,  to  the  nerv'e  terminations  in  connection  with  salivary 
glands  but  extends  to  the  nerve  terminations  in  connection  with  many  other 
glands  in  the  ahmentary  canal  and  skin.  Even  though  the  dose  of  atropin 
be  large,  lo  to  15  milligrams  for  a  dog,  its  action  is  confined  to  the  terminal 
nerve  fibers  in  connection  with  the  gland  cells,  for  when  the  chorda  tympani 
is  stimulated  the  blood-vessels  around  the  gland  dilate  as  usual,  a  fact  which 
indicates  that  the  submaxillary  ganglion  gives  off  fibers  of  a  vaso-dilatator  as 


156  TEXT-BOOK  OF  PHYSIOLOGY. 

well  as  a  secretor  character.  Unless  the  dose  of  atropin  be  largely  increased, 
e.g.,  loo  milligrams,  it  fails  to  depress  the  conductivity  of  the  terminals  of 
the  sympathetic  nerve  fibers. 

Pilocarpin. — Pilocarpin  in  small  doses,  from  2  to  5  milligrams,  hypo- 
dermatically  causes  in  the  cat  a  free  flow  of  saliva  which  may  amount  to  half 
a  liter  or  more  in  the  course  of  several  hours.  In  human  beings  its  effect  on 
the  flow  of  saliva  is  equally  marked.  Ringer  reports  that  in  two  patients, 
after  taking  a  medicinal  dose,  the  amount  of  saliva  discharged  was  622  c.c. 
and  764  c.c.  respectively.  Since  division  of  the  nerves  both  pre-  and  post- 
ganglionic, does  not  diminish  or  abolish  the  secretion,  the  inference  is  that 
the  pilocarpin  exerts  a  stimulating  action  on  the  nerve  endings  in  connection 
with  the  gland  cells.  This  inference  is  strengthened  by  the  fact  that  the 
pilocarpin  effect  is  antagonized  and  the  secretion  checked  by  a  suitable  dose 
of  atropin,  the  seat  of  the  action  of  which  is  known.  The  two  alkaloids  thus 
appear  to  be  in  physiologic  antagonism  in  their  action  on  these  nerve  termi- 
nations. The  action  of  pilocarpin  is  not  limited  to  the  salivary  glands,  but 
extends  to  glands  found  in  the  alimentary  canal,  respiratory  passages,  and 
skin. 

DEGLUTITION. 

Deglutition  is  that  part  of  the  digestive  process  which  is  concerned  in 
the  transference  of  the  food  from  the  mouth  through  the  pharynx  and 
esophagus  into  the  stomach.  This  is  an  extremely  complex  act  and  involves 
the  action  of  a  large  number  of  structures,  all  of  which  are  made  to  act  in 
proper  sequence  under  the  coordinating  influence  of  the  nerve  system.  The 
deglutitory  canal  consists  of  the  mouth,  phar}'nx,  and  esophagus,  each  of 
which  presents  certain  anatomic  features  on  which  its  physiologic  action 
depends. 

The  cavity  of  the  mouth  communicates  posteriorly  with  the  pharynx  by 
a  narrow  orifice,  the  isthmus  of  the  fauces.  This  orifice  is  bounded  above 
by  the  soft  palate,  laterally  by  the  anterior  and  posterior  half  arches,  and 
below  by  the  tongue. 

The  pharynx  is  an  oval-shaped  cavity  extending  from  the  base  of  the 
skull  to  the  lower  border  of  the  cricoid  cartilage,  a  distance  of  about  12  centi- 
meters. (See  Fig.  72.)  Its  walls  are  formed  mainly  by  three  pairs  of 
muscles — the  superior,  middle,  and  inferior  constrictors — each  consisting 
of  red,  striated  muscle-fibers,  and  hence  capable  of  rapid  and  energetic 
contractions.  Superiorly  the  pharynx  is  attached  to  and  supported  by  the 
basilar  process  of  the  occipital  bone;  inferiorly  it  becomes  continuous  with 
the  esophagus.  The  anterior  wall  of  the  pharynx  is  imperfect  and  presents 
openings  which  communicate  with  the  nasal  chamber?,  the  mouth,  and  the 
larynx.  The  lateral  wall  on  either  side  presents  the  opening  of  the  Eustachian 
tube  which  leads  directly  into  the  cavity  of  the  middle  ear.  The  interior  of 
the  pharynx  is  lined  by  mucous  membrane.  The  pharynx  is  partially  separ- 
ated from  the  mouth  by  the  velum  pendulum  palati,  a  muscular  structure 
attached  above  to  the  hard  palate;  its  lower  edge  or  border  is  directed 
downward  and  backward  and  presents  in  the  middle  line  a  conical  process, 
the  uvula.     On  either  side  the  palate  presents  two  curved  arches,  the  anterior 


DIGESTION. 


157 


and  posterior,  formed  respectively  by  the  palato-glossei  and  palato-pharyngei 
muscles.  The  superior  laryngeal  aperture  is  placed  just  beneath  the  base 
of  the  tongue.  It  is  triangular  in  shape,  wide  in  front,  narrow  behind,  and 
directed  downward  and  backward.  It  is  bounded  above  by  a  thin  plate  of 
cartilage,  the  epiglottis,  placed  just  behind  the  tongue  and  so  arranged  that 
it  can  easily  be  depressed  and  elevated. 

The  esophagus,  the  continuation  of  the  deglutitory  canal,  extends  down- 
ward from  the  lower  border  of  the  cricoid  cartilage  for  a  distance  of  from 


Fig.  68. — \'ertical  Section  of  the  Nasal  Fossa  .■vmd  Mouth,  i.  Left  nares.  2.  Lateral 
cartilage  of  the  nose.  3.  Portion  of  the  internal  alar  cartilage  forming  the  skeleton  of  the  lower 
part.  4.  Superior  meatus.  5.  ISIiddle  meatus.  6.  Inferior  meatus.  7.  Sphenoidal  sinuses. 
8.  External  boundary  of  the  posterior  nares.  9.  Internal  elliptical  opening  of  the  Eustachian 
tube.  10.  Soft  palate.  11.  Vestibule  of  the  mouth.  12.  Vault  of  palate.  13.  Genioglossus 
muscle.  14.  Geniohyoid  muscle.  15.  Cut  margin  of  the  mylohyoid  muscle.  16.  Anterior 
pillar  of  the  palate  (anterior  half-arch),  presenting  a  triangular  figure  with  the  base  inferiorly, 
covering  partly  the  tonsil.  17.  Posterior  pillar  (posterior  half-arch)  of  the  palate.  18.  Tonsil. 
19.  Follicular  (mucous)  glands  at  the  base  of  the  tongue.  20.  Cavity  of  the  lar}-nx.  21.  \'entricle 
of  the  larynx.  22.  Epiglottitis.  23.  Cut  os  hyoides.  24.  Cut  thyroid  cartilage.  25.  Thyrohyoid 
membrane.  26.  Section  of  posterior  portion  of  the  cricoid  cartilage.  27.  Section  of  the  anterior 
portion  of  the  same  cartilage.     28.  Crico-thyroid  membrane. — (Sappey.) 

2  2  to  25  centimeters,  to  a  point  opposite  the  ninth  thoracic  vertebra,  where 
it  expands  into  the  stomach.  Its  walls  are  composed  of  an  internal  or 
mucous  and  an  external  or  muscle  coat,  united  by  areolar  tissue.  The 
muscle  coat  consists  of  an  external  layer  of  longitudinal  fibers  arranged  in 
three  bands  and  of  an  internal  layer  composed  of  fibers  arranged  circularly 
in  the  upper  part  and  obliquely  in  the  lower  part  of  the  esophagus.  In  the 
upper  third  the  fibers  are  striated;  in  the  middle  third  they  are  a  mixture  of 
both  striated  and  non-striated;  in  the  lower  third  they  are  entirely  non- 
striated. 

The  muscle  fibers  surrounding  the  esophago-gastric  orifice  are  arranged 
in  the  form  of  and  play  the  part  of  a  sphincter  muscle,  and  for  this  reason 


158  TEXT-BOOK  OF  PHYSIOLOGY. 

may  be  termed  the  sphincter  cardicB  muscle.     By  its  action  it  prevents  a 
return  under  normal  conditions  of  food  into  the  esophagus. 

The  deglutitive  act  may  be  for  convenience  divided  into  three  stages,  viz. : 

1.  The  passage  of  the  food  from  the  mouth  into  the  pharynx. 

2.  The  passage  of  the  food  through  the  pharynx  into  the  esophagus. 

3.  The  passage  of  the  food  through  the  esophagus  into  the  stomach. 

In  the  first  stage  the  bolus  of  food  is  placed  on  the  superior  surface  of  the 
tongue.  The  mouth  is  then  closed  and  respiration  is  momentarily  sus- 
pended. The  tip  of  the  tongue  is  placed  against  the  posterior  surfaces  of  the 
teeth.  The  tongue,  by  reason  of  its  intrinsic  musculature,  then  arches  from 
before  backward  against  the  roof  of  the  mouth  and  pushes  the  bolus  of  food 
through  the  isthmus  of  the  fauces  into  the  pharynx.  This  completes  the 
first  stage.  It  is  a  voluntary  effort  and  accomplished  partly  by  the  tongue, 
though,  as  shown  by  Meltzer,  mainly  by  the  mylohyoid  muscles. 

The  second  and  third  stages,  or  the  passage  of  the  food  through  the 
pharynx  and  esophagus  into  the  stomach,  have  been  attributed  until  quite 
recently  entirely  to  peristaltic  movements  of  their  musculature.^  It  has  been 
stated  that  with  the  passage  of  the  food  through  the  isthmus  of  the  fauces  the 
posterior  wall  of  the  pharynx  advances  and  seizes  the  food,  which,  in 
consecjuence  of  a  rapid  peristaltic  movement  running  through  the  constrictor 
muscles  from  above  downward  is  transferred  to  the  esophagus;  that  with  the 
entrance  of  the  food  into  the  esophagus  a  similar  peristalsis,  varying  in  rapidity 
in  different  sections  in  consequence  of  a  change  in  the  character  of  its 
musculature,  gradually  transfers  the  food  into  the  stomach.  There  can  be 
but  slight  doubt  that  by  this  method  the  bolus  of  food,  especially  if  it  is  of  firm 
consistence  and  of  a  size  sufficient  to  distend  the  esophagus,  is  transferred 
into  the  stomach,  but  that  it  is  the  exceptional  rather  than  the  usual  method 
has  been  demonstrated  by  Kronecker,  Falk,  and  Meltzer. 

In  1880  the  first  of  these  experimenters  made  the  observation  that  the 
sensation  in  the  stomach  following  the  swallowing  of  a  mouthful  of  cold  water 
occurred  too  quickly  to  be  explained  by  the  prevalent  belief  that  its  transfer- 
ence was  caused  by  ordinary  peristalsis,  the  rate  of  progression  of  which  was 
known  to  be  slow.  Falk  then  discovered  the  fact,  by  introducing  through 
the  mouth  into  the  pharynx  a  tube  connected  externally  with  a  water  mano- 
meter, that  during  the  act  of  swallowing  there  is  a  sudden  rise  of  pressure 
equal  to  about  twenty  centimeters  of  water. 

These  experiments  demonstrated  that  at  the  beginning  of  deglutition 
there  is  a  sudden  rise  of  pressure,  the  result  of  a  quickly  acting  force  resident 
in  the  mouth  or  pharynx,  in  consequence  of  which  the  food  is  rapidly  thrown 
down  into  the  stomach,  peristalsis  playing  no  part  in  the  process.  The 
proof,  however,  of  these  statements  was  furnished  by  Meltzer.  This  ob- 
server introduced  into  the  pharynx  and  esophagus  rubber  tubes,  the  ends  of 
which  were  provided  with  thin-walled  rubber  balloons  which  could  be 
distended  with  air.  The  outer  ends  of  the  tubes  were  connected  with 
Marey's  recording  tambours.  Any  compression  of  the  balloon  would  be 
followed  by  the  passage  of  the  air  into  the  tambour  and  an*  elevation  of  the 

*  Peristalsis  may  be  defined  as  a  progressive  wave-like  movement  which  passes  over  different 
portions  of  the  walls  of  the  alimentary  canal.  Its  effect  physiologically  is  the  propulsion  of  its 
solid  and  semisohd  contents.  It  is  characterized  by  a  contraction  of  the  muscle-fibers  behind 
the  object  and  an  inhibition  or  relaxation  of  the  muscle-fibers  in  front  of  it.     (Bayliss  and  Starling.) 


DIGESTION. 


159 


lever.  With  one  balloon  in  the  pharynx  and  the  other  in  the  esophagus  at 
varying  depths,  and  the  recording  levers  of  the  tambours  applied  against 
the  surface  of  a  revolving  cylinder,  it  became  possible,  with  the  addition  of  a 
chronograph,  to  obtain  a  graphic  representation  of  the  time  relations  of 
simultaneous  and  successive  compressions  of  the  two  balloons. 

It  was  found  as  the  result  of  many  experiments  that  no  matter  how  deep 
the  position  of  the  esophageal  balloon,  it  was  compressed  almost  simultane- 
ously with  the  pharyngeal  balloon,  as  shown  by  the  rise  of  the  levers  on 
swallowing  a  mouthful  of  water.  The  interval  of  time  between  the  rise  of 
the  two  levers  did  not  amount  to  more  than  the  tenth  of  a  second.  The 
inference  was  that  the  water  was  projected  or  shot  down  the  pharynx  and 
esophagus  in  this  period  of  time,  and  in  its  passage  compressed  both  balloons 
practically  at  the  same  instant.  The  same  was  found  to  be  true  when  small 
masses  of  more  consistent  food  were  swallowed. 

The  curves  of  the  entire  deglutitive  act  recorded  by  the  two  levers  are, 
however,   different   in   form.     (See  Fig.    69.)     The   pharyngeal   curve,    i, 


r 

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3_ 

' —      W  \^ 

Fig.  69. — TiLA.cixG  of  the  Act  of  Deglutition,  i.  A  indicates  the  compression  of  the 
elastic  bag  caused  by  the  bolus  projected  by  the  contraction  of  the  mylohyoid  muscles.  B.  Con- 
traction of  the  pharynx.  2.  Line  marking  seconds.  3.  Tracing  of  the  bag  in  the  esophagus 
12  cm.  from  the  teeth.  C.  Compression  of  the  bag  by  the  bolus  corresponding  to  A.  D.  Com- 
pression by  the  residues  of  the  bolus  carried  on  by  the  contraction  of  the  pharynx,  B.  E.  Contrac- 
tion of  the  esophagus. — (Landois  and  Stirling.) 

presents  two  crests,  the  first.  A,  being  due  to  the  compression  caused  by  the 
passage  of  the  bolus,  the  second,  B,  due  to  the  compression  exerted  by  the 
contraction  of  the  pharyngeal  muscles.  The  interv^al  of  time  between  these 
two  crests  amounts  to  not  more  than  0.3  second.  In  the  esophageal  curv-e, 
3,  the  elevation,  C,  corresponds  to  the  elevation.  A,  and  is  likewise  due  to 
the  compression  exerted  by  the  bolus.  The  interval  of  time  between  the 
beginning  of  the  first  and  second  curves  was  not  more  than  o.i  second, 
regardless  of  the  depth  to  which  the  esophageal  balloon  was  plunged.  At  a 
later  period  a  second  rise  of  the  lever  was  recorded;  the  time  of  its  appear- 
ance, height,  duration,  etc.,  were  found  to  increase  with  the  depth  of  the 
balloon. 

These  facts  demonstrate  that  deglutition  consists  of  two  phases:  (i)  a 
rapid  rise  of  pressure  in  the  pharynx,  as  a  result  of  which  the  bolus  is  sud- 
denly shot  dowh  to  the  lower  end  of  the  esophagus;  (2)  a  peristaltic  contrac- 
tion of  the  musculature  of  the  canal,  which,  acting  as  a  supplementary  force, 
carries  onward  any  particles  of  food  in  the  canal  and  forces  the  bolus  through 
the  closed  sphincter  cardicB  at  the  end  of  the  esophagus. 


i6o  TEXT-BOOK  OF  PHYSIOLOGY. 

The  immediate  cause  of  the  sudden  rise  of  pressure  was  shown  by 
Meltzer  to  be  the  contraction  of  the  mylohyoid  muscles.  When  the  nerves 
going  to  these  muscles  were  divided  in  a  dog,  deglutition  was  practically 
abolished.  These  muscles  are  probably  assisted  in  their  action  by  the 
contraction  of  the  hyoglossus  muscles  as  well  as  the  tongue  itself. 

It  was  also  demonstrated  in  these  experiments  that  the  contraction  of 
the  esophagus  did  not  partake  of  the  character  of  ordinary  peristalsis.  It 
was  found  that  the  esophagus  contracted  in  three  distinct  segments,  corre- 
sponding in  all  probability  to  the  difference  in  the  character  of  their  muscular 
fibers.  The  first  segment,  about  six  centimeters  in  length,  was  found  to 
begin  to  contract  about  1.2  seconds  after  the  beginning  of  the  first  curve  and 
acts  for  2  seconds;  the  second  segment,  about  twelve  centimeters  in  length, 
beginning  to  contract  about  1.8  seconds  or  3  seconds  after  the  beginning  of 
the  first  section,  and  lasting  for  from  5  to  7  seconds;  the  third  segment, 
six  centimeters  in  length,  contracting  from  6  to  7  seconds.  The  beginning 
and  the  end  of  the  contraction  for  each  segment  occurred  simultaneously 
throughout  its  entire  extent.  If,  however,  a  series  of  deglutitory  acts  follow 
each  other  in  quick  succession,  there  is  an  inhibition  of  the  peristaltic  con- 
tractions until  after  the  final  swallow. 

An  examination  of  the  action  of  the  esophagus  during  deglutition,  made 
by  Cannon  and  Moser  with  x-rays  and  the  fluoroscope,  disclosed  the  fact 
that  the  method  of  food  transmission  varied  in  different  animals.  In  the 
cat  and  dog  the  transmission  was  effected  by  peristalsis  alone.  The  time 
required  for  the  food  to  reach  the  stomach  varied  in  the  cat  from  nine  to 
twelve  seconds  and  in  the  dog  from  four  to  five  seconds.  The  descent  of 
the  bolus  was  more  rapid  in  the  upper  than  in  the  lower  part  of  the  esophagus. 
In  man,  liquids  descended  rapidly,  at  the  rate  of  several  feet  a  second,  in 
consequence  of  the  rapid  and  energetic  contraction  of  the  mylohyoid  muscles. 
A  peristaltic  contraction,  passing  over  the  entire  esophagus,  was  necessary 
to  the  passage  of  solid  and  semisolid  food  through  it. 

Closure  of  the  Posterior  Wares  and  Larynx.— Because  of  the  rapid 
rise  of  pressure  in  the  deglutitory  canal  during  the  act  of  swallowing  it  is 
essential  that  the  openings  into  the  nasal  and  laryngeal  cavities  be  closed  to 
prevent  the  entrance  of  food  into  them,  which  would  otherwise  take  place. 
Under  normal  circumstances  this  is  done  so  effectually  that  it  is  seldom  that 
any  portion  of  the  food,  liquid  or  solid,  ever  enters  the  nasal  chambers 
or  the  cavity  of  the  larynx.  The  mechanism  by  which  these  openings  are 
closed  is  as  follows: 

At  the  moment  the  food  passes  into  the  pharynx  the  posterior  nasal  open- 
ings are  closed  against  the  entrance  of  the  food  by  a  septum  formed  by  the 
pendulous  veil  of  the  palate  and  the  posterior  half  arches.  The  palate  is 
drawn  upward  and  backward  by  the  levator  palati  muscles,  until  it  meets 
the  posterior  wall  of  the  pharynx,  which  at  this  moment  advances.  At  the 
same  time  it  is  made  tense,  by  the  action  of  the  tensor  palati  muscles.  (Fig. 
70).  This  septum  is  completed  by  the  advance  toward  the  middle  line  of  the 
posterior  half  arches  caused  by  the  contraction  of  the  muscles,  the  palato- 
pharyngei,  which  compose  them.  When  these  structures  are  impaired  in 
their  functional  activity,  as  in  diphtheritic  paralysis  and  ulcerations,  there 
is  not  infrequently  a  regurgitation  of  food,  especially  liquids,  into  the  nose. 


DIGESTION. 


i6i 


The  larynx  is  equally  protected  against  the  entrance  of  food  during 
deglutition  under  normal  circumstances.  That  this  accident  occasionally 
happens,  giving  rise  to  severe  spasmodic  coughing,  and  even  in  extreme 
cases  to  suffocation,  is  abundantly  shown  by  the  records  of  cHnical  medicine. 
Usually  it  does  not  occur,  for  the  following  reasons:  just  preceding  and  dur- 
ing the  act  of  deglutition  there  is  a  complete  suspension  of  the  act  of  inspira- 
tion, by  which  particles  of  food  might  otherwise  be  drawn  into  the  larynx; 
at  the  same  time  the  larynx  is  always  drawn  well  up  under  the  base  of  the 
tongue  and  its  entrance  closed  by  the 
downward  and  backward  movement 
of  the  epiglottis. 

The  action  here  attributed  to  the 
epiglottis  has  been  denied  by  Stuart 
and  McCormick.  These  observers 
had  the  opportunity  of  looking  into 
a  naso-pharynx  which  had  been  laid 
open  by  a  surgical  operation  for  the 
removal  of  a  morbid  growth.  In  this 
patient,  the  epiglottis,  at  the  time  of 
deglutition,  was  always  more  or  less 
erect  and  closely  applied  to  the  base 
of  the  tongue.  So  complete  was  this 
that  the  food  passed  over  its  posterior 
or  inferior  surface  for  a  certain  dis- 
tance. In  no  instance  was  it  ever  ob- 
served to  fold  backward  like  a  lid. 

Because  of  the  possibility  that  this 
position  of  the  epiglottis  was  due  to 
pathologic  causes,  Kanthack  and  An- 
derson instituted  a  new  series  of  ex- 
periments with  a  view  of  determining 
the  action  of  the  epiglottis.  As  a  result  of  many  experiments  on  animals 
and  of  observations  on  themselves,  these  observers  reaffirm  the  gener- 
ally accepted  view,  that  under  normal  conditions,  the  entrance  of  the  larynx 
is  always  closed  by  the  epiglottis  after  the  manner  of  a  lid. 

In  addition  to  the  downward  and  backward  movement  of  the  epiglottis 
and  the  ascent  of  the  larynx  under  the  base  of  the  tongue,  it  is  also  certain 
from  the  observations  of  Meltzer  that  the  larynx  is  protected  from  the  en- 
trance of  food,  in  the  rabbit  at  least,  by  the  closure  of  the  glottis  itself.  This 
experimenter  noticed,  while  observing  the  interior  of  the  larynx,  both  from 
above,  through  an  opening  in  the  hyothyreoid  membrane,  and  from  below, 
through  an  opening  in  the  trachea,  that  when- an  act  of  deglutition  was  excited 
by  touching  the  soft  palate  with  a  sound,  there  was  simultaneously  with  the 
contraction  of  the  mylohyoid  muscles,  a  firm  closure  of  the  glottis.  This 
was  accomplished  by  an  approximation  of  the  true  vocal  bands,  a  close 
approximation  and  a  downward  and  forward  movement  of  the  arytenoid 
cartilages,  until  they  almost  touched  the  anterior  wall  of  the  thyreoid  carti- 
lage. This  movement  preceded  the  ascent  of  the  larynx.  When  the  larynx 
was  separated  from  all  surrounding  structures  with  the  exception  of  the 


^VLcoryjiX' 
Trctciiea/ 


Fig.  70. — Diagram  Showing  the  Man- 
ner or  Closure  of  the  Posterior  Nares 
AND  Larynx  during  Deglutition. —  {Lan- 
dois  and  Stirling) 


i62  TEXT-BOOK  OF  PHYSIOLOGY. 

laryngeal  nerves,  a  touch  of  the  palate  excited  the  same  phenomenon.  Under 
such  circumstances  the  closure  of  the  glottis  must  have  been  due  to  the  con- 
traction of  its  own  intrinsic  muscles  and  in  consequence  of  a  reflex  action 
through  the  inferior  laryngeal  nerves. 

The  Nerve  Mechanism  of  Deglutition. — Deglutition  is  almost  exclu- 
sively a  reflex  act  throughout  its  entire  extent,  and  requires  for  its  inaugura- 
tion merely  a  stimulus  to  some  portion  of  the  mucous  membrane  of  the  deglu- 
titory  canal.  The  first  stage  is  primarily  voluntary,  but  from  inattention  to 
the  process  may  become  secondarily  reflex.  The  origin  and  course  of  the 
afferent  ners'es,  stimulation  of  which  excite  reflexly  the  movements  of  the 
pharynx  and  esophagus,  however,  are  practically  unknown.  In  the  rabbit 
deglutition  can  be  excited  by  stimulating  the  anterior  central  part  of  the  soft 
palate;  in  man  it  has  not  yet  been  possible  to  locate  an  area  stimulation  of 
which  will  give  rise  to  a  reflex  deglutitory  act.  Though  electric  stimulation 
of  the  superior  laryngeal  nerve  will  cause  reflex  deglutitory  movements,  it  is 
obvious  that  the  terminals  of  this  nerve  cannot  be  the  source  of  the  natural 
afferent  impulses.  Stimulation  of  the  glosso-pharyngeal  nerve  causes  an 
inhibition  of  the  movements. 

The  center  from  which  emanate  ner\-e  impulses  which  excite  the  various 
muscles  to  action  has  been  located  experimentally  in  the  medulla  oblongata 
just  above  the  alse  cinereae.  The  efferent  nerA'es  comprise  branches  of  the 
facial,  hypoglossal,  motor  filaments  of  the  third  division  of  the  fifth  nerve, 
motor  filaments  of  the  glosso-pharyngeal  and  vagus  nerves  derived  in  all 
probability  directly  from  the  medulla  oblongata.  Inasmuch  as  the  different 
mechanisms  of  this  reflex,  act  not  only  in  a  coordinate  but  sequential  manner, 
it  would  appear  as  if  the  deglutition  center  sent  out,  in  response  to  the  nerve 
impulses  coming  from  a  single  peripheral  area,  a  series  of  ners-e  impulses 
successively  to  successive  portions  of  the  canal,  through  the  groups  of  nerve- 
cells  corresponding  to  the  origins  of  the  efferent  nerves.  That  this  orderly  and 
progressive  peristalsis  usually  observed  is  due  to  a  sequence  of  changes  in  the 
central  nerve  system  is  shown  by  the  fact,  that  if  the  esophagus  is  divided  or 
a  ring  of  it  excised,  the  extremity  in  connection  with  the  stomach  will  exhibit 
a  well-marked  peristalsis  after  a  short  inten-al,  when  an  act  of  deglutition  is 
excited  in  the  customary  manner.  The  efferent  nen-e  fibers,  which  stimulate 
the  esophageal  muscles  to  action  are  contained  in  the  trunk  of  the  vagi  nen^es 
for  after  their  division  the  peristalsis  is  abolished. 

In  addition  to  this  primary  reflex  mechanism,  the  esophagus  appears  to 
possess  a  secondary  reflex  mechanism  consisting  of  a  series  of  reflex  arcs, 
whose  afferent  and  efferent  paths  are  found  in  the  trunk  of  the  vagus  and 
both  connected  with  successive  portions  of  the  esophagus.  The  first  mechan- 
ism is  temporarily  suspended  during  deep  anesthesia  while  the  second  per- 
sists.    (Meltzer.) 

Though  the  peristalsis  of  the  esophagus  is  excited  by  nen-e  impulses 
coming  through  the  vagus  nerves  and  is  abolished  by  their  division,  Cannon 
has  shown  by  means  of  the  Rontgen  rays  that  this  effect  for  the  lower  portion 
of  the  esophagus,  at  least  in  the  cat  and  monkey,  is  of  a  temporary  duration 
only,  lasting  from  one  to  several  days,  after  which  a  peristalsis  again  develops 
with  sufficient  vigor  to  force  food  through  the  cardiac  orifice  into  the  stom- 
ach.    The  muscle  coat  of  this  portion  of  the  esophagus  is  composed  of  non- 


DIGESTION. 


163 


striated  muscle-fibers,  is  supplied  with  a  myenteric  nerve  plexus  and  resem- 
bles lower  portions  of  the  alimentary  canal.  It  is  capable  of  developing  a 
peristalsis  merely  in  response  to  the  pressure  of  food  within  and  independent 
of  extrinsic  nerves. 

GASTRIC  DIGESTION. 

After  the  food  has  passed  through  the  esophagus  it  is  received  by  the 
stomach,  where  it  is  retained  for  a  variable  length  of  time,  during  which 
important  changes  are  induced  in  its  physical  and  chemic  composition. 
The  disintegration  of  the  food  inaugurated  by  mastication  and  insalivation 
is  still  further  carried  on  in  the  stomach  by  the  solvent  action  of  the  acid  fluid 
there  present,  until  the  entire  mass  is  reduced  to  a  liquid  or  semi-liquid 
condition. 

The  stomach  is  the  dilated  and  highly  specialized  portion  of  the  alimen- 
tary canal  intervening  between  the  esophagus  and  small  intestine.     When 


oblique:  fibers  of 
muscle  coat 


POSITION  OF  THE 
SPHINCTER   CARDIAE 

ESOPHAGO-GASTRIC 

x^  orifice;the  cardia. 

FUNDUS 


DUODENUM 

PYLORUS 

ANTRUM 

SPHINCTER 
ANTRI  PYLORICl 


CIRCULAR    FIBERS 

OF 
MUSCLE   COAT 


PREANTRAL  OR  CARDIAC    REGION 

Fig.  71. — ^Anatomic  Features  of  xhe  Stomach. 

moderately  distended  with  food,  it  is  somewhat  conical  or  pyriform  in  shape 
and  slightly  curved  on  itself.  It  is  situated  obliquely  and  in  some  individuals 
almost  vertically  in  the  upper  part  of  the  abdominal  cavity,  extending  from 
the  left  hypochondrium  to  the  right  of  the  epigastrium.  The  dimensions 
and  capacity  of  the  stomach  undergo  considerable  periodic  variation  accord- 
ing to  the  extent  to  which  it  is  distended  by  food.  In  the  average  condition 
it  measures  in  its  long  diameter  from  25  to  35  centimeters,  in  its  vertical 
diameter  at  the  cardia  15  centimeters,  in  its  antero-posterior  diameter  from 
II  to  12  centimeters.  The  capacity  of  the  stomach  varies  from  1500  to  1700 
c.c.  In  the  empty  condition  its  walls  are  contracted  and  partly  in  contact, 
and  the  entire  organ  is  drawn  up  into  the  upper  part  of  the  abdominal  cavity. 
The  opening  through  which  the  food  passes  into  the  stomach  is  known  as 
the  esophagogastric  orifice  or  the  cardia.  The  opening  through  which  it 
passes  into  the  intestine  is  known  as  the  pylorus,  the  pyloric  or  gastro- 
duodenal  orifice.     Between  these  two  orifices  the  stomach  along  its  upper 


i64  TEXT-BOOK  OF  PHYSIOLOGY. 

border  presents  a  curve  and  along  its  lower  border  a  much  larger  curve, 
known  as  the  lesser  and  greater  curvatures  respectively.  The  extreme  left 
end  of  the  stomach  is  termed  the  fundus.  Toward  the  pyloric  orifice  there 
is  a  region  of  the  stomach  included  between  the  pylorus  and  a  line  uniting  a 
small  indentation  on  the  lesser  curvature  with  a  point  or  angle  almost  op})o- 
site  on  the  greater  curvature,  known  as  the  antrum.  The  region  included 
between  the  ill-defined  limits  of  the  fundus  and  the  antrum  is  known  as 
the  preantral  or  cardiac  region. 

The  walls  of  the  stomach  are  formed  by  four  distinct  coats  united  by 
areolar  tissue  and  named,  from  without  inward,  as  the  serous,  muscle, 
submucous,  and  mucous. 

The  external  or  serous  coat  is  thin  and  transparent  and  formed  by  a 
reduplication  of  the  general  peritoneal  membrane. 

The  middle  or  muscle  coat  consists  of  three  layers  of  non-striated  muscle- 
fibers,  named  from  their  direction  the  longitudinal,  circular,  and  oblique. 
The  longitudinal  fibers  are  most  abundant  along  the  lesser  curvature 
and  are  a  continuation  of  those  of  the  esophagus;  over  the  remainder  of 
the  stomach  they  are  thinly  scattered,  but  toward  the  pyloric  orifice  they 
are  more  numerous  and  form  a  tolerably  thick  layer  which  becomes  con- 
tinuous with  the  fibers  of  the  small  intestine.  The  circular  fibers  form  a 
complete  layer  encircling  the  entire  organ,  with  the  exception,  perhaps,  of  a 
portion  of  the  fundus.  The  fibers  of  this  coat  cross  the  longitudinal  fibers 
at  right  angles.  At  the  lower  end  of  the  esophagus  and  surrounding  the 
cardia  the  circular  muscle  fibers  form  a  true  sphincter  which  is  known  as 
the  sphincter  cardice.  At  the  juntion  of  the  antrum  with  the  preantral  region 
the  circular  fibers  are  arranged  in  a  well-defined  bundle  termed  the  sphincter 
antri  pylorici.  In  the  pyloric  region  the  circular  fibers  are  more  closely 
arranged,  forming  thick  well-defined  rings  termed  the  antral  muscles.  At 
the  pyloric  opening  the  circular  fibers  are  again  crowded  together  and 
form  a  distinct  muscle  band — the  sphincter  pylori — which  projects  for  some 
distance  into  the  interior  of  the  stomach.  It  has  been  stated  by  Riidinger 
that  the  inner  fibers  of  the  longitudinal  coat  become  connected  with  this 
circular  band  and  constitute  a  distinct  muscle,  the  dilatator  pylori.  The 
oblique  fibers  are  most  distinct  over  the  cardiac  portion  of  the  stomach, 
but  extend  from  left  to  right  as  far  as  the  junction  of  the  middle  and  last 
thirds  of  the  stomach.  They  are  continuations  of  the  circular  fibers  of  the 
esophagus. 

The  submucous  coat  consists  of  loose  areolar  tissue  carrying  blood-vessels, 
nerves,  and  lymphatics.  It  serves  to  unite  the  muscle  to  the  mucous 
coat.  Its  inner  surface  bears  a  thin  layer  of  muscular  tissue,  the  muscularis 
mucoscB,  which  supports  the  mucous  membrane. 

The  internal  or  mucous  coat  is  loosely  attached  to  the  muscular  coat.  In 
the  empty  and  contracted  state  of  the  stomach  it  is  thrown  into  longitudinal 
folds,  or  rugae,  which  are,  however,  obliterated  when  the  organ  is  distended 
with  food.  The  mucous  membrane  in  adult  life  is  smooth  and  velvety  in 
appearance,  gray  in  color,  and  covered  with  a  layer  of  mucus.  Its  average 
thickness  is  about  one  millimeter.  The  surface  of  the  membrane  is  covered 
with  a  layer  of  columnar  epithelial  cells.  At  the  pylorus  there  is  a  circular 
involution  of  the  mucous  membrane  which  is  known  as  the  pyloric  valve. 


DIGESTION. 


165 


This  is  strengthened  by  fibrous  tissue  and  embraced  by  the  sphincter  muscle 
previously  described. 

Gastric  Glands. — The  surface  of  the  mucous  membrane  when  examined 
with  a  low  magnifying  power  presents  throughout  innumerable  depressions 
polygonal  in  shape  and  separated  by  slightly  elevated  ridges.  At  the  bottom 
of  these  spaces  are  to  be  seen  small  orifices,  which  are  the  mouths  of  the 
glands  embedded  in  the  mucous  membrane.  A  vertical  section  of  the 
gastric  walls  shows  not  only  the  position  and  the  appearance  of  the 
glands,  but  the  relation  of  the  various  tissues  which  enter  into  the  formation 
of  these  walls.  An  examination  of  the  mucous  membrane  in  dift"erent 
regions  of  the  stomach  reveals  the  presence  of  two 
m  ^"A^SL  distinct  types  of  glands,  which  from  their  situation  are 
^  r^  termed  preantral  or  cardiac,  and  pyloric,  which  differ 
not  only  in  histologic  structure,  but  also  in  function. 
Both  types  extend  through  the  entire  thickness  of  the 
mucosa. 

The  preantral  or  cardiac  glands  are  formed  by  an 
involution  of  the  basement  membrane  of  the  mucosa 
and  lined  by  epithehal  cells.  Each  gland  may  be  said 
to  consist  of  a  short  duct,  or  neck,  and  a  body,  or  fun- 
dus (Fig.  72).  The  latter  portion  is  wavy  or  tortuous 
and  frequently  subdivided  into  as  many  as  four  dis- 


/  ^ 
Fig.  72. — Preantral 
OR  Cardl^c  Gl.'\xd.  m 
Mouth  of  the  duct:  w,  neck; 
/.  fundus;  c,  central  cells; 
p,  parietal  cells. 


Fig.  73-  — iLcriox  of  Fundus  Gland 
OF  }^IousE.  Left  upper  half  drawn  after 
an  alcohol  preparation,  right  upper  half 
after  a  Golgi  preparation.  The  entire 
lower  portion  is  a  diagrammatic  combi- 
nation of  both  preparations.      (St-ilir.') 


tinct  and  separate  tubules.  The  duct  is  lined  by  columnar  epithehal  cells 
similar  to  those  covering  the  surface  of  the  mucosa.  The  lumen  of  the 
gland  is  bordered  by  epithelial  cells,  cuboid  in  shape,  and  consisting  of  a 
granular  protoplasm  containing  a  distinct  spherical  nucleus.  These  cells 
are  generally  spoken  of  as  the  chief  or  central  cells.  In  addition  to  the 
chief  cells,  the  preantral  or  cardiac  glands  contain  a  second  variety  of 
cell,  which  is  of  a  larger  size,  of  a  triangular  or  oval  shape,  and  consisting  of  a 
finely  granular  protoplasm.  From  their  situation  in  and  just  beneath  the 
gland  wall  they  have  been  termed  parietal  or  border  cells.     Each  parietal 


i66 


TEXT-BOOK  OF  PHYSIOLOGY. 


cell  appears  to  be  surrounded  and  penetrated  by  a  system  of  passages  which 
open  into  the  lumen  of  the  gland  by  means  of  a  delicate  cleft  or  canaliculus 
(Fig.  78).  Glands  with  these  histologic  features  are  most  abundant  in  the 
middle  zone  of  the  stomach. 

The  pyloric  glands  are  also  formed  by  an  involution  of  the  mucous  mem- 
brane and  lined  by  epithelial  cells  (Fig.  74).  The  ducts  are  much  longer 
than  the  ducts  of  the  fundic  glands.  At  its  extremity  each  duct  becomes 
branched,  gi\ang  rise  to  a  number,  from  2  to  10,  of  short  tubes,  each  of  which 
has  a  large  lumen  and  communicates  with  the  duct  by  a  narrow  short  neck. 
The  ducts  are  lined  throughout  by  columnar  epi- 
thelium. According  to  Mall,  the  total  number  of 
openings  on  the  surface  of  the  mucous  membrane  of 
the  dog's  stomach  is  somewhat  over  1,000,000,  and 
the  total  number  of  blind  tubes  opposite  the  muscu- 
laris  mucosae  exceeds  16,500,000.  According  to 
Sappey,  the  surface  of  the  mucous  membrane  of  the 
human  stomach  presents  over  5,000,000  orifices  of 
gastric  glands. 

Blood-vessels,  Nerves,  and  Lymphatics. — The 
blood-vessels  of  the  stomach  after  entering  the  mucosa 
break  up  into  a  number  of  branches  which  are  dis- 
tributed to  the  muscle  and  mucous  coats.  The 
branches  to  the  latter  soon  form  a  capillary  network 
with  oblong  meshes  which  not  only  surround  the 
tubules  but  form  a  network  just  beneath  the  surface 
of  the  mucosa.  Veins  gradually  arise  from  the 
capillaries  which  empty  into  the  larger  veins  of  the 
mucosa.  The  glands  are  also  supported  by  proc- 
esses of  smooth  muscle-fibers  passing  up  from  the 
muscularis  mucosae. 

The  nerve-fibers  distributed  to  the  stomach  are 
derived  from  the  vagus  and  the  sympathetic  branches 
of  the  solar  plexus.  After  piercing  the  serous  coat 
the  fibers  form  or  unite  with  a  plexus  of  fibers  situa- 
ted between  the  circular  and  longitudinal  layers  of 
the  muscle-coat.  At  the  nodal  points  of  this  plexus 
large  nen^e-ganglion  cells  are  to  be  found,  the  whole 
forming  the  mechanism  known  as  Auerbach's  plexus.  A  similar  plexus 
of  cells  and  fibers  in  more  or  less  intimate  anatomic  connection  with  the 
foregoing  is  found  between  the  muscle  and  submucous  coats,  and  is  known 
as  Meissner's  plexus.  From  this  plexus  fine  nerve  filaments  are  distrib- 
uted to  muscle-fibers,  blood-vessels,  and  glands.  In  the  latter  structure  ter- 
minal arborizations  have  been  detected  in  close  contact  with  the  secreting 
cells  themselves. 

The  lymphatics,  which  are  c|uite  numerous,  originate  in  the  meshes  of  the 
mucosa.  The  larger  trunks  enter  lymph-glands  lying  along  the  greater  and 
lesser  cur\'atures  of  the  stomach. 

Gastric  Fistulae. — The  general  process  of  digestion,  as  it  takes  place  in 
the  stomach,  has  been  studied  in  human  beings  and  animals  with  a  fistula  in 


Fig.  74.  —  Pyloric 
Gland  of  the  Stomach. 
m,  Mouth  of  duct;  «,  neck. 


DIGESTION, 


167 


the  walls  of  the  stomach  and  abdomen,  the  result  either  of  accident  or  of 
necessary  surgical  or  experimental  procedures. 

The  earhest  observations  on  gastric  digestion  were  made  by  Dr.  Beau- 
mont on  Alexis  St.  Martin,  who,  as  the  result  of  a  gunshot  wound,  was  left 
with  a  permanent  fistulous  opening  into  the  fundus  of  the  stomach.  This 
opening  two  years  after  the  accident  was  about  two  and  a  half  inches  in 
circumference  and  usually  closed  from  within  by  a  fold  of  mucous  mem- 
brane which  prevented  the  escape  of  the  food.  This  valve  could  be  readily 
displaced  by  the  finger  and  the  interior  of  the  stomach  exposed  to  view. 
After  the  complete  recovery  of  St.  Martin,  Dr.  Beaumont  during  the  years 
between  1825  and  183 1  at  intervals  made  numerous  experiments  on  the 
nature  of  gastric  digestion.  As  the  result  of  an  admirable  series  of  investi- 
gations it  was  estabhshed  that  the  digestion  of  the  food  is  largely  a  chemic 
act,  due  to  the  presence  of  an  acid  fluid  secr.eted  by  the  mucous  membrane; 
that  this  fluid  is  secreted  most  abundantly  after  the  introduction  of  food  into 
the  stomach;  that  difl'erent  articles  of  food 
possess  varying  degrees  of  digestibility;  that 
the  duration  of  digestion  varies  according  to 
the  nature  of  the  food,  exercise,  mental  states, 
etc.,  and  that  the  process  is  aided  by  contin- 
uous movements  of  the  muscle  walls. 

Since  Dr.  Beaumont's  time  the  establishing 
of  a  gastric  fistula  in  human  beings  has  been 
necessitated  by  pathologic  conditions  of  the 
esophagus.  After  recovery  these  cases  offered 
fair  facilities  for  the  study  of  the  process  when 
the  food  was  introduced  through  the  opening. 
Similar  fistulae  have  been  established  in  both 
carnivorous  and  herbivorous  animals  with  a 
view  of  studying  the  process  as  it  takes  place 
in  them.  The  results  obtained  in  these  in- 
stances in  many  respects  corroborate  those 
obtained  by  Dr.  Beaumont,  though  many  new 
facts,  unobser^-ed  by  him,  have  been  brought 
to  light. 

Much  additional  information  as  to  the  mode  of  secretion  and  the  char- 
acteristics of  the  gastric  juice  has  been  obtained,  since  the  introduction  of  two 
new  procedures  by  Pawlow.  The  first  consists  in  establishing  a  gastric 
fistula  and  subsequently  dividing  the  esophagus  in  the  neck,  and  then  so 
adjusting  the  divided  ends  that  they  heal  separately  into  an  angle  of  the  skin 
incision.  The  second  procedure  consists  in  forming  a  diverticulum  or  pouch 
out  of  the  cardiac  end  of  the  stomach  which  opens  on  the  surface  of  the  ab- 
domen but  is  separated  from  the  rest  of  the  stomach  by  a  thin  septum  formed 
of  two  layers  of  mucous  membrane.  (Fig.  75.)  The  serous  and  muscle-coats 
of  this  pouch  are  in  direct  continuity  with  the  large  stomach  and  all  possess 
the  same  vascular  and  nerve  connections.  Because  of  this  fact  this  miniature 
stomach,  about  one-tenth  the  size  of  the  natural  stomach,  exhibits  the  same 
phenomena,  so  far  as  the  secretion  of  the  gastric  juice  is  concerned,  as  the 
large  stomach  does.     The  phenomena  which  are  observed  in  it  may  be  taken 


Fig.  75. — Diagram  Showing 
THE  Relation  of  the  Natural 
Stomach  to  the  Miniature 
Stomach  or  Pouch  made  Ac- 
cording TO  THE  Procedure  De- 
vised BY  Pawlow.  V.  The  nat- 
ural stomach.  5.  The  miniature 
stomach,  e,  e.  The  septum  formed 
by  the  mucous  membrane.  A,  A. 
The  abdominal  walls. 


i68  TEXT-BOOK  OF  PHYSIOLOGY. 

as  an  indication  as  to  the  phenomena  which  are  taking  place  in  the  natural 
stomach. 

By  the  first  procedure  it  is  possible  to  feed  an  animal  with  different  kinds 
of  food  and  to  observe  the  effects  of  psychic  states  on  the  secretion  of  gastric 
juice.  As  the  swallowed  food  is  discharged  from  the  lower  end  of  the 
divided  esophagus  the  appetite  continues,  and  hence  the  animal  will  eat  for 
several  hours.  By  the  second  procedure  it  is  possible  to  collect  gastric  juice 
from  the  miniature  stomach  and  to  study  the  effects  on  its  quantity  and 
quality  produced  by  psychic  states,  mastication,  different  articles  of  food, 
and  by  the  process  of  digestion  itself  as  it  goes  on  in  the  large  stomach.  In 
both  instances  the  juice  is  obtained  free  from  admixture  with  saliva  or  food. 

Gastric  Juice. — The  gastric  juice  obtained  from  the  human  stomach 
free  from  mucus  and  other  impurities  is  a  clear,  colorless  fluid  with  a  con- 
stant acid  reaction,  a  slightly  saline  and  acid  taste,  and  a  specific  gravity 
varying  from  1.002  to  1.005.  The  juice  obtained  from  the  dog's  stomach 
possesses  essentially  the  same  characteristics,  though  its  acidity  as  well  as  its 
specific  gravity  are  slightly  greater.  When  kept  from  atmospheric  influences, 
it  resists  putrefactive  change  for  a  long  period  of  time,  undergoes  no  apparent 
change  in  composition,  and  loses  none  of  its  digestive  power.  It  will  also 
prevent  and  even  arrest  putrefactive  change  in  organic  matter.  The  chemic 
composition  of  the  gastric  juice  has  never  been  satisfactorily  determined, 
owing  to  the  fact  that  the  secretion  as  obtained  from  fistulous  openings  has 
not  been  absolutely  normal.  The  following  analyses  represent  the  com- 
position of  a  sample  obtained  by  Schmidt  from  the  stomach  of  a  woman  who 
had  a  fistula,  but  who  was  nevertheless  in  good  health;  also  the  composition 
of  the  juice  from  a  dog: 

COMPOSITION  OF  GASTRIC  JUICE. 

Human.  Dog. 

Water 994.04  973.06 

Organic  matter 3  ■  19  ^7  •  ^3 

Hydrochloric  acid o .  20  3-34 

Calcium  chlorid 0.06  0.26 

Sodium  chlorid i .  46  2  .  50 

Potassium  chlorid o  •  55  112 

Calcium  phosphate ]  i-73   ' 

Magnesium  phosphate J-   0.12  023 

Ferric  pjhosphate .J  o  .08 

Ammonium  chlorid o  .47 

The  organic  matter  present  in  gastric  juice  is  a  mixture  of  mucin  and  a 
protein,  products  of  the  metabolic  activity  of  the  epithelial  cells  on  the  sur- 
face of  the  mucous  membrane  and  of  the  chief  or  central  cells  of  the  gastric 
glands  respectively.  Associated  with  the  protein  material  are  two  ferment 
or  enzyme  bodies,  termed  pepsin  and  rennin.  As  is  the  case  with  other 
enzymes,  their  true  chemic  nature  is  practically  unknown. 

Pepsin,  though  present  in  gastric  juice,  is  not  present  as  such  in  the  chief 
cells  of  the  glands,  but  is  derived  from  a  zymogen,  propepsin  or  pepsinogen, 
when  the  latter  is  treated  with  hydrochloric  acid.  This  antecedent  com- 
pound is  related  to  the  granules  observed  in  and  produced  by  the  cell  proto- 
plasm during  the  period  of  rest.  Though  pepsin  is  largely  produced  by  the 
central  cells  of  the  preantral  glands,  it  is  also  produced,  though  in  less  amount. 


DIGESTION.  169 

by  the  cells  of  the  pyloric  glands.  Pepsin  is  the  chief  proteolytic  or  proteo- 
clastic  agent  of  the  gastric  juice  and  exerts  its  influence  most  energetically 
in  the  presence  of  hydrochloric  acid  and  at  a  temperature  of  about  40°  C. 
Other  acids— ?.^.,  phosphoric,  nitric,  lactic,  etc.— are  also  capable  of  exciting 
it  to  activity,  though  with  less  intensity. 

Rcnnin  or  pexin  is  present  in  the  gastric  juice  not  only  of  man  and  all  the 
mammalia,  but  also  of  birds  and  even  fish.  In  its  origin  from  a  zymogen 
substance ;  in  its  relation  to  an  acid  medium  and  an  optimum  temperature 
it  bears  a  close  resemblance  to  pepsin.  Its  specific  action  is  the  coagulation 
of  milk,  a  condition  due  to  a  transformation  of  soluble  caseinogen  into  a 
solid  flaky  body,  casein. 

Hydrochloric  acid  is  the  agent  which  gives  to  the  gastric  juice  its  normal 
acidity.  Though  the  juice  frequently  contains  lactic,  acetic,  and  even  phos- 
phoric acids,  it  is  generally  believed  that  they  are  the  result  of  fermentation 
changes  occurring  in  the  food,  the  result  of  bacterial  action.  The  percentage 
of  hydrochloric  acid  has  been  the  subject  of  much  discussion.  The  analysis 
of  human  gastric  juice  made  by  Schmidt  shows  a  percentage  of  0.02,  while ' 
that  of  the  dog  is  0.34.  It  is  probable,  however,  that  the  low  percentage  of 
HCl  in  human  gastric  juice  was  due  to  the  admixture  with  saliva.  At  pres- 
ent it  is  believed  from  analyses  made  for  cHnical  purposes  that  the  acid  is 
present  to  the  extent  of  at  least  0.2  per  cent.  This  degree  of  acidity  is  not 
constant  during  the  entire  process  of  digestion.  In  the  earher  as  well  as  in 
the  later  stages  it  is  much  less. 

The  immediate  origin  of  the  hydrochloric  acid  is  difficult  of  explanation. 
That  it  is  derived,  however,  primarily  from  the  chlorids  of  the  food  and 
secondarily  from  the  chlorids  of  the  blood-plasma  has  been  established  by 
direct  experiment.  If  all  the  chlorids  be  removed  from  the  food  and  all 
the  chlorids  be  withdrawn  from  the  animal  tissues  by  the  administration  of 
various  diuretics — e.g.,  potassium  nitrate — there  will  be  a  total  disappearance 
of  hydrochloric  acid  from  the  stomach.  On  the  addition  of  sodium  or  potas- 
sium chlorids  to  the  food,  there  is  at  once  a  reappearance  of  the  acid. 

As  to  the  nature  of  the  process  by  which  the  acid  is  formed,  nothing 
definite  is  known.  Various  theories  of  a  chemic  and  physical  character 
have  been  offered,  all  of  which  are  more  or  less  unsatisfactory.  As  no  hydro- 
chloric acid  is  found  either  in  the  blood  or  lymph,  the  most  plausible  \'iew  as 
to  its  origin  is  that  w^hich  regards  it  as  one  of  the  products  of  the  metaboHsm 
of  the  gland-cells,  and  more  particularly  of  the  parietal  or  border  cells,  and 
w'hich  for  this  reason  have  been  termed  acid-producing  or  oxyntic  cells. 
From  the  chlorids  furnished  by  the  blood  the  chlorin  is  derived,  which, 
uniting  with  hydrogen,  forms  the  HCl.  The  base  set  free  returns  to  the 
blood,  which  in  part  accounts  for  its  increased  alkaHnity  during  digestion  as 
well  as  the  diminished  acidity  of  the  urine.  The  acid  thus  formed  passes 
through  the  canahculi,  which  penetrate  and  surround  the  cells,  into  the 
lumen  of  the  gland. 

Hydrochloric  acid  exerts  its  influence  in  a  variety  of  ways.  It  is  the 
main  agent  in  the  derivation  of  pepsin  and  rennin  or  pexin  from  their  ante- 
cedent zymogen  compounds,  pepsinogen  and  pexinogen  (Warren) ;  it  imparts 
actiAity  to  these  ferments;  it  prevents  and  even  arrests  fermentative  and 
putrefactive  changes  in  the  food  by  destroying  microorganisms;  it  softens 


I70  TEXT-BOOK  OF  PHYSIOLOGY. 

connective  tissue,  it  dissolves  and  acidifies  the  proteins,  thus  making  possible 
the  subsequent  action  of  pepsin. 

The  inorganic  salts  of  the  gastric j'uice  are  probably  only  incidental  and 
play  no  part  in  the  digestive  process. 

Mode  of  Secretion. — The  observations  of  Dr.  Beaumont  and  the  experi- 
ments of  many  physiologists  have  made  it  certain  that  the  secretion  of  the 
gastric  juice  is  intermittent  and  not  continuous,  that  it  is  only  on  the  intro- 
duction and  digestion  of  the  food  that  the  normal  amount  is  poured  out. 
During  the  intervals  of  digestive  activity  the  stomach  is  practically  free  from 
all  traces  of  the  juice.  The  mucous  membrane  is  pale  and  covered  w^ith  a 
layer  of  mucus  having  an  alkaline  or  neutral  reaction.  The  introduction, 
however,  of  small  portions  of  food  or  irritation  with  a  glass  rod  causes  a 
change  in  the  appearance  of  the  mucous  membrane.  At  the  points  of 
irritation  the  membrane  becomes  red  and  vascular  and  in  a  few  minutes 
small  drops  of  a  secretion  make  their  appearance;  these  coalesce  and  run 
down  the  sides  of  the  stomach. 

The  statements  of  Beaumont  and  many  subsequent  investigators  that 
the  secretion  thus  obtained  is  gastric  juice  have  been  apparently  disproved 
by  Pawlow,  who  asserts  that  it  is  only  an  alkaline  mucous  the  function  of 
which  is  protective  in  character.  According  to  this  investigator,  mechanic 
stimulation  is  incapable  of  exciting  the  secretion. 

The  primary  stimulus  to  gastric  secretion,  according  to  Pawlow,  is  a 
psychic  state  induced,  on  the  one  hand,  by  the  sight  or  the  odor  of  food 
especially  if  the  animal  is  hungry  and  the  food  appetizing;  and  on  the  other 
hand  by  the  mastication  of  food  which  is  agreeable  to  the  animal.  Thus 
when  a  dog  was  tempted  by  the  sight  of  food,  the  secretion  made  its  appear- 
ance at  the  end  of  six  minutes  and  during  the  time  of  the  experiment,  which 
lasted  for  an  hour  and  a  half,  80  cubic  centimeters  of  the  juice  were  obtained. 
This  is  known  as  psychic  or  appetite  juice.  The  character  of  a  psychic 
state,  however,  greatly  influences  the  amount  of  the  juice  secreted.  Agree- 
able emotions  increase,  depressing  emotions  inhibit  it.  Again  when  a  dog 
with  a  divided  esophagus  and  a  gastric  fistula  was  subjected  to  sham  feeding, 
mastication  continued  for  five  or  six  hours  during  which  time  700  cubic 
centimeters  of  juice  were  obtained  from  the  stomach.  Similar  results  have 
been  obtained  in  human  beings  with  an  occluded  esophagus  and  a  gastric 
fistula.  It  is  evident  from  these  facts  that  the  secretion  of  gastric  juice  is 
favorably  influenced  by  the  sight  and  odor  of  appetizing  food,  by  exhilarating 
emotional  states  and  thorough  mastication. 

As  a  result  of  the  psychic  states  induced  by  the  sight  and  odor  of  food  and 
of  the  taste  of  food  during  mastication,  nerve  impulses  not  only  descend  from 
the  brain  but  are  also  transmitted  from  the  mouth  through  afferent  nerves, 
to  some  central  mechanism;  and  that  from  this  mechanism,  nerve  impulses 
must  in  turn  be  discharged  to  be  transmitted  through  efferent  nerve  fibers 
which  are  distributed  to  the  epithelium  of  the  gastric  glands.  Experimental 
investigations  render  it  probable  that  the  central  mechanism  is  located  in  the 
medulla  oblongata  and  that  the  efferent  path  for  the  secretor  fibers  lies  in  the 
trunk  of  the  vagus  nerve.  Though  this  nerv^e  has  been  the  subject  of  much 
experimentation,  the  results  which  have  been  obtained  have  not  been  uni- 
form.    The  investigations  of  Pawlow  seem  to  be  the  most  reliable.     He 


DIGESTION.  171 

found  that  after  division  of  the  nen'e,  secretion  was  arrested,  and  that  stim- 
ulation of  the  peripheral  ends  with  induced  electric  currents  at  the  rate  of  one 
or  two  per  second,  caused  after  a  latent  period  of  several  minutes'  duration 
a  flow  of  gastric  juice.  Coincidently  with  the  development  of  the  psychic 
secretion  there  is  a  dilatation  of  the  gastric  blood-vessels  and  an  increase  in 
the  supply  of  blood  to  the  gastric  glands.  Whether  this  is  due  to  the  action 
of  vaso-dilatator  fibers  or  to  an  inhibition  of  the  action  of  vaso-constrictor 
fibers  is  uncertain. 

Though  the  secretion  of  the  gastric  juice  can  be  initiated  by  these  means, 
the  amount  secreted  is  but  small  compared  with  the  quantity  secreted  after 
digestion  has  begun.  Then  it  is  that  the  blood-vessels  dilate  to  their  full 
capacity  and  furnish  for  several  hours  the  requisite  materials  for  the  pro- 
duction of  the  juice  on  a  relatively  large  scale.  That  some  factor  is  active 
in  keeping  up  the  secretion  in  the  stomach,  is  apparent  from  the  in- 
crease in  the  quantity  and  the  change  in  the  quality  of  the  juice  secreted  by 
the  miniature  stomach. 

The  secondary  stimulus  to  the  gastric  secretion  is  in  all  probability 
chemic  in  character  and  developed  in  the  stomach  or  in  its  walls  during 
digestive  activity,  inasmuch  as  the  secretion  takes  place  independent  of 
nerve  influences  and  after  division  of  all  afferent  and  eft'erent  ner\-es  that 
pass  from  and  to  the  stomach.  On  the  assumption  that  this  factor  might 
be  developed  in  the  walls  of  the  stomach  itself,  Edkins  conducted  a  series  of 
experiments,  the  results  of  which  lead  to  the  inference  that  there  is  developed 
in  the  pyloric  mucous  membrane,  by  the  action  of  certain  articles  of  food, 
e.g.,  dextrin,  meat  broths,  soups,  etc.,  or  by  the  first  products  of  digestive 
activity,  a  chemic  agent,  which  is  absorbed  by  the  blood,  is  carried  to  the  glands 
throughout  the  stomach  and  which,  on  reaching  the  glands,  stimulates  their 
cells  in  a  specific  manner.  For  this  reason  it  has  been  called  the  gastric 
hormone  or  the  gastric  secretin.  Whatever  the  agent  or  the  mechanism  may 
be,  there  is  not  only  an  increase  in  the  quantity  but  a  change  in  the  quality 
of  the  juice  in  accordance  with  the  character  of  the  food;  in  other  words, 
there  is  an  adaptation  of  the  juice  to  the  kind  of  food  to  be  digested.  Thus 
the  protein  of  bread  causes  a  secretion  of  five  times  more  pepsin  than  the 
same  amount  of  the  protein  of  milk,  while  the  protein  of  meat  causes  a 
secretion  of  25  per  cent,  more  pepsin  than  milk.  Meat  extract  and  bouillon 
have  a  very  stimulating  effect  on  the  quantity  of  juice  produced,  while  alkalies 
have  an  inhibitor  effect 

Histologic  Changes  in  the  Gastric  Cells  during  Secretion. — During 
the  periods  of  rest  and  secretor  activity  the  cells  of  the  gastric  glands  undergo 
changes  in  histologic  structure  which  are  believed  to  be  connected  with  the 
production  of  the  enzymes,  pepsin  and  rennin,  and  the  acid.  In  the  resting 
period  the  protoplasm  of  the  chief  or  central  cells  of  the  preantral  or  cardiac 
glands  becomes  crowded  with  large  and  well-defined  granules,  which  during 
the  period  of  secretory  activity  largely  disappear,  so  much  so  that  only  the 
luminal  border  of  the  cell  is  occupied  by  them,  the  outer  border  being  clear 
and  hyaline  in  appearance.  The  parietal  cells  during  rest  are  large  and 
finely  granular,  but  after  secretion  they  are  smaller  in  size  though  still  granu- 
lar.    (See  Fig.  76,  A  and  B.) 

The  cells  of  the  pyloric  glands,  though  containing  granules,  do  not  show 


172  TEXT-BOOK  OF  PHYSIOLOGY. 

any  marked  difference  between  the  resting  and  active  conditions.  According 
to  some  observers  they  contain  pepsinogen;  according  to  others,  mucin. 
The  epitheUal  cells  lining  the  ducts  of  the  pylorus  and  fundus  glands,  if  not 
identical  with  the  epithelial  cells  on  the  surface  of  the  mucous  membrane, 
pass  by  transitional  forms  into  them.  Among  these  cells  are  found  many 
goblet  cells  which  secrete  a  portion  of  the  mucin  found  in  the  stomach  and 
gastric  juice.  In  the  period  of  rest  the  protoplasm  of  the  epithelial  cells 
absorbs  and  assimilates  from  the  surrounding  lymph-spaces  material  which 
eventually  makes  its  reappearance  as  a  product  of  metabolism  in  the  form  of 
granules  and  hydrochloric  acid.  With  the  onset  of  digestive  actiiity  there 
is  a  dilatation  of  the  blood-vessels,  an  increase  in  the  blood-supply,  a  stimu- 
lation through  the  nerve-supply  of  the  cells,  and  an  output  of  a  fluid  to  which 
the  name  gastric  juice  is  given. 


.iftV'«° 


,.- /"- 


(j 


A  B 

Fig.  76. — Secretions  of  Deep  Ends  of  Fundus  Glands  of  the  Cat  in  Different  Secre- 
tive Phases.  X  1000. — (Bensley).  4.  From  a  fasting  stomach.  The  chief  cells  are  filled  with 
large  zymogen  granules;  nuclei  near  the  outer  ends  of  cells.  Gentian- violet  preparation. 
b  b  b.  Border  cells.  B.  Six  hours  after  an  abundant  meal  of  raw  flesh.  The  chief  cells  e.xhibit 
two  zones,  the  inner  occupied  by  large  zymogen  granules,  the  out.er  by  a  deeply  staining,  obscurely 
fibrillar  element,  prozymogeu:  the  nuclei  lie  at  the  junction  of  the  two  zones,  b  b  b.  Border 
cells,  pr.  Prozymogen.  c.  Mucin-secreting  cells,  similar  to  those  found  in  the  neck  of  the  gland. 
Gentian-violet  preparation. — {Henimeter  after  Bensley.) 

The  Physiologic  Action  of  Gastric  Juice. — In  the  study  of  the  physi- 
ology of  gastric  digestion  as  it  takes  place  under  normal  conditions  it  is 
important  to  bear  in  mind  that  the  foods  introduced  into  the  stomach  are 
heterogeneous  compounds  consisting  of  both  nutritive  and  non-nutritive 
materials,  and  that  before  the  former  can  be  digested  and  utilized  for  nutri- 
tive purposes  they  must  be  freed  from  their  combinations  with  the  latter. 
This  is  accomplished  by  the  solvent  action  of  the  gastric  juice,  which  in 
virtue  of  the  chemic  activity  of  its  constituents  on  proteins,  gradually  disinte- 
grates the  food  and  reduces  it  to  the  liquid  or  semilicjuid  condition. 

The  nature  of  this  change  and  the  respective  influence  which  the  acid 
and  pepsin  exert  can  be  studied  with  almost  any  form  of  protein.  A  most 
convenient  form,  however,  is  fibrin  obtained  from  blood  by  whipping  and 
thoroughly  freed  from  corpuscles  by  washing  under  a  stream  of  water.  The 
chemic  features  of  proteins,  as  well  as  the  typical  forms  contained  in  the 


DIGESTION.  173 

different  articles  of  food,  have  been  considered  in  connection  with  the  chemic 
composition  of  the  body  and  the  composition  of  foods  (see  pages  15  and  119). 
For  purposes  of  experimentation  artificial  gastric  juice  may  be  employed. 
This  is  as  effective  as  the  normal  secretion  and  in  no  essential  respect  differs 
from  it.  A  glycerin  extract  of  the  mucous  membrane  acidulated  with  0.2 
per  cent,  hydrochloric  acid  is  probably  the  best. 

If  the  small  pieces  of  fibrin  be  suspended  in  clear  gastric  juice  and  kept 
at  a  temperature  of  104°  F.  (40°  C.)  for  an  hour  or  two,  they  will  be  dissolved 
and  will  entirely  disappear,  giving  rise  to  a  slightly  opalescent  mixture.  In 
the  early  stages  of  the  process  the  fibrin  becomes  swollen  and  transparent 
and  partly  dissolved.  If  at  this  time  the  solution  be  carefully  neutralized, 
the  dissolved  portion  can  be  regained  in  the  form  of  acid  fibrin — a  fact  which 
indicates  that  the  first  effect  of  the  gastric  juice  is  the  acidification  of  the 
protein.  This  having  been  accomplished,  the  pepsin  becomes  operative, 
and  in  a  varying  length  of  time  transforms  the  acid-protein  into  a  new  form 
of  protein,  termed  peptone  which  dift'ers  from  all  other  forms  of  protein 
in  being  soluble  in  both  acids  and  alkaUes  and  non-coagulable  by  heat. 
In  the  transformation  of  acid-protein  into  peptone  it  is  possible  to  isolate 
by  the  addition  of  magnesium  sulphate  and  ammonium  sulphate  inter- 
mediate bodies  to  which  the  term  proteoses  has  been  given,  and  which 
differ  somewhat  in  their  solubility.  The  proteoses  are  termed,  from  the 
order  in  which  they  make  their  appearance,  primary  and  secondary.  The 
primary  proteoses  are  precipitated  by  magnesium  sulphate,  the  secondary 
by  ammonium  sulphate.  This  supposed  change  produced  by  gastric  juice 
is  represented  by  the  following  scheme: 

Protein. 
Acid-protein 
Proteose  (primary) 
Proteose  (secondary) 
Peptone. 

From  the  fact  that  when  peptones  are  subjected  to  the  prolonged  action 
of  pancreatic  juice  there  arise  compounds  such  as  leucin,  tyrosin,  aspartic 
acid,  arginin,  etc.,  it  was  beheved  that  two  kinds  of  peptones  were  formed 
out  of  a  simple  protein  one  of  which  succumbed  to  the  destructive  action  of 
pancreatic  juice,  while  the  other  resisted  it;  for  this  reason  the  latter  was 
•termed  anti-  and  the  former  hemi-peptone.  The  two  were  included  under 
the  term  ampho-peptone.  It  is  generally  admitted  now,  however,  that  there 
is  but  one  kind  of  peptone  formed  from  any  given  protein,  which  under  the 
influence  of  pancreatic,  and  intestinal  juice  as  well,  is  reduced  by  hydrolysis, 
through  successive  stages  to  amino-acids  or  perhaps  only  to  the  antecedent 
stage,  in  which  two  or  more  amino-acids  yet  remain  united  forming  sub- 
stances known  as  peptids. 

Nearly  all  forms  of  protein  are  in  a  similar  manner  transformed  into 
peptones  by  gastric  juice.  Beyond  this  stage,  however,  there  does  not  seem 
to  be  any  further  change,  peptones  apparently  being  the  final  products  of 
gastric  digestion.     The  intimate  nature  of  this  change  is  practically  un- 


174  TEXT-BOOK  OF  PHYSIOLOGY. 

known,  but  there  are  reasons  for  thinking  that  it  is  a  process  of  hydra- 
tion, attended  by  cleavage,  with  increasing  solubiHty  of  the  resulting 
products. 

Characters  of  Peptones.— The  peptones  resulting  from  the  digestion  of 
different  proteins,  though  resembling  each  other  in  many  respects,  yet  possess 
different  chemic  characteristics,  as  shown  by  their  reaction  to  various  chemic 
reagents.  Though  having  some  resemblance  to  the  proteins  from  which 
they  are  derived,  they  are  to  be  distinguished  from  them  by  the  following 
general  characteristics: 

1.  They  are  not  coagulable  either  by  heat  or  by  nitric  acid. 

2.  They  are  soluble  in  water,  either  hot  or  cold,  and  in  acid  and  alkaline 

solutions, 

3.  They  are  diffusible,   passing   through  animal  membranes  with  great 

rapidity.     It  has  been  demonstrated  that  peptones  diffuse  about  twelve 

times  as  rapidly  as  the  proteins  from  which  they  are  derived. 

From  the  foregoing  facts  it  may  be  inferred  that  in  the  digestion  of  pro- 
teins there  is  a  progressive  diminution  in  the  size  of  the  molecules  through  a 
series  of  hydrolytic  changes.  The  molecules  of  the  proteins,  which  from 
various  causes  are  coagulated,  are  transformed  into  smaller  molecules  which 
are  non-coagulable,  soluble,  and  diffusible. 

On  liquid  fat  and  hydrated  starch  gastric  juice  has  no  appreciable  action. 
It  has  apparently  been  demonstrated,  however,  that  when  fat  in  the  emulsi- 
fied  state,  the  state  in  which  it  exists  in  milk,  is  introduced  into  the  stomach 
it  undergoes  a  cleavage  into  fat  acids  and  glycerin,  in  a  manner  similar  to 
that  which  fat  undergoes  in  the  intestine  under  the  action  of  pancreatic 
juice,  as  will  be  stated  in  a  future  paragraph.  This  presupposes  the 
existence  of  a  ferment  to  which  the  name  lipase  has  been  given.  Though 
the  action  of  saliva  on  starch  is  interfered  with  and  even  checked  by  a  small 
percentage  of  hydrochloric  acid  it  is  certain  from  the  results  of  recent  experi- 
ments, that  starch  digestion  continues  for  from  twenty  minutes  to  a  half 
hour  or  longer,  for  the  reason  that  the  acid  as  fast  as  it  is  secreted  combines 
with  the  proteins  and  is  thus  rendered  inoperative  and  for  the  reason  also 
that  the  food  is  largely  retained  in  the  extreme  fundic  end  of  the  stomach 
where  the  gastric  juice  is  not  abundant.  After  the  above-mentioned  period, 
free  acid  makes  its  appearance  when  salivary  digestion  ceases. 

Notwithstanding  the  fact  that  dilute  solutions  of  hydrochloric  acid  (0.3 
per  cent.)  will  promptly  invert  cane-sugar  to  dextrose  and  levulose,  and  that 
gastric  juice  will  accomplish  the  same  result  in  test-tubes,  there  is  no  strong 
evidence  for  the  belief  that  the  inversion  of  cane-sugar  takes  place  to  any 
marked  extent  in  the  stomach  under  normal  conditions. 

Action  of  Gastric  Juice  on  Foods. — The  action  of  gastric  juice  on 
proteins  affords  a  key  to  its  action  in  the  reduction  of  foods  to  the  liquid  or 
semiliquid  condition.  It  is  evident  that  it  will  be  most  active  in  the  digestion 
of  food  consisting  largely  of  protein  materials,  such  as  meat,  eggs,  milk,  etc. 
Meat  is  disintegrated  first  by  the  conversion  of  the  proteins  of  the  connective 
tissue,  which  have  been  more  or  less  gelatinized  by  cooking,  into  peptones. 
The  sarcolemma  of  the  muscle-fibers  which  have  been  thus  separated  is  in  a 
similar  manner  attacked  and  converted  into  peptones.  The  true  muscle 
or  sarcous  substance,  consisting  largely  of  myosin,  undergoes  a  corresponding 


DIGESTION.  175 

change.  If  the  quantity  of  meat  be  not  too  large  and  the  gastric  juice  be 
secreted  in  proper  amount,  it  is  possible  that  all  the  meat  will  be  digested  in 
the  stomach.  It  is  quite  probable,  however,  that  this  is  not  the  case  and  that 
a  portion  of  the  semidigested  meat  passes  into  the  intestine,  where  its  final 
solution  is  effected. 

The  white  of  egg,  especially  when  slightly  boiled,  is  much  more  readily 
digested  than  when  raw  or  firmly  coagulated  by  prolonged  boihng.  In  either 
condition,  however,  the  connective  tissue  is  dissolved  and  peptonized,  after 
which  the  native  albumin  undergoes  the  same  change.  The  yolk  of  the  egg 
consists  largely  of  fat  held  in  suspension  by  a  protein  substance,  vitellin, 
which  is  also  capable  of  transformation  into  peptone. 

Adipose  tissue  is  similarly  reduced.  The  protein  of  the  connective  tissue 
and  of  the  fat  vesicles  is  dissolved  and  peptonized  and  the  fat-drops  set 
free. 

Milk  undergoes  a  peculiar  change  in  composition  before  its  chief  protein 
constituent,  caseinogen,  can  be  transformed  into  peptone.  The  caseinogen 
in  the  presence  of  calcium  salts  is  always  in  the  soluble  state.  When  acted 
on  by  the  gastric  juice,  the  caseinogen  undergoes  a  chemic  change  by  reason 
of  which  it  combines  with  calcium  salts  and  is  then  transformed  into  a  solid 
compound  casein.  This  change  is  due  to  the  presence  and  activity  of  the 
enzyme,  rennin.  The  necessity  for  this  change  in  the  process  of  digestion, 
however,  is  not  apparent.  The  coagulated  casein  presents  itself  in  the  form 
of  a  flocculent  curd,  which  is  finer  in  human  than  in  cow's  milk,  and  hence 
more  easily  digestible.  After  its  production,  the  casein  is  acidified  by  the 
hydrochloric  acid  and  then  converted  by  the  pepsin  into  peptone. 

Vegetables,  though  consisting  of  a  woody  or  cellulose  framework,  undergo 
a  partial  disintegration  in  the  stomach.  When  they  are  boiled  and  dis- 
integrated by  the  teeth,  the  gastric  juice  is  enabled  to  penetrate  the  frame- 
work and  dissolve  and  peptonize  the  various  protein  constituents.  As  a 
general  rule,  the  vegetable  proteins  are  more  diflicult  of  digestion  than  the 
animal  proteins. 

Duration  of  Gastric  Digestion. — The  length  of  time  the  food  remains 
in  the  stomach  and  the  relative  digestibility  of  different  articles  of  food  were 
carefully  studied  by  Dr.  Beamount  on  St.  Martin,  and  though  the  results 
obtained  by  him  may  not  be  absolutely  correct,  ■viewed  in  the  light  of  recent 
knowledge  of  the  digestive  process,  yet  in  the  main  they  have  been  corrob- 
orated in  various  ways.  As  a  result  of  many  observations  Dr.  Beaumont 
came  to  the  conclusion  that  the  average  length  of  time  an  ordinary  meal 
consisting  of  meat,  bread,  potatoes,  etc.,  remained  in  the  stomach  under- 
going digestion  was  about  three  and  a  half  hours,  the  duration  of  the  pro- 
cess, however,  being  increased  when  an  excessive  quantity  of  food  w^as 
taken  or  the  quantity  and  quality  of  the  gastric  juice  impaired  by  abnormal 
conditions  of  the  system.  As  soon  as  the  food  is  liquefied  by  the  gastric 
juice  that  portion  not  absorbed  by  the  gastric  vessels  passes  into  the  intes- 
tines, this  continuing  for  two  to  three  hours  until  the  stomach  is  completely 
emptied.  The  relative  digestibiHty  of  the  different  foods  was  also  made  the 
subject  of  many  experiments  by  Dr.  Beaumont.  After  repeating  and 
verifying  his  observations  made  under  varying  conditions,  he  summed  up 
his  results  in  a  table,  of  which  the  following  is  an  abstract,  in  which  the 


176  TEXT-BOOK  OF  PHYSIOLOGY. 

mode  of  preparation  and  the  time  required  for  the  digestion  of  different 
foods  are  exhibited: 

TABLE  SHOWING  THE  DIGESTIBILITY  OF  VARIOUS  ARTICLES  OF  FOOD. 

Hours.      Minutes.  Hours.      Minutes 

Eggs,  whipped i              20  Soup,  barley,  boiled i  30 

Eggs,  soft  boiled 3  .  .  Soup,  bean,  boiled 3 

Eggs,  hard  boiled 3  30  Soup,  chicken,  boiled 3 

Oysters,  raw 2              55  Soup,  mutton,  boiled 3  30 

Oysters,  stewed 3             30  Sausage 3  20 

Lamb,  broiled 2             30  Green  corn,  boiled 3  45 

Veal,  broiled 4              .  .  Beans,  boiled 2  30 

Pork,  roasted 5              15  Potatoes,  roasted 2  30 

Beefsteak,  broiled 3              .  .  Potatoes,  boiled 3  30 

Turkey,  roasted 2              25  Cabbage,  boiled 4  30 

Chicken,  boiled 4              .  .  Turnips,  boiled 3  30 

Chicken,  fricasseed 2             45  Beets,  boiled 3  45 

Duck,  roasted 4             .  .  Parsnips,  boiled 2  30 


The  time  required  for  the  stomach  to  discharge  any  given  article  of 
food  has  been  shown  by  Cannon  to  depend  partly  on  its  chemic  composition 
and  partly  on  its  capacity  for  absorbing  hydrochloric  acid.  From  an 
examination  of  the  stomach  and  duodenum  of  the  cat  by  means  of  Rontgen 
rays  and  the  fluoroscopic  screen,  after  the  administration  of  equal  quantities, 
25  c.c,  of  pure  protein,  fat,  and  carbohydrate,  mixed  with  5  grams  of  bismuth, 
it  became  possible  to  determine  the  rate  at  which  they  left  the  stomach  from 
the  length  of  the  food  masses  in  the  duodenum  and  small  intestine  as  indi- 
cated by  the  shadows  on  the  screen,  at  intervals  of  half  an  hour  or  longer. 
The  duration  of  the  observations  extended  over  a  period  of  seven  hours. 

When  a  pure  protein,  e.g.,  boiled  beef  free  from  fat,  boiled  haddock,  or 
the  white  meat  of  fowls  is  administered,  foods  which  not  only  excite  the 
flow  of  gastric  juice  but  readily  absorb  hydrochloric  acid,  the  pylorus  remains 
closed  for  some  time,  scarcely  any  protein  leaving  the  stomach  during  the  first 
half  hour.  Shortly  after  this  when  free  hydrochloric  acid  makes  its  appear- 
ance, the  signal  for  the  relaxation  of  the  sphincter,  the  pylorus  opens  from 
time  to  time  and  the  passage  of  the  protein  into  the  duodenum  begins  and 
gradually  increases  in  rapidity  until  the  maximum  speed  is  attained,  about 
two  hours  after  ingestion;  from  this  time  on,  the  speed  of  discharge  gradually 
diminishes  until  the  end  of  the  observation  period. 

When  fat,  e.g.,  beef,  mutton,  or  pork  fat,  is  administered,  they  remain 
in  the  stomach  for  some  time  and  when  they  begin  to  leave,  the  rate  of  dis- 
charge is  so  slow  that  they  are  digested  and  absorbed  almost  as  fast  as 
discharged  and  hence  seldom  accumulate  in  the  small  intestine.  These 
compounds  delay  the  secretion  of  gastric  juice  and  therefore  free  hydrochloric 
acid,  the  presence  of  which  appears  to  be  necessary  for  the  relaxation  of  the 
pyloric  sphincter.  When  carbohydrates,  e.g.,  starch  paste,  boiled  rice, 
boiled  mashed  potatoes,  are  administered  their  discharge  begins  shortly 
after  their  entrance  into  the  stomach;  they  pass  out  rapidly,  the  velocity  of 
discharge  reaching  its  maximum  at  the  end  of  two  hours,  after  which  the 
speed  declines  to  the  end  of  the  observation  period.  The  reason  for  the 
early  and  rapid  discharge  is  to  be  found  in  the  fact  that  while  the  carbo- 
hydrates excite  the  secretion  of  gastric  juice  they  do  not  absorb  the  hydro- 
chloric acid  to  any  appreciable  extent.     A  combination  of  equal  quantities 


DIGESTION. 


177 


of  protein  and  carbohydrate  varies  the  rate  of  discharge  of  each  separately. 
Thus  under  these  circumstances  the  carbohydrates  are  not  discharged  so 
rapidly  nor  are  the  proteins  detained  so  long  as  usual;  a  combination  of  fat 
with  either  protein  or  carbohydrate  delays  the  time  of  discharge  of  both. 
From  these  facts  it  may  be  inferred  that  the  time  any  given  food  remains  in 
the  stomach  will  depend  on  its  chemic  composition  or  the  relative  amounts 
of  its  contained  protein,  fat,  and  carbohydrate  principles. 

Movements  of  the  Stomach.— During  the  period  of  gastric  digestion 
the  muscle  walls  of  the  stomach  become  the  seat  of  a  series  of  movements, 

peristaltic  in  character,  which  not  only  incorpo- 
rate the  gastric  juice  with  the  food,  but  also  serve 
to  eject  the  liquefied  portions  of  the  food  into 
the  small  intestine. 

The  movements  of  the  human  stomach  as 
described  by  Beaumont,  as  well  as  the  movements 
of  the  dog's  stomach  as  stated  by  different  ob- 
servers are  not  in  agreement  in  all  respects,  and 
are,  moreover,  open  to  question  for  the  reason 
that  they  were  not  observed  under  strictly  physio- 
logic conditions.  The  more  recent  investigations 
of  Cannon  have  thrown  new  light  on  this  sub- 
ject.    By  means  of  the  Rontgen  rays  he  has  been 


Left 


Fig.  77. — Shadow  Sketches 
OF  THE  Outlines  of  the 
Stomach  of  a  Cat  Immedi- 
ately after  a  Meal  (ii.o), 
AND  AT  Various  Intervals 
Afterward  (at  12.0,  at  2.0, 
3.30,  4.30). — (ir.  B.  Cannon.) 


Post 


Fig.  78. — The  cardiac  portion  is  all  that 
part  to  the  left,  as  the  stomach  lies  in  the 
body,  of  WX.  The  cardia  is  at  C.  The 
pylorus  is  at  P,  and  the  pyloric  portion 
is  the  part  between  P  and  WX.  This 
has  two  divisions:  the  antrum,  between 
P  and  YZ,  and  the  pre-antral  part,  between 
WX  and  YZ.  The  lesser  curvature  is  on  the 
top  of  the  outline  between  C  and  P,  and  the 
greater  curvature  between  the  same  points 
along  the  lower  border. — {Amer.  Jour,  of 
Physiology,  Cannon.) 


enabled  to  study  the  movements  in  the  living  animal  and  under  normal 
conditions.  The  animal  (the  cat)  was  fed  with  bread  and  milk,  to  which 
was  added  subnitrate  of  bismuth.  This  substance,  being  opaque,  rendered 
the  movements  of  the  stomach  walls  visible  on  the  fluorescent  screen. 
With  paper  placed  over  the  screen  it  was  possible  to  sketch  the  change 
in  shape  that  the  stomach  undergoes  at  different  periods  of  the  digestive 
act.  Some  of  these  changes  are  represented  in  Fig.  77.  The  anatomic 
features  of  the  cat  stomach  of  interest  in  this  connection  are  represented 
in  Fig.  78. 


178  TEXT-BOOK  OF  PHYSIOLOGY. 

These  investigations  show  that  different  portions  of  the  stomach  walls 
exhibit  different  forms  of  activity,  which  for  convenience  of  description  are 
separately  described  by  Cannon  as  follows: 

I.  The  Movements  of  the  Pyloric  Part. — Within  five  minutes  after 
a  cat  has  finished  a  meal  of  bread  there  is  \dsible  near  the  duodenal  end 
of  the  antrum  a  slight  annular  contraction  which  moves  peristaltically  to 
the  pylorus;  this  is  followed  by  several  waves  recurring  at  regular  intervals. 
Two  or  three  minutes  after  the  first  movement  is  seen,  very  slight  constric- 
tions appear  near  the  middle  of  the  stomach,  and,  pressing  deeper  into  the 
greater  curvature,  course  slowly  toward  the  pyloric  end.  As  new  regions 
enter  into  constriction,  the  fibers  just  previously  contracted  become  relaxed, 
so  that  there  is  a  true  moving  wave,  with  a  trough  between  two  crests.  When 
a  wave  swings  round  the  bend  in  the  pyloric  part,  the  indentation  made  by 
it  deepens;  and  as  digestion  goes  on  the  antrum  elongates  and  the  constrictions 
running  over  it  grow  stronger,  but,  until  the  stomach  is  nearly  empty,  they 
do  not  entirely  divide  the  cavity.  After  the  antrum  has  lengthened,  a  wave 
takes  about  thirty-six  seconds  to  move  from  the  middle  of  the  stomach  to 
the  pylorus.  At  all  periods  of  digestion  the  waves  recur  at  intervals  of  almost 
exactly  ten  seconds.  It  results  from  this  rhythm  that  when  one  wave  is 
just  beginning  several  others  are  already  running  in  order  before  it.  Be- 
tween the  rings  of  constriction  the  stomach  is  bulged  out,  as  shown  in  the 
various  outlines  in  Fig.  77. 

Movements  of  the  Pyloric  Sphincter. — During  the  first  ten  or  fifteen 
minutes  after  the  first  constriction  of  the  antrum  the  pylorus  is  tightly 
closed.  After  this  period  it  opens  at  irregular  intervals  to  permit  the  passage 
of  liquefied  food  which  is  ejected  by  peristaltic  waves  for  a  distance  of  two 
or  three  centimeters  into  the  duodenum.  The  frequency  with  which  the 
pylorus  opens  depends  apparently  on  the  degree  to  which  the  food  is  softened. 
When  the  food  is  hard,  the  pylorus  closes  more  tightly  and  remains  closed  a 
longer  period  than  when  it  is  soft. 

The  physiologic  cause  for  the  relaxation  or  inhibition  of  the  sphincter 
pylori  appears  to  be  the  presence  of  free  acid  at  the  pylorus;  its  contract'on, 
the  presence  of  acid  in  the  duodenum.  With  the  neutralization  of  the  acid 
-in  the  duodenum,  its  influence  on  the  sphincter  muscle  is  weakened,  after 
which  the  muscle  again  becomes  susceptible  to  the  inhibitor  influence  of  the 
acid  within  the  stomach.  It  is  probably  for  this  reason  that  carbohydrates, 
which  do  not  absorb  the  acid,  are  discharged  from  the  stomach  early;  that 
the  proteins,  which  postpone  the  appearance  of  free  acid,  are  retained  longer 
and  that  fats,  which  check  the  secretion  of  gastric  juice  are  discharged 
slowly  (Cannon).  It  should  be  emphasized,  however,  that  the  relaxation 
and  contraction  of  the  pyloric  sphincter,  due  to  the  action  of  free  acid  on 
the  gastric  and  duodenal  sides,  respectively,  can  take  place  independently 
of  the  nerve  system. 

The  Activity  of  the  Cardiac  Portion. — As  digestion  proceeds,  the  pre- 
antral  part  of  the  stomach  elongates  and  assumes  the  shape  of  a  tube, 
which  becomes  the  seat  also  of  peristaltic  constriction  waves.  As  a  result, 
some  of  the  food  is  gradually  forced  into  the  antrum  to  succeed  that  which 
has  been  prepared  and  ejected  into  the  duodenum.  As  the  pre-antral  tube 
is  emptied  of  its  contents  the  longitudinal  and  circular  fibers  of  the  fundus 


DIGESTION.  179 

steadily  contract  and  gradually  force  its  contents  into  the  tubular  portion. 
This  continues  until  the  fundus  is  completely  emptied.  The  changes  in 
shape  which  the  cardiac  portion  undergoes  during  digestion  are  represented  in 
Fig.  ?>2.  The  fundus  acts  as  a  reservoir  for  the  food  and  forces  out  its 
contents  a  little  at  a  time  as  the  antral  mechanism  is  ready  to  receive  them. 
Since  peristaltic  movements  are  absent  from  the  cardiac  portion  the  food  is 
not  mixed  with  gastric  juice,  and  therefore  salivary  digestion  can  continue 
for  a  considerable  period.  There  is  no  evidence  of  a  circulation  of  food 
in  the  stomach  as  sometimes  described.  On  the  contrary,  the  movement 
through  the  pre-antral  tube  and  antrum  is  in  general  a  progressive  though 
an  oscillating  one.  As  the  constriction  waves  rapidly  pass  over  the  food  it  is 
advanced  toward  the  pyloric  opening,  but  as  this  is  closed  the  food  is  forced 
backward  through  the  advancing  constricted  ring  for  a  variable  distance. 

The  effect  of  the  constriction  waves  is  to  mix  the  food  with  the  gastric 
juice,  triturate  and  soften  it.  So  soon  as  this  is  effected,  the  pylorus  relaxes, 
when  the  advancing  constriction  wave  expels  it  into  the  intestine.  With  its 
expulsion  room  is  afforded  for  an  additional  quantity  of  food,  and  hence 
there  is  a  general  advance  of  the  food  mass  toward  the  pylorus. 

Though  these  observations  were  made  on  the  cat,  evidence  is  accumu- 
lating which  goes  to  show  that  in  human  beings  the  walls  of  the  stomach 
exhibit  constriction  waves  which  are  similar  in  all  respects  to  those  above 
described. 

The  Nerve  Mechanism  of  the  Stomach. — In  preceding  paragraphs 
it  was  stated  that  during  the  period  of  gastric  digestion  the  food  is  retained 
in  the  stomach  because  of  the  closure  of  the  cardia  (the  esophago-gastric 
orifice)  and  of  the  pylorus  (the  gastro-duodenal  orifice)  both  orifices  being 
tightly  closed  by  the  tonic  contraction  of  sphincter  m.uscles;  that  both  sphinc- 
ters relax  from  time  to  time,  the  one  to  permit  the  entrance  of  food  into  the 
stomach  for  further  digestion,  the  other  to  permit  the  exit  of  food  into 
the  intestine  after  its  more  or  less  complete  digestion,  after  which  in  both 
instances  the  sphincters  again  contract  and  close  the  orifices;  that  the 
pyloric  or  antral  muscles  are  vigorously  active  throughout  the  digestive 
period,  triturating  the  food,  mixing  it  with  gastric  juice,  and  finally  driving 
it  through  the  temporarily  open  pylorus  into  the  intestine. 

These  separate  but  related  groups  of  muscle-fibers,  by  reason  of  their 
endowments,  and  possibly  by  virtue  of  the  presence  of  local  ner\'e  mechan- 
isms, exhibit  activities  which  are  independent  of  the  central  nerve  system. 
Thus  the  isolated  stomach -of  the  dog  and  of  other  animals  as  well,  if  kept 
warm  and  moist,  will  exhibit  rhythmic  movements  for  a  period  of  time  vary- 
ing from  an  hour  to  an  hour  and  a  half.  Though  nerve-cells  and  ner\^e- 
fibers  (Auerbach's  plexus)  are  present  in  the  walls  of  the  stomach  between 
the  layers  of  muscle-fibers,  it  is  not  believed  that  they  are  the  immediate 
sources  of  the  stimulus  to  the  contraction,  though  they  may  act  as  a  coordinat- 
ing mechanism.  The  stimulus  in  all  probability  develops  in  the  muscle- 
fiber  itself  and  is  therefore  myogenic. 

The  sphincter  car  dice  muscle  surrounding  the  esophago-gastric  orifice 
is  always,  under  normal  conditions,  tonically  contracted  and  the  orifice 
closed.  This  contraction  is  partly  due  to  inherent  causes  as  shown  by  the 
fact  that  it  persists  for  from  24  hours  to  several  days  after  division  of  all 


i8o  TEXT-BOOK  OF  PHYSIOLOGY. 

nerves  distributed  to  it.  The  contraction  may  be  so  pronounced  as  to  offer 
considerable  resistance  not  only  to  the  passage  of  food  but  even  to  the  intro- 
duction of  a  sound  into  the  stomach.  (Cannon.)  That  the  normal  con- 
traction is  under  the  influence  of  the  central  nerv^e  system  is  shown  by  the 
effects  which  follow  stimulation  of  the  peripheral  end  of  the  divided  vagus. 
If  it  is  stimulated  with  weak  induced  currents,  the  contraction  is  somewhat 
inhibited  and  the  orifice  enlarged;  if  it  is  stimulated  with  strong  currents  the 
contraction  is  markedly  increased.  Apparently  there  are  in  the  vagus  two 
sets  of  efferent  nerv^e-fibers,  one  of  which  augments,  while  the  other  inhibits 
the  contraction,  and  corresponding  to  the  nerves  there  must  be  in  the  medulla 
oblongata  two  centers  from  which  they  arise,  an  augmentor  and  an 
inhibitor. 

Observation  has  shown  that  at  the  beginning  of  each  act  of  deglutition, 
there  is  an  inhibition  of  the  sphincter  muscle,  and  if  the  acts  follow  each 
other  in  quick  succession,  the  inhibition  and  relaxation  are  increased. 
(Meltzer.)  With  the  passage  of  the  food  into  the  stomach  the  tonic  con- 
traction again  supervenes.  These  effects  also  follow  stimulation  of  the 
glosso-pharyngeal  nerve.  Whether  the  sphincter  inhibition  is  the  result  of 
an  inhibition  of  the  center  which  maintains  the  tonus,  or  a  stimulation  of 
an  inhibitor  center,  is  uncertain. 

It  has  recently  been  reported  by  Cannon  that  a  similar  inhibition  or 
relaxation  of  the  musculature  of  the  cardiac  end  of  the  stomach  is  occasioned 
by  each  act  of  deglutition  and  that  it  continues  and  increases  if  the  acts 
follow  each  other  in  quick  succession.  As  the  bolus  descends  the  esophagus 
and  before  it  reaches  its  termination  there  is  a  relaxation  of  the  musculature  of 
the  cardiac  end,  a  fall  of  intragastric  pressure,  an  enlargement  of  the  stomach 
capacity  and  hence  a  readier  receptivity  of  the  bolus.  That  this  inhibition 
is  caused  by  impulses  descending  the  vagus  is  shown  by  the  effects  which 
follow  a  moderate  stimulation  of  the  vagus  nerve  and  by  the  fact  that  it 
does  not  take  place  if  the  vagus  nerves  are  divided.  To  this  inhibition  and 
enlargement  of  the  cardiac  end  of  the  stomach  the  term  receptive  relaxa- 
tion has  been  given. 

The  degree  of  activity  of  both  the  pyloric  sphincter  and  the  antral  muscles 
is  modified  also  by  the  central  nerve  system  either  in  the  way  of  inhibi- 
tion or  augmentation  and  in  response  to  gastric  stimulation.  The  nerves 
more  especially  concerned  in  the  maintenance  and  regulation  of  the  gastric 
contractions  are  the  vagi  and  the  splanchnics.  The  afferent  fibers  through 
which  nerve  impulses  pass  to  the  nerve  centers  are  in  all  probability  con- 
tained in  the  trunk  of  the  vagus  nerve;  the  efferent  fibers  through  which 
nerve  impulses  from  the  centers  reach  the  stomach,  are  contained  partly  in 
the  trunk  of  the  vagus  and  partly  in  the  trunk  of  the  splanchnic  nerve. 

If  the  vagus  nerves  are  divided  in  the  neck,  there  is  a  loss  of  muscle  tonus 
though  the  contractions  do  not  wholly  disappear.  Stimulation  of  the  per- 
ipheral end  of  one  divided  vagus  is  followed  by  an  augmentation  in  the 
vigor  of  the  contraction  of  the  antral  muscles,  an  increase  in  the  tone  of  the 
fundus  muscles,  as  well  as  an  increase  in  the  contraction  of  the  sphincter 
pylori  and  sphincter  cardiae.  Though  this  is  the  usual  result  there  may  be  a 
primary  relaxation  or  inhibition  of  short  duration  of  one  or  all  of  these 
structures  before  the  augmentation  occurs.   May  states  that  this  was  always 


DIGESTION.  i8i 

the  case  in  his  experiments.  A  similar  inhibition  may  be  brought  about 
reflexly  by  stimulation  of  the  central  end  of  a  divided  vagus.  This  result 
will  not  be  produced  if  the  opposite  vagus  has  previously  been  divided.  The 
vagi,  therefore,  contain  both  inhibitor  and  augmentor  nerve-fibers  for  the 
gastric  musculature,  though  the  augmentor  nerves  largely  preponderate. 

Stimulation  of  the  peripheral  end  of  a  divided  splanchnic  is  followed  by 
an  inhibition  of  the  peristalsis  and  a  loss  of  tone.  Morat,  however,  has 
observed  a  primary  opposite  effect.  The  splanchnic  nerves,  therefore, 
also  contain  both  inhibitor  and  augmentor  fibers  for  the  gastric  musculature 
though  the  inhihitor  libers  largely  preponderate.  From  these  facts  it  would 
appear  that  the  gastric  muscles  receive  both  inhibitor  and  augmentor  fibers 
from  two  different  sources. 

The  conditions  necessary  for  the  development  of  the  gastric  peristalsis 
are  (i)  a  condition  of  tonicity  of  the  musculature,  i.e.,  a  slight  degree  of  con- 
traction whereby  the  muscle  is  shortened;  (2)  intragastric  pressure.  When 
these  two  conditions  are  mutually  adapted  the  musculature  acc|uires  a  cer- 
tain degree  of  tension  whereupon  the  peristalsis  arises.  An  excess  or  de- 
ficiency of  internal  pressure  as  well  as  a  loss  of  tonicity  prevents  peristalsis. 
The  peristalsis  has  no  necessary  fixed  point  of  origin  but  arises  at  that 
portion  of  the  stomach  in  which  the  two  factors  previously  mentioned  bear 
a  certain  relation  one  to  the  other.  From  their  origin  the  peristaltic  waves 
pass  toward  the  pylorus  as  a  result  of  increased  internal  pressure.  The 
necessary  degree  of  the  preliminary  tonus  is  imparted  to  the  musculature 
by  nerve  impulses  descending  the  vagi.  If  these  nerves  are  cut,  the  tonus  is 
lost  and  peristalsis  fails  to  develop.  When  once  the  peristalsis  is  well 
developed  in  digestion,  division  of  the  vagi  has  no  effect.     (Cannon.) 

INTESTINAL  DIGESTION. 

The  physical  and  chemic  changes  which  the  food  principles  undergo  in 
the  small  intestine,  and  which  collectively  constitute  intestinal  digestion, 
are  probably  more  important  and  complex  than  those  taking  place  in  the 
stomach,  for  the  food  is,  in  this  situation,  subjected  to  the  solvent  action  of 
the  pancreatic  and  intestinal  juices,  as  well  as  to  the  action  of  the  bile,  each 
of  which  exerts  a  transforming  influence  on  one  or  more  substances  and 
still  further  prepares  them  for  absorption  into  the  blood. 

To  rightly  appreciate  the  physiologic  actions  of  the  digestive  juices 
poured  into  the  intestine,  the  nature  of  the  partially  digested  food  as  it 
comes  from  the  stomach  must  be  kept  in  mind.  This  consists  of  water, 
inorganic  salts,  acidified  proteins,  proteoses,  peptones,  starch,  maltose, 
liquefied  fat,  saccharose,  lactose,  dextrose,  cellulose,  and  the  indigestible 
portion  of  meats,  cereals,  and  fruits.  Collectively  they  are  known  as  chyme. 
As  this  acidified  mass  passes  through  the  duodenum  its  contained  acids 
excite  a  secretion  and  discharge  of  the  intestinal  fluids:  e.g.,  pancreatic 
juice,  bile,  and  intestinal  juice.  Inasmuch  as  these  fluids  are  alkaline  in 
reaction  they  exert  a  neutralizing  and  precipitating  influence  on  various 
constituents  of  the  chyme.  As  soon  as  this  has  taken  place,  gastric  diges- 
tion ceases  and  those  chemic  changes  are  inaugurated  which  eventuate  in 
the  transformation  of  all  the  remaining  undigested  nutritive  materials  into 


i82  TEXT-BOOK  OF  PHYSIOLOGY. 

absorbable  and  assimilable  compounds  which  collectively  constitute  intes- 
tinal digestion, 

THE  SMALL  INTESTINE. 

The  small  intestine,  in  which  this  stage  of  digestion  takes  place,  is  a 
convoluted  tube,  measuring  about  seven  meters  in  length  and  3.5  cm.  in 
diameter,  and  extending  from  the  pyloric  orifice  of  the  stomach  to  the  begin- 
ning of  the  large  intestine. 

The  intestine  consists  of  four  coats:  viz.,  serous,  muscle,  submucous, 
and  mucous. 

The  serous  coat  is  the  most  external  and  is  formed  by  a  reflection  of 
the  general  peritoneal  membrane.  It  is,  however,  wanting  in  the  duodenal 
portion. 

The  muscle  coat,  situated  just  beneath  the  former,  surrounds  the  entire 
intestine.  It  is  composed  of  non-striated  fibers,  which  are  mol-e  abundant 
and  better  developed  in  the  upper  than  in  the  lower  portions  of  the  intestine. 
The  muscle  coat  consists  of  two  layers  of  fibers:  (i)  an  external  or  longitudinal, 
and  (2)  an  internal  or  circular  layer.  The  longitudinal  fibers  are  most 
marked  at  that  border  of  the  intestine  free  from  peritoneal  attachment, 
though  they  form  a  thin  layer  all  over  the  intestine.  The  circular  fibers  are 
much  more  numerous,  and  completely  encircle  the  intestine  throughout  its 
entire  extent.  It  has  been  demonstrated  that  at  the  junction  of  the  ileum 
and  colon,  and  surrounding  the  orifice,  the  ileo-colic,  common  to  both,  the 
muscle-fibers  are  arranged  in  the  form  of,  and  play  the  part  of,  a  sphincter 
muscle,  which  has  been  termed  the  ileo-colic  sphincter.  It  is  usually 
in  a  state  of  tonic  contraction  and  regulates  the  passage  of  materials  from 
the  small  into  the  large  intestine,  and  possibly  also  in  the  reverse  direction 
under  special  circumstances. 

The  submucous  coat  consists  of  areolar  tissue  and  serves  to  unite  the 
muscle  with  the  mucous  coat.  A  thin  layer  of  muscle-fibers,  the  musculoxis 
mucosa,  is  placed  on  its  inner  surface. 

The  mucous  coat  is  soft  and  velvety  in  appearance  and  covered  by  a 
single  layer  of  columnar  epithelium.  Its  entire  surface  is  covered  with 
small  conical  projections  termed  villi.  Throughout  its  entire  extent, 
with  the  exception  of  the  lower  portion  of  the  ileum  and  the  duodenum, 
the  mucous  membrane  presents  a  series  of  transverse  folds — the  valvulae 
conniventes,  or  valves  of  Kirkring.  These  folds  vary  from  one-fourth  to 
half  an  inch  in  width  and  extend  one-half  to  two-thirds  of  the  distance  around 
the  interior  of  the  bowel.  Each  valve  consists  of  two  layers  of  the  mucous 
membrane  permanently  united  by  fibrous  tissue.  It  is  beheved  that  the 
valves  retard  to  some  extent  the  passage  of  the  food  through  the  intestine 
and  present  a  greater  surface  for  absorption. 

Blood-vessels,  Nerves,  and  Lymphatics. — The  blood-vessels  of 
the  small  intestine,  which  are  very  numerous,  are  derived  mainly  from 
the  superior  mesenteric  artery.  After  penetating  the  intestinal  walls  the 
smaller  vessels  ramify  in  the  submucous  coat  and  send  branches  to  the 
muscle  and  mucous  coats,  supplying  all  their  structures  with  blood.  After 
circulating  through  the  capillary  vessels  the  blood  is  returned  by  small 
veins  which  subsequently  unite  to  form  the  superior  mesenteric  vein,  which. 


DIGESTION.  183 

uniting  with  the  splenic  and  gastric  veins,  forms  the  portal  vein.  The 
nerves  are  derived  from  the  lower  part  of  the  solar  plexus.  The  branches 
follow  the  blood-vessels  and  become  associated  with  two  plexuses,  one 
(Auerbach's)  lying  between  the  muscle  coats,  the  other  (Meissner's)  lying 
in  the  submucous  coat.  To  this  nerve  net,  composed  of  nerve  cells  and 
nerve  processes,  found  in  connection  with  the  muscle  coats  of  the  stomach, 
of  the  small  and  of  the  large  intestine  as  well,  the  term  myenteric  plexus 
has  been  given.  The  lymphatics,  which  originate  in  the  mucous  and  muscle 
coats,  are  very  abundant.  They  unite  to  form  those  vessels  seen  in  the 
mesentery  and  empty  into  the  thoracic  duct. 

Intestinal  Glands. — The  gland  apparatus  of  the  intestine  by  which 
the  intestinal  juice  is  secreted  consists  of  the  duodenal  (Brunner's)  and  the 
intestinal  (Lieberkiihn's)  glands. 

The  duodenal  glands  are  situated  beneath  the  mucous  membrane  and 
open  by  a  short  wide  duct  on  its  free  surface.  They  are  racemose  glands 
lined  by  nucleated  epithelium.  The  secretion  of  these  glands  is  clear, 
slightly  viscid,  and  alkaline.  Its  chemic  composition  and  functions  are 
unknown. 

The  intestinal  glands  or  follicles  are  distributed  throughout  the  entire 
mucous  membrane  in  enormous  numbers.  They  are  formed  mainly  by 
an  inversion  of  the  mucous  membrane  and  hence  open  on  its  free  surface. 
Each  tubule  consists  of  a  thin  basement  membrane  lined  by  a  layer  of  spheric 
epithelial  cells,  some  of  which  undergo  distention  by  mucin  and  become 
converted  into  mucous  or  goblet  cells.  The  epithelial  secreting  cells  consist 
of  granular  protoplasm  containing  a  well-defined  nucleus.  The  intestinal 
follicles  constitute  the  apparatus  which  secretes  the  chief  portion  of  the  in- 
testinal juice. 

Intestinal  Juice. — Owing  to  its  admixture  with  other  secretions  and 
to  the  profound  disturbance  of  the  digestive  function  caused  by  the  establish- 
ment of  intestinal  fistulae,  this  fluid  has  rarely  been  obtained  in  a  state  of 
purity  or  in  quantities  sufficient  for  accurate  analyses  or  for  experimental 
purposes.  Its  physiologic  properties  and  functions  are  therefore  imperfectly 
known.  Various  attempts  have  been  made  by  physiologists,  by  the  employ- 
ment of  different  methods,  to  obtain  this  secretion.  The  method  usually 
employed  is  that  of  Thiry  and  Vella.  This  consists  in  dividing  the  intestine 
at  two  places,  about  eight  or  ten  inches  apart,  restoring  the  continuity  of  the 
intestine,  and  then  uniting  the  two  ends  of  the  resected  portion  to  the  edges  of 
two  openings  in  the  abdominal  walls.  The  resected  portion,  being  supplied 
with  blood-vessels  and  nerves,  maintains  its  nutrition  and  secretes  a  more  or 
less  normal  juice. 

When  obtained  from  a  dog  under  these  circumstances  the  intestinal 
juice  is  watery  in  consistence,  slightly  opalescent,  light  yellow  in  color, 
alkaline  in  reaction,  with  a  specific  gravity  of  i.oio.  Chemic  analysis 
reveals  the  presence  of  proteins,  mucin,  and  sodium  carbonate. 

The  intestinal  juice  obtained  by  Tubbey  and  Manning  from  a  small 
portion  of  the  human  intestine  (ileum)  was  opalescent,  occasionally  brownish 
in  color,  alkaline,  and  had  a  specific  gravity  of  1.006.  On  the  addition  of 
hydrochloric  acid,  carbonic  acid  was  given  off,  showing  the  presence  of 
carbonates.     It  contained  proteins  and  mucins. 


1 84 


TEXT-BOOK  OF  PHYSIOLOGY. 


PANCREAS. 

The  pancreas  is  a  long  flattened  gland,  situated  deep  in  the  abdominal 
cavity,  lying  just  behind  the  stomach.  It  measures  from  fifteen  to  twenty 
centimeters  in  length,  six  in  breadth,  and  two  and  a  half  in  thickness.  It  is 
usually  divided  into  a  head,  body,  and  tail.     The  head  is  directed  to  the  right 


Pancreatic  ducts. 


Common  bile  duct- 


Tail. 


Fig.  79. — Pancreas  and  Duodenum  Removed  from  the  Body  and  Seen  from 
Behind.     The  Gland  is  Cut  to  Show  the  BvcTS.—(Latidois  and  Stirling.) 

side  and  is  embraced  by  the  curved  portion  of  the  duodenum;  the  tail  is  di- 
rected to  the  left  side  and  extends  as  far  as  the  spleen  (Fig.  79) .  The  pancreas 
communicates  w^ith  the  intestine  by  means  of  a  duct.  This  duct  commences 
at  the  tail  and  runs  transversely  through  the  body  of  the  gland.  As  it  ap- 
proaches the  head  of  the  gland  it  gradually  increases  in  size  until  it  measures 

about  two  or  three  millimeters 
in  diameter.  It  then  curves 
downward  and  forward  and 
opens  into  the  duodenum.  In 
its  course  through  the  gland  it 
receives  branches  which  enter 
it  nearly  at  right  angles.  The 
pancreas  is  richly  supplied  with 
blood-vessels  and  nerves,  the 
latter  coming  from  the  solar 
plexus. 

Histologic  Structure.— In 
its  structure  the  pancreas  re- 
sembles the  salivary  glands.  It 
consists  of  a  connective-tissue 
framework  which  divides  the 
gland  tissue  into  lobules.  Each 
lobule  is  composed  of  a  number 
of  acini  or  alveoli,  more  or  less 
elongated  or  tubular  in  shape.  Each  acinus  gives  origin  to  a  small  duct 
which,  uniting  with  adjoining  ducts,  forms  the  lobular  duct,  which  becomes 
tributary  to  the  main  duct.  The  acinus  is  lined  by  a  layer  of  cylindric 
epithelial  cells  characterized  by  a  difference  in  structure  between  their  cen- 


FiG.  80.  Fig.  81. 

One  Saccule  of  the  Pancreas  of  the  Rabbit 
in  Different  States  of  Activity.  Fig.  80. — After 
a  period  of  rest,  in  which  case  the  outlines  of  the  cells 
are  indistinct  and  the  inner  zone — i.  e.,  the  part  of  the 
cells  (a)  next  the  lumen  (c) — is  broad  and  filled  with 
fine  granules.  Fig.  8i . — After  the  gland  has  poured 
out  its  secretion;  when  the  cell  outlines  (d)  are  clearer, 
the  granular  zone  (a)  is  smaller,  and  the  clear  outer 
zone    is    wider. — (Kiihne  and  Lea.) 


DIGESTION. 


185 


tral  and  peripheral  ends  (Fig.  80).  The  central  end,  that  bordering  the 
lumen  of  the  acinus,  is  dark  in  appearance  and  filled  with  dark  granules, 
while  the  peripheral  end  is  clear  and  homogeneoi^s.  The  relative  depth  of 
these  two  zones  varies  according  to  the  functional  activity  of  the  gland. 
During  the  intervals  of  digestion  the  granular  layer  is  very  deep  and  oc- 
cupies almost  the  entire  cell;  after  active  digestion  the  granular  layer  is 
very  narrow,  while  the  clear  zone  is  largely  increased  in  depth  (Fig.  81.) 
The  blood-vessels  of  the  pancreas  are  arranged  around  the  acini  in  a 
manner  similar  to  that  observed  in  the  salivary  glands.  The  ultimate  ter- 
minations of  the  nerves  in  the  epithelium  are  probably  by  means  of  the 
usual  end-tufts. 

The  Islands  of  Langcrhans. — Throughout  the  body  of  the  pancreas  and 
especially  in  the  outer  extremity  there  are  found  between  and  among  the 
acini  collections  of  globular  cells  arranged  in  the  form  or  rods  or  columns, 
separated  from  the  acini  and  from  one  another  by  layers  of  connective 


Fig.  82. — Section  of  Human 
Pancreas,  including  Several  Acini 
AND  Two  Ducts.  The  Cells  Pre- 
sent A  Central  Granular  and  a 
Peripheral  Clear  Zone. — (Pier sol.) 


Fig.  83. — Section  of  Human  Pan- 
creas SHOWING,  a,  a,  Island  of 
Langerhans,  and  b,  the  Usual  Acini.— 
{Piersol.) 


tissue  in  which  ramify  large  tortuous  capillary  blood-vessels.  These  colum- 
nar bodies,  seen  in  cross-section  in  Fig.  ?)2),  have  been  named,  after  their 
discoverer,  the  islands  of  Langerhans. 

Embryologic  investigations  have  shown  that  these  cells  are  outgrowths 
from  the  primitive  acini,  to  which  they  remain  attached  for  some  time  by 
means  of  a  foot-stalk.  This  subsequently  becomes  constricted  by  the 
connective  tissue  and  the  cells  become  completely  detached.  The  cells 
then  assume  the  columnar  arrangement,  after  which  vascularization  takes 
place. 

From  the  fact  that  complete  extirpation  of  the  pancreas  as  well  as  its 
various  diseases  is  followed  by  serious  disturbances  of  the  carbohydrate 
metabolism  it  has  been  suggested  that  the  islands  of  Langerhans  have  a  func- 
tion separate  and  distinct  from  that  of  the  glandular  portion  of  the  pancreas; 
that  they  secrete  a  specific  material  which  partakes  of  the  nature  of  an 
internal  secretion  which  is  absorbed  by  the  blood  circulating  around  them 
and  carried  to  different  tissues.     The  effect  on  the  metabolism  of  the  body 


i86  TEXT-BOOK  OF  PHYSIOLOGY. 

which  follows  extirpation  of  the  pancreas  will  be  referred  to  in  a  subsequent 
chapter. 

Pancreatic  Juice. — The  pancreatic  juice  may  be  obtained  by  intro- 
ducing a  silver  cannula,  through  an  opening  in  the  abdominal  wall,  into  the 
duct,  and  securing  it  by  a  ligature.  In  a  short  time  the  juice  flows  from 
the  distal  end  of  the  cannula,  when  it  can  be  collected.  According  to 
Bernard,  normal  juice  can  be  obtained  only  during  the  first  twenty-four 
hours  of  the  experiment.  The  juice  obtained  from  a  temporary  fistula  is 
clear,  slightly  opalescent,  viscid,  of  a  decidedly  alkaline  reaction,  and  has  a 
specific  gravity  (in  the  dog)  of  1.040.  When  cooled  to  0°  C,  it  assumes  a 
gelatinous  consistence.  At  100°  C.  it  completely  coagulates.  When  obtained 
from  a  permanent  fistula,  the  juice  is  watery  and  the  solid  constituents  are 
very  much  diminished  in  amount. 

The  chemic  composition  of  the  pancreatic  juice  of  the  dog  as  determined 
by  Schmidt  is  as  follows:  water,  900.76;  organic  matter,  90.44;  inorganic 
salts,  8.80.  Of  the  inorganic  salts,  sodium  carbonate  is  probably  the  most 
essential,  as  it  is  this  salt  which  gives  to  the  juice  its  alkaline  reaction. 

Human  pancreatic  juice  obtained  from  a  fistula  of  the  duct  was  found  to 
be  clear  and  limpid,  resembling  water,  alkaline  in  reaction  and  with  a  sp.  gr. 
of  1.007.  The  total  solids  of  two  specimens  amounted  to  about  1.270  and 
1.244,  grams  in  100  grams  of  the  juice.  The  amount  of  juice  collected 
varied  from  420  c.c.  to  884  c.c.  daily. 

Mode  of  Secretion. — The  secretion  of  the  juice  is,  in  the  rabbit  and 
dog  at  least,  almost  continuous  during  a  period  of  twenty-four  hours  after  a 
single  average  meal,  though  the  rate  of  flow  varies  considerably  during  this 
period.  Shortly  after  the  food  enters  the  stomach  the  flow  of  the  pancreatic 
juice  begins,  and  steadily  increases  in  amount  until  about  the  third  hour, 
when  it  reaches  its  maximum;  after  this  period  the  flow  diminishes  until  the 
sixth  hour,  when  it  again  increases  for  about  an  hour.  It  then  gradually 
diminishes  until  it  ceases  entirely.  During  the  period  of  secretory  activity 
the  blood  supply  is  very  much  increased,  from  a  dilatation  of  the  blood- 
vessels. 

The  secretion  and  discharge  of  the  pancreatic  juice  is  associated  with 
the  introduction  of  food  into  the  stomach  and  its  early  passage  into  the  duo- 
denum and  is  brought  about  by  the  action  of  a  primary  and  a  secondary 
stimulus. 

The  primary  stimulus  is  the  discharge  of  nerve  impulses  from  nerve- 
cells  in  the  medulla  oblongata  and  their  transmission  by  efferent  nerves 
in  the  trunk  of  the  vagus  nerve  to  the  cells  of  the  acini.  It  is  probable 
that  the  impressions  made  by  the  food  on  the  terminal  filaments  of  the 
afferent  fibers  in  the  vagus  nerve  develop  nerve  impulses  which,  when  trans- 
mitted to  the  medulla,  occasion  the  discharge  of  nerve  impulses  that  not 
only  excite  the  secretion  but  increase  the  blood  supply  as  well.  The  vaso- 
motor nerve  impulses  reach  the  blood-vessel  supplied  to  the  gland,  by 
way  of  the  great  splanchnic  nerve  and  the  post-gangHonic  fibers  from  the 
semilunar  ganglion.  That  the  vagus  nerve  contains  secretor  fibers  for  the 
pancreas  has  been  established  by  Pawlow.  This  investigator  states  that 
the  vagus  nerve  contains  two  classes  of  fibers  for  the  pancreas,  secreto-motor 
and  secreto-inhibitor,  as  well  as  vaso-dilatator  fibers  for  the  blood-vessels,  and- 


DIGESTION.  187 

therefore  the  effects  of  stimulation  are  often  contradictory  and  confused,  but  if 
the  nerve  be  divided  and  time  given  for  the  degeneration  of  the  secreto- 
inhibitor  and  vaso-dilatator  nerves,  usually  a  period  of  four  or  five  days,  then 
stimulation  of  the  peripheral  end  of  the  nerve  with  induced  electric  currents 
is  follovv'ed  after  a  latent  period  of  two  or  three  minutes  by  a  discharge  of 
the  juice.  Stimulation  of  the  splanchnic  nerve  under  similar  conditions  also 
gives  rise  to  a  secretion. 

Inasmuch  as  various  agents,  such  as  mineral  and  organic  acids,  placed 
on  the  duodenal  mucous  membrane  excite  the  flow,  it  is  quite  possible  that 
the  passage  of  the  acid  contents  of  the  stomach  through  the  duodenum 
acts  as  a  powerful  stimulus  to  this  nerve  mechanism.  But  as  the  secretion 
and  discharge  of  the  juice  is  excited  by  the  same  ccfhditions  after  the  division 
of  all  related  nerves,  other  explanations  have  been  sought  for  and  found 
in  a  secondary  stimulus  discovered  by  Bayliss  and  Starling. 

The  secondary  stimulus  is  chemic  in  character  and  developed  in  the 
glands  of  the  mucous  membrane  of  the  duodenum  by  the  action  of  the  acids 
of  the  chyme,  that  is,  of  the  digested  foods,  coming  through  the  pylorus. 

These  investigators  made  the  discovery  that  if  an  extract  of  the  gland 
portion  of  the  duodenal  mucous  membrane,  made  with  hydrochloric  acid  0.4 
per  cent,  is  injected  into  the  blood  it  evokes  a  profuse  discharge  of  pancreatic 
juice.  As  hydrochloric  acid  alone  will  not  produce  this  effect  they  assumed 
that  the  extract  contained  an  agent  that  excited  or  aroused  the  pancreas  to 
secretor  activity  and  to  which  therefore  they  gave  the  name  secretin.  This 
agent  resists  the  temperature  that  usually  destroys  enzymes  and  therefore 
is  not  regarded  as  a  member  of  this  class  of  agents.  Since  hydrochloric  acid 
appears  to  be  necessary  to  the  development  of  secretin,  the  further  assumption 
has  been  made  that  it  is  a  derivative  of  a  preexisting  compound  to  which  the 
name  prosecretin  is  given.  The  secretin  thus  developed  is  absorbed  into  the 
blood  and  carried  eventually  to  the  pancreas  and  brought  into  relation  with 
the  cells  on  which  it  exerts  its  stimulating  action.  To  an  agent  of  this  class 
Starling  has  given  the  name  hormone. 

Histologic  Changes  in  the  Cells  during  Secretor  Activity. — 
Reference  has  already  been  made  to  the  fact  that  the  cells  lining  the  acini 
consist  of  two  zones:  an  outer  one,  clear  and  homogeneous;  and  an  inner  one, 
dark  and  granular.  The  position  of  the  nucleus  of  the  cell  varies,  being  at 
one  time  in  the  outer,  at  another  time  in  the  inner,  zone.  If  the  pancreas  be 
examined  microscopically  during  the  intervals  of  digestion,  it  will  be  observed 
that  the  inner  zone  is  broad,  highly  granular,  occupying  nearly  the  entire  cell, 
while  the  outer  zone  is  narrow  and  clear.  If,  however,  the  gland  be  examined 
shortly  after  a  period  of  active  secretion,  the  reverse  conditions  will  be 
observed;  that  is,  the  inner  zone  will  be  narrow,  containing  relatively  few 
granules,  while  the  outer  zone  will  be  clear  and  wide.  This  change  in  the 
cell  has  been  witnessed  in  the  pancreas  of  the  living  animal — rabbit — by 
Kiihne  and  Lea.  They  observ- ed  that  as  soon  as  digestion  set  in,  the  granules 
of  the  broad  inner  zone  began  to  pass  toward  the  lumen  of  the  acinus  and  to 
disappear  gradually  as  the  secretion  was  poured  out,  while  the  outer  zone 
increased  in  width  until  almost  the  entire  cell  became  clear  and  homogeneous. 
(See  Fig.  81.)  After  secretion  ceased  the  granules  again  made  their  appear- 
ance, the  result,  in  all  probability,  of  metabolic  activity. 


i88  TEXT-BOOK  OF  PHYSIOLOGY. 

Physiologic  Action  of  Pancreatic  Juice. — Experimental  investi- 
gations have  demonstrated  the  fact  that  pancreatic  juice  is  the  most  complex 
in  its  physiologic  action  of  all  the  digestive  fluids.  In  virtue  of  its  contained 
enzymes,  pancreatic  juice  acts: 

1.  On  starch.  When  normal  pancreatic  juice  or  a  glycerin  extract  of 
the  gland  is  added  to  a  solution  of  hydrated  starch,  the  latter  is  speedily 
transformed  into  maltose,  passing  through  the  intermediate  stage  of  dextrin. 
The  process  is  in  all  respects  similar  to  that  observed  in  the  digestion  of 
starch  by  saliva.  Pancreatic  juice,  however,  is  more  energetic  in  this  respect 
than  saliva.  The  enzyme  which  effects  this  change  is  termed  amylopsin. 
When  the  starch  which  escapes  salivary  digestion  passes  into  the  small 
intestine  and  mingles  with  pancreatic  juice,  it  is  very  promptly  converted  into 
maltose  by  the  action  or  in  the  presence  of  this  enzyme. 

2.  On  protein.  When  protein  compounds  are  subjected  to  the  action 
of  pancreatic  juice,  they  are  transformed  into  peptones  which  do  not  differ 
in  essential  respects  from  those  formed  by  the  action  of  gastric  juice.  The 
intermediate  stages,  however,  are  believed  to  be  somewhat  different.  The 
enzyme  which  effects  this  change  is  termed  trypsin. 

When  fibrin,  for  example,  is  added  to  trypsin  in  a  solution  rendered 
alkaline  by  sodium  carbonate,  it  does  not  swell  and  become  translucent, 
as  it  does  when  treated  with  hydrochloric  acid  and  pepsin.  On  the  con- 
trary, it  becomes  corroded  on  the  surface,  fragile,  and  in  a  short  time  under- 
goes solution.  The  first  product  is  a  compound  termed  alkali-protein. 
After  solution  has  taken  place,  various  chemic  changes  are  initiated  which 
eventuate  in  the  production  of  peptone  and  certain  nitrogenized  bodies, 
leucin,  tyrosin,  aspartic  acid,  etc.  The  intermediate  stages  in  this  process 
have  not  been  satisfactorily  determined.  At  no  time  during  artificial 
pancreatic  digestion  is  there  any  evidence  of  the  presence  of  the  primary 
proteoses.  The  secondary  proteoses,  however,  are  usually  present.  It 
will  be  recalled  that  when  the  peptone  of  peptic  digestion  is  subjected  to  the 
action  of  trypsin  a  portion  of  it  is  decomposed  into  leucin  and  tyrosin,  while 
another  portion  presumably  is  not  so  decomposed,  for  which  reason  the 
latter  was  called  anti-  and  the  former,  /zt'mi-peptone.  It  is  now  believed  that 
anti-peptone  is  not  a  peptone  at  all,  but  a  compound  termed  carnic  acid, 
which  can  be  decomposed  into  simpler  nitrogen-holding  bodies  such  as 
leucin,  tyrosin,  arginin,  etc. 

The  action  of  trypsin  on  proteins  in  an  alkaline  medium  may  be  illustrated 
by  the  following  scheme: 

Protein 

Alkali-protein 

Secondary  proteoses 

Peptone 


Leucin  Tyrosin  Aspartic  acid  Arginin         Ammonia 

When  the  proteins  which  have  escaped  digestion  in  the  stomach  pass 
into  the  small  intestine  and  mingle  with  the  pancreatic  juice,  they  are 


DIGESTION.  189 

doubtless  digested  in  the  course  of  the  intestinal  canal,  passing  through  the 
stages  just  described.  As  leucin  and  tyrosin  are  found  in  the  intestine  during 
digestion,  it  is  probable  that  a  portion  of  the  peptone  undergoes  decompo- 
sition into  these  bodies;  but  as  to  the  extent  to  which  this  takes  place  or  in 
how  far  it  is  a  necessary  process  under  normal  conditions  is  yet  a  subject  of 
investigation.  It  is  certain  that  it  takes  place  when  there  is  an  excess  of 
protein  food  or  when  for  any  reason  digestion  is  prolonged  or  absorption  is 
delayed. 

Though  the  view  that  the  final  stage  in  the  digestion  of  proteins  is 
the  formation  of  peptones,  which  in  due  time  are  absorbed  and  synthesized 
into  blood  albumin,  has  been  generally  accepted,  there  is  an  ever  increasing 
evidence  that  it  is  not  wholly  true,  and  that  the  final  stage  may  be  the  forma- 
tion of  the  nitrogen-holding  compounds  above  mentioned;  in  other  words, 
that  the  cleavage  of  the  proteins  is  far  more  complete  than  has  heretofore 
been  assumed.  Indeed  it  has  been  asserted  that  they  are  reduced,  if  not  to 
their  ultimate  constituents,  the  amino-  and  diamino-acids,  at  least  to  one  or 
more  of  the  different  polypeptid  stages.  Ever  since  the  discovery  by  Cohn- 
heim  of  the  existence  in  the  intestinal  juice  of  a  substance  termed  by  him 
erepsin,  which  is  capable  of  splitting  proteoses  and  peptones  into  simple 
nitrogen-holding  compounds,  there  has  been  slowly  developing  the  idea  that 
normally  during  intestinal  digestion  the  proteoses  and  peptones  are  reduced 
by  this  agent  to  leucin,  tyrosin,  histidin,  arginin,  aspartic  acid,  etc.,  which 
in  turn  are  absorbed  and  synthesized  to  blood  or  tissue  albumin.  The 
discovery  by  \'ernon  of  erepsin  in  pancreatic  juice  lends  further  support  to 
this  view. 

3.  On  fat.  If  pancreatic  juice  be  added  to  a  perfectly  neutral  fat — 
olein,  palmitin,  or  stearin — and  kept  at  a  temperature  of  about  100°  F. 
(38°  C),  it  will  at  the  end  of  an  hour  or  two  be  partially  decomposed  into 
glycerin  and  the  particular  fat  acid  indicated  by  the  name  of  the  fat  used 
— e.g.,  oleic,  palmitic,  stearic.  The  oil  will  then  exhibit  an  acid  reaction. 
The  reaction  is  represented  in  the  following  formula: 

C3H,(C,,H330,)3     +     3H,0      =     3CXSH3.O,        +     C3H,(HO)3 
Triolein.  Water.  Oleic  Acid.  Glycerin. 

If  to  this  acidified  oil  there  be  added  an  alkali,  e.g.,  potassium  or  sodium 
carbonate,  the  latter  will  at  once  combine  with  the  fat  acid  to  form  a 
salt  known  as  a  soap.     The  reaction  is  expressed  in  the  following  equation : 

Sodium  Carbonate.  Oleic  Acid.  Sodium  Oleate.  Carbonic  Acid. 

NaoCOa  4-  CisH3^02        =         2NaCjsH3302  +  HjCOj 

Coincident  with  the  formation  of  the  soap  the  remaining  neutral  oil  under- 
goes division  into  drops  of  microscopic  size,  which  float  in  the  soap  solution, 
forming  what  has  been  termed  an  emulsion,  which  is  white  and  creamy 
in  appearance.  The  action  of  the  pancreatic  juice  may  then  be  said  to 
consist  in  the  cleavage  of  the  neutral  fats  into  fatty  acids  and  glycerin,  after 
which  the  formation  of  the  soap  and  the  division  of  the  fat  takes  place  spon- 
taneously. The  enzyme  which  produces  the  cleavage  of  the  neutral  fats 
has  been  termed  steapsin  or  lipase.  The  extent  to  which  the  cleavage  of  the 
fat  takes  place  in  the  intestine  has  not  been  definitely  determined.  There  are 
some  who  think  the  amount  is  relatively  small,  while  others  consider  that 


I90  TEXT-BOOK  OF  PHYSIOLOGY. 

it  is  large,  practically  all  of  the  fat  undergoing  this  decomposition,  with 
the  formation  of  soap  and  glycerin  prior  to  their  absorption. 

According  to  Pawlow,  the  relative  amounts  of  the  pancreatic  enzymes 
produced,  are  conditioned  by  the  character  and  amounts  of  the  food  principles 
consumed.  Thus,  if  chyme  contains  an  excess  of  either  starch,  protein,  or 
fat,  there  is  a  corresponding  increase  in  the  amount  of  either  amylopsin, 
trypsin,  or  steapsin  produced.  The  pancreas  apparently  adapts  its  activities 
to  the  character  of  the  food.  Though  it  is  probable  that  each  enzyme  is  a 
derivative  of  a  special  zymogen,  it  is  positively  known  that  this  is  the  case 
only  with  trypsin.  This  enzyme  is  a  derivative  of  the  zymogen,  trypsinogen, 
the  production  of  which  is  thought  to  be  the  special  function  of  secretin. 
The  pancreatic  juice  at  the  moment  of  its  discharge  into  the  intestine  does 
not  contain  trypsin  but  trypsinogen.  The  transformation  of  the  latter  into 
the  former  is  accomplished,  according  to  Pawdow,  by  a  special  activating 
ferment  secreted  by  the  epithelium  of  the  small  intestine  and  termed  entero- 
kinase. 

The  rapidity  with  which  pancreatic  juice  in  the  presence  of  bile  and 
hydrochloric  acid  (under  conditions  such  as  are  present  in  the  duodenum) 
can  develop  sufhcient  fatty  acid  to  form  an  emulsion  was  determined  by 
Rachford  to  be  two  minutes.  The  activity  of  steapsin  is  thus  shown  to  be 
very  great. 

Physiologic  Action  of  the  Intestinal  Juice. — The  part  played 
by  the  intestinal  juice  in  the  digestive  process  is  yet  a  subject  of  discussion, 
as  the  results  obtained  by  different  observers  are  in  some  respects  con- 
tradictory, due  to  the  fact  that  animals,  including  human  beings,  have 
been  the  subjects  of  experimentation.  Notwithstanding  the  actions  of 
saliva,  gastric  and  pancreatic  juice,  there  yet  remain  in  the  food  saccharose, 
maltose,  and  lactose,  three  forms  of  sugar  which  are  believed  by  most  to 
observers  to  be  non-assimilable  and  therefore  require  some  change  before 
they  can  be  absorbed  and  assimilated.  .An  extract  of  the  intestinal  mucous 
membrane  or  the  intestinal  juice  of  a  dog,  added  to  a  solution  of  saccharose, 
will  in  a  very  short  time  convert  it  into  dextrose  and  levulose,  which  together 
constitute  invert  sugar.  The  enzyme  by  which  this  inversion  is  produced, 
though  nothing  definite  is  known  as  to  its  nature,  has  been  termed  invertase 
or  saccharase.  Tubbey  and  Manning  state  that  the  human  intestinal  juice 
as  obtained  by  them  has  the  same  action.  In  the  case  of  intestinal  fistulas 
reported  by  Busch,  which  were  supposed  to  be  located  in  the  upper  third  of 
the  intestine,  it  was  found  that  when  saccharose  was  introduced  into  the 
lower  opening,  it  was  not  inverted  but  appeared  in  the  feces  unchanged. 

Maltose  is  also  rapidly  transformed  into  dextrose.  Lactose  appears 
to  be  unaffected  by  the  pure  juice.  As  it  is  non-assimilable  it  has  been 
supposed  to  undergo  conversion  into  dextrose  and  galactose  while  passing 
through  the  epithehal  cells  of  the  intestinal  mucosa.  In  either  case  the 
transformation  is  brought  about  by  two  ferments  known  respectively  as 
maltase  and  lactase. 

The  intestinal  juice  also  contains  the  two  ferments  cntcrokinase  and 
erepsin.  The  former  activates  the  trypsinogen  of  the  pancreatic  juice  and 
converts  it  into  trypsin;  the  latter  acts  on  the  peptones  and  reduces  them  to 
amino-acids  and  peptids'. 


DIGESTION.  191 

THE  LIVER. 

The  liver  is  a  highly  vascular  conglomerate  gland  situated  in  the  right 
hypochondriac  region  and  connected  with  the  intestine  by  a  duct. 

Inasmuch  as  the  liver  performs  several  functions  related  to  both  secretion 
and  excretion,  a  consideration  of  its  structure  and  its  various  functions  will 
be  deferred  to  a  subsequent  chapter.  In  this  connection  only  the  bile,  and  its 
physical  properties  and  chemic  composition  in  relation  to  the  digestive 
process,  will  be  considered. 

The  bile  is  a  product  of  the  secretor  activity  of  the  liver  cells.  As 
it  is  poured  into  the  intestine  in  man  and  most  mammals  at  a  point  corre- 
sponding to  the  orifice  of  the  pancreatic  duct,  and  most  abundantly  at  the 
time  the  food  is  passing  through  the  duodenum,  it  is  usually  regarded  as  a 
digestive  fluid  possessing  an  influence  favorable  if  not  necessary  to  the  com- 
pletion of  the  general  digestive  process. 

Anatomic  Relations  of  the  Biliary  Passages.— After  its  forma- 
tion by  the  liver  cells  the  bile  is  conveyed  from  the  liver  by  the  bile  capillaries, 
which  unite  finally  to  form  the  main  hepatic  duct.  This  duct  emerges 
from  the  liver  at  the  transverse  fissure.  At  a  distance  of  about  5  centimeters 
it  is  joined  by  the  cystic  duct,  the  distal  extremity  of  which  expands  into  a 
pear-shaped  reservoir,  the  gall-bladder  in  which  the  bile  is  temporarily 
stored  (Fig.  60).  The  duct  formed  by  the  union  of  the  hepatic  and  cystic 
ducts,  the  common  bile-duct,  passes  downward  and  forward  for  a  distance 
of  about  7  centimeters,  pierces  the  walls  of  the  intestine  and  passes  obHquely 
through  its  coats  for  about  a  centimeter  and  opens  into  a  small  receptacle, 
the  ampulla  of  Vater.  The  ampulla  in  turn  opens  on  a  small  papilla  into 
the  intestine.  The  walls  of  the  biliary  passages  are  composed  of  a  mucous 
membrane  internally,  a  fibrous  and  muscular  coat  externally.  The  termina- 
tion of  the  common  bile-duct  is  provided  with  a  distinct  band  of  circularly 
disposed  muscle-fibers,  which  when  in  action  completely  close  the  orifice 
and  prevent  the  discharge  of  bile.  It  may  therefore  be  regarded  as  a  true 
sphincter  muscle.  Small  racemose  glands  are  embedded  in  the  mucous 
membrane  of  the  main  ducts. 

Physical  Properties  and  Chemic  Composition  of  Bile. — The  bile 
obtained  directly  from  the  liver  through  a  cannula  inserted  into  the  hepatic 
duct  is  always  thin  and  watery,  while  that  obtained  from  the  gall-bladder  is 
more  or  less  viscid  from  admixture  with  mucin,  the  degree  of  the  viscidity 
depending  on  the  length  of  time  it  remains  in  this  reservoir.  The  specific 
gravity  of  human  bile  varies  within  normal  limits  from  i.oio  to  1.020. 
The  reaction  is  invariably  alkaline  in  the  human  subject  when  first  dis- 
charged from  the  liver,  but  may  become  neutral  in  the  gall-bladder.  The 
alkaHnity  depends  on  the  presence  of  sodium  carbonate  and  sodium  phos- 
phate. When  fresh,  it  is  inodorous;  but  it  readily  undergoes  putrefactive 
changes,  and  soon  becomes  offensive.  Its  taste  is  decidedly  bitter.  When 
shaken  with  water,  it  becomes  frothy — a  condition  which  lasts  for  some 
time  and  which  is  due  to  the  presence  of  mucin.  In  ox  bile  the  mucin  is 
replaced  by  a  nucleo-proteid. 

The  color  of  bile  obtained  from  the  hepatic  duct  is  variable,  usually  a 
shade  between  a  greenish-yellow  and  a  brownish-red.     In  different  animals 


192  TEXT-BOOK  OF  PHYSIOLOGY. 

the  color  varies.  In  the  herbivorous  animals  it  is  usually  green;  in  the  car- 
nivorous animals  it  is  orange  or  brown.  In  man  it  is  green  or  a  golden 
yellow.  The  colors  are  due  to  the  presence  of  pigments.  Microscopic 
examination  fails  to  show  the  presence  of  structural  elements. 

Human  bile  obtained  from  an  accidental  biliary  fistula  was  shown  by 
Jacobson  to  contain  the  following  ingredients,  viz. : 

COMPOSITION  OF  HUM.\X  BILE. 

Water 977  .40 

Sodium  glycocholate 9-94 

Sodium  taurocholate a  trace 

Cholesterin o  •  54 

Free  fat o .  10 

Sodium  palmitate  and  stearate i  36 

Lecithin o  .  04 

Organic  matter,  and  pigments  bilirubin  and  biliverdin 2  .26 

Sodium  chlorid 5-45 

Potassium  chlorid -. 0.28 

Sodium  phosphate ^  -33 

Calcium  phosphate o  -37 

Sodium  carbonate °  •  93 

1000.00 

In  this  analysis  the  solid  ingredients  constitute  22.6  parts  per  1000,  of  which 
two-thirds  are  organic  and  one-third  inorganic.  The  amount  of  solid  varies 
according  to  the  animal  from  which  the  bile  is  obtained. 

Sodium  Glycocholate  and  Taurocholate. — Of  the  various  ingredients 
of  the  bile  none  are  more  important  than  these  two  salts,  usually  known  as 
the  bile  salts.  The  sodium  glycocholate  is  found  most  abundantly  in  the 
bile  of  herbivora,  the  sodium  taurocholate  in  the  bile  of  the  carnivora.  These 
salts  are  compounds  of  sodium  and  glycocholic  and  taurocholic  acids. 
When  separated  from  the  sodium,  the  acids  will  crystallize  in  the  form  of 
fine  acicular  needles.  Under  the  influence  of  hydrating  agents,  such  as 
dilute  acids  and  alkalies,  both  acids  will  undergo  cleavage  into  their  re- 
spective components — e.g.,  glycocoll  and  cholaUc  acid,  taurine  and  cholalic 
acid.  Glycocoll  and  taurine  are  crystallizable  nitrogenized  compounds 
known  chemically  as  amido-acetic  and  amido-isethionic  acids  respectively. 
The  bile  salts  are  produced  in  the  liver  by  a  true  act  of  secretion,  as  they 
are  not  found  in  any  of  the  tissues  and  fluids  of  the  body.  After  being  dis- 
charged into  the  intestine  they  undergo  chemic  changes,  after  which  they 
can  no  longer  be  recognized.  In  all  probability  they  are  resorbed  into  the 
blood  and  play  some  ulterior  part  in  the  nutrition  of  the  body. 

The  presence  of  the  bile  salts  can  be  demonstrated  by  the  employment 
of  Pettenkofer's  test  or  reaction.  It  was  shown  by  this  investigator  that  if 
to  a  solution  of  bile  salts  a  small  quantity  of  a  10  per  cent,  solution  of  cane- 
sugar  be  added  and  subsec^uently  a  small  quantity  of  strong  sulphuric  acid, 
a  brilliant  red  color  appears  which  soon  passes  into  a  rich  purple.  To 
secure  the  best  results  in  the  performance  of  this  test  care  should  be  exercised 
to  keep  the  temperature  below  70°  C;  the  characteristic  colors  appear  to 
be  due  to  the  action  of  the  sulphuric  acid  on  the  cane-sugar  by  which  a 
substance,  furfurol,  is  produced,  which  in  turn  reacts  with  the  cholalic  acid. 
This  test  can  be  applied  to  bile  directly;  thus  if  to  bile  in  a  test-tube  cane- 
sugar  be  added  and  the  mixture  thoroughly  shaken,  a  portion  of  the  bile 


DIGESTION.  193 

becomes  quite  frothy.  If  now  sulphuric  acid  be  carefully  added,  the  red 
and  purple  colors  present  themselves  at  once  in  the  white  froth — an  indica- 
tion that  the  bile  salts  are  distributed  through  it. 

Cholesterin.^Cholesterin  is  a  constant  ingredient  of  bile,  though  it  is 
not  confined  to  this  fluid,  as  its  presence  has  been  determined  in  the  crystal- 
line lens,  blood-corpuscles,  nerve-tissue,  and  various  pathologic  fluids.  It 
is  an  organic  non-nitrogenized  compound  resem- 
bling the  fats  in  some  particulars,  but  differing 
from  them  in  not  being  capable  of  saponification 
with  alkalies.  It, presents  itself  in  the  form  of 
thin  transparent  rectangular  crystals,  insolublo 
in  water  but  soluble  in  ether  and  boiling  alcohel 
(Fig.  84).  It  is  held  in  solution  in  bile  by  the  bile 
salts.  If  they  are  deficient  in  amount,  the  choles-  Fig.  84.— Cholesterix 
terin  may  pass  out  of  solution,  collect  around  some  Crystals. —  (Lcih^o/^  and 
foreign  matter,  and  form  a  gall-stone.  Choles-  "^  '"^ 
terin  is  largely  a  product  of  the  metabolism  of  ner^'e-tissue,  from  which  it 
is  absorbed  by  the  blood,  carried  to  the  liver,  and  excreted.  In  the  intes- 
tine it  is  converted  into  stercorin  and  discharged  from  the  body  in  the  feces. 

Bilirubin,  Biliverdin. — These  two  pigments  imp^irt  to  the  bile  its 
red  and  green  colors  respectively.  Bilirubin  is  present  in  the  bile  of  human 
beings  and  the  carnivora,  biliverdin  in  the  bile  of  the  herbivora.  As  the 
former  pigment  readily  undergoes  oxidation  in  the  gall-bladder,  giving  rise 
to  the  latter  pigment,  almost  any  specimen  of  bile  may  present  anv  shade 
of  color  between  red  and  green.  Bilirubin  is  regarded  as  a  derivative  of 
hematin,  one  of  the  cleavage  products  of  hemoglobin,  the  coloring-matter 
of  the  blood.  In  the  liver  the  hematin  combines  wdth  water,  loses  its  iron, 
and  is  changed  to  bilirubin.  By  continuous  oxidation  there  are  formed 
biliverdin,  bilicyanin,  and  choletelin.  x\fter  their  discharge  into  the  intes- 
tine the  bile  pigments  are  finally  reduced  to  hydrobilirubin  or  an  allied  sub- 
stance, stercobilin,  which  becomes  one  of  the  constituents  of  the  feces.  An 
oxidation  of  the  bilirubin  can  be  produced  by  nitroso-nitric  acid.  If  this 
agent  is  added  to  a  thin  layer  of  bile  on  a  porcelain  surface,  a  series  of  colors 
will  rapidly  succeed  one  another,  commencing  with  green  and  passing  to 
blue,  orange,  purple,  and  yellow.  This  is  the  basis  of  the  well-known  test 
for  bile  pigments  suggested  by  Gmelin. 

Lecithin. — Lecithin  is  regarded,  because  of  its  physical  properties  and 
chemic  composition,  as  a  complex  fat.  When  pure  it  presents  itself  gener- 
ally as  a  white  crystalline  powder,  though  very  frequently  as  a  white  waxy 
mass  which  is  soluble  in  either  and  alcohol.     Its  chemic  formula  is  C,,H„„- 

•  •11  44        90 

NPOy.  Lecithin  is  widely  distributed  throughout  the  body,  being  found  in 
blood,  lymph,  red  and  w^hite  corpuscles,  nervx-tissue,  yolk  of  egg,  semen, 
milk,  and  bile.  It  is  readily  decomposed,  yielding  with  various  reagents 
glyco-phosphoric  acid,  a  fat  acid  (stearic),  and  the  alkaloid,  cholin.  Leci- 
thin has  been  regarded  as  one  of  the  decomposition  products  of  nerve-tissue, 
removed  from  the  blood  by  the  liver  and  thus  becoming  one  of  the  constitu- 
ents of  the  bile,  in  which  it  is  held  in  solution  by  the  bile  salts. 

The  Mode  of  Secretion  and  Discharge  of  Bile. — The  manner  in  which 
the  bile  flows  from  the  liver  into  the  main  hepatic  ducts,  the  variations  in  the 


194  TEXT-BOOK  OF  PHYSIOLOGY. 

rate  of  its  discharge  into  the  intestine,  as  well  as  the  total  quantity  secreted 
daily,  have  been  approximately  determined  by  fistulous  openings  either 
in  the  hepatic  ducts  or  in  the  gall-bladder.  Although  the  liver  presents 
some  physiologic  peculiarities,  there  is  no  reason  to  believe  that  the  condi- 
tions of  secretion  therein  are  different  from  those  in  any  other  secretor 
organ,  or  that  any  other  structure  than  the  cell  is  engaged  in  this  process. 
As  shown  by  chemic  analysis,  the  bile  consists  of  compounds,  some  of  which, 
like  the  bile  salts,  are  formed  in  the  liver  cells,  out  of  material  furnished  by 
the  blood  by  a  true  act  of  secretion,  while  others,  such  as  cholesterin  and 
lecithin,  principles  of  waste,  are  merely  excreted  from  the  .blood  to  be  finally 
eliminated  from  the  body.  The  bile  is  thus  a  compound  of  both  secretory 
and  excretory  principles. 

The  flow  of  bile  from  the  liver  is  continuous  but  subject  to  considerable 
variation  during  the  twenty-four  hours.  The  introduction  of  food  into  the 
stomach  at  once  causes  a  slight  increase  in  the  flow,  but  it  is  not  until 
about  two  hours  later  that  the  amount  discharged  reaches  its  maximum; 
after  this  period  it  gradually  decreases  up  to  the  eighth  hour,  but  never 
entirely  ceases.  During  the  intervals  of  digestion  though  a  small  quantity 
passes  into  the  intestine,  the  main  portion  is  diverted  into  the  gall-bladder, 
because  of  the  closure  of  the  common  bile-duct  by  the  sphincter  muscle  near 
its  termination,  where  it  is  retained  until  required  for  digestive  purposes. 
When  acidulated  food  passes  over  the  surface  of  the  duodenum,  there  is  an 
increase  in  the  secretion  or  at  least  the  discharge  of  bile,  and  as  this  takes 
place  after  the  nerves  distributed  to  the  liver  are  divided,  the  assumption  is 
that  an  agent,  possibly  secretin,  is  developed  in  the  duodenal  mucous  mem- 
brane, which,  absorbed  into  the  blood,  is  ultimately  distributed  to  the  liver 
cells  and  by  which  they  are  excited  to  activity.  At  the  same  time  there  is 
excited,  through  reflex  action,  a  contraction  of  the  muscle  walls  of  the  gall 
bladder  and  ducts,  a  relaxation  of  the  sphincter,  and  a  gush  of  bile  into  the 
intestine,  the  discharge  continuing  intermittently  until  digestion  ceases  and 
the  intestine  is  emptied  of  its  contents. 

The  storage  and  the  discharge  of  bile,  brought  about  by  the  alternate 
contraction  and  relaxation  of  the  muscle  walls  of  the  gall-bladder  and  of 
the  sphincter  are  regulated  by  the  nerve  system.  As  the  result  of  his  experi- 
ments Doyon  concludes,  that  during  the  intervals  of  intestinal  digestion  the 
vagus  nerve  is  carrying  nerve  impulses  which  on  the  one  hand  augment  the 
contraction  of  the  sphincter  and  inhibit  the  contraction  of  the  w^alls  of  the 
gall-bladder,  thus  establishing  the  conditions  for  the  storage  of  bile;  but. 
when  intestinal  digestion  is  inaugurated  the  splanchnic  nerve  carries  nerve 
impulses  which  inhibit  the  sphincter  and  augment  the  contraction  of  the 
walls  of  the  gall-bladder,  thus  establishing  the  condition  for  the  discharge  of 
the  bile. 

The  total  quantity  of  bile  secreted  daily  has  been  estimated  to  be  from 
500  to  800  grams. 

Physiologic  A.ction  of  Bile. — ^Notwithstanding  our  knowledge  of  the 
complex  composition  of  bile,  the  quantity  discharged  daily,  and  the  time 
and  place  of  its  discharge,  its  exact  relation  to  the  digestive  process  has  not 
been  fully  determined.  No  specific  action  can  be  attributed  to  it.  It  has 
but  a  slight,  if  any,  diastatic  action  on  starch.     It  is  without  influence  on 


DIGESTION.  19s 

proteins  or  on  fats  directly.  But  indirectly  and  by  virtue  of  the  bile  salts  it 
contains,  it  plays  an  important  part  in  increasing  the  action  of  the  pancreatic 
enzymes.  Thus  the  amylolytic  or  amyloclastic  power  of  the  pancreatic 
juice  is  almost  doubled  and  the  same  is  true  for  its  proteoclastic  power, 
while  its  lipoclastic  or  fat-splitting  power  is  tripled. 

The  bile  salts  also  dissolve  insoluble  soaps  which  may  be  formed  during 
digestion  and  thus  favors  the  digestion  of  fat.  If  it  be  excluded  from  the 
intestine  there  is  found  in  the  feces  from  22  to  58  per  cent,  of  the  ingested 
fats.  At  the  same  time  the  chyle,  instead  of  presenting  the  usual  white 
creamy  appearance,  is  thin  and  slightly  yellow.  The  manner  in  which  the 
bile  promotes  fat  digestion  is  yet  a  subject  of  investigation.  If  all  the  fat 
is  converted  into  fatty  acid  and  glycerin,  with  the  formation  of  soaps,  as 
seems  probable,  the  action  of  the  bile  becomes  more  apparent  from  the  fact, 
already  stated,  thai:  it  dissolves  and  holds  in  solution  the  soaps  so  formed 
which  v^^ould  be  necessary  to  their  absorption  by  the  epithelial  cells.  This 
action  has  been  attributed  to  the  presence  of  the  bile  salts.  As  an  aid  to 
digestion  the  bile  has  been  regarded  as  important,  for  the  reason  that  its 
entrance  into  the  intestine  is  attended  by  a  neutralization  and  precipitation 
of  the  proteins  which  have  not  been  fully  digested  and  are  yet  in  the  stage 
of  acid-albumin.  In  this  way  gastric  digestion  is  arrested  and  the  foods  are 
prepared  for  intestinal  digestion. 

Though  bile  possesses  no  antiseptic  properties  outside  the  body,  itself 
undergoing  putrefactive  changes  very  rapidly,  it  has  been  believed  that  in 
the  intestine  it  in  some  way  prevents  or  retards  putrefactive  changes  in  the 
food.  There  can  be  no  doubt  that  if  the  bile  is  prevented  from  entering 
the  intestine  there  is  an  increase  in  the  formation  of  gases  and  other  products 
which  impart  to  the  feces  certain  characteristics  which  are  indicative  of 
putrefaction.  As  to  the  manner  in  which  bile  retards  this  process  nothing 
definite  can  be  stated.  It  has  been  supposed  to  be  a  stimulant  to  the 
peristaltic  movements  of  the  intestine,  inasmuch  as  these  movements 
diminish  when  bile  is  diverted  from  the  intestine. 

Though  no  definite  nor  specific  action  on  any  of  the  different  classes  of 
food  principles  can  be  attributed  to  the  bile,  there  is  abundant  evidence  to 
show  that  its  presence  in  the  alimentary  canal  during  digestion  is  essential 
to  the  maintenance  of  the  nutrition  of  the  body.  That  the  bile  as  a  whole, 
or  at  least  part  of  its  constituents,  favorably  influences  digestion  and  general 
nutrition  is  evident  from  the  phenomena  which  follow  its  total  exclusion 
from  the  intestine,  as  when  the  common  bile-duct  is  ligated  and  a  fistula  of 
the  gall-bladder  is  established.  The  following  phenomena  were  observed 
in  a  young  dog  so  prepared  by  Professor  Flint.  During  the  first  five  days 
succeeding  the  operation  the  abdomen  was  tumid  and  there  was  some  rum- 
bling in  the  bowels.  Though  the  animal  ate  every  day,  the  discharge  of 
fecal  matter  became  infrequent,  the  matter  passed  being  grayish  in  color  and 
highly  offensive.  After  two  weeks  the  alvine  discharges  took  place  three 
and  four  times  daily.  For  four  days  the  weight  remained  normal;  afterward 
it  began  to  diminish,  and  from  this  time  the  animal  continued  to  lose  strength 
and  weight  until  its  death,  thirty-eight  days  after  the  operation.  Ten  days 
after  the  operation  the  appetite,  which  had  been  very  good,  increased,  but 
did  not  become  ravenous  until  a  few  days  before  death.    The  animal  usually 


196  TEXT-BOOK  OF  PHYSIOLOGY. 

ate  about  a  pound  and  a  half  of  beef-heart  daily,  but  always  refused  fat. 
There  was  an  absence  at  all  times  of  jaundice,  fetor  of  the  breath,  and  fall- 
ing of  the  hair.  Post-mortem  examination  showed  that  the  bile-duct  was 
obliterated,  and  there  was  no  evidence  that  any  bile  could  have  passed  into 
the  intestine.  The  results  of  this  and  similar  cases  go  to  show  that  that  por- 
tion of  the  bile  which  is  secretory  in  character  is  essential  to  digestion  and 
the  nutrition  of  the  body — that,  though  large  quantities  of  food  are  con- 
sumed, progressive  diminution  of  weight  takes  place  until  nearly  40  per 
cent,  of  the  body  is  consumed.  In  some  instances  the  breath  becomes 
fetid  and  there  is  a  falling  of  the  hair,  showing  some  profound  disturbance 
of  the  general  nutritive  process. 

The  Movements  of  the  Intestine. — The  movements  of  the  intestine 
have  been  studied  by  means  of  the  Rontgen  rays  by  Cannon.  The  method 
adopted  was  to  mix  with  the  food  subnitrate  of  bismuth,  which  being  opaque 
rendered  the  movements  of  the  intestinal  contents  and  thereby  the  move- 
ments of  the  intestinal  walls  visible  on  the  fluorescent  screen.  There 
investigations  revealed  the  presence  of  two  forms  of  activity,  one  of  which  is 
more  or  less  stationary  and  due  to  rhythmic  contraction  of  circular  muscle- 
fibers,  the  other  progressive,  passing  from  above  downward  and  due  to  the 
contraction  of  circular  and  longitudinal  muscle-fibers.  The  former  activity, 
which  is  by  far  the  more  common,  results  in  a  division  of  the  intes- 
tinal contents  into  small  segments  and  for  this  reason  was  termed  by  Can- 
non rhythmic  segmentation;  the  latter  activity  is  the  well-known  peristaltic 
wave. 

Rhythmic  Segmentation. — When  the  abdominal  cavity  is  investigated  by 
the  method  above  mentioned,  it  is  observed  that  after  the  food  has  passed 
into  the  intestine  and  formed  a  more  or  less  consistent  mass  of  variable 
length,  bands  of  circular  muscle-fibers,  situated  at  regular  distances  one 
from  another,  begin  to  contract  and  divide  a  mass  of  food  into  segments, 
after  which  they  at  once  relax  to  be  followed  by  contraction  of  other  bands  in 
the  segments  of  the  intestines  overlying  the  segments  of  food.  The  result  is 
again  a  division  of  the  food  into  two  new  segments  (Fig.  85).  The  lower 
half  of  each  segment  then  unites  with  the  upper  half  of  the  segment  of  food 
below  to  commingle  with  it  and  expose  new  surfaces  of  the  food  mass  to' 
contact  with  the  actively  absorbing  mucosa.  The  continual  repetition  of 
this  process  results  in  a  thorough  mixing  of  the  food  with  the  digestive  juices. 
From  the  manner  in  w^hich  these  contractions  make  their  appearance  it  would 
seem  that  the  mere  presence  of  a  segment  of  food  in  the  lumen  of  the  intestine 
is  sufficient  to  excite  the  overlying  fibers  to  activity. 

In  certain  regions  of  the  intestine  rhythmic  segmentation  may  continue 
for  half  to  three-quarters  of  an  hour  without  moving  the  food  forward  to  any 
marked  extent.  In  the  cat  the  segmentation  may  proceed  at  the  rate  of 
thirty  divisions  a  minute. 

Peristalsis. — After  the  food  has  been  prepared  by  the  process  described 
in  the  foregoing  paragraph,  it  is  then  slowly  carried  downward  by  what  is 
known  as  the  vermicular  or  peristaltic  wave.  This  wave  is  characterized  by 
a  contraction  of  the  circular  fibers  behind  the  mass  of  food  and  a  relaxation 
of  the  fibers  in  advance  of  it.  The  result  is  a  movement  forward  of  the  food, 
and  as  it  moves  it  is  followed  by  a  ring  of  constriction  and  preceded  by  a 


DIGESTION. 


197 


ring  of  relaxation  or  inhibition.  The  rate  of  movement  of  the  peristaltic 
wave  is  usually  extremely  slow. 

After  the  peristaltic  wave  has  advanced  the  food  a  variable'distance,  it 
disappears  and  the  food  comes  to  rest.  By  this  procedure  the  incoming  food 
from  the  stomach  is  readily  accommodated  in  the  duodenal  portion  of  the 
intestine.  With  the  disappearance  of  the  peristaltic  wave,  rhythmic  seg- 
mentation again  arises  in  the  portion  of  the  intestine  corresponding  to  the 
new  situation  of  the  segment  of  food.  This  in  turn  is  succeeded  by  another 
peristaltic  wave  which  advances  the  food  to  a  more  distant  region  of  the 
intestine.  This  continues  until  at  the  end  of  gastric  digestion  a  more  or  less 
continuous  column  of  food  occupies  the  lumen  of  the  small  intestine  from  the 
stomach  to  the  ileo-cecal  valve. 

In  addition  to  this  characteristic  physiologic  movement  it  has  also  been 
observed  by  different  experimenters  that  the  intestine  manifests  under  special 
circumstances  two  other  forms  of  moving  waves,  waves  moving  downward 


B 


/  \     /  \    /  \     /   \     /   \ 


\    /  \    /   \    /   \    /   \    / 


D 


P"iG.  85. — The  Divisive  or  Segimextixg  Movements  of  the  S-\la.ll  Intestine.  A,  surface 
of  a  portion  of  the  intestine,  showing  six  constrictions  which  divide  the  contents  into  five 
segments,  as  shown  in  B:  as  these  constrictions  pass  away  new  ones  come  in  between  them  and 
divide  each  segment  of  the  contents  into  two,  the  adjoining  halves  of  neighboring  segments 
fusing  to  make  the  new  segments  sho-svn  in  C.  Repetition  of  this  process  results  in  the  condition 
shown  in  D .—{Modified,  after  Hough  and  Sedgewick,  "  The  Human  Mechanism.") 

as  well  as  upward  from  their  point  of  origin,  but  without  being  produced  by 
an  inhibition  or  relaxation.  These  waves  are  therefore  not  regarded  as  true 
peristaltic  waves.  To  avoid  confusion,  the  term  diastalsis  has  recently  been 
employed  (Cannon)  to  designate  the  true  peristaltic  movement,  viz. :  pro- 
gressive contraction  preceded  by  inhibition,  and  the  terms  katastalsis  and 
anastalsis  to  designate  the  descending  and  ascending  contractions  respectively, 
that  occur  without  a  forerunning  inhibition. 

Rush  Peristalsis. — Under  conditions  that  are  perhaps  not  stricdy 
physiologic,  a  rapid  and  far-reaching  peristalsis  is  developed  which  may  pass 
over  the  intestine,  from  the  duodenum  to  the  cecum  without  stopping  in  the 
course  of  15  seconds,  in  the  rabbit,  and  which  has  been  designated  rush 
peristalsis.     It  is   characterized  by  a  wave  of   constriction  preceded  by  a 


198  TEXT-BOOK  OF  PHYSIOLOGY. 

completely  inhibited  long  section  of  intestine.  The  contents  of  the  intestine 
are  carried  along  with  extreme  rapidity  and  vigor.  The  contractions  may 
be  increased  by  purgative  salts,  ergot,  barium  chlorid,  etc.,  the  inhibition 
may  be  increased  by  calcium  chlorid,  magnesium  chlorid,  etc.  A  combina- 
tion of  ergot  and  calcium  chlorid  develops  in  the  rabbit  a  pronounced  rush 
peristalsis  (Metzger).  This  movement  appears  to  be  under  the  control  of 
the  central  nerve  system  as  it  fails  to  develop  after  division  of  the  vagus  nerves. 

Bayliss  and  Starling  state,  from  observations  made  on  the  exposed 
intestine  of  a  dog,  that  in  addition  to  the  usual  peristaltic  movement  the 
intestinal  coils  exhibit  a  swaying  or  pendulum  movement  accompanied  by 
slight  waves  of  contraction  which  may  arise  apparently  at  any  point  and 
pass  down  the  intestine.  These  contractions  may  occur  from  ten  to  twelve 
times  a  minute  and  travel  at  a  rate  varying  from  two  to  five  centimeters  a 
second.  In  how  far  this  movement  represents  the  normal  movement  as  it 
takes  place  under  physiologic  conditions  and  as  observed  by  Cannon,  remains 
for  further  investigations  to  decide. 

The  Nerve  Mechanism  of  the  Intestine. — The  causes  of  these  two 
forms  of  intestinal  activity,  rhythmic  segmentation  and  peristalsis,  have  been 
the  subject  of  much  investigation.  Because  of  the  presence  of  a  network  or 
plexus  (Auerbach's  and  Meissner's)  of  nerve-cells  and  nerve-fibers  in  the 
walls  of  the  intestines  and  in  close  relation  to  the  muscle  cells,  the  so-called 
myenteric  plexus  and  because  of  the  fact  that  the  intestines  will  contract  for 
some  time  after  removal  from  the  body  of  the  animal,  it  has  been  difficult  to 
decide  whether  the  contractions  are  myogenic  or  neurogenic. 

As  the  rhythmic  contractions  continue  though  the  peristaltic  are  abol- 
ished by  the  introduction  of  nicotin  into  the  blood,  an  agent  which  tempo- 
rarily paralyses  peripheral  nerve-cells,  it  was  concluded  by  Bayliss  and 
Starling  that  the  rhythmic  contractions  are  myogenic  and  that  the  peristaltic 
contractions  are  reflex  in  character,  the  coordination  being  carried  out  by 
the  local  nerve  mechanisms  and  initiated  by  stimulation  of  the  intestine. 
This  observation  has  been  corroborated  by  Cannon  who  has  shown  that  if 
the  myenteric  plexus  is  divided  by  incisions  extending  around  the  intestine, 
at  intervals  of  1.5  to  2  cm.  for  a  distance  of  45  cm.,  incisions  reaching  to  the 
submucous  coat,  the  peristaltic  movement  is  totally  abolished  though 
rhythmic  segmentation  develops  as  usual  and  continues  for  long  periods. 
As  the  segmentation  activity  continues  after  interruption  of  the  myenteric 
plexus,  the  inference  is  justifiable  that  it  is  purely  myogenic  and  is  the  re- 
sponse to  a  stimulus  within  the  intestine  such  as  distension  by  food,  when 
the  intestinal  wall  possesses  the  requisite  degree  of  tonicity. 

Though  the  orderly  and  coordinated  contractions  and  relaxations  of  the 
muscle  coat,  which  constitutes  a  peristaltic  movement,  are  mediated  by  the 
myenteric  plexus  nevertheless  the  contraction  may  be  augmented  or  inhibited 
by  the  central  nerve  system  through  the  vagus  and  splanchnic  nerves. 

Stimulation  of  the  vagus  is  followed  by  an  augmentation  of  the  contrac- 
tion, though  not  infrequently  there  is  a  primary  inhibition  of  short  duration. 
Stimulation  of  the  splanchnic  is  followed  by  a  relaxation  or  inhibition  of  the 
contraction,  though  according  to  some  observers  there  is  at  times  an  opposite 
effect. 

The  extent  to  which  the  contraction  is  initiated  or  augmented  by  stimula- 


DIGESTION.  199 

tion  of  the  vagus  depends  upon  the  extent  to  which  the  contraction  at  the 
moment  is  inhibited  by  the  splanchnic.  Thus  after  the  splanchnics  are 
divided,  stimulation  of  the  vagus  causes  a  much  more  pronounced  contrac- 
tion than  would  otherwise  be  the  case;  a  fact  that  indicates  that  the 
splanchnic  nerve-center  is  in  a  state  of  tonus  or  tonic  activity  and  therefore 
exerting  a  constant  inhibitor  effect  on  the  muscle-fibers.  Stimulation  of  the 
peripheral  end  of  the  divided  splanchnic  causes  an  arrest  or  inhibition  of 
a  preexisting  contraction. 

The  nerve-centers  regulating  the  contraction  and  relaxation  of  the 
muscle  walls  of  the  intestine  are  doubtless  excited  to  activity  by  nerve  impulses 
transmitted  through  afferent  nerves,  probably  the  vagus,  from  the  mucous 
surface  of  the  small  intestine.  These  centers  are  also  influenced  by  nerve 
impulses  descending  from  the  cerebrum,  though  the  route  they  take  is  not 
clearly  defined.  It  is  well  known  that  mental  states  markedly  influence  the 
contraction  in  one  direction  or  another. 

It  has  also  been  experimentally  determined  that  the  introduction  of 
various  acids  and  gases  into  the  intestinal  canal  is  followed  by  an  increase  in 
the  contraction.  It  is  probable  therefore  that  the  gases,  acids,  and  perhaps 
other  compounds  as  well,  developed  by  bacterial  action  also  act  as  excitants 
to  muscle  activity. 

The  Large  intestine. — The  large  intestine  is  that  portion  of  the  ali- 
mentary canal  situated  between  the  termination  of  the  ileum  and  the  anus. 
It  varies  in  length  from  four  and  a  half  to  five  feet,  in  diameter  from  one  and 
a  half  to  two  and  a  half  inches.  It  is  divided  into  the  cecum,  the  colon  (sub- 
divided into  an  ascending,  transverse,  and  descending  portion,  including  the 
sigmoid  flexure),  and  the  rectum. 

The  cecum  is  situated  in  the  right  iliac  fossa.  It  is  that  dilated  portion 
of  the  large  intestine  below  the  orifice  of  the  small  intestine.  The  posterior 
and  inner  wall  presents  a  small  opening  which  leads  into  a  narrow  round 
process  about  four  inches  in  length — the  vermiform  appendix.  The  opening 
of  the  small  intestine  into  the  cecum  is  narrow  and  elongated  and  bordered 
by  two  folds  of  mucous  membrane  strengthened  by  fibrous  and  muscle- 
tissue.  These  folds  constitute  the  so-called  ileo-cecal  valve.  When  the 
cecum  is  distended  the  margins  of  these  folds  are  approximated  and  effectually 
prevent  the  return  of  material  into  the  small  intestine. 

The  closure  of  this  opening  is  now  attributed  to  the  activity  of  a  sphincter 
muscle,  ihe  ileo-colic,  the  contraction  of  which  is  regulated  by  the  nerve 
system. 

The  colon  ascends  to  the  under  surface  of  the  liver,  where  it  bends  at  a 
right  angle,  crosses  the  abdominal  cavity  to  the  spleen,  bends  again,  and 
descends  to  the  left  iliac  fossa.  At  this  point  it  turns  upon  itself  to  form  the 
sigmoid  flexure.  The  rectum  is  a  dilated  pouch,  situated  within  the  true 
pelvis.  It  measures  from  15  to  18  centimeters  in  length.  Within  an  inch 
of  its  termination  at  the  anus  it  presents  a  constriction  formed  by  a  circular 
band  of  smooth  muscle-fibers  known  as  the  internal  sphincter.  The  margin 
of  the  anus  is  also  surrounded  by  bands  of  striated  muscle-fibers  known 
collectively  as  the  external  sphincter. 

The  walls  of  the  large  intestine  consist  of  three  coats:  viz.,  serous, 
muscular,  and  mucous. 


200  TEXT-BOOK  OF  PHYSIOLOGY. 

The  serous  is  a  reflection  of  the  general  peritoneal  membrane. 

The  muscle  is  composed  of  both  longitudinal  and  circular  fibers.  The 
longitudinal  fibers  are  collected  into  three  narrow  bands  which  are  situated 
at  points  equidistant  from  one  another.  At  the  rectum  they  spread  out  so 
as  to  surround  it  completely.  As  the  longitudinal  bands  are  shorter  than 
the  intestine  itself,  its  surface  becomes  sacculated,  each  sac  being  partially 
separated  from  adjoining  sacs  by  narrow  constrictions.  The  circular  fibers 
are  arranged  in  the  form  of  a  thin  layer  over  the  entire  intestine.  Between 
the  sacculi,  however,  they  are  more  closely  arranged.  In  the  rectum  they 
are  well  developed,  and  at  a  point  an  inch  above  the  anus  they  form,  as 
stated  above,  the  internal  sphincter. 

The  mucous  membrane  of  the  large  intestine  possesses  neither  villi  nor 
valvulae  conniventes.  It  contains  a  large  number  of  tubules  consisting  of  a 
basement  membrane  lined  by  columnar  epithelium.  They  resemble  the 
follicles  of  Lieberkiihn.  The  secretion  of  these  glands  is  thick  and  viscid 
and  contains  a  large  quantity  of  mucin. 

Contents  of  the  Large  Intestine. — As  a  result  of  the  actions  of  saliva, 
of  gastric,  intestinal,  and  pancreatic  juice,  and  of  the  bile,  the  food  is  disin- 
tegrated and  liquefied.  The  nutritive  principles,  protein,  starches,  sugars, 
and  fats,  undergo  chemic  changes  and  are  transformed  into  amino-acids  and 
peptids,  dextrose,  soap  and  glycerin,  fat  acids,  under  which  forms  they  are 
absorbed.  After  the  more  or  less  complete  digestion  and  absorption  of  these 
nutritive  substances  the  residue  of  the  food,  comprising  the  indigestible  and 
undigested  matter,  passes  out  of  the  small  intestine  into  the  large  intestine 
and  forms  a  portion  of  its  contents.  This  residue  consists  of  the  hard  parts 
of  the  cereals,  vegetable  seeds,  cellulose,  etc.,  the  quantity  and  variety  of 
which  depend  on  the  nature  of  the  food.  These  substances,  passing  into 
the  large  intestine  along  with  the  excrementitious  matter  of  the  bile,  become 
incorporated  with  the  mucous  secretions  and  assist  in  the  formation  of  the 
feces. 

Under  the  influence  of  a  peristaltic  movement  similar  to  that  wit- 
nessed in  the  small  intestine,  all  this  excrementitious  matter,  deprived  by 
absorption  of  the  excess  of  its  contained  water  and  nutritive  material,  is 
gradually  carried  downward  to  the  sigmoid  flexure,  where  it  accumulates 
prior  to  its  extrusion  from  the  body.  The  effects  of  the  peristaltic  waves  are 
to  some  extent  interfered  with  by  anti-peristaltic  waves  which  beginning  in  the 
transverse  colon  run  toward  and  to  the  cecum.  An  anti-peristaltic  wave 
occurs  in  the  cat  about  every  fifteen  minutes  and  lasts  for  about  five  minutes. 
The  intestinal  contents  are  thereby  driven  back  toward  the  cecum.  The 
effect  is  a  still  further  admixture  with  the  secretions  and  exposure  to  the 
absorbing  mucosa.  There  is  some  evidence  also  that  the  anti-peristaltic 
waves  may  force  some  of  the  liquefied  contents  through  the  ileo-colic  opening 
into  the  small  intestine  because  of  the  relaxation  of  the  ilio-colic  sphincter 
muscle.  It  is  questionable  if  this  ascending  movement  is  a  true  peristalsis 
inasmuch  as  the  advancing  contraction  is  apparently  not  preceded  by  an  area 
of  inhibition  or  relaxation.  It  resembles  rather  the  corresponding  move- 
ment manifested  by  the  small  intestine  to  which  the  term  anastalsis  has  been 
given,  and  is  propagated  along  the  muscle  coat  independently  of  the  myen- 
teric plexus. 


DIGESTION.  20I 

The  Nerve  Mechanism  of  the  Large  Intestine. — The  nerve  mechan- 
ism of  the  large  intestine  involves  both  motor  and  inhibitor  nerves.  The 
motor  nerves  comprise  both  pre-  and  postganglionic  fibers;  the  former 
have  their  origin  in  the  spinal  cord,  from  which  they  emerge  in  the  third  and 
fourth  sacral  nerves  and  pass  by  way  of  the  pelvic  nerve  to  ''the  pelvic 
ganglia  around  the  cells  of  which  their  fibers  arborize;  the  latter  (post- 
ganglionic) fibers  emerge  from  the  cells  of  these  gangHa  and  are  distributed 
to  circular  and  longitudinal  muscle-fibers  of  the  intestinal  wall. 

The  inhibitor  fibers  also  comprise  both  pre-  and  postgangHonic  fibers; 
the  former  have  their  origin  in  the  lumbar  region-  of  the  spinal  cord,  from 
which  they  emerge  in  the  second  to  the  fifth  lumbar  nerves;  they  then  pass  into 
and  through  the  sympathetic  chain  and  the  inferior  splanchnic  nerves  to  the 
inferior  mesenteric  ganglion  around  the  cells  of  which  they  arborize;  the 
postganglionic  fibers  pass  directly  to  the  muscle-fibers  of  the  intestinal  wall. 
Stimulation  of  the  pelvic  nerve  with  induced  electric  currents  causes  con- 
traction of  the  muscle-fibers;  stimulation  of  the  hypogastric  nerves  causes 
an  inhibition  of  the  contraction. 

Intestinal  Fermentation. — Owing  to  the  favorable  conditions  in  the 
intestine  for  fermentative  and  putrefactive  processes — e.g.,  heat,  moisture, 
oxygen,  and  the  presence  of  various  microorganisms — the  food,  when  con- 
sumed in  excessive  quantity  or  when  acted  on  by  defective  secretions,  under- 
goes a  series  of  decomposition  changes  which  are  attended  by  the  production 
of  gases  and  various  chemic  compounds.  Dextrose  and  maltose  are  partially 
reduced  to  lactic  acid;  this  to  butyric  acid,  carbon  dioxid,  and  hydrogen. 
Fats  are  reduced  to  glycerol  and  fatty  acids,  the  glycerol,  according  to  the 
organisms  present,  yields  succinic  acid,  carbon  dioxid,  and  hydrogen.  The 
proteins  under  the  prolonged  action  of  the  erepsin  of  the  intestinal  juice  are 
reduced,  with  the  production  of  leucin  and  tyrosin.  These  crystalline 
compounds  are  in  turn  reduced  to  simpler  forms.  The  former  yields  valer- 
ianic acid,  ammonia,  and  carbon  dioxid;  the  latter,  tyrosin,  gives  rise  to  indol, 
which  is  the  antecedent  of  indican,  found  in  the  urine.  This  compound  is 
discharged  in  part  in  the  feces  though  it  is  in  part  absorbed  into  the  portal 
blood  and  carried  direct  to  the  liver  where  it  is  oxidized  to  indoxyl  and  com- 
bined or  conjugated  with  potassium  sulphate  forming  the  salt  potassium 
indoxyl  sulphate  or  indican.  after  which  it  enters  the  blood,  is  carried  to  and 
eliminated  by  the  kidneys.  The  presence  of  this  salt  in  the  urine  can  be 
demonstrated  by  adding  hydrochloric  acid  with  a  small  quantity  of  potassium 
chlorate;  after  this  is  done  the  indican  combines  with  water  and  under- 
goes a  cleavage  into  indoxyl  and  potassium  sulphate;  the  former  then 
combines  with  oxygen  and  gives  rise  to  indigo  blue.  The  extent  to  which 
the  indican  is  present  is  taken  as  a  measure  of  the  extent  of  intestinal 
putrefaction. 

Skatol,  another  derivative  of  the  protein  molecule,  the  result  of  bacterial 
decomposition,  passes  in  part  into  the  feces  and  gives  to  them  the  character- 
istic odor.  It  is  also  in  part  absorbed  and  oxidized  to  skatoxyl,  after  which 
it  combines  with  potassium  sulphate  to  form  potassium  skatoxyl  sulphate. 
It  is  eliminated  in  the  urine  by  the  kidneys. 

The  Feces. — The  feces  is  a  term  applied  to  the  mass  of  material  ejected 
from  the  rectum  through  the  anus.     They  are  characterized  by  consistency, 


202  TEXT-BOOK  OF  PHYSIOLOGY. 

color  and  odor.  The  origin  and  the  nature  of  this  material  have  both  a 
physiologic  and  a  clinic  interest. 

The  consistency  varies  from  day  to  day  from  liquid  to  solid,  depending 
partly  on  the  character  of  the  food,  the  rapidity  with  which  it  is  transported 
through  the  intestine  and  hence  the  extent  to  which  absorption  of  water  in 
the  large  intestine  takes  place.  On  a  meat  diet  the  consistency  is  firm;  on  a 
vegetable  diet  it  is  apt  to  be  soft.  The  amount  discharged  from  day  to  day 
on  a  mixed  diet  varies  from  120  to  170  grams  containing  from  30  to  42  grams 
of  dry  matter.  On  a  meat  diet  alone  the  quantity  diminishes;  on  a  vege- 
table diet,  especially  if  the  articles  of  food  are  rich  in  cellulose,  the  quantity 
will  increase  considerably  beyond  the  customary  amount. 

The  color  on  a  mixed  diet  varies  from  a  light  yellow  to  black.  The 
usual  brown  color  is  due  to  the  pigment  urobilin  or  stercobilin,  a  derivative 
of  the  pigments  of  the  bile.  On  a  meat  diet  the  color  deepens  until  it  becomes 
quite  black  due  to  the  presence  of  sulphid  of  iron,  the  result  of  the  union 
of  sulphuretted  hydrogen  with  the  iron  derived  from  hematin  contained  in 
the  meat.  On  a  vegetable  diet  the  color  lightens  and  may  become  slightly 
yellow.  If  the  contents  of  the  intestine  are  carried  forward  too  rapidly,  the 
time  may  be  insufficient  for  a  complete  reduction  of  the  bile  pigments,  hence 
they  appear  in  the  feces  imparting  to  them  a  green  color.  If  there  is  an 
obstruction  to  the  discharge  of  bile  into  the  intestine  the  feces  may  become 
yellow  or  clay-colored. 

The  odor  is  characteristic  and  due  to  the  presence  of  skatol  and  allied 
bodies  produced  by  the  putrefaction  of  proteins  by  bacterial  action.  Sul- 
phuretted hydrogen  also  contributes  to  the  odor. 

The  chemic  composition  of  the  feces  is  complex.  They  consist  of  water, 
mucin,  an  indigestible  residue  of  food,  decomposition  products,  excretions 
from  the  intestinal  glands,  and  inorganic  salts.  The  residue  of  the  food 
usually  consists  of  the  denser  portions  of  the  connective  tissue  of  meats  and 
the  cellulose  of  vegetables  and  cereals.  When  the  latter  are  eaten  in  large 
amounts  the  cellulose  residue  is  increased  and  by  its  mechanic  stimulation 
increases  the  peristalsis  and  hastens  the  transfer  of  the  feces  through  the 
intestine.  The  decomposition  products  are  derived  from  protein,  fat,  and 
carbohydrate  food  by  bacterial  action  and  include  skatol,  indol,  fat  acids, 
soaps,  xanthin,  ammonia,  sulphuretted  hydrogen,  etc.  The  excretion  from 
the  intestine  itself  contributes  a  considerable  portion  to  the  fecal  mass.  The 
inorganic  salts  include  phosphates  of  calcium  and  magnesium  together  with 
various  sodium  and  potassium  compounds. 

Defecation. — Defecation  is  the  final  act  of  the  digestive  process  and 
consists  in  the  expulsion  of  the  indigestible  residue  of  the  food  and  its  asso- 
ciated compounds  from  the  intestine.  This  act  usually  takes  place  in  the 
human  being  but  once  in  twenty-four  hours,  as  the  diet  contains  but  a 
minimum  quantity  of  indigestible  matter.  Previous  to  their  expulsion  the 
feces  which  have  accumulated  in  the  sigmoid  flexure  must  pass  downward 
into  the  rectum.  In  so  doing  they  develop  the  sensation  which  leads  to  the 
act  of  defecation.  The  descent  of  the  feces  is  accomplished  by  the  peristaltic 
contraction  of  the  intestinal  wall.  Coincident  with  the  passage  of  the  feces 
into  the  rectum  there  is  a  relaxation  of  the  sphincter  muscles  and  a  contrac- 
tion of  the  longitudinal  and  circular  muscle  fibers,  in  consequence  of  which 


DIGESTION.  203 

the  feces  are  expelled.  These  complex  muscle  actions  are  also  aided  by  the 
voluntary  contractions  of  the  diaphragm  and  abdominal  muscles. 

Nerve  Mechanism  of  Defecation. — The  act  of  defecation  is  primarily 
reflex  though  somewhat  influenced  by  voluntary  efforts.  The  reflex  charac- 
ter of  the  act  is  especially  noticeable  in  young  children  in  whom  by  reason  of 
the  imperfect  development  of  the  brain  there  is  a  lack  of  volitional  control. 
During  the  intervals  of  defecation  the  anal  orifice  is  tightly  closed  by  the 
tonic  contraction  of  the  internal  non-striated  sphincter  and  the  external 
striated  sphincter  muscles,  thus  preventing  the  escape  of  gases  or  semi-liquid 
material.  The  tonic  contraction  of  both  muscles  is  maintained  by  the 
activity  of  nerve-centers  located  in  the  lumbar  region  of  the  spinal  cord. 
The  circular  and  longitudinal  fibers  of  the  rectum  proper  are  at  the  same 
time  in  a  relaxed  or  inhibited  condition,  the  result  of  an  inhibition,  or  a  want 
of  stimulation,  of  their  governing  nerve-center  or  centers  in  the  lumbar  region 
of  the  spinal  cord.  When  the  desire  to  evacuate  the  bowel  is  experienced, 
impressions  are  being  made  by  the  feces  on  the  afferent  nerves  in  the  mucous 
membrane  of  the  sigmoid  flexure  and  of  the  rectum.  The  nerve  impulses 
thus  developed  are  transmitted  to  the  defecation  or  rectal  nerv^e-centers  in 
the  spinal  cord  and  to  the  cerebrum  and  influence  in  one  direction  or  an- 
other their  activities.  If  the  act  of  defecation  is  to  take  place  there  is  an 
inhibition  of  the  nerv^e-centers  maintaining  the  tonus  or  contraction  of  the 
two  sphincter  muscles  and  a  stimulation  of  the  nerve-centers  exciting  or 
augmenting  the  contraction  of  the  rectal  muscles  with  the  result  of  a  dis- 
charge of  the  fecal  mass.  In  their  expulsive  efforts,  these  latter  muscles 
are  assisted  by  the  contraction  of  the  diaphragm,  abdominal,  and  other 
muscles  in  response  to  volitional  efforts.  After  the  expulsion  of  the  feces, 
there  is  a  return  to  the  former  condition,  namely,  a  relaxation  or  inhibition 
of  the  rectal  muscles  and  a  contraction  of  the  sphincter  muscles.  If  the  act  of 
defecation  is  to  be  suppressed,  the  controlling  influence  of  the  nerve-center 
on  the  contraction  of  the  external  sphincter  may  by  an  act  of  volition  be 
strengthened  and  the  action  of  the  reflex  mechanism  for  a  while  antagonized. 

The  efferent  nerv^e-fibers  for  the  external  sphincter  muscle  have  their  origin 
in  the  spinal  cord  from  which  they  pass  by  way  of  the  third  and  fourth  sacral 
nerves,  the  pelvic  nerve  and  the  inferior  hemorrhoidal  nerve  directly  to  the 
muscle. 

The  efferent  nerve-fibers,  for  the  longitudinally  and  circularly  arranged 
muscle  fibers  of  the  rectum,  including  the  specialized  portion,  the  internal 
sphincter,  have  their  origin  in  nerve-cells  in  the  lumbo-sacral  region  of  the 
spinal  cord  and  pass  to  their  destination  by  two  paths.  The  fibers  in  the 
first  path  leave  the  spinal  cord  by  way  of  the  second  to  the  fifth  lumbar 
nerves,  then  pass  into  and  through  the  sympathetic  chain,  through  the  inferior 
splanchnics  to  the  inferior  mesenteric  ganglion  around  the  cells  of  which 
their  terminal  branches  arborize;  from  the  cells  of  this  ganglion  new  fibers 
emerge  which  pass  through  the  hypogastric  nerve  to  the  muscles.  The 
fibers  of  the  second  path  leave  the  spinal  cord  by  way  of  the  second  to  the 
fourth  sacral  nerves,  then  pass  into  the  pelvic  or  erigens  nerve  to  small 
gangha  along  the  sides  of  the  rectum  around  the  cells  of  which  tlieir 
terminal  branches  arborize;  from  the  cells  of  these  ganglia  new  nerve- 
fibers  emerge  which  pass  directly  to  the  muscles.     In  both  paths  the  nerves 


204  TEXT-BOOK  OF  PHYSIOLOGY. 

coming  from  the  cord  are  preganglionic,  those  coming  from  the  gangha, 
postgangHonic. 

The  central  mechanism  that  excites  and  coordinates  the  activities  of  the 
rectal  muscles  is  situated  in  the  lumbo-sacral  region  of  the  spinal  cord  and 
is  designated  the  recto-anal  center. 

The  afferent  nerve  fibers  that  excite  the  central  mechanism  to  activity, 
are  contained  in  both  the  nerve  paths  described  in  foregoing  paragraphs 
and  enter  the  spinal  cord  in  the  dorsal  roots  of  the  lumbar  and  spinal  nerves. 
Although  the  anatomic  relations  of  the  various  nerves  composing  this  mechan- 
ism are  fairly  well  known,  their  physiologic  actions  are  not  clearly  defined. 
The  results  of  experimental  methods  of  investigation  are  neither  uniform  nor 
in  accord.  The  want  of  accord  lies  partly  in  anatomic  peculiarities  of  the 
animal  the  subject  of  the  investigation,  and  partly  perhaps  in  the  character 
of  the  stimulus  employed. 

Stimulation  of  the  pelvic  nerve  causes,  in  the  dog  at  least,  a  peristaltic 
contraction  of  the  circular  fibers  of  the  rectum.  Stimulation  of  the  hypo- 
gastric nerve  causes  an  inhibition  or  relaxation  of  the  circular  fibers  of  the 
rectum  and  of  the  internal  sphincter  as  well.  Inasmuch  as  these  two  groups 
of  fibers  have  opposite  functions  it  may  be  assumed  that  the  nerve  centers 
controlling  them,  both  motor  and  inhibitor,  are  double  centers  and  that  they 
can  be  made  to  act  separately  and  alternately. 

Recalling  the  events  that  take  place,  it  may  be  assumed  that  peripheral 
stimulation  of  the  afferent  nerves  develops  nerve  impulses  which,  when 
transmitted  to  the  cord  cause  i,  a  stimulation  of  the  motor  center,  a  dis- 
charge of  nerve  impulses  through  the  pelvic  nerve  to  the  rectal  muscles 
calling  forth  a  contraction;  2,  a  stimulation  of  the  inhibitor  center,  a  dis- 
charge of  nerve  impulses  through  the  hypogastric  ner\^e  to  the  internal 
sphincter  and  perhaps  the  external  sphincter  as  well,  calling  forth  their 
relaxation  or  inhibition.  With  the  discharge  of  the  feces  the  former  condi- 
tion is  re-established.  A  stimulus,  the  nature  of  which  is  not  fully  known, 
causes  a  stimulation  of  the  inhibitor  center  for  the  rectal  muscles  and  a 
stimulation  of  the  motor  center  for  the  sphincters,  the  nerve  impulses  reaching 
the  muscles  through  the  hypogastric  and  pelvic  nerves  respectively. 


CHAPTER  XL 
ABSORPTION. 

Absorption  is  a  process  by  which  nutritive  material  from  the  tissue 
spaces,  from  the  serous  cavities,  from  the  interior  of  the  lungs  and  from  the 
mucous  surfaces  of  the  body,  and  waste  materials  from  the  tissues  are  trans- 
ferred into  the  blood. 

The  absorption  of  nutritive  materials  from  the  tissue  spaces  and  from  the 
serous  cavities  may  be  regarded  as  an  act  of  resorption  or  a  return  to  the 
blood  of  nutritive  material  which  has  passed  through  the  walls  of  the  capil- 
lary blood-vessels  in  excess  of  that  needed  for  purposes  of  nutrition,  and 
which  if  not  returned  would  lead  to  an  accumulation  and  the  development  of 
edematous  conditions;  the  absorption  of  oxygen  from  the  lungs  is  essential 
to  the  maintenance  of  nutritive  activity,  to  the  oxidation  of  foods  and  the 
liberation  of  the  energy;  the  absorption  of  new  nutritive  materials  from  the 
mucous  surfaces  of  the  entire  alimentary  canal,  but  more  especially  from 
that  of  the  small  intestine,  materials  that  have  been  produced  out  of  the 
foods  by  the  digestive  process,  is  essential  to  the  maintenance  of  the 
quantity  and  quality  of  the  blood. 

The  absorption  of  the  products  of  metabolism,  of  carbon  dioxid,  urea  and 
other  nitrogen-holding  compounds  from  the  tissues  into  the  blood  is  essential 
to  the  continuance  of  their  activities  as  well  as  a  necessary  preliminary  to 
their  elimination  from  the  body. 

The  anatomic  mechanisms  involved  in  the  absorptive  process  are,  pri- 
marily, the  tissue  or  lymph-spaces,  the  lymph-  and  blood-capillaries;  second- 
arily, the  lympJi-vessels  and  the  veins. 

Tissue  or  Lymph-spaces ;  Lymph-capillaries. — Everywhere  through- 
out the  body,  in  the  connective-tissue  system  and  in  the  interstices  of  the 
several  structures  of  which  an  organ  is  composed,  are  found  spaces  or  clefts 
of  irregular  shape  and  size,  determined  largely  by  the  structure  of  the  organ 
in  which  they  are  found,  which  have  been  termed  tissue  or  lymph-spaces, 
from  the  fact  that  they  contain  a  clear  fluid,  the  lymph.  These  spaces  are 
devoid  for  the  most  part  of  any  endothelial  lining,  but  as  they  communicate 
more  or  less  freely  one  with  another,  there  is  a  circulation  of  lymph  through 
them  and  around  the  islets  of  tissue  (Fig.  86).  In  addition  to  the  connective- 
tissue  lymph-spaces,  different  observers  have  described  special  spaces  or 
clefts  in  organs  such  as  the  kidney,  liver,  spleen,  testicle,  and  in  all  secreting 
glands  between  their  basement  membrane  and  the  surrounding  blood- 
vessels, all  of  which  contain  a  greater  or  less  quantity  of  lymph.  Within  the 
brain,  spinal  cord,  bone,  and  other  tissues  it  has  been  shown  that  the  smallest 
blood-vessels  and  capillaries  are  bounded  and  limited  by  a  cylindrical  sheath 
containing  lymph,  which  is  known  as  a  perivascular  lymph-space.  A 
similar  sheath  surrounds  the  smallest  nerve-bundles  and  fibers,  enclosing  a 
perineural  lymph-space.     The  large  serous  cavities  of  the  body,  pleural, 

20S 


2o6 


TEXT-BOOK  OF  PHYSIOLOGY. 


peritoneal,  pericardial,  etc.,  are  also  to  be  regarded  as  lymph-spaces.  The 
surfaces  of  these  cavities,  however,  are  covered  with  a  layer  of  endothelial 
cells  with  sinuous  margins.  At  intervals  between  these  cells  are  to  be  found 
small  free  openings  which  have  received  the  name  of  stomata. 

The  lymph-capillaries  in  which  the  lymph-vessels  proper  take  their 
origin  are  arranged  in  the  form  of  plexuses  of  quite  irregular  shape.  In 
most  situations  they  are  intimately  interwoven  with  the  blood-vessels,  from 
which  they  can  be  readily  distinguished  by  their  larger  caliber  and  irregular 
expansions.  The  wall  of  the  lymph-capillary  is  formed  by  a  single  layer  of 
endothelial  cells  with  characteristic  sinuous  outlines.  These  capillaries  anas- 
tomose very  freely  one  with 
another  and  communicate,  on 
the  one  hand,  with  the  lymph- 
spaces  and  on  the  other  with 
the  lymph-vessels  proper.  It 
was  formerly  believed  that  the 
communication  of  the  lymph- 
capillary  with  the  tissue  space 
was  a  direct  one,  the  lymph 
flowing  from  the  latter  into  the 
former  through  an  open  passage- 
way. Recent  investigation 
would  indicate  that  this  histo- 
logic arrangement  does  not  exist 
but  that  on  the  contrary  the 
lymph-capillaries  are  closed  ves- 
sels and  that  the  tissue  space  and 
the  interior  of  the  lymph-capil- 
lary are  separated  one  from  the 
other  by  a  thin  partition  of  endo- 
thehal  cells.  As  the  shape,  size, 
etc.,  of  both  lymph-spaces  and 
capillaries  are  determined  largely 
by  the  nature  of  the  tissue  in 
which  they  are  found,  it  is  not  always  possible  to  separate  one  from  the 
other.  Their  function,  however,  may  be  regarded  as  similar:  viz.,  the 
reception  and  collection  of  the  excess  of  lymph  which  has  transuded  through 
the  walls  of  the  blood-vessels  and  its  transmission  onward  into  the  regular 
lymph-vessels. 

The  blood-capillaries  not  only  permit  of  a  transudation  of  the  liquid 
nutritive  material  from  the  blood  through  their  delicate  walls,  but  are  also 
engaged,  if  not  in  the  resorption  of  a  portion  of  this  transudate,  at  least  in  the 
absorption  of  waste  products  resulting  from  tissue  metabolism. 

Lymph-vessels. — The-  lymph-vessels  constitute  a  system  of  minute, 
delicate,  transparent  vessels  found  in  nearly  all  the  organs  and  tissues  of  the 
body,  and  take  their  origin  from  the  lymph-capillaries  and  spaces  above 
described  (Figs.  87  and  88.)  From  their  origin  they  gradually  converge 
toward  the  trunk  of  the  body,  and  finally  empty  into  the  thoracic  duct.  In 
their   course   they   anastomose   very   freely   with   adjoining   vessels.     The 


Fig.  86. — Origin  of  Lymph- vessels  from  the 
Central  Tendon  of  the  Diaphragm  Stained 
WITH  Nitrate  of  Silver,  j.  The  lymph-spaces 
and  lymph-canals,  communicating  at  x  with  the 
lymphatics,  a.  Origin  of  the  lymphatics  by  the 
confluence  of  several  juice  canals.  B.  Capillary 
blood-vessels. — {Landois  and  Stirling.) 


ABSORPTION. 


207 


diameter  of  a  lymph- vessel  varies  from  i  to  2  mm.  After  the  lymph- vessels 
have  emerged  from  the  lymph-capillaries  they  acquire  three  distinct  coats, 
each  of  which  possesses  definite  histologic  features. 

The  internal  coat  is  composed  of  a  delicate  lamina  of  longitudinally  dis- 
posed elastic  fibers  covered  with  a  layer  of  flattened  nucleated  endothelial 
cells  with  wavy  outlines. 

The  middle  coat  consists  of  white  fibrous  tissue  arranged  longitudinally 
and  of  non-striated  muscle  and  elastic  fibers  arranged  transversely. 

The  external  coat  consists  of  practically  the  same  structures,  though  the 
muscle-fibers  are  longitudinally  disposed. 

The  lymph-vessels  are  provided  with  valves  which  are  so  numerous  and 
located  at  such  short  inter\'als  as  to  give  the  vessels  a  beaded  appearance. 


Fig.  87.— Lymph- vessels  and  Lvmph-nodes  of  the  Head  and  Neck. 


These  valves  are  arranged  in  pairs  and  consist  of  two  semi-lunar  folds  with 
their  concavities  directed  toward  the  larger  vessels.  They  are  formed  by  a 
reduplication  of  the  lining  membrane,  which  is  strengthened  by  fibrous  tissue 
derived  from  the  middle  coat. 

Lymph-nodes,  or  glands.^ — In  their  course  toward  the  thoracic  duct  the 
lymph-vessels  pass  through  a  number  of  small  pisiform  bodies  termed  lymph- 
nodes  or  glands.  These  are  exceedingly  abundant  in  some  situations,  as  the 
cervical,  axillar}-,  and  inguinal  regions,  and  the  abdominal  cavity.  As  the 
lymph-vessels  approach  a  gland  they  divide  into  a  number  of  branches  before 
entering  it,  known  as  the  afferent  vessels.     From  the  opposite  side  of  the 

i 


208 


TEXT-BOOK  OF  PHYSIOLOGY. 


gland  the  lymphatics  again  emerge  as  efferent  vessels  to  unite  to  form  larger 
trunks.  A  section  of  a  gland  shows  that  it  consists  of  an  outer  dense  cortical 
and  an  inner  soft  pulpy  medullary  portion.  Each  gland  is  covered  exter- 
nally by  a  dense  membrane  of  fibrous  tissue  containing  in  its  meshes  non- 
striated  muscle-fibers.  From  the  inner  surface  of  this  membrane  there  pass 
inward  septa  of  connective  tissue  which,  as  they  converge  toward  the  center 
of   the   gland,  divide  its  outer  zone  into  small  conical  compartments  or 

alveoli.  When  the  septa  reach  the  medul- 
lary portion,  they  subdivide  and  form 
bands  or  cords  which  interlace  in  every 
direction  and  constitute  a  loose  meshwork 
the  spaces  of  which  communicate  with  one 
another  and  with  the  alveoli  (Fig.  89). 
Within  the  meshes  of  this  framework  the 
proper  gland  substance  is  contained.  In 
the  cortical  compartments  it  is  moulded 
into  pear-shaped  masses;  in  the  medullary 
meshwork  it  assumes  the  form  of  rounded 
cords  which  are  connected  with  one  another. 
In  both  regions,  however,  it  is  separated 
from  the  septa  by  a  space  termed  a  lymph 
sinus,  through  which  the  lymph  flow^s  as  it 
passes  through  the  gland.  The  lymph 
sinus  is  crossed  by  a  network  of  retiform 
connective  tissue  which  offers  considerable 
resistance  to  the  passage  of  the  lymph. 
The  gland  substance  consists  also  of  a 
framework  of  retiform  connective  tissue  in 
the  meshes  of  which  large  numbers  of 
lymph-corpuscles  are  contained.  The 
gland  substance  is  separated  from  the 
lymph  sinus  by  a  dense  layer  of  a  reti- 
culum, which,  however,  does  not  prevent 
lymph  and  even  corpuscles  from  passing 
through  it  into  the  lymph  sinus. 

The  lymph-glands  are  abundantly  sup- 
plied with  blood-vessels.  The  arteries  enter 
the  gland  at  the  hilum,  penetrate  into  the 
medullary  substance,  and  terminate  in  a  fine 
capillary  plexus  which  is  supported  by  the 
connective  tissue.  The  veins  arising  from 
this  plexus  leave  the  gland  also  at  the 
hilum. 

The  lymph-vessels  which  enter  a  gland 
first  ramify  in  the  investing  membrane  and 
then  open  directly  into  the  lymph  sinus.  The  vessels  which  leave  the  gland 
are  also  in  communication  with  the  sinus.  After  the  lymphatics  enter  the 
gland  they  lose  their  external  and  middle  coats,  retaining  only  the  internal 
or  endothelial  coat,    which    lines  the    inner  surface  of  the  lymph  sinus. 


Fig.  88. — Lymph-vessels  of  the 
Arm. — {Denver.) 


ABSORPTION. 


209 


The  current  of  lymph,  therefore,  is  from  the  afferent  vessels  through  the 
lymph  sinus  into  the  efferent  vessels.  In  addition  to  this  primary  current, 
there  is  a  secondary  current  flowing  from  the  capillary  blood-vessels  out- 
ward and  into  the  sinus,  which  carries  with  it  large  numbers  of  lymph- 
corpuscles.  It  is  quite  probable  that  the  movement  of  the  lymph  through 
this  complicated  system  of  passages  is  aided  by  the  contraction  of  the 
muscle-fibers  in  the  capsule  of  the  gland. 

The  lymph-corpuscles  or  lymphocytes  originate  for  the  most  part  in  the 
gland  substance  of  the  cortical  alveoli.  In  this  situation  there  are  groups  of 
cells,  so-called  germ  centers,  which  di\'ide  very  rapidly  by  mitosis  and  give 
rise  constantly  to  groups  of  young  cells  which  soon  find  their  way  into  the 
lymph  stream. 

The  Thoracic  Duct. — The  thoracic  duct  is  the  general  trunk  of  the 
lymph  system,  into  which  the  vessels  of  the  lower  extremities,  of  the  abdom- 
inal organs,  of  the  trunk,  of  the  left  arm,  and  of  the  left  side  of  the  head 


Fig.  89. — Dlagrammatic  Section  of  a  Lymph-node.  a.  /.,  Afferent;  e.  /.,  efferent  lymph- 
vessel,  C.  Cortical]  substance.  M.  Recticular  cords  of  the  medulla.  /.  s.  Lymph  sinus,  c. 
Capsule,  with  trabeculoe,  tr. — (Landois  and  Stirling.) 

empty'their  contents.  It  is  about  fifty  centimeters  in  length  and  four  milli- 
meters in  diameter.  It  extends  upward  from  the  third  lumbar  vertebra 
along  the  vertebral  column  to  the  seventh  cervical  vertebra,  where  it  empties 
into  the  venous  system  at  the  junction  of  the  internal  jugular  and  subclavian 
veins  on  the  left  side.  The  thoracic  duct  wall  has  the  same  general  layers 
as  the  wall  of  the  lymph-vessel:  \'iz.,  an  internal  or  endothelial;  a  middle 
elastic  and  muscular;  an  external  or  fibrous.  It  is  also  provided  with 
numerous  valves. 

The  lymph-vessels  of  the  right  side  of  the  head,  of  the  right  arm,  and 
a  portion  of  the  right  side  of  the  trunk  terminate  in  the  right  thoracic  duct, 
which  is  about  25  to  30  mm.  in  length  and  which  empties  into  the  venous 
system  at  the  junction  of  the  internal  jugular  and  subclavian  veins  on  the 
right  side.  The  general  arrangement  of  the  lymphatic  system  is  diagram- 
matically  shown  in  Fig.  90. 
14 


2IO 


TEXT-BOOK  OF  PHYSIOLOGY. 


LYMPH. 


Lymph  is  the  clear  fluid  found  within  the  tissue  spaces  and  within  the 
lymph-vessels.  Inasmuch  as  there  are  reasons  for  the  view  that  lymph 
varies  in  composition,  as  well  as  in  function,  in  these  different  regions  it 
will  be  found  conducive  to  clearness  to  designate  the  lymph  found  in  the 
tissue  spaces  as  intercellular  lymph,  and  that  found  in  the  lymph-vessels  as 
intravascular  lymph. 


Fig.  90. — Diagram  Showing  the  Course  of  the  Main  Trunks  of  the  Absorbent  System. 
The  lymph-vessel  of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC)  at  the  recep- 
taculum  chyli  (RC),  where  the  thoracic  duct  begins.  The  superficial  vessels  are  shown  in  the 
diagram  on  the  right  arm  and  leg  (S),  and  the  deeper  ones  on  the  arm  to  the  left  (D).  The 
glands  are  here  and  there  shown  in  groups.  The  small  right  duct  opens  into  the  veins  on  the 
right  side.  The  thoracic  duct  opens  into  the  union  of  the  great  veins  of  the  left  side  of  the  neck 
(T). — {Yeo^s  "  Text-book  of  Physiology  J') 

The  Physical  Properties  of  Lymph. — Whether  obtained  from  tissue 
spaces  or  from  lymph-vessels,  the  lymph  presents  practically  the  same  physical 
properties.  The  lymph  obtained  from  the  thoracic  duct  during  the  intervals 
of  digestion  or  from  one  of  the  large  trunks  of  the  leg  is  a  clear,  colorless  or 
slightly  opalescent  fluid  having  an  alkaline  reaction  and  a  specific  gravity 
of  1.020  to  1.040.  Examined  microscopically  it  is  seen  to  hold  in  suspen- 
sion a  large  number  of  corpuscles  similar  to  those  seen  in  the  lymph-glands 


ABSORPTION.  211 

and  to  the  white  corpuscles  of  the  blood.  Their  number  has  been  estimated 
at  about  8000  per  cubic  millimeter,  though  this  count  will  vary  within  wide 
limits  according  as  the  lymph  examined  has  passed  through  a  larger  or 
smaller  number  of  glands.  The  lymph-corpuscle  consists  of  a  small  quan- 
tity of  protoplasm  in  which  is  embedded  a  distinct  nucleus.  Some  of 
these  lymphocytes  contain  distinct  granules,  more  or  less  refractive,  which 
impart  to  the  corpuscle  a  granular  appearance.  When  withdrawn 
from  the  vessels  lymph  undergoes  a  spontaneous  coagulation,  though 
the  coagulum  is  never  as  firm  as  that  observed  in  the  coagulation  of  the 
blood.  The  cause  of  the  coagulation  is  the  appearance  of  fibrin.  x\fter 
a  variable  length  of  time  the  coagulum  separates  into  a  liquid  and  a  solid 
portion,  the  serum  and  the  clot. 

The  Chemic  Composition  of  Lymph. — Although  the  lymph  obtained 
from  the  tissue  spaces,  from  the  lymph-vessels,  as  well  as  from  the  so-called 
serous  cavities  has  the  same  general  chemic  characteristics,  there  is  reason 
for  the  view  that  it  varies  in  its  ultimate  composition  according  as  it  is 
derived  from  one  region  of  the  body  or  from  another.  The  needs  of  any 
individual  tissue  as  well  as  the  character  of  its  metabolic  products  will 
in  all  probability  change  not  only  its  normal  composition,  but  also  the 
relative  amounts  of  its  normal  constituents. 

Chemic  analysis  has  shown  that  the  lymph  from  the  thoracic  duct  con- 
tains from  3.4  to  4.1  per  cent,  of  proteins  (serum-albumin,  fibrinogen), 
0.046  to  0.13  per  cent,  of  substances  soluble  in  ether  (probably  fat),  o.i 
per  cent,  of  sugar,  and  from  0.8  to  0.9  per  cent,  of  inorganic  salts,  of  which 
sodium  chlorid  (0.55  per  cent.)  and  sodium  carbonate  (0.24  per  cent.)are 
the  most  abundant  (Munk).  There  are  usually  in  most  specimens  small 
quantities  of  potassium,  calcium,  and  magnesium  salts.  Fibrinogen  is 
seldom  present  beyond  o.i  per  cent.,  which  will  account  for  the  feeble  and 
slow  coagulation.  Lymph  contains  both  free  oxygen  and  carbon  dioxid. 
Of  the  former,  however,  there  is  but  a  small  percentage;  of  the  latter,  about 
45  vols,  per  cent.,  partially  in  the  free  state  and  partially  combined  with 
sodium.  Urea  is  also  present  in  very  small  amounts.  This  analysis  indi- 
cates that  lymph  resembles  blood-plasma  in  the  character  of  its  constituents, 
though  their  relative  quantities  vary  considerably.  With  the  exception 
that  it  contains  no  red  corpuscles,  lymph  may  be  regarded  as  a  diluted 
blood. 

The  Production  of  Lymph. — Though  blood  is  the  common  reservoir  of 
nutritive  material,  the  latter  is  not  available  for  nutritive  purposes  as  long 
as  it  is  confined  within  the  blood-vessels.  The  capillary  wall,  thin  as  it  is, 
and  composed  of  but  a  single  layer  of  endothelial  cells,  would  be  sufficient 
to  prevent  its  utilization  by  the  tissues,  if  it  W' ere  not  permeable  to  the  liquid 
portion  of  the  blood.  As  this  is  the  case,  however,  it  is  found  that  as  the 
blood  flows  through  the  capillary  vessels  a  portion  of  the  blood-plasma 
passes  through  the  capillary  wall  and  is  received  into  the  tissue-spaces, 
where  it  comes  into  intimate  contact  with  the  tissue-cells. 

The  forces  concerned  in  the  passage  of  the  constituents  of  the  blood- 
plasma  through  the  capillary  wall  have  been  the  subject  of  much  investigation. 
According  to  some  investigators,  diffusion,  osmosis,  and  filtration  are  suffi- 
cient to  account  for  ?11  the  Dhenomena.     For  a  consideration  of  the  phenom- 


212  TEXT-BOOK  OF  PHYSIOLOGY. 

ena  of  diffusion,  osmosis,  and  filtration  the  reader  is  referred  to  paragraphs 
at  the  end  of  this  chapter.  It  is  assumed  that  the  capillary  wall,  being  an 
animal  membrane,  is  freely  permeable  to  water  and  crystalloid  bodies  gener- 
ally; less  so,  however,  to  colloid  bodies,  such  as  the  proteins  of  the  blood- 
plasma;  moreover,  it  is  further  assumed  that  the  physiologic  conditions  of 
the  capillary  walls  are  such  as  not  only  to  permit  of  the  passage  of  the  con- 
stituents of  the  blood  into  the  tissue  spaces,  but  also  the  passage  of  the  con- 
stituents of  the  intercellular  lymph  into  the  blood,  according  to  laws  similar 
at  least  to  those  determining  the  passage  of  substances  through  animal 
membranes  as  determined  experimentally.  The  force  giving  rise  to  filtra- 
tion is  the  difference  of  pressure  between  that  exerted  by  the  blood  within 
the  capillary  vessels  and  that  exerted  by  the  fluid  in  the  tissue  spaces;  hence 
any  increase  or  decrease  of  this  difference  of  pressure  is  attended  by  an 
increase  or  decrease  in  the  production  of  lymph.  Thus  compression  of  the 
veins  of  a  part  which  interferes  with  the  outflow  of  blood  from  the  capillaries, 
or  a  dilatation  of  the  arterioles  which  increases  the  inflow  of  blood  to  them 
will  increase  the  capillary  pressure  and  therefore  the  production  of  lymph. 
The  reverse  conditions  will,  of  course,  diminish  the  intracapillary  pressure 
and  lymph  production.  Hemorrhages  which  lower  the  general  blood-pres- 
sure may  so  lower  the  capillary  pressure  as  not  only  to  stop  the  flow  of 
lymph  to  the  tissues,  but  may  give  rise  to  a  filtration  current  from  the  tissues 
into  the  blood. 

The  quantitative  composition  of  the  lymph  compared  with  that  of  the 
blood  indicates  that  it  is  produced  by  diffusion,  osmosis,  and  filtration.  In 
the  lymph  the  concentration  of  the  inorganic  salts  is  practically  the  same  as 
in  the  blood;  the  concentration  of  the  proteins,  however,  is  somewhat  less. 
These  facts  are  in  accordance  with  what  is  known  regarding  the  diffusibility 
of  both  crystalloids  and  colloids  through  animal  membranes. 

According  to  other  investigators,  the  production  of  lymph  is  not  so 
much  due  to  intracapillary  pressure  as  it  is  to  the  specialized  activities 
of  the  endothelial  cells,  activities  which  indicate  that  lymph  is  a  secre- 
tion the  composition  of  which  varies  in  different  situations  by  virtue  of  a 
difference  in  the  molecular  structure  of  the  endothelial  cells.  As  is  the  case 
with  many  of  the  secreting  cells  of  the  body,  the  injection  of  various  sub- 
stances into  the  blood  apparently  increases  the  activity  of  the  endothelial 
cells,  as  shown  by  an  increased  lymph  production  without  any  appreciable 
increase  of  intracapillary  pressure.  Thus  it  has  been  shown  that  after  the 
injection  into  the  blood  of  sugar,  sodium  chlorid,  sodium  sulphate,  urea,  etc., 
there  is  an  increase  in  the  flow  of  lymph  from  the  thoracic  duct.  The  lymph, 
however,  under  these  circumstances  is  richer  in  water  than  is  normally  the 
case.  As  the  blood  at  the  same  time  increases  its  percentage  of  water,  it  is 
assumed  that  the  water  is  extracted  from  the  tissues,  by  reason  of  an  increased 
percentage  of  salts  in  the  tissue  spaces  due  to  increased  activity  of  the  endo- 
thelial cells.  A  higher  percentage  of  salts  in  the  lymph  than  in  the  blood  is 
difficult  to  account  for  on  the  diffusion-filtration  theory.  The  injection  of 
peptones,  albumin,  the  extract  of  the  muscles  of  the  leech,  crab,  mussel,  etc., 
is  also  followed  by  an  increase  in  the  amount  of  lymph  discharged  from  the 
thoracic  duct;  but  in  this  instance  the  lymph  possesses  a  higher  degree  of 
concentration,  being  richer  not  only  in  inorganic  but  also  in  organic  con- 


ABSORPTION.  213 

stituents.  The  cause  of  this  increase  in  both  the  quantity  and  quality 
of  the  lymph  is  believed  to  be  an  increased  activity  in  the  secreting  power  of 
the  endothelial  cells. 

The  more  recent  experiments  of  Starling  indicate  that  in  addition  to  the 
difference  of  pressure  between  the  blood  in  the  capillaries  and  the  lymph  in 
the  tissue  spaces,  a  new  factor  must  be  considered  and  that  is,  the  permea- 
bility of  the  capillary  wall.  This  he  finds  to  vary  considerably  in  different 
parts  of  the  vascular  apparatus,  being  greatest  in  the  capillaries  of  the 
liver,  less  in  the  capillaries  of  the  intestines  and  least  in  the  capillaries  of 
the  extremities.  It  also  varies  doubtless  in  all  other  situations.  The 
increase  in  the  production  of  lymph  by  the  injection  of  peptones,  extract  of 
muscles  of  the  leech,  the  crab,  etc..  Starling  explains  by  the  assumption  that 
these  substances  alter  the  properties  of  the  capillary  wall  and  thus  increase 
its  permeability.  The  difference  of  pressure,  therefore,  between  blood  and 
lymph  taken  in  connection  with  the  degree  of  permeability  of  the  capillary 
wall  will  account  for  the  production  of  lymph  in  all  regions  of  the  body. 

Another  factor  which  has  been  invoked  to  account  for  the  passage  of  the 
constituents  of  lymph  through  the  capillary  wall,  is  an  increased  concentra- 
tion of  the  intercellular  lymph,  the  result  of  an  accumulation  of  metabolic 
products,  and  hence  an  increase  in  the  osmotic  pressure,  which  would  lead 
to  an  increase  in  the  passage  of  the  constituents  of  the  blood  into  the  lymph. 
The  activity  of  a  tissue  would  thus  indirectly  lead  to  the  formation  of 
lymph.  It  is  possible  that  all  these  facts  may  be  otherwise  interpreted; 
the  subject  is  yet  a  matter  of  investigation. 

The  Functions  of  Intercellular  Lymph. — The  origin  and  composition 
of  lymph,  its  situation  and  relation  to  the  tissue  cells  "indicate  that  its  func- 
tion is  to  provide  the  tissue  cells  with  those  nutritive  materials  necessary  to 
their  growth,  repair,  and  functional  activities,  and  to  receive  from  the  tissue 
cells  the  waste  products  of  their  metabolism  prior  to  their  removal  by  the  > 
blood-  and  lymph- vessels. 

The  necessity  for  the  production  of  lymph  becomes  apparent  when  the 
chemic  changes  which  the  tissues  undergo  at  all  times  are  considered.  Thus 
whether  in  a  state  of  relative  rest  or  in  a  state  of  activity,  disintegrative 
changes  are  constantly  taking  place  and  always  in  direct  proportion  to  the 
degree  and  continuance  of  the  activity.  If  the  tissues  are  to  continue  in  the 
performance  of  their  customary  activities,  it  is  essential  that  repair  and 
restoration  be  at  once  established.  This  is  made  possible  by  the  presence 
of  lymph,  and  by  the  power  which  living  material  possesses  of  absorbing 
from  the  lymph  the  necessary  nutritive  materials,  of  assimilating  them  and 
transforming  them  into  material  like  unto  itself  and  endowing  them  with 
its  own  physiologic  properties. 

Coincidently  with  the  loss  of  nutritive  material,  the  lymph  receives  the 
products  of  the  metabolism  of  the  tissues  and  hence  changes  in  composition. 
Should  this  change  in  composition  continue  for  any  length  of  time,  the 
lymph  would  lose  its  restorative  character  and  become  destructive  to  tissue 
vitality.  Therefore  it  is  essential  that  the  nutritive  material  be  renewed  as 
rapidly  as  consumed  and  the  waste  products  be  carried  away  as  rapidly  as 
produced.  Both  these  conditions  are  fulfilled  by  the  blood-  and  lymph- 
vessels. 


214  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Absorption  of  Intercellular  Lymph. — From  the  fact  that  lymph 
is  being  discharged  from  the  thoracic  duct  into  the  blood,  more  or  less  con- 
tinually, it  is  evident  that  lymph  is  being  absorbed  from  the  intercellular 
spaces;  from  which  fact  it  may  be  inferred  that  the  production  of  lymph  is  a 
continuous  process  and  that  it  is  passing  through  the  capillaries  in  amounts 
greater  than  is  necessary  for  the  immediate  needs  of  the  tissues.  Should 
this  excess  accumulate  there  would  soon  arise  the  condition  of  edema  and 
an  interference  with  the  functional  activities  of  the  tissues.  Therefore 
so  soon  as  the  accumulation  attains  a  certain  volume  it  is  absorbed  in  large 
measure  by  the  lymph-capillaries  and  transmitted  to  the  lymph-vessels  and 
thoracic  duct.  Because  of  the  general  belief  that  the  lymph-capillaries 
were  in  open  communication  with  the  tissue  spaces  it  was  assumed  that 
the  absorption  of  lymph  and  its  flow  through  the  lymph-vessels  was  the 
result  of  a  difference  of  pressure  between  the  lymph  in  the  tissue  spaces  and 
the  blood  in  the  innominate  veins.  But  if  the  lymph-capillaries  are  closed 
vessels,  as  recent  investigations  indicate,  then  additional  factors,  in  explana- 
tion of  lymph  absorption,  must  be  sought  for. 

It  is  quite  possible  under  even  normal  conditions  of  pressure  in  the  tissue 
spaces  that  some  of  the  more  diffusible  constituents  of  the  lymph  are 
absorbed  by  the  capillary  blood-vessels.  As  to  whether  the  relatively  feebly 
diffusible  colloids  are  so  resorbed  is  as  yet  a  matter  of  investigation. 

ABSORPTION  OF  FOODS. 

The  most  important  of  the  absorbing  surfaces,  especially  in  its  relation 
to  the  absorption  of  new  material,  is  the  mucous  membrane  of  the  alimentary 
canal,  and  more  particularly  that  portion  lining  the  small  intestine,  provided 
as  it  is  with  specialized  absorbing  structures — the  villi.  Though  certain 
substances  can  be  absorbed  from  the  mouth,  it  is  not  probable  that  any  food 
is  so  absorbed.  From  the  changes  which  the  food  principles  undergo  in  the 
stomach  it  might  naturally  be  inferred  that  their  absorption  would  promptly 
follow.  Experimental  researches  have  demonstrated,  however,  that  this 
takes  place,  if  at  all,  but  to  a  slight  extent.  If,  however,  solutions  of  inor- 
ganic salts,  sugars,  and  peptones  possessing  a  concentration  of  at  least  5  per 
cent. — a  degree  of  concentration  seldom  realized  under  normal  conditions — 
are  introduced  into  the  stomach,  their  absorption  will  be  effected,  the  rate  of 
absorption  following  in  a  general  way  the  increase,  within  limits,  in  concen- 
tration. Water  is  practically  not  absorbed  from  the  stomach.  The  absorp- 
tion of  the  products  of  digestion — i.e.,  dextrose,  levulose,  peptones,  amino- 
acids,  soaps,  glycerin,  fat  acids,  salts,  along  with  water,  in  which  for  the 
most  part  they  are  held  in  solution — is  therefore  limited  very  largely  to  the 
small  intestine,  and  is  accomplished  by  the  villous  processes  projecting 
from  the  surface  of  the  mucous  membrane. 

Structure  of  the  Villi. — The  villi  are  small  filiform  or  conical  processes, 
from  0.5  to  I  mm.  in  length,  and  from  0.2  to  0.5  mm.  in  breadth,  covering 
the  surface  of  the  mucous  membrane  from  the  pyloric  orifice  to  the  upper 
surface  of  the  ileo-cecal  valve.  Each  villus  consists  of  a  basement  mem- 
brane (see  Fig.  91)  supporting  tall  columnar  epithelial  cells.  Each  cell  is 
composed  of  granular- bioplasm  containing  a  distinct  nucleus.     At  its  free 


ABSORPTION. 


215 


extremity  a  narrow  border  of  the  cell  presents  a  striated  appearance,  as  if  it 
were  composed  of  small  rods  embedded  in  some  cement  substance.  Goblet 
or  mucin-holding  cells  are  also  to  be  found  among  the  columnar  cells.  The 
body  of  the  villus,  that  portion  within  the  basement  membrane,  consists  of  a 
reticulated  connective  tissue  supporting  arteries,  capillaries,  veins,  and 
lymphoid  corpuscles.  In  the  center  of  the  villus  there  is  usually  a  single, 
though  at  times  a  double,  club-shaped  lymph-capillary,  the  walls  of  which 
are  cooatosed  of  endothelial  cells  with  sinuous  margins.     This  capillary 


Fig.  91. — Longitudinal  Sec- 
tion OF  A  Villus  from  In- 

TESTINE     OF     THE     DOG,     HIGHLY 

Magnified,  a.  Columnar  epi- 
thelium containing  goblet-ccUs  (b) 
and  migratory  leukocytes  {h).  c. 
Basement  membrane,  d.  Plate- 
like connective-tissue  elements  of 
core,  e,  e.  Blood-vessels.  /.  .\b- 
sorbcnt  radical  or  lacteal. — 
{Pier  sol.) 


Fig.  92. — Section  of  Injected  Small 
Intestine  of  Cat.  a,  b.  Mucosa,  g. 
\'illi.  i.  Their  absorbent  vessels,  h.  Sim- 
ple follicles,  c.  Muscularis  mucosae,  j.  Sub- 
mucosa.  g,  e'.  Circular  and  longitudinal 
layers  of  muscle.  /.  Fibrous  coat.  .\11  the 
dark  lines  represent  blood-vessels  filled  with 
the  injection  mass. — (Piersol.) 


probably  begins  by  a  blind  extremity  and  opens  at  the  base  of  the  villus  into 
the  subjacent  lymph- vessels.  The  communicating  orifice  is  guarded  by  a 
valve.  It  is  also  surrounded  by  a  layer  of  non-striated  muscle-libers, 
arranged  longitudinally,  derived  from  the  muscularis  mucosae  and  attached 
to  the  apex  of  the  body  of  the  villus. 

The  arteries  which  penetrate  the  villi  are  derived  from  those  of  the  sub- 
mucous coat  of  the  intestine,  which  are  the  ultimate  branches  of  the  intestinal 
artery,  and  serve  the  purpose  of  delivering  nutritive  material  to  the  capillary 
plexus  (Fig.  92).  While  passing  through  the  latter  a  portion  of  the  blood- 
plasma  transudes  through  the  capillary  walls  into  the  spaces  of  the  reticu- 


2l6 


TEXT-BOOK  OF  PHYSIOLOGY. 


latcd  tissue,  constituting  lymph.  At  the  same  time  products  of  tissue  metab- 
oUsm  pass  through  the  capillary  walls  into  the  blood.  The  blood  then  passes 
into  the  venules,  which,  leaving  the  villus  at  its  base,  unite  with  the  veins  of 
the  submucous  coat  to  form  the  intestinal  veins.  These  finally  unite  with 
the  gastric  and  splenic  veins  to  form  the  portal  vein,  which  enters  the  liver 
at  the  transverse  fissure  (Fig.  93).  The  excess  of  lymph  within  the  villus 
passes  into  the  club-shaped  lymph-capillary,  to  be  finally  carried  by  the 
lymph-vessels  of  the  mesentery  into  the  thoracic  duct.  During  the  intervals 
of  digestion  and  in  the  absence  of  food  from  the  intestine  there  is,  of  course, 

no  absorption  of  food  nor  the  re- 
moval from  the  villus  of  anything  but 
the  excess  of  lymph  and  metabolic 
products. 

Function  of  the  Villi.— The  villi, 
and  especially  the  epithelial  cells  cover- 
ing them,  are  the  essential  agents  in 
the  absorption  of  the  products  of 
digestion.  It  is  by  the  activity  of 
these  cells  that  the  new  materials  are 
taken  out  of  the  alimentary  canal  and 
transferred  into  the  lymph-spaces  in 
the  interior  of  the  villi,  from  which 
they  are  subsequently  removed  by 
the  blood-vessels  and  lymph-vessels. 
As  to  the  mechanism  by  which  the 
epithelial  cells  accomplish  this  result, 
nothing  definite  can  be  asserted.  In- 
asmuch as  the  absorption  of  food  does 
not  take  place  in  accordance  with  the 
laws  of  osmosis  as  at  present  under- 
stood, it  has  been  suggested  that  the 
cells  possess  a  "selective  action"  de- 
pendent on  their  organization  and 
physiologic  activity,  an  activity  which 
is  to  a  great  extent  conditioned  and 
limited  by  the  degree  of  diffusibility  of  the  substances  to  be  absorbed. 

Absorption  of  Water  and  Inorganic  Salts. — Water  and  inorganic  salts 
after  their  absorption  from  the  intestine  and  transference  into  the  lymph- 
spaces  of  the  villi  pass  through  the  walls  of  the  capillary  blood-vessels  and 
are  carried,  by  the  blood  of  the  portal  vein,  into  and  through  the  liver  into 
the  blood  of  the  general  circulation.  Unless  water  be  present  in  excessive 
amounts,  there  is  no  appreciable  absorption  of  water  by  the  lymph-vessels. 
Absorption  of  Sugar. — As  previously  stated,  all  the  carbohydrates,  with 
the  exception  possibly  of  lactose,  are  transformed  by  the  digestive  fluids  into 
either  dextrose  or  levulose,  under  which  forms  they  are  absorbed  by  the 
epithelial  cells.  It  is  possible,  however  that  soluble  dextrin  may  also  be 
absorbed.  Whatever  the  form  under  which  the  carbohydrates  are  absorbed, 
they  never  leave  the  epithelial  cells  except  as  dextrose  and  levulose.  Direct 
experimentation  has  shown  that  the  sugars  are  taken  up  by  the  capillary 


Fig.  93.  — Diagram  of  the  Portal 
Vein  (pv)  arising  in  the  Alimentary 
Tract  .and  Spleen  (s),  and  Carrying  the 
Blood  from  These  Organs  to  the  Liver. 
— {Veo's   "Text-book  of  Physiology^') 


ABSORPTION.  217 

blood-vessels  and  carried  direct  to  the  liver.  Analysis  of  the  blood  of  the 
portal  vein  after  the  ingestion  of  large  quantities  of  sugar  my  reveal  an 
increase  to  0.25  per  cent.;  while  after  the  injection  of  sugar  into  the  intestine 
the  percentage  may  rise  as  high  as  0.4  per  cent.  As  chemic  analysis  of 
lymph  obtained  from  the  thoracic  duct  shows  no  increase  in  the  percentage 
of  sugar  beyond  that  normally  present  (o.i  per  cent.),  it  is  assumed  that 
sugar  is  not  removed  from  the  villi  by  the  lymph-vessels. 

On  reaching  the  liver  a  large  portion  of  the  sugar  passes  from  the  blood 
stream  through  the  walls  of  the  capillaries  into  surrounding  lymph  spaces 
and  comes  into  direct  relation  with  the  liver  cells.  Then  through  the 
agency  of  an  enzyme,  the  sugar  is  dehydrated,  converted  into  starch  and 
stored  for  a  variable  length  of  time  in  the  liver  cells  in  the  form  of  minute 
granules  which  can  be  readily  seen  with  the  aid  of  the  microscope.  Under 
this  form  the  carbohydrate  material  is  retained  until  the  necessity  arises 
for  its  return  to  the  blood,  and  this  happens,  when  the  percentage  of  sugar  in 
the  blood  falls  below  the  normal,  viz.,  0.05  to  0.15  per  cent.  Under  such 
circumstances  the  necessary  amount  of  the  liver  starch  is  hydrated,  con- 
verted into  sugar,  and  passed  into  the  blood  in  quantities  sufficient  to  restore 
the  normal  percentage.  The  apparent  necessity  for  this  temporary  storage 
of  sugar  in  the  liver  is  to  prevent  its  too  rapid  entrance  into  the  arterial 
blood  and  hence  a  rise  in  the  percentage  far  beyond  that  which  is  normal. 
Should  this  occur  a  condition  known  as  hyperglycemia  would  result  and 
as  a  consequence  an  elimination  of  the  excess  by  the  kidneys  giving  rise  to 
the  condition  known  as  glycosuria. 

Absorption  of  the  Products  0}  Protein  Digestion. — For  the  reason 
that  the  proteins  are  for  the  most  part  transformed  through  hydration  and 
cleavage  by  the  action  of  the  gastric  and  pancreatic  enzymes  into  peptones 
and  for  the  further  reason  that  the  peptones  are  diffusible  bodies,  it  was 
formerly  believed  that  they  represented  the  final  stages  in  the  digestion  of 
the  proteins,  and  as  such  were  absorbed  out  of  the  intestinal  contents  by 
the  action  of  the  epithelium  covering  the  villi.  Though  the  production  of  pep- 
tones was  believed  to  be  a  necessary  process  before  the  absorption  of  protein 
material  could  be  effected,  yet  it  was  apparently  demonstrated  by  the  results 
of  experimentation  that  unchanged  native  protein,  e.g.,  egg-albumin  and 
partially  digested  proteins,  e.g.,  proteoses,  were  also  absorbed  though  in 
far  less  amounts.  It  has  also  been  demonstrated  that  native  proteins  can 
be  absorbed  by  the  mucous  membrane  of  the  large  intestine.  Inasmuch 
as  chemic  analysis  has  failed  to  detect  more  than  a  trace  of  either  peptone  or 
native  protein  in  the  portal  blood  or  in  the  lymph  of  the  thoracic  duct,  it 
must  be  assumed  that  the  epithelium  after  absorbing  must  also  synthesize 
them  into  some  form  of  coagulable  protein  (plasma-albumin)  which  is 
readily  assimilable  by  the  blood.  That  such  a  reconversion  is  necessary 
would  appear  from  the  fact  that  the  introduction  of  peptones  even  in  small 
amounts  into  the  blood  is  followed  by  their  elimination  unchanged  in  the 
urine.  When  injected  into  the  blood  in  large  amounts,  they  act  as  toxic 
agents,  giving  rise  to  a  fall  of  blood-pressure,  a  diminished  coagulability  of 
the  blood,  coma,  and  death. 

After  passing  through  the  epithelium  into  the  spaces  of  the  villi  the  recon- 
structed or  synthesized  plasma-protein  molecules  are  removed  by  the  blood- 


2i8  TEXT-BOOK  OF  PHYSIOLOGY. 

vessels  and  carried  direct  to  the  liver.  Even  though  there  is  no  appreciable 
increase  in  the  amount  of  protein  in  the  portal  blood  during  digestion,  there 
is  every  reason  to  think  that  this  is  the  route  by  which  it  reaches  the  general 
circulation.  Ligation  of  the  thoracic  duct  does  not  interfere  to  any  appre- 
ciable extent  with  protein  absorption  nor  with  the  normal  elimination  of  urea 
nor  with  the  weight  of  the  animal. 

The  foregoing  statements  are  based  on  the  view  that  the  final  stage  in 
the  digestion  of  proteins  is  the  formation  of  peptones.  There  are  reasons 
however  for  believing  that  the  change  is  more  far-reaching  and  complete,  and 
that  the  peptones  in  turn  are  disintegrated  and  reduced  to  still  less  complex 
bodies  represented  by  polypeptids,  peptids,  and  even  amino-acids.  (See 
page  189.)  The  extent  to  which  this  disintegration  proceeds  will  doubtless 
depend  on  the  quantity  and  variety  of  proteins  consumed. 

If  the  reduction  of  the  protein  molecule  to  this  fragmentary  condition  is 
the  outcome  of  protein  digestion,  as  recent  investigations  indicate,  then  the 
problem  of  absorption  is  transferred  to  these  fragmentary  bodies  rather  than 
to  the  peptone  molecule.  Inasmuch  as  the  presence  of  the  peptids  and  the 
amino-acids  in  the  blood  of  the  portal  vein  has  not  been  demonstrated 
beyond  question,  the  supposition  is  that  after  their  absorption  by  the  intes- 
tinal epithelium,  they  are  synthesized  and  a  protein  molecule  constructed, 
similar  to,  if  not  identical  with,  the  plasma-albumin.  This  view  renders 
it  much  easier  to  understand  how  out  of  the  different  proteins,  vary- 
ing widely  in  their  composition,  the  specific  proteins  of  the  blood  are  con- 
structed. It  is  only  necessary  to  assume  that  the  epithelial  cell  selects  from 
the  variety  of  fragments  presented  to  it,  only  those  which  are  necessary  to 
the  formation  of  the  plasma-albumin  and  the  plasma-globulin,  and  to  syn- 
thesize them  to  these  characteristic  compounds. 

The  plasma-albumin  thus  becomes  the  common  protein  out  of  which 
each  tissue  constructs  the  particular  kind  of  protein  characteristic  of  it, 
thus  bringing  about  repair  and  growth.  Whether  this  is  accomplished  by 
the  simple  incorporation  of  the  protein  molecule  directly  or  whether  it  must 
be  first  reduced  to  amino-acids  before  the  tissues  can  construct  their  own 
protein  is  unknown.  That  the  plasma-albumin  bears  an  intimate  relation 
to  the  nutritive  activities  of  the  tissues  is  apparent  from  the  decline  in  the 
general  nutrition  and  a  marked  loss  of  body  weight,  when  in  consequence  of 
diseases  of  the  kidney  it  escapes  in  the  urine.  An  alternate  assumption 
however  is  conceivable,  viz.,  that  the  amino-acids,  though  not  readily  demon- 
strable in  the  blood  of  the  portal  vein,  are  nevertheless  absorbed  as  such,  pass 
through  the  liver,  enter  the  blood  of  the  general  circulation,  and  are  carried 
direct  to  the  tissue-cells  in  which  they  are  directly  synthesized  into  the  form 
of  protein  characteristic  of  them.  The  plasma-albumin  might  then  be 
regarded  as  a  protein  surplus  to  be  called  upon  if  the  protein  ingested 
should  be  insufficient. 

Many  facts  in  the  physiologic  chemistry  of  the  body  raise  the  question 
as  to  what  percentage  of  the  amino-acids  is  utilized  for  tissue  repair  and 
growth  and  what  percentage  for  heat  production.  If  the  protein  require- 
ments of  Chittenden,  viz.,  58  to  60  grams  only,  are  necessary  for  repair  and 
growth,  then  approximately  one-half  the  amino-acids  ordinarily  produced 
are  used  for  heat  production.     The  manner  of  disposal  of  these  unused 


ABSORPTION.  219 

(that  is  unused  for  tissue  repair  and  growth)  fragments  of  protein 
disintegration  is  doubtless  varied;  a  large  portion  is  undoubtedly  ab- 
sorbed by  the  epithelial  cells  of  the  villi  and  mucosa  after  which  they 
are  deprived  of  NH,  (the  amino-acid  nitrogen)  or  deaminized;  the  NH, 
is  then  converted  into  ammonia,  combined  with  carbon  dioxid  to 
form  ammonium  carbonate,  carried  to  the  liver,  and  changed  into  urea. 
That  this  is  very  probably  the  case  is  rendered  likely  from  the  presence  of  a 
large  quantity  of  ammonia  in  the  mucous  membrane  of  the  intestine  and 
in  the  blood  of  the  portal  vein,  in  which  after  a  meal  rich  in  protein  it  may 
be  four  times  as  great  as  in  the  arterial  blood.  The  remainder  of  the  amino- 
acid  molecule  is  changed  into  sugar  or  fat  and  subsequently  utilized  by  the 
organism  for  heat  production.  The  dynamic  portion  of  the  amino-acid  is 
this  deaminized  remainder.  Another  portion  is  acted  on  by  intestinal 
bacteria,  and  converted  into  simpler  compounds,  after  which  they  are 
eliminated  in  the  feces  or  absorbed  and  carried  to  the  liver  where  they 
undergo  other  changes  and  eventually  appear  in  the  urine. 

Absorption  of  Fat. — As  previously  stated,  there  are  two  views  as  to  the 
changes  which  fats  undergo  during  digestion.  According  as  the  one  or  the 
other  is  accepted  will  depend  the  view  as  to  the  nature  of  the  absorptive 
process.  If  it  be  assumed  that  the  final  stage  in  the  digestion  of  fat  is  a 
purely  physical  one,  the  production  of  an  emulsion  in  which  the  fats  present 
themselves  as  fine  granules,  it  is  difiicult  to  give  any  satisfactory  explanation 
of  the  mechanism  by  which  the  epithelial  cells  take  them  up.  Various 
theories  have  been  advanced  to  explain  the  process,  but  none  are  free  from 
serious  objections.  This  view  of  fat  absorption  has  largely  been  based  on 
the  observation  that  during  digestion  fatty  granules  can  be  seen  in  all  por- 
tions of  the  cell  apparently  passing  toward  the  interior  of  the  villus. 

If,  on  the  contrary,  it  be  admitted  that  the  final  stage  in  the  digestion  of 
fats  is  the  formation  of  soaps  and  glycerin,  both  of  which  are  soluble,  their 
absorption  can  more  readily  be  accounted  for.  According  to  this  view, 
the  soaps  and  glycerin  are  again  synthesized  by  a  process  the  reverse  of  that 
which  is  brought  about  by  the  pancreatic  enzyme,  with  the  appear- 
ance of  minute  granules  of  fat.  That  this  is  the  more  probable  view  as  to 
the  mechanism  of  fat  absorption  is  evident  from  the  fact  that  when  animals 
are  fed  with  alkaline  soaps  and  glycerin,  or  with  fatty  acids  alone,  globules 
of  fat  are  found  in  the  epithelial  cells  and  in  the  interior  of  the  villus. 

With  the  passage  of  the  fat-granules  into  the  interior  of  the  villus  they 
at  once  enter  the  lymph-radicle  and  become  constituents  of  the  lymph- 
stream,  to  which  they  impart  a  white,  milky  appearance.  If  the  abdomen 
of  an  animal  in  full  digestion  be  opened,  the  lymph-vessels  of  the  mesentery 
present  themselves  as  distinct  white  threads.  An  examination  of  the  fluid 
they  contain,  known  as  chyle,  shows  the  presence  of  fat-granules  of  micro- 
scopic size.  With  the  passage  of  the  chyle  into  the  thoracic  duct  it  also 
presents  the  same  milky  appearance.  For  this  reason  the  lymphatics  of 
the  mesentery  were  erroneously  termed  lacteals.  The  chyle  as  obtained 
from  these  lymph-vessels  possesses  the  same  qualitative  though  not  quanti- 
tative composition  as  lymph,  the  difference  being  mainly  in  the  large  excess 
of  fat  in  the  former.  Indeed,  chyle  may  be  regarded  as  lymph  with  the  ad- 
dition of  fat. 


220  TEXT-BOOK  OF  PHYSIOLOGY. 

Routes  for  the  Absorbed  Food.^ — Physiologic  experiments  have  dem- 
onstrated that  the  agents  concerned  in  the  removal  of  the  products  of 
digestion  after  their  absorption  from  the  interior  of  the  villus  are: 

1.  The  veins  of  the  gastro-intestinal  tract,  which  converge  to  form  the 

portal  vein. 

2.  The  lymph-vessels  of  the  small  intestine,  which  converge  to  empty  into 

the  thoracic  duct. 

The  products  of  digestion  find  their  way  into  the  general  circulation  by 
these  two  routes,  as  follows:  (See  Fig.  94). 

The  water,  inorganic  salts,  proteins,  and  sugar  after  entering  the  blood- 
vessels of  the  villus  are  carried  by  the  blood  of  the  intestinal  veins  directly 
into  the  liver  by  the  portal  vein;  after  circulating  through  the  capillaries  of 
the  liver  and  being  influenced  by  the  liver  cells,  they  are  discharged  by  the 
hepatic  veins  into  the  inferior  or  ascending  vena  cava. 

The  fats  after  entering  the  lymph-radicle  of  the  villus  are  carried  by  the 
lymph-stream  of  the  intestinal  lymph-vessels  and  emptied  into  the  recep- 
taculum  chyli  from  which  they  ascend  into  the  thoracic  duct,  by  which 
they  are  discharged  into  the  blood  at  the  junction  of  the  left  subclavian  and 
internal  jugular  veins. 

Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The  force 
which  primarily  determines  the  movement  of  the  lymph  has  its  origin  in  the 
beginnings  of  the  lymph-vessels,  the  lymph-spaces,  and  depends  on  a  dif- 
ference in  pressure  here  and  at  the  termination  of  the  thoracic  duct.  The 
rise  of  pressure  in  the  lymph-spaces  is  due  to  the  continual  production  of 
lymph,  either  by  filtration  or  secretor  activity  of  the  capillary  walls.  As 
soon  as  the  pressure  rises  above  that  in  the  thoracic  duct  a  forward  move- 
ment of  lymph  takes  place.  Other  things  being  equal,  the  rate  of  move- 
ment will  be  proportional  to  the  difference  of  pressure.  The  first  movement 
of  the  chyle,  its  passage  from  the  lymph-capillary  in  the  villus  into  the  sub- 
jacent lymph-vessel,  has  been  attributed  to  a  shortening  of  the  villus  and  a 
compression  of  the  capillary  by  the  contraction  of  the  non-striated  muscle- 
fibers  by  which  it  is  surrounded.  With  the  entrance  of  the  chyle  into  the 
subjacent  lymph-vessel  there  is  a  distention  of  the  vessel  and  a  rise  in  pres- 
sure. When  the  muscle-fibers  relax,  regurgitation  is  prevented  by  the 
closure  of  the  valves  at  the  base  of  the  villus.  The  elastic  tissue  of  the 
lymph-vessel  now  recoils  and  forces  the  chyle  toward  the  thoracic  duct. 
After  the  emptying  of  the  lymph-capillary  the  conditions  as  far  as  pressure 
is  concerned  are  favorable  for  the  absorption  of  new  material.  The  rhythmic 
contractions  of  the  intestinal  wall  undoubtedly  aid  in  the  movement  of  lymph 
and  chyle.  It  is  quite  possible  that  the  walls  of  the  general  lymphatic  system 
aid  the  forward  movement  of  lymph  by  more  or  less  rhythmic  contractions 
of  their  contained  muscle-fibers. 

Inasmuch  as  the  lymph-vessels  lie  in  situations  in  which  they  are  sub- 
ject to  compression  by  muscles  during  contraction,  it  is  probable  that  the 
fluid  in  the  vessels  will  be  forced  onward  toward  the  thoracic  duct  at  each 
compression,  a  backward  movement  being  prevented  by  the  closure  of  the 
valves  which  are  everywhere  present  in  the  vessels.  Experimental  observa- 
tions have  demonstrated  the  truth  of  this  supposition.  Alternate  contraction 
and  relaxation  of  the  muscles  of  the  leg  will,  in  an  animal  at  least,  increase 


ABSORPTION, 


''"P"'-     %-th.d. 
inf.v.c, 

h.v.  /   '' 


Fig.  94. — Diagram  Showing  the  Routes  by  which  the  Absorbed  Foods  Reach 
THE  Blood  of  the  General  Circulatiom  (G.  Bachman).  I.  i..  Loop  of  small  intestine; 
int.,  v.,  intestinal  veins  converging  to  form  in  part,  p.  ic,  the  portal  vein,  which  enters  the 
liver  and  by  repeated  branchings  assists  in  the  formation  of  the  hepatic  capillary  plexus; 
h.  u.  the  hepatic  veins  carrying  blood  from  the  liver  and  discharging  it  into,  inf.  v.  c,  the 
inferior  vena  cava;  hit.  I.  v.,  the  intestinal  lymph  vessels  converging  to  discharge  their 
contents,  chyle,  into  rec.  c.  the  receptaculum  chyli,  the  lower  expanded  part  of  the  thoracic 
duct;  //;.  c?..  the  thoracic  duct  discharging  lymph  and  chyle  into  the  blood  at  the  junction 
of  the  internal  jugular  and  subclavian  veins;  sup.  v.  c,  the  superior  vena  cava. 


222  TEXT-BOOK  OF  PHYSIOLOGY. 

considerably  the  flow  as  well  as  the  production  of  lymph  from  the  thoracic 
duct.     Massage  has  a  similar  influence. 

The  respiratory  movements  also  aid  the  flow  of  both  lymph  and  chyle 
from  the  thoracic  duct  and  larger  lymph-vessels  into  the  venous  blood. 
During  inspiration  the  intrathoracic  pressure  (that  is,  the  positive  pressure 
exerted  by  the  air  in  the  lungs  on  the  intrathoracic  viscera,  e.g.,  heart,  veins, 
thoracic  duct,  etc.,  which  is  less  by  about  6  millimeters  of  mercury  than 
the  pressure  in  the  lungs)  decreases.  The  decrease  is  proportional  to  the 
extent  of  the  inspiration.  With  this  decrease  of  pressure,  the  thoracic  duct 
expands  and  its  internal  pressure  falls.  As  the  intra-abdominal  portion 
of  the  thoracic  duct  and  its  tributaries  are  subjected  to  a  higher  pressure, 
practically  that  of  the  atmosphere  the  lymph  in  these  vessels  is  forced,  by 
reason  of  the  difference  in  pressure  between  these  two  regions,  into  the 
intrathoracic  portion  of  the  duct.  Daring  expiration,  the  rise  of  the  in- 
trathoracic pressure  to  its  former  value  leads  to  a  compression  of  the  thoracic 
duct  and  causes  the  lymph  to  be  discharged  rapidly  into  the  blood-stream. 
A  regurgitation  of  the  lymph  is  prevented  by  the  closure  of  the  numerous 
valves  throughout  the  course  of  the  duct. 

DIFFUSION.     OSMOSIS.     FILTRATION. 

As  these  three  factors  are  believed  to  play  an  important  part  in  many  physiologic 
processes,  it  is  essential  to  a  better  understanding  of  these  processes,  that  certain 
elementary  facts  relating  to  these  three  factors  be  known. 

Diffusion. — By  diffusion  is  meant  the  gradual  and  spontaneous  mixture  of 
the  molecules  of  two  or  more  liquids,  or  of  two  or  more  gases,  when  brought 
into  contact  with  each  other,  wdthout  the  application  of  an  external  force.  The 
reason  for  both  processes  lies  in  the  fact  that  the  molecules  of  a  liquid  and  of  a 
gas  are  in  constant  motion,  in  consequence  of  which  a  mutual  interpenetration  of 
the  molecules  takes  place,  which  continues  until  a  condition  of  homogeneity  is 
established. 

Again,  when  a  soluble  substance,  inorganic  or  organic,  is  placed  in  water, 
the  molecules  of  the  substance  will  at  once  begin  to  separate  themselves  and 
to  diffuse  throughout  the  water  until  the  solution  becomes  homogeneous,  and 
notwithstanding  the  fact  that  the  dissolved  substance  possesses  weight,  the 
solution  remains  homogeneous.  The  force  of  gravity  is  overcome  by  the  force 
of  diffusion. 

The  velocity  with  which  the  molecules  of  a  substance  will  diffuse  through 
a  solvent  like  water,  varies  considerably.  The  experiments  of  Graham  show 
that  if  the  molecules  of  a  given  weight  of  hydrochloric  acid  diffuse  completely 
in  a  unit  of  time,  the  molecules  in  the  same  weight  of  sodium  chlorid,  cane-sugar, 
albumin  and  caramel,  will  require  for  their  diffusion  2.33,  7,  48,  and  98  units  of 
time  respectively. 

Osmosis. — Osmosis  may  be  defined  as  the  passage  of  the  molecules  of  water 
through  an  intervening  membrane.  If  the  water  on  one  side  of  the  membrane, 
parchment  for  example,  contains  in  solution  substances  such  inorganic  salts, 
their  molecules  will  also  pass  through  the  membrane  though  the  time  required 
for  this  to  take  place  may  be  much  longer  than  in  the  case  of  the  water  molecules. 
The  passage  of  the  dissolved  substance  through  the  membrane  though  usually 
included  under  the  term  osmosis  is  more  properly  termed  dialysis. 

If  the  two  volumes  of  water  on  opposite  sides  of  the  membrane  are  the  same 
in  amount,  and  if  the  one  volume  contains  a  salt  in  solution,  the  salt  molecules 


ABSORPTION.  223 

will  continue  to  pass  through  the  membrane  until  the  water  on  both  sides  contain 
the  same  number  of  molecules,  or,  in  other  words,  until  it  is  homogeneous  in  com- 
position. The  time  required  for  their  passage  being  longer  than  the  time  required 
for  the  passage  of  the  water  molecules,  there  will  be  (owing  to  factors  which  will 
be  explained  later),  a  temporary  increase  in  the  volume  of  the  water  originally  con- 
taining the  salt,  but  in  time  the  two  volumes  wnll  again  become  equal.  Certain 
other  substances  which  may  be  in  solution,  such  as  albumin,  starch,  etc.,  will  not 
pass  across  a  membrane,  because  of  the  large  size  of  their  molecules.  Graham 
termed  all  those  substances  which  by  virtue  of  the  small  size  of  their  molecules 
pass  through  membranes,  crystalloids,  and  all  those  which  by  virtue  of  the  large  size 
of  their  molecules  do  not  pass  through  membranes  or  to  a  very  slight  extent,  colloids. 

It  was  stated  in  the  foregoing  paragraph  that  if  two  equal  volumes  of  water 
are  separated  by  a  parchment  septum,  one  of  which  contains  in  solution  an 
inorganic  salt,  the  molecules  of  the  salt-free  water  will  osmose  through  the  septum 
into  salt-containing  water,  more  rapidly  than  they  will  in  the  opposite  direction, 
and  as  a  result,  there  will  be  a  temporary  increase  in  the  volume  of  the  water 
containing  the  salt.  If  the  membrane  were  impermeable  to  the  salt  molecules, 
the  difference  in  the  two  volumes  of  the  water  would  be  far  more  permanent  and 
striking.  The  reason  assigned  for  this  is  that  the  molecules  of  the  salt  exert  a 
pressure  against  the  outer  layer  of  the  water  molecules  and  these  in  turn  against 
the  membrane,  in  consequence  of  which  there  is  a  more  rapid  osmosis  of  the 
water  molecules  towards  the  salt  than  in  the  reverse  direction.  To  this  pressure 
is  applied  the  term 

Osmotic  Pressure. — Osmotic  pressure  may  be  defined  as  the  pressure  exerted 
by  the  molecules  of  the  substance  in  solution  against  the  outer  layer  of  the  mole- 
cules of  the  solvent.  If  the  solvent  is  enclosed  by  an  elastic  membrane  it  is 
expanded  and  in  consequence  there  is  an  osmosis  of  a  surrounding  solvent  towards 
and  through  it.  The  reason  for  this  pressure  lies  in  the  fact  that,  when  the  mole- 
cules of  a  substance  are  separated  a  certain  distance,  as  they  are  when  in  solution, 
they  repel  one  another  as  do  the  molecules  of  a  gas  and  in  their  flight  strike 
against  the  outer  layer  of  the  solvent.  The  pressure  of  the  molecules  of  a  substance 
in  solution  is  therefore  comparable  to  the  pressure  of  the  molecules  of  a  gas. 

Three  methods  may  be  employed  for  measuring  the  force  of  the  osmotic 
pressure  of  different  substances,  viz.:  i.  Physical.  2.  The  determination  of  the 
freezing  point.     3.  By  calculation. 

I.  Physical  Method. — For  the  purpose  of  measuring  osmotic  pressure  by 
physical  methods,  it  is  customary  to  make  use  of  an  apparatus  similar  to  that 
represented  in  Fig.  95,  which  consists  of  an  earthenware  ^vessel  (a),  into  the 
upper  open  end  of  which  a  tall  vertical  glass  tube  has  been  hermetically  sealed. 
The  pores  of  the  earthenware  vessel  have  been  filled  by  a  membrane  made  by 
precipitating  ferrocyanid  of  copper  within  them.  This  membrane  is  freely 
permeable  to  water,  but  impermeable  to  certain  substances  in  solution,  e.g., 
cane-sugar.  Such  a  membrane,  which  permits  the  passage  of  the  molecules  of 
the  solvent  but  not  the  molecules  of  the  dissolved  substance,  is  termed  a  semi- 
permeable membrane,  and  its  use  is  absolutely  necessitated  when  it  is  desired 
to  obtain  the  actual  pressure  exerted  by  any  given  substance  in  solution.  An 
apparatus  of  this  character  is  termed  an  osmometer. 

When,  therefore,  the  osmometer  containing  a  solution  of  cane-sugar  is  placed 
in  the  vessel  {b)  containing  water,  the  following  phenomena  occur,  viz.:  an  ascent 
of  the  cane-sugar  solution  in  the  vertical  glass  tube,  and  a  descent  of  the  level 
of  the  water  in  the  vessel  b.  These  phenomena  continue  until  the  level  of  the 
fluid  in  the  glass  tube  reaches  a  certain  height,  when  it  becomes  stationary,  and 
no  further  efl"ect  takes  place. 

In  explanation  of  the  foregoing  phenomena  it  may  be  said  that  the  molecules 


224 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  sugar  strike  or  press  against  the  outer  layer  of  the  molecules  of  the  solvent, 
which  at  all  points  are  in  contact  with  the  rigid  walls  of  the  earthenware  vessel, 
except  at  the  open  extremity  of  the  vertical  glass  tube.  Inasmuch  as  the  rigid 
walls  of  the  osmometer  prevent  any  outward  displacement  of  the  molecules  of  the 
water,  the  force  of  the  impact  of  the  sugar  molecule  is  directed  against  the  molecules 
at  the  extremity  of  the  vertical  tube  which  are  in  consequence  pressed  or  pushed 
upward  a  certain  distance.  Because  of  the  loss  of  energy  due  to  the  impact,  the 
sugar  molecule  does  not  rebound  with  the  same  velocity,  and  hence  time  is  per- 
mitted for  the  molecules  of  the  water  to  pass  into  the 
sugar  solution,  to  occupy  the  space,  and  thus  maintain 
the  level  of  the  fluid  in  the  vertical  tube.  (For  the  reason 
that  the  osmometer  is  permeable  to  water,  the  molecules 
will  pass  outward  as  well  as  inward  though  more  will 
pass  in  a  unit  of  time  in  the  latter,  than  in  the  former 
direction,  until  equilibrium  is  established.)  The  pressure 
of  the  sugar  molecules  continuing,  the  level  of  the  fluid  in 
the  glass  tube  continues  to  rise  and  the  level  of  the  fluid  in 
the  vessel,  b,  continues  to  fall  until  the  force  of  gravity 
prevents  any  further  upward  movement  of  the  molecules 
of  sugar  against  the  outer  film  of  the  molecules  of  the 
water.  The  difference  in  the  level  of  the  two  fluids  ex- 
pressed in  millimeters  of  mercury  is  taken  as  a  measure 
of,  and  equal  to,  the  pressure  of  the  sugar  in  solution. 
A  I  per  cent,  solution  of  cane-sugar  at  a  temperature 
of  from  13°  C.  to  16°  C,  as  determined  by  this  method, 
exerts  an  osmotic  pressure  of  about  535  mm.  Hg.;  a  2 
per  cent,  solution  exerts  an  osmotic  pressure  approxi- 
mately twice  this  amount. 

Experiments  made  with  this  and  similar  osmometers 
show — 

1.  That  the  osmotic  pressure  of  any  substance  in  solu- 
tion is  proportional  to  the  concentration,  providing 
the  temperature  is  constant. 

2.  That  when  the  concentration  is  constant  the  osmotic 
pressure  rises  with,  and  is  proportional  to,  the  tempera- 
ture. 

3.  That  when  different  substances  are  present  in  the 
same  solvent  the  osmotic  pressure  is  equal  to  the  sum 

of  the  individual  or  partial  pressures. 

That  whatever  the  nature  of  the  substance  in  solution 

it  will  exert  the  same  osmotic  pressure,   providing 

always  the  same  number  of  molecules  are  present; 

molecular   weights   in    grams   per   liter  of  different  substances 


Sofution 
CaneSuffar 


V-ct 


Heater 


4- 


Fi5    95 .—  An  Osmometer. 


hence    the 

exert  the  same  osmotic  pressure  at  the  same  temperature. 
Because  of  the  fact  that  when  certain  substances,  e.g.,  many  inorganic  salts, 
many  acids  and  bases,  are  dissolved,  some  of  their  molecules  undergo  ionization, 
i.e.,  separation  into  parts  which  are  charged  with  electricity,  and  hence  the  two 
together,  molecules  and  ions,  exert  a  greater  osmotic  pressure  than  would  other- 
wise be  the  case;  and  because  of  the  further  fact,  that  it  is  extremely  difficult  to 
obtain  absolutely  semipermeable  membranes,  uniform  results  are  not  obtained  by 
the  employment  of  the  three  methods;  therefore,  the  osmometric  methods  as  well 
as  the  calculation  or  arithmetic  method  have  been  largely  discarded  and  the 
method  based  on  the  determination  of  the  freezing  point  has  been  adopted. 

2.  The  Determination  of  the  Freezing  Point. — Because  of  the  difficulty  in  obtain- 


ABSORPTION.  225 

ing  the  exact  osmotic  pressure  by  means  of  the  osmometer  as  stated  above,  reliance 
is  now  placed  on  the  mathematic  relation  known  to  exist  between  osmotic  pressure 
and  the  freezing  point.  Thus  the  freezing  point  of  water  holding  any  substance 
in  solution  is  lower  than  water  itself  and  is  indeed  proportional  to  the  number 
of  molecules  dissolved.  As  a  standard  of  comparison  it  is  customary  to  employ 
a  gram-molecule  of  a  substance  dissolved  in  one  liter  of  water.  (A  gram-molecule 
is  the  quantity  of  a  substance  expressed  in  grams  equal  to  its  molecular  weight.) 
The  lowering  of  the  freezing  point  of  a  gram-molecule  solution  below  that  of  water 
is  constant,  viz.,  1.87°  C.  The  osmotic  pressure  therefore  of  such  a  solution,  as 
determined  by  calculation  (see  below),  is  equal  to  22.38  atmospheres,  or  17,008 
mm.  of  Hg. 

Therefore  it  is  only  necessary  to  determine  by  means  of  a  differential  thermom- 
eter the  lowering  of  the  freezing  point  in  degrees  centigrade,  which  is  usually  expres- 
sed by  the  symbol  A-  Then  the  osmotic  pressure  is  equal  to  A  divided  by  1.87° 
C,  and  multiplied  by  22.38  atmospheres,  or  17,008  mm.  of  Hg.  Thus  if  the 
freezing  point  of  any  solution  was  found  to  be  0.83°  C.  lower  than  water,  its 
osmotic  pressure  would  be  0.83^1.87X22.38  atmospheres  or  9.847  atmospheres 
=  7,483  mm.  Hg.  If  any  two  solutions  have  the  same  freezing  point  they  contain 
the  same  number  of  molecules  and  hence  have  the  same  osmotic  pressure.  Blood 
plasma  has  a  freezing  point  of  0.56°  C.  Experimentally  it  has  been  determined 
that  the  freezing  point  of  water  is  lowered  to  the  same  level,  when  it  contains  so- 
dium chlorid  to  the  extent  of  0.95  per  cent.  Hence  these  two  fluids  have  the  same 
osmotic  pressure  and  are  isotonic;  each  exerts  a  pressure  of  6.696  atmospheres. 

For  this  reason  the  sodium  chlorid  solution  can  be  employed  for  preserving, 
for  a  time  at  least,  the  form  of  blood  corpuscles  or  other  living  mammalian  cells, 
from  which  it  may  be  inferred,  that  the  contents  of  the  cells  have  an  osmotic  pres- 
sure approximately  equal  to  that  of  the  blood  plasma  or  the  salt  solution.  If  the 
salt  solution  has  a  lower  concentration  and  hence  a  lower  osmotic  pressure,  water 
will  osmose  into  the  corpuscle  and  cause  a  discharge  of  its  hemoglobin  content. 
Such  a  fluid  is  said  to  be  hypo-isotonic.  If,  on  the  contrary,  the  salt  solution  has  a 
higher  concentration  and  hence  a  higher  osmotic  pressure,  water  will  osmose  from 
the  corpuscle  causing  a  shrinkage  and  crenation  of  the  corpuscle.  Such  a  fluid 
is  said  to  be  hyperisotonic. 

3.  By  Calculation. — The  osmotic  pressure  may  also  be  obtained  by  calculation 
based  on  the  known  pressure  exerted  by  a  gram-molecule  of  hydrogen — 2  grams — 
when  compressed  to  a  volume  of  one  liter.  It  is  well  known  that  i  gram  of  hydro- 
gen at  0°  C.  and  at  an  atmospheric  pressure  of  760  mm.  Hg.  occupies  a  volume  of 
II. 19  liters,  and  that  2  grams  under  the  same  conditions  will  occupy  a  volume  of 
22.38  liters,  and  that  when  the  two  grams,  that  is,  one  gram-molecule  is  com- 
pressed to  a  volume  of  i  liter  the  molecules  will  exert  a  pressure  equal  to  that  of 
22.38  liters  or  22.38  atmospheres  or  17,008  mm.  of  Hg.  Since  a  gram-molecule  of 
any  substance  dissolved  in  i  liter  of  water  contains  the  same  number  of  molecules 
as  a  gram-molecule  of  hydrogen  compressed  to  one  liter,  they  have  the  same 
osmotic  pressure. 

From  this  it  is  possible  to  calculate  the  osmotic  pressure  of  an  electrolyte,  if 
the  percentage  composition  of  the  substance  in  solution  be  know^n.  Let  it  be 
supposed,  for  example,  that  it  is  desirable  to  know  the  osmotic  pressure  of  a  i  per 
cent,  solution  of  cane-sugar.  The  procedure  is  as  follows:  A  gram-molecule  of 
cane-sugar  (Cjj  H22  Oj^)  contains  342  grams;  a  i  per  cent,  solution  contains  10 
grams  to  the  liter;  hence  its  osmotic  pressure  is  io-=-342X  22.38  atmospheres  or 
0.65  atmosphere  which  is  equal  to  494  mm.  of  Hg. 

Filtration, — Filtration  may  be  defined  as  the  passage  of  water,  and  of  all  sub- 
stances dissolved  in  it,  through  a  membrane  as  a  result  of  a  difference  of  hydro- 
static pressure  on  the  two  sides.     The  difference  between  the  two  pressures  con- 


226  TEXT-BOOK  OF  PHYSIOLOGY. 

stitutes  the  force  of  iiilration,  and  hence  the  greater  the  difference,  the  greater  will 
be  the  amount  of  fluid  filtered. 

With  any  given  artificially  prepared  animal  membrane  the  quantities  of  water 
and  crystalloids  in  general  which  pass  through  the  membrane  are  proportional 
to  the  filtration  force,  and  hence  the  filtrate  will  have  a  concentration  similar  to, 
if  not  identical  with,  that  of  the  original  solution.  The  passage  of  colloids  in  solu- 
tion will  be  proportional  to  the  permeability  of  the  membrane  and  an  increase  in 
the  filtration  force.  The  filtrate,  however,  will  have  a  lower  degree  of  concentra- 
tion than  the  original  solution  for  the  reason  that  as  the  pressure  rises  the  quantity 
of  water  filtered  increases  in  a  greater  ratio  than  the  quantity  of  colloid  filtered. 

Physiologic  Applications. — In  the  animal  body  the  fluids  are  separated  by 
delicate  membranes  through  which  the  constituents  of  the  fluids,  inorganic  and 
organic,  are  continually  passing.  Thus  prepared  foods  in  the  intestine  pass  through 
the  intestinal  wall  into  blood-  and  lymph-vessels;  the  constituents  of  the  blood 
pass  through  the  wall  of  the  capillary  vessel  into  the  tissue  spaces  from  which  they 
pass  (a)  through  the  walls  of  various  glands  to  take  part  in  the  formation  of  their 
secretion;  (b)  through  the  sarcolemma  into  the  interior  of  the  muscle  fiber;  (c)  through 
the  limiting  surface  of,  and  into  the  interior  of  all  other  tissue  cells.  The  waste 
products,  the  result  of  tissue  and  food  metabolism,  pass  from  the  interior  of  cells 
through  their  limiting  membranes  or  surfaces  into  the  tissue  spaces;  thence  through 
the  wall  of  the  capillary  vessel  into  the  blood  and  finally  through  the  wall  of  the 
capillary  vessel  and  the  epithelium  of  the  lung,  the  kidney,  the  liver,  etc.,  to  take 
part  in  the  formation  of  the  excretions.  These  and  other  processes  are  believed 
to  be  accomplished  by  the  factors,  diffusion,  osmosis,  and  filtration. 

The  statements  that  have  been  made  in  foregoing  paragraphs  in  reference  to 
diffusion,  osmosis,  and  filtration  have  been  based  on  the  results  of  experiments 
which  have  been  made  with  non-living  membranes,  and  under  conditions  purely 
physical ;  and  though  it  is  quite  true  that  in  the  animal  body  the  fluids  are  separated 
by  membranes  more  or  less  permeable  to  all  their  constituents,  and  that  all  pass, 
through  these  membranes,  it  is  possible  that  the  facts  which  have  been  obtained 
experimentally  are  not  strictly  paralleled  in  the  living  body,  and  hence  not  strictly 
applicable  to  the  elucidation  of  physiologic  processes.  Nevertheless  there  are 
reasons  for  thinking  that  a  thorough  understanding  of  these  factors  will  eventually 
throw  much  light  on  the  intimate  nature  of  the  process  by  which  organic  as  well  as 
inorganic  substances  in  solution  pass  through  animal  membranes  in  the  living 
condition. 


CHAPTER  XII. 
THE  BLOOD. 

The  blood  is  a  highly  complex  nutritive  fluid,  the  presence  and  proper 
circulation  of  which  in  the  living  organism  are  essential  to  the  maintenance 
and  activity  of  all  physiologic  mechanisms.  The  escape  of  the  blood  from 
the  vessels,  especially  in  the  higher  animals,  is  followed  by  a  loss  of  the 
physiologic  activities  of  all  the  tissues  within  a  short  period  of  time.  The 
immediate  dependence  of  the  functional  activities  of  the  tissues  and  organs 
on  the  presence  of  the  blood  can  be  demonstrated  by  the  following  experi- 
ment: If  the  nozzle  of  a  syringe,  adapted  to  the  size  of  the  animal,  be  intro- 
duced through  the  jugular  vein  into  the  right  side  of  the  heart  and  the  blood 
be  suddenly  withdrawn,  there  is  an  immediate  cessation  in  the  activity  of 
all  the  organs;  the  return  of  the  blood  to  the 'vessels  within  a  limited  period 
of  time  is  promptly  followed  by  a  renewal  of  their  acti\dty. 

Though  contained  within  a  practically  closed  system  of  vessels,  the  blood 
is  brought  into  intimate  relation  with  all  the  tissue  elements  through  the 
intermediation  of  the  capillaries.  As  the  blood  flows  through  these  delicate 
vessels,  portions  of  its  soluble  nutritive  constituents,  including  oxygen,  are 
given  up  to  the  tissues,  by  which  they  are  utilized  for  growth,  repair,  and 
functional  activity.  At  the  same  time  the  tissues  yield  up  to  the  blood  a 
series  of  decomposition  or  katabolic  products,  resulting  from  their  activity, 
which  vary  in  quantity  and  quality  according  as  the  blood  traverses  the 
muscles,  nerves,  glands,  or  other  tissues. 

The  blood  may  be  regarded,  therefore,  as  a  reservoir  of  nutritive  materials 
prepared  by  the  digestive  apparatus  and  absorbed  from  the  intestinal  canal; 
of  oxygen,  absorbed  from  the  respiratory  surface  of  the  lungs;  of  katabolic 
products,  produced  by  and  absorbed  from  the  tissues.  Though  the  blood 
varies  in  composition  in  different  parts  of  the  body  in  consequence  of  the 
introduction  of  both  nutritive  material  and  katabolic  products,  it  neverthe- 
less presents  certain  average  physical,  morphologic,  and  chemic  properties 
which  distinguish  it  as  an  individual  tissue. 

Constituents  of  Blood. — A  microscopic  examination  of  the  biood  as  it 
flows  through  the  capillary  vessels  of  the  web  of  the  frog  or  the  mesentery  of 
the  rabbit  shows  that  it  is  not  a  homogeneous  fluid,  but  that  it  consists  of  two 
distinct  portions:  viz.,  (i)  a  clear,  transparent,  slightly  yellow  fluid,  the 
plasma  or  liquor  sanguinis;  (2)  small  particles  termed  corpuscles  floating  in 
it,  of  which  there  are  two  varieties,  the  red  or  the  erythrocytes  and  the  white 
or  the  leukocytes.  By  appropriate  methods  it  can  be  shown  that  a  third  cor- 
puscle, colorless  in  appearance  and  smaller  in  size  than  the  ordinary  white 
corpuscle,  is  present  in  the  blood-stream  and  known  as  the  blood- platelet 
or  plaque.  The  dift'erent  Constituents  can  be  roughly  separated  by  appropriate 
means  when  the  blood  is  withdrawn  from  the  body.  If  the  blood  of  the 
horse  is  allowed  to  flow  directly  into  a  tall  cylindric  glas5  vessel,  surrounded 

227 


228  TEXT-BOOK  OF  PHYSIOLOGY. 

by  ice,  it  separates  in  the  course  of  a  few  hours  into  three  distinct  layers  in 
accordance  with  their  specific  gravities.  The  lower  layer  is  dark  red  and 
consists  of  the  red  corpuscles;  the  middle  layer  is  grayish  in  color  and  consists 
of  the  white  corpuscles;  the  upper  layer  is  clear  and  transparent  and  consists 
of  the  plasma.  The  red  corpuscles  occupy  almost  one-half,  the  white  one- 
fortieth,  the  plasma  a  trifle  more  than  one-half  of  the  height  of  the  entire 
blood-column,  which  indicates  approximately  the  different  volumes  of  each. 
The  same  result  can  be  obtained  with  human  blood  by  the  use  of  the  centri- 
fuge or  hematocrit. 

PHYSICAL  PROPERTIES  OF  BLOOD. 

1.  Color. — Within  the  blood-vessels  two  kinds  of  b'ood  are  distinguished 
— the  arterial,  the  color  of  which  is  a  bright  scarlet,  and  the  venous,  the  color 
of  which  is  a  dark  bluish-red  or  purple.  '  The  cause  of  the  color  as  well  as 
the  difference  in  color  is  the  presence  in  the  red  corpuscles  of  a  coloring- 
matter,  hemoglobin,  in  different  degrees  of  combination  with  oxygen.  The 
intensity  of  the  color  in  either  kind  of  blood  is  dependent  on  the  thickness  of 
the  blood-stream,  for  in  the  finest  capillaries,  as  seen  under  the  microscope, 
there  is  an  almost  total  absence  of  color.  As  the  arterial  blood  passes  into 
and  through  the  systemic  capillaries,  the  hemoglobin  yields  up  a  portion  of 
its  oxygen  to  the  tissues  and  changes  in  color,  though  the  change  is  not 
appreciable  by  the  eye.  On  passing  into  the  veins,  however,  the  blood- 
stream soon  presents  its  characteristic  dark  bluish-red  color,  which  deepens 
as  it  approaches  the  lungs.  (3n  passing  into  and  through  the  capillary 
vessels  of  the  lungs  the  hemoglobin  absorbs  a  new  volume  of  oxygen,  changes 
in  color,  and  on  emerging  from  the  lungs  the  blood  presents  its  characteristic 
scarlet  color. 

2.  Opacity. — Owing  to  the  fact  that  the  corpuscles -have  a  refracting 
power  different  from  the  plasma,  the  blood,  even  in  thin  layers,  is  opaque. 
The  repeated  refractions  and  reflections  which  light  undergoes  in  passing 
through  plasma  and  corpuscles  is  attended  by  such  a  dissipation  that  it  is 
impossible  to  see  printed  matter  through  it.  That  the  opacity  is  due  to  the 
shape  of  the  corpuscles  rather  than  to  their  contained  coloring-matter  is 
evident  from  the  fact  that  when  the  hemoglobin  is  caused  to  separate  from 
the  corpuscles  by  the  addition  of  chemic  reagents,  the  blood,  though  it 
deepens  in  color,  at  once  becomes  transparent. 

3.  Odor. — When  freshly  drawn  the  blood  possesses  a  peculiar  charac- 
teristic odor  which  has  been  attributed  to  the  presence  of  a  volatile  fatty  acid 
in  combination  with  an  alkaline  base.  The  intensity  of  the  odor  may  be 
increased  by  the  addition  of  concentrated  sulphuric  acid,  by  means  of  which 
the  volatile  acid  is  set  free. 

4.  Specific  Gravity. — The  specific  gravity  of  blood  lies  within  the  limits 
of  1. 051  and  1.059,  averaging  in  man  1.056  and  in  woman  1.053.  Normally, 
variations  from  these  values  are  only  temporary  and  are  connected  with 
variations  in  physiologic  processes.  The  specific  gravity  is  diminished  by 
the  ingestion  of  liquids  and  abstinence  from  solid  food.  It  is  increased  by 
abstinence  from  liquids,  by  the  ingestion  of  dry  food,  and  by  the  elimination 
of  large  quantities  of  water  by  the  lungs,  skin,  and  kidneys. 

Inasmuch  as  the  specific  gravity  of  the  blood  varies  from  the  normal  in 


THE  BLOOD.  229 

one  direction  or  the  other  in  certain  pathologic  states,  it  is  deemed  desirable 
for  clinical  purposes  to  determine  the  extent  of  this  variation.  Among  the 
methods  suggested  for  this  purpose  that  of  Hammerschlag  is  the  one  most 
generally  resorted  to.  It  is  based  on  the  principle,  that  a  fluid  in  which  a 
drop  of  blood  neither  rises  nor  falls  must  have  the  same  specific  gravity  as 
the  blood  itself.  As  the  specific  gravity  of  the  blood  varies  in  different 
pathologic  states  it  is  essential  that  the  fluid  employed  is  of  such  a  character 
that  its  specific  gravity  can  be  quickly  varied  in  one  direction  or  the  other. 
To  meet  this  indication  a  fluid,  a  mixture  of  chloroform  (specific  gravity 
1.526)  and  benzol  (specific  gravity  0.889)  is  first  prepared  in  such  propor- 
tions that  the  mixture  has  a  specific  gravity  of  about  1.040.  With  a  pipette, 
a  drop  of  blood  is  then  placed  in  the  mixture.  If  the  drop  rises  the  specific 
gravity  of  the  mixture  is  greater  than  that  of  the  blood.  Benzol  is  then 
gradually  added  until  the  drop  remains  stationary.  At  this  moment  the 
specific  gravity  of  the  mixture  is  the  same  as  that  of  the  blood.  If  the 
drop  falls  the  specific  gravity  of  the  mixture  is  less  than  that  of  the  blood. 
Chloroform  is  then  gradually  added  until  the  drop  remains  stationary. 
At  this  moment  the  specific  gravity  of  the  mixture  is  the  same  as  that  of 
the  blood.  In  either  case  the  specific  gravity  of  the  mixture  is  determined 
with  a  suitable  hydrometer  and  the  figure  observed  attributed  to  the  blood. 

5.  Reaction. — The  reaction  of  the  blood  has  usually  been  stated  as 
alkaline  from  its  effect  on  litmus  paper.  Thus,  if  blood  is  permitted  to 
remain  for  a  few  seconds  on  slightly  reddened  glazed  litmus  paper  and  then 
washed  off,  a  distinct  blue  color  presents  itself  against  a  red  or  violet  back- 
ground. The  alkalinity  thus  indicated  has  been  attributed  to  disodium 
phosphate,  Na,  HPO^,  and  sodium  carbonate,  Na,  CO3.  The  degree  of  the 
alkalinity  is  measured  by  the  amount  of  a  standard  acid  necessary  to  be 
added  before  the  indicator  used  shows  an  acid  reaction.  According  to 
V.  Jaksch  the  alkalinity  corresponds  to  from  260  to  300  milligrams  of  sodium 
hydrate,  XaOH,  for  every  100  c.c.  of  blood,  according  to  Lowy  from  300  to 
325  milligrams.  The  hitherto  unavoidable  error  in  these  estimates  is  about 
30  milligrams.  The  alkalinity  from  this  point  of  view  varies  but  within 
narrow  limits  in  physiologic  conditions.  It  is  increased  in  the  early  stages 
of  digestion  and  decreased  in  the  later  stages  as  well  as  after  prolonged 
exercise. 

In  accordance  with  the  ideas  of  physical  chemistry  the  acidity  of  a  fluid 
is  dependent  on  the  presence  of  hydrogen  ions,  H+,  and  the  alkalinity  on  the 
presence  of  hydroxyl  ions,  (OH).  The  reaction  of  any  fluid,  containing  a 
number  of  chemic  compounds  in  solution,  will  be  dependent  therefore  on  the 
relative  proportions  of  hydrogen  ions  and  hydroxyl  ions  that  make  their 
appearance. 

If  the  hydrogen  ions  are  in  excess  the  fluid  is  acid,  if  the  hydroxyl  ions 
are  in  excess  the  fluid  is  alkaline  in  reaction.  Tested  by  the  methods  of 
physical  chemistry  blood  and  lymph  are  found  to  possess  these  opposite 
ions  in  equal  degree  and  therefore  are  neither  acid  nor  alkaline  but  neutral 
in  reaction. 

6.  Temperature. — The  temperature  varies  from  36.78°  C.  (98.2°  F.)  in 
the  superior  vena  cava  to  39.7°  C.  (103.4°  F.)  in  the  hepatic  vein,  the  mean 
being  about  38°  C.  (100°  F.). 


230  TEXT-BOOK  OF  PHYSIOLOGY. 

7.  Viscosity. — \lscosity  may  be  defined  as  the  resistance  to  the  move- 
ment of  the  molecules  of  a  fluid  homogeneous  body  among  themselves.  In 
accordance  with  the  degree  of  this  resistance,  w^hich  may  also  be  spoken  of 
as  internal  friction,  will  the  fluid  at  a  given  temperature  be  mobile  or  viscous. 
Viscosity  varies  partly  with  the  nature  of  the  fluid  and  partly  on  its  tempera- 
ture. Thus  at  the  same  temperature  water,  syrup,  and  pitch  possess  different 
degrees  of  viscosity.  A  rise  in  temperature  of  1°  C.  diminishes  the  viscosity 
about  2  per  cent.  In  all  discussions  relating  to  the  viscosity  of  fluids,  that 
of  distilled  water  is  taken  as  a  standard  and  regarded  as  unity. 

Blood  as  a  fluid  is  regarded  by  physiologists  as  possessing  viscosity, 
though  the  definition  in  the  foregoing  paragraph  is  not  strictly  applicable, 
as  it  is  not  a  homogeneous  but  a  heterogeneous  fluid  consisting  of  plasma 
the  molecules  of  which  show  an  inner  friction  and  of  corpuscles  which  also 
show  friction.  Blood  having  a  complex  composition  as  compared  with 
water  has  naturally  a  greater  degree  of  viscosity  or  internal  friction.  Experi- 
mental investigations  render  it  certain  that  the  observed  viscosity  is  depend- 
ent on  the  corpuscular  elements  to  a  greater  extent  than  on  the  composition 
of  the  plasma.     About  two-thirds  of  the  viscosity  is  due  to  the  corpuscles. 

The  viscosity  of  blood  as  compared  with  water  may  be  determined  by 
permitting  the  two  fluids  to  flow  through  capillary  tubes  of  corresponding 
caliber  under  a  steadily  acting  pressure  and  then  determining  the  volume 
that  flows  through  each  in  a  given  time  and  comparing  one  with  the  other. 
Normal  human  blood  is  thus  found  to  possess  a  viscosity  4.5  times  that  of 
distilled  water  at  body  temperature.  Dog's  blood  has  a  viscosity  6  times 
that  of  water.  If  the  temperature  of  blood  is  raised  the  viscosity  diminishes. 
Recalling  the  statement  that  the  viscosity  is  closely  connected  with  the 
presence  of  red  corpuscles  it  would  be  expected  that  either  an  increase  or 
decrease  in  their  number  would  change  the  viscosity  in  one  direction  or 
another.  In  a  case  of  polycythemia  in  which  the  red  corpuscle  count  was 
11,000,000  per  cubic  millimeter  the  viscosity  was  between  3  and  4  times  the 
normal.  In  certain  other  pathologic  states  of  the  blood  characterized  by  a 
diminution  in  the  number  of  red  corpuscles  the  viscosity  diminished  one-half 
or  more.  The  ingestion  of  meat  raises  the  \dscosity,  while  the  ingestion  of 
fats  and  carbohydrates  diminishes  it. 

The  determination  of  the  viscosity  for  clinical  purposes  is  accomplished 
by  the  use  of  special  forms  of  apparatus  termed  viscosimeters.  These  for 
the  most  part  consist  of  a  capillary  tube  through  which  the  blood  is  caused 
to  flow  under  the  influence  of  a  constant  positive  or  negative  pressure.  The 
distance  the  blood  flows  in  a  unit  of  time,  compared  with  that  of  water, 
represents  the  degree  of  viscosity.  Among  these  apparatus  those  of  Hess 
and  Determann  are  generally  employed,  descriptions  of  which  will  be  found 
in  works  on  diagnosis. 

8.  Coagulability. — Within  a  few  minutes  after  the  blood  is  withdrawn 
from  the  vessels  of  a  living  animal  it  begins  to  lose  its  fluidity,  becomes  some- 
what viscid,  arid  if  left  undisturbed  passes  rapidly  into  a  semisolid  or  jelly- 
like state.  To  this  change  in  the  physical  condition  of  the  blood  the  term 
coagulation  has  been  applied.  The  blood,  during  the  progress  of  coagula- 
tion, not  only  assumes  the  shape  of  the  vessel  in  which  it  is  contained,  but 
becomes  so  firmly  adherent  to  its  walls  that  it  may  be  inverted  without  the 


THE  BLOOD. 


231 


coagulum  becoming  dislodged.  If  a  portion  of  such  a  jelly-like  mass  be 
examined  microscopically,  it  will  be  found  to  be  penetrated  in  all  directions 
by  a  felt-work  of  extremely  line  delicate  fibrils,  which,  having  made  their 
appearance  before  the  corpuscles  have  had  time  to  settle  to  the  bottom  of 
the  fluid,  have  entangled  them  in  the  meshes  so  that  the  entire  mass 
retains  its  characteristic  color.  These  fibrils  are  collectivelv  known  as  fibrin 
(Fig.  96). 

If  the  coagulated  blood  be  allowed  to  remain  undisturbed,  a  clear, 
transparent,  slightly  yellowish  fluid  makes  its  appearance  on  the  surface  of 
the  mass,  which  as  it  accumulates  forms  a  layer  of  varying  degrees  of  thick- 
ness. Within  a  few  hours  the  blood-mass  detaches  itself  from  the  sides  of 
the  vessel  in  consequence  of  the  retraction  of  the  fibrils,  while  at  the  same 


^ 


^'I'V/'i^v::'^ 


Fig.  96. — DiAGR.\M  to  Illustrate  the  Process  of  Coagulation,  i.  Fresh  blood,  plasma, 
and  corpuscles.  2.  Coagulating  blood  (birth  of  fibrin).  3.  Coaglulated  blood  (clot  and  serum). — 
(Waller.) 


time  the  clear  fluid  increases  in  amount  and  accumulates  along  the  sides 
and  bottom  of  the  vessel.  The  shrinkage  in  the  volume  of  the  red  coagulum 
and  the  increase  of  the  volume  of  the  clear  fluid  which  is  expressed  from  its 
meshes  continue  for  a  period  varying  from  ten  to  fifteen  hours,  according 
to  certain  external  conditions.  The  blood  has  now  become  separated  into 
two  distinct  portions:  viz.,  a  solid  contracted  red  mass,  termed  dot,  and  a 
clear  fluid,  termed  serum.  The  clot  consists  of  the  fibrin  containing  in  its 
meshes  the  red  and  white  corpuscles;  the  serum  consists  of  all  the  constitu- 
ents of  the  plasma  except  the  antecedents  of  the  fibrin.  The  stages  of  coagu- 
lation are  shown  in  Fig.  96. 

If  the  blood  coagulates  slowly  the  red  corpuscles,  owing  to  their  greater 
specific  gravity,  subside  to  the  bottom  of  thebloodmass,  giving  to  it  a  deeper 
color;  the  white  corpuscles,  owing  to  their  lesser  specific  gravity,  remain  near 
the  surface  of  the  clot  and  give  to  it  a  more  or  less  whitish  appearance,  pro- 
ducing the  so-called  buffy  coat.  In  certain  inflammatory  conditions  the 
coagulating  power  of  the  blood  is  much  diminished,  and  the  corpuscles, 
having  time  to  subside,  a  well-developed  buffy  coat  is  formed  which  at  one 
time  had  much  interest  for  pathologists.  As  the  contraction  of  the  fibrin 
takes  place  more  actively  in  the  center,  there  being  here  less  resistance  than 
at  the  sides  of  the  coagulum,  the  upper  surface  usually  becomes  depressed 
or  cupped. 

Coagulation  of  Plasma. — Clear  plasma  may  be  obtained  by  means  of 
the  centrifuge  from  blood  to  which  sufficient  magnesium  sulphate  has  been 
added  to  prevent  coagulation,  or  from  horse's  blood  which  has  been  allow^ed 
to  flow  into  a  tall  vessel  surrounded  by  a  cooling  mixture  so  as  to  prevent 
coagulation  and  thus  permit  the  red  corpuscles  to  subside.     If  such  plasma 


232  TEXT-BOOK  OF  PHYSIOLOGY. 

is  subjected  to  room-temperature,  it  very  shortly  undergoes  coagulation, 
exhibiting  practically  the  same  phenomena  as  blood  itself.  After  a  variable 
length  of  time  it  also  separates  into  a  soft,  colorless  coagulum  or  clot  consisting 
of  fibrin,  and  a  clear  fluid,  the  serum.  The  presence  of  the  red  corpuscles 
is  therefore  not  essential  to  the  process  of  coagulation. 

Rapidity  of  Coagulation. — The  rapidity  with  which  the  blood  coagu- 
lates varies  in  different  classes  of  animals  under  the  same  conditions:  e.g., 
the  blood  of  the  pigeon  coagulates  immediately;  that  of  the  dog,  in  from  one 
to  three  minutes;  that  of  the  horse,  in  from  five  to  thirteen  minutes;  that  of 
man,  in  from  four  to  seven  minutes.  The  time,  however,  can  be  lengthened 
or  shortened  by  either  changing  the  external  conditions  or  by  altering 
temporarily  the  normal  composition  of  the  blood. 

Coagulation  is  retarded  or  prevented  by  the  following  agents,  viz.:  (i) 
A  low  temperature,  especially  that  of  melting  ice.  (2)  The  addition  of 
magnesium  sulphate  (i  volume  of  a  25  per  cent,  solution  to  3  volumes  of 
blood) ;  of  sodium  sulphate  (i  volume  of  a  saturated  solution  to  7  volumes  of 
blood).  (3)  The  addition  of  potassium  oxalate  (i  volume  of  a  i  per  cent, 
solution  to  3  volumes  of  blood).  (4)  The  injection  into  the  circulating 
blood  of  commercial  peptone.     (5)  The  mouth  secretion  of  the  leech. 

Coagulation  is  hastened  by  the  following  agents,  viz.:  (i)  gradually 
increasing  the  temperature  from  38°  C.  to  50°  C.  (2)  The  addition  of 
water  in  not  too  large  amounts.  (3)  The  presence  of  foreign  bodies.  (4) 
Agitation  of  the  blood — e.g.,  stirring. 

Fibrin  and  Defibrinated  Blood. — If  freshly  drawn  blood  is  stirred  with 
a  bundle  of  twigs  or  glass  rods  for  a  few  minutes,  the  fibrin  collects  on  them 
in  the  form  of  thick  bundles  or  strands;  after  washing  it  with  water  the 
entangled  corpuscles  are  removed,  when  the  fibrin  assumes  its  natural  white 
appearance.  The  strands  can  be  resolved  into  a  large  number  of  delicate 
fibers  which  possess  extensibility  and  retractility,  and  are  therefore  elastic. 
The  chemic  features  of  fibrin  have  already  been  considered  (see  page  18). 
The  remaining  fluid,  similar  in  its  physical  appearance  to  the  blood,  is 
termed  defibrinated  blood.  When  such  blood  is  allowed  to  remain  at  rest  for 
a  few  days,  the  remaining  red  corpuscles  gradually  sink  to  the  bottom  of  the 
fluid,  above  which  will  be  found  the  clear  serum. 

COMPOSITION  OF  PLASMA  AND  SERUM. 

Plasma. — The  plasma  obtained  by  any  of  the  methods  previously 
described  is  a  clear,  colorless,  transparent,  slightly  viscid  fluid,  with  a  specific 
gravity  of  1.026  to  1.029.  It  is  composed  largely  of  water  holding  in  solution 
proteins,  sugar,  fatty  matter,  inorganic  salts,  urea,  cholesterin,  lecithin,  etc. 
In  composition  it  is  cjuite  complex,  containing  as  it  does  not  only  the  nutritive 
materials  derived  from  the  digestion  of  the  food,  but  also  the  substances 
resulting  from  the  disintegration  of  the  tissues  consequent  on  their  functional 
activity. 

Serum. — The  serum  is  the  clear,  transparent,  slightly  yellow  fluid  ex- 
pressed from  the  coagulated  blood  during  the  contraction  of  the  fibrin.  It 
consists  practically  of  the  ingredients  of  the  plasma,  with  the  exception  of. 
those  substances  which  entered  into  the  formation  of  fibrin.  The  average 
composition  of  plasma  is  shown  in  the  following  table: 


THE  BLOOD.  233 

COMPOSITION  OF  PLASMA. 

Water 90 .  00 

{Serum-albumin 4  •  50 

Paraglobulin 3  •  4° 

Fibrinogen 0.30 

Fatty  matters 0-25 

Sugar o .  10 

Extractives .  o .  60 

Inorganic  salts o  .85 

100.00 

Serum-albumin. — Of  the  protein  constituents  of  the  blood,  serum- 
albumin  is  the  most  abundant,  existing  to  the  extent  of  from  4  to  5  per  cent. 
From  its  similarity  to  egg-albumin  it  is  regarded  as  holding  an  important 
position  as  a  nutritive  agent,  for  it  is  out  of  this  common  protein  that  in  all 
probability  each  individual  tissue  elaborates  the  special  protein  characteristic 
of  it,  since  during  starvation  the  albumin  steadily  diminishes  in  amount.  As 
it  passes  through  the  walls  of  the  capillary  vessels  it  is  found  in  the  lymph, 
pericardial  fluid,  and  similar  secretions  in  various  parts  of  the  body,  as  well 
as  in  various  pathologic  transudates.  It  is  also  present  in  serum.  While 
circulating  in  the  lymph-spaces  the  serum-albumin  is  utilized  in  replacing 
the  proteins  which  have  undergone  disintegration  during  tissue  metabolism. 
Its  supply  in  the  blood  is  maintained  by  the  absorption  of  peptones  or  simpler 
products  of  protein  digestion,  e.g.,  amino-acids,  which  are  formed  from  the 
proteins  of  the  food  and  which  during  the  time  of  absorption  are  changed  in 
some  unknown  way  into  serum-albumin.  It  is  readily  obtained  from  plasma 
or  serum  by  saturating  either  of  these  fluids  with  magnesium  sulphate,  when 
all  the  proteins  except  serum-albumin  are  precipitated.  After  their  removal 
the  remaining  fluid  is  subjected  to  a  temperature  of  from  70°  to  75°  C,  when 
the  serum-albumin  is  precipitated  in  a  coagulable  form,  after  which  it  can 
be  removed  and  its  chemic  features  determined. 

Paraglobulin. — This  protein,  though  present  in  plasma,  is  best  obtained 
from  serum  when  this  fluid  is  saturated  with  magnesium  sulphate.  As  the 
line  of  saturation  is  approached  the  fluid  becomes  turbid,  and  after  a  few 
minutes  a  fine  white  precipitate  occurs.  It  can  then  be  collected  on  a  filter, 
dried,  and  its  chemic  properties  determined.  In  its  reactions  it  resembles 
the  various  members  of  the  globulin  class.  The  amount  varies  from  2  to  4 
per  cent,  in  the  blood  of  man.  As  to  the  physiologic  importance  or  ante- 
cedents of  paraglobulin  nothing  is  definitely  known.  Its  constant  presence 
in  the  blood  would  indicate  that  it  plays  an  equally  important,  though  per- 
haps different,  part  with  serum-albumin  in  the  nutrition  of  the  body. 

Fibrinogen. — This  protein  can  be  obtained  from  plasma,  lymph, 
pericardial,  and  peritoneal  fluids,  as  well  as  from  hydrocele  fluid.  It  is, 
however,  not  to  be  obtained  from  serum,  as  it  is  removed  from  the  blood 
during  the  formation  of  solid  fibrin.  It  is  normally  present  in  the  blood  in 
very  small  quantity,  amounting  to  not  more  than  0.22  to  0.33  parts  per 
hundred.  Fibrinogen  may  be  obtained  from  plasma  which  has  been  pre- 
vented from  coagulating,  by  the  addition  of  magnesium  sulphate  in  certain 
quantities  or  by  the  addition  of  a  saturated  solution  of  sodium  chlorid.  In 
a  few  minutes  a  flaky  precipitate  occurs.  By  repeated  washing  and  pre- 
cipitation with  sodium-chlorid  solutions  of  varying  strength,  the  fibrinogen 
may  be  obtained  in  a  pure  state.     The  history  of  fibrinogen  is  unknown, 


234  TEXT-BOOK  OF  PHYSIOLOGY. 

though  there  is  some  experimental  evidence  for  the  belief  that  it  is  produced 
in  the  liver  though  out  of  what  has  not  been  determined.  Beyond  the  fact 
that  it  contributes  to  the  occasional  formation  of  fibrin  there  is  no  positive 
knowledge  either  as  to  its  origin,  its  nutritive  value,  or  its  final  disposition  in 
the  blood  under  normal  conditions. 

Fat. — The  plasma  as  well  as  the  serum  contains  a  very  small  quantity 
of  fat  in  the  form  of  microscopic  globules.  Though  the  percentage  is  nor- 
mally not  above  0.25,  yet  just  after  a  meal  rich  in  fatty  matter  the  amount 
may  be  so  great  as  to  give  to  the  blood  a  milky  or  opalescent  appearance. 
Within  a  few  hours,  however,  this  excess  of  fat  disappears  from  the  blood, 
though  its  immediate  disposition  is  unknown.  Soaps  or  alkaline  salts  of  the 
fat  acids,  though  formed  during  the  digestion  of  fats,  are  not  present  in 
the  blood.     Lecithin  and  cholesterin  are  present  in  very  small  quantities. 

Sugar. — Sugar  is  present  in  the  blood  in  the  form  of  dextrose,  and  is 
now  regarded  as  a  normal  constituent.  The  amount  is  about  i  part  per 
thousand,  though  it  may  be  present  to  the  extent  of  15  parts  per  thousand. 
Beyond  this,  the  excess  soon  appears  in  the  urine. 

Extractives. — The  blood  contains  a  series  of  nitrogenized  bodies,  such 
as  urea,  uric  acid,  creatin,  creatinin,  xanthin,  etc.,  which  result  from  the 
katabolic  changes  in  nerve-  and  muscle-tissues  as  well  as  from  subsequent 
chemic  combinations  and  decompositions.  Though  constantly  absorbed 
from  the  tissues,  they  seldom  accumulate  beyond  a  small  amount,  since 
they  are  constantly  being  eliminated  from  the  blood  by  the  various  ex- 
cretory organs. 

Inorganic  Salts.— The  inorganic  salts  of  the  plasma  are  chiefly  sodium 
and  potassium  chlorids,  sulphates,  and  phosphates,  together  with  calcium 
and  magnesium  phosphates.  Of  the  salts,  sodium  chlorid  is  the  most 
abundant,  amounting  to  0.56  parts  per  hundred.  Calcium  phosphate  is 
present  in  small  quantity — 2  parts  per  1000.  This  salt  is  not  present  to 
the  same  extent  in  serum  for  the  reason  that  it  became  a  constituent  of 
fibrin  at  the  time  of  coagulation.  In  other  respects  serum  differs  but 
slightly  from  plasma  in  the  proportions  of  its  saline  constituents. 

HISTOLOGY  OF  THE  RED  CORPUSCLES  OR  ERYTHROCYTES. 

The  histologic  features  of  the  red  corpuscles  are  readily  observed  in  a 
drop  of  freshly  drawn  blood  when  examined  microscopically.  The  field  of 
the  microscope  will  be  seen  to  be  crowded  with  red  corpuscles  floating  in  a 
clear  transparent  fluid — the  plasma.  Here  and  there  will  also  be  seen  a  white 
corpuscle,  round  or  irregular  in  shape,  and  granular  in  appearance.  Within 
a  short  time  a  characteristic  phenomenon  takes  place:  viz.,  the  arranging  of 
the  corpuscles  in  the  form  of  columns  of  varying  lengths,  resembling  rolls  of 
coins.  These  rolls  interlace  with  each  other  at  all  angles  and  form  a  network 
in  the  meshes  of  which  lie  individual  red  and  white  corpuscles.  (See  Fig. 
97.)  The  cause  of  this  tendency  of  the  corpuscles  to  adhere  to  one  another 
is  not  definitely  known.  Since  it  does  not  take  place  in  circulating  blood,  and 
since  it  is  to  a  great  extent  prevented  by  defibrinating  the  blood,  it  has  been 
supposed  to  be  dependent  on  the  formation  of  some  adhesive  substance  con- 
nected with  the  formation  of  fibrin. 


THE  BLOOD. 


^35 


Color. — When  viewed  by  transmitted  light,  a  single  corpuscle  is  slightly 
yellow  or  greenish  in  color;  but  when  a  number  are  grouped  together,  the 
color  deepens  and  the  corpuscles  appear  red.  In  either  case  the  color  is  due 
to  the  presence  in  the  corpuscle  of  a  specific  coloring-matter,  hemoglobin. 

Shape. — The  red  corpuscle  is  a 
circular,  flattened  disk  with  rounded 
edges.  Each  surface  is  perfectly 
smooth  and  presents  a  shallow  de- 
pression in  its  center,  so  that  it  is 
also  biconcave.  A  longitudinal  sec- 
tion of  a  corpuscle  would  present, 
when  viewed  edgewise,  an  outline 
similar  to  that  of  Fig.  98.  This 
difference  in  the  thickness  of  the 
peripheral  and  central  poitions  of 
the  corpuscle  gives  rise  to  differences 
in  optical  appearances  when  ex- 
amined microscopically.  At  a  certain 
distance  of  the  object-glass  the  cor- 
puscle presents  in  its  peripheral  por- 
tion a  bright  rim,  and  in  its  central 
portion  a  dark  spot  If  the  objective 
be  brought  nearer  and  the  center 
accurately  focused,  the  reverse  ap- 


FiG.  97. — Corpuscles  from  Human 
Subject.  A  few  colorless  corpusck^s  are 
seen  among  ,the  colored  discs,  many  of 
which  are  arranged  in  rouleaux. — (Fitiike.) 


Fig.  98.— Ideal  Tra>:sverse 
Section  of  a  Human  Red  Corpus- 
cle. (Magnified  5000  times.)  a,  b. 
Diameter,     c,  d.  Thickness. 


pearance  obtains;  the  central  portion  becomes  bright  and  the  peripheral 
portion  dark.  The  cause  of  this  difference  in  optical  appearance  is  the 
unequal  distribution  of  the  transmitted  Hght  in  consequence  of  the  shape 

of  the  corpuscle. 

Size. — The  diameter  of  a  typical  well- 
developed  red  corpuscle  under  normal  con- 
ditions is  0.0075  rnni.,  its  greatest  thick- 
ness is  0.0019  mm.  Though  this  may  be 
assumed  as  the  average  diameter,  there 
is  a  small  percentage  of  distinctly  sriialler 
and  a  small  percentage  of  distinctly  larger 
corpuscles.  The  following  table  shows  the  results  of  measurement  made 
by  different  observers: 

Xormal  Limits.  Average  Diameter. 

Welcker diameter  o  .0045-0 .0095  mm o  .0070  mm. 

Hayem diameter  o .0060-0. 0088  mm o .0075  mm. 

Gram diameter  0.0067-0.0093  mm 0.0078  mm. 

Melassez o  .0076  mm. 

0.00747  (,./,M,  inch) 

Structure. — The  red  corpuscle  of  man  as  well  as  of  all  other  mammals 
possesses  neither  a  nucleus  nor  a  limiting  membrane,  but  appears  to  consist 
of  a  homogeneous  substance  more  or  less  semisolid  in  consistence.  Under 
the  influence  of  certain  reagents  the  corpuscle  separates  into  two  distinct 
portions:  viz.,  a  colorless  protoplasmic  stroma  and  a  coloring-matter  which 
diffuses  into  the  surrounding  liquid.  The  presence  of  the  former  can  be 
demonstrated  by  the  addition  of  iodin,  which  imparts  to  it  a  faint  yellow  color. 


0 


236  TEXT-BOOK  OF  PHYSIOLOGY. 

The  stroma  is  elastic,  and  determines  not  only  the  shape  of  the  corpuscle  but 
gives  to  it  the  properties  of  extensibility  and  retractility. 

The  foregoing  is  the  classic  and  generally  accepted  view  as  to  the  shape, 

size,  and  structure  of  the  red  corpuscle.     Nevertheless  recent  investigations 

render  it  probable  that  the  statements  were  based  on  observations  of  the 

corpuscles  under  artificial  rather  than  natural  conditions,  and  therefore  not 

<--.  ov  strictly  true.     For  many  years 

-o        j  ")  j         '  C\        ty  histologists  from  time  to  time 

i;i>5      ^^CD    ^  Cx?i       (T)  have  stated  that  the  red  cor- 

^^^^  '  puscle  is  not  circular  and  bi- 

f^^^    \\/         V  concave  in  shape,  in  the  cir- 

^-'''  -^.^  culating     blood,     but     bell- 

"7  shaped,  similar  to  that  shown 

in  Fig.  99.     It  was  not  until 
CN     vO  1902,  after  the  publication  of 

Cq)    Q)  ^  Weidenreich's     investigations 

X?,    ^  K^  \^  that  this  view  began  to  receive 

r^'    w  more     attention     than     had 

^9  C^  hitherto     been     accorded    it. 

c.  d.  Weidenreich   preserved   in   a 

Fig.  99.— The  Shape  of  the  Red  Corpuscle      moist  chamber  a  hanging  drop 
IX    Different     Mammals.    {Weidenreich.)    a.   Man.      of  human    blood,  and  on  ex- 
og-    c.     ig.      .     a   3it.  amination  found  that  the  red 

corpuscles  were  bell-shaped  though  the  depth  of  the  bell  cavity  varied 
considerably.  An  examination  of  the  capillary  circulation  in  the  omentum 
of  the  rabbit  revealed  the  fact  that  the  corpuscles  in  their  natural  medium 
were  also  bell-shaped.  The  circular  biconcave  shape  they  ordinarily  pre- 
sent when  a  drop  of  blood  is  examined  microscopically  he  attributes  to  cool- 
ing, evaporation  and  concentration  of  the  drawn  blood.  Experimentally 
it  was  shown  that  when  blood  was  added  to  0.6  to  0.65  per  cent,  solution 
of  sodium  chlorid  all  the  corpuscles  were  bell  shaped;  but  if  the  solution 
was  increased  or  decreased  in  strength,  this  form  was  at  once  changed. 

The  dimensions  of  the  bell-shaped  cell  according  to  Weidenreich  are  as 
follows : — 

Greatest  diameter 7      microns  o  .007    mm. 

Diameter  of  cavity 3      microns  c  .003    mm. 

Height  of  bell 4      microns  o .  004    mm. 

Height  of  cavity 2.5  microns  0.0025  mm. 

Thickness  of  wall  at  apex 2      microns  o  .002    mm. 

Thickness  of  wall  at  base 1.5  microns  o .0015  mm. 

The  foregoing  observations  have  been  confirmed  by  many  subsequent 
investigators.  Thus  Lewis  states  that  if  a  drop  of  blood  is  placed  immediately 
on  a  warm  slide  and  examined,  the  corpuscles  exhibit  the  bell  shape,  but  as 
the  slide  cools  they  gradually  become  biconcave  disks  of  the  conventional 
form.  He  also  observed  that  the  corpuscles  in  the  capillary  blood-vessels 
of  the  omentum  of  the  guinea-pig  were  bell-shaped  and  presenting  an 
appearance  similar  to  that  shown  in  Fig.  loo.  Radasch  found  on  examina- 
tion of  fetal  tissues  such  as  the  spleen,  kidney,  liver,  placenta,  etc.,  that  the 
great  majority  of  the  corpuscles  in  all  situations  presented  the  bell  shape 
rather  than  the  circular  biconcave  shape.     This  observer  is  of  the  opinion 


THE  BLOOD. 


237 


that  the  bell  shape  can  not  be  due  to  the  action  of  the  fixatives  employed  in 
the  preparation  of  the  tissues. 

The  structure  of  the  corpuscle,  according  to  Weidenreich,  differs  also 
from  that  usually  stated.  He  asserts  that  the  corpuscle  is  surrounded  by  a 
structureless,  colorless  membrane  enclosing  a  colored  but  not  nucleated 
semi-fluid  mass,  which  consists  chemically  of  protein  material,  lecithin, 
cholesterin,  inorganic  salts  and  hemoglo- 
bin. There  is  no  evidence  of  the  existence 
of  a  stroma  in  the  adult  state. 

Number  of  Red  Corpuscles.^ — In  any 
given  specimen  of  blood  the  corpuscles  are 
so  numerous  and  the  spaces  between  them 
so  small  that  it  seems  almost  impossible  to 
determine  their  number.  This,  however, 
has  been  accomplished  for  a  cubic  milli- 
meter of  blood  by  various  observers  em- 
ploying different  methods  with  compara- 
tively uniform  results.  The  average  normal 
number  of  corpuscles  in  one  cubic  milli- 
meter of  blood  is,  for  men,  5,000,000;  and  for  women,  4,500,000.  This  value, 
however,  will  vary  within  slight  limits,  with  variations  in  the  activity  of  physio- 
logic processes  and  to  a  large  extent  at  times  in  pathologic  states  of  the  blood 
or  body.  The  number  is  increased  in  the  cutaneous  veins  by  all  influences 
which  cause  a  diminution  in  the  quantity  of  water  in  the  blood — e.g.,  co- 
pious sweating,  acute  watery  diarrhea,  fasting,  abstinence  from  liquids;  the 
number  is  diminished  by  influences  which  dilute  the  blood — e.g. ,  the  ingestion 
of  liquids,  the  absorption  of  fluids  from  the  tissue  spaces,  etc.  But  it  is  well 
to  remember  that  these  influences  which  produce  changes  in  the  number  of 
corpuscles  per  cubic  millimeter  do  not  necessarily  produce  corresponding 
changes  in  the  total  number  of  red  corpuscles  in  the  body.  In  women 
lactation,  menstruation,  and  the  act  of  parturition  diminish  the  number. 
High  altitudes  apparently  increase  the  number  of  corpuscles,  as  shown  by 
their  increase  in  the  blood  of  the  peripheral  vessels.  Whether  this  is  an 
indication  that  there  is  a  corresponding  increase  of  the  total  number  in  the 
general  volume  of  the  blood  is  uncertain.  The  following  table  will  show  the 
increase  in  the  count  per  cubic  millimeter  at  different  altitudes : 


Fig.  100. —  Red  Corpusclk s 
Sketched  while  Circulating  in 
THE  Vessels  of  the  Omentum  of  a 
Guinea-pig.  {F.  T.  Lewis  in  Stohr's 
Histology.) 


Place 


Height  Above  Sea-level 


Christiania 

GottingeiT 

Tubingen 

Zurich 

Auerbach 

Reibaldsgriin. . . 

Arosa 

The  Cordilleras. 


o  meter 
148  meters 
314  meters 
414  meters 
425  meters 
700  meters 
1800  meters 
4392  meters 


Red  CeUs 

Author 

4,974,000 

Laache. 

5,225,000 

Schaper. 

5,322,000 

Reinert. 

5,752,000 

Steirlin. 

5,748,000 

Koppe. 

5,900,000 

Koppe. 

7,000,000 

Egger. 

8,000,000 

Viault. 

(Koppe.) 

This  increase  in  the  number  of  corpuscles  takes  place,  according  to 

Viault's  observations,  within  two  or  three  weeks,  and  is  apparently  not  con- 


238 


TEXT-BOOK  OF  PHYSIOLOGY. 


nected  with  either  diet  or  mode  of  life,  but  rather  with  diminished  atmos- 
pheric, if  not  oxygen,  pressure.  On  returning  to  sea-level  there  is  a  gradual 
reduction,  without  any  apparent  destruction  of  the  corpuscles,  to  their  normal 
number.     The  reason  for  these  variations  is  not  clear. 

The  method  of  counting  corpuscles  introduced  by  Vierordt  and  Welcker 
has  been  modified  by  different  observers,  and  especially  by  Thoma.  On 
account  of  the  great  number  of  corpuscles  in  i  cubic  millimeter  of  blood,  it 
becomes  necessary  for  purposes  of  enumeration  that  the  blood  be  diluted  a 
definite  number  of  times  and  that  the  diluted  mixture  be  placed  in  a  counting 
chamber  possessing  a  definite  capacity.  By  means  of  the  pipette  or  melang- 
eur  of  Potain  and  the  counting  chamber  of  Thoma  both  these  objects  are 
attained. 

The  pipette  consists  of  a  capillary  tube  (Fig.  loi)  provided  with  an  enlarge- 
ment containing  a  freely  movable  small  glass  ball,  E.  One  end  of  the  tube,  S, 
is  pointed,  while  to  the  other  end  is  attached  a  rubber  tube,  G,  for  the  purpose 
of  faciUtating  the  introduction  of  the  blood  and  the  diluting  fluid.  The  capillary 
tube,  which  is  accurately  calibrated,  carries  marks,  0.5,  i,  loi,  which  signify  that 


Fig. 


loi. — Hemocytometer.     a,  Surface;  b,  section  view;  c,  squares  on  the  surface  of  B  magnified- 
^1/,  G,  S,  mouth  piece,  rubber  tube  and  pipette. 


if  the  tube  be  filled  with  blood  up  to  the  mark  i  and  the  diluting  fluid  be 
sucked  into  the  tube  up  to  the  mark  10 1,  the  blood  will  be  diluted  100  times. 
If  the  blood  be  sucked  up  to  the  mark  0.5  and  the  diluting  fluid  to  loi,  then  the 
blood  will  be  dfluted  200  times.  In  using  the  pipette  the  point  is  introduced 
into  a  drop  of  blood  derived  from  a  small  wound  in  the  skin  of  the  lobe  of  the 
ear  or  finger  and  sucked  into  the  tube  by  introducing  the  end,  M,  of  the  rubber 
tube  into  the  mouth.  The  tube  is  then  quickly  inserted  into  a  solution  which 
will  preserve  the  shape  and  size  of  the  corpuscles,  such  as  Gowers's  sodium  sulphate 
solution,  sp.  gr.  1.025,  or  a  3  per  cent,  sodium  chlorid  solution,^  and  the  fluid 

'  Various  solutions  have  been  devised  for  diluting  blood,  any  one  of  which  may  be  employed,  e.g.: 
Hayem's  Fluid:  Toisson's  Fluid: 


Hydrarg.bichlor. 0.5  gm. 

Sodii  sulphat 5  .0  gm. 

Sodii  chlorid 2.0  gm. 

Aquse  destillat 200.0  gm. 


Aquae  destillat 160.00  parts. 

Glycerinae 30 .00  parts 

Sodii  sulphat 8.00  parts. 

Sodii  chlorid, i  .00  part. 

Methyl- violet, 0-025  part. 


Gower's  Fluid: 

Sodii  sulphat gr.  104 

Acid,  acetic 5j 

Aquae  dest q.  s.  ad   3iv. 


THE  BLOOD. 


239 


This  group  is  separated  from 


!^  ^T  ^^^  T7^  ^  .°  ^  1  °  o  T*"T  "T~" 


sucked  into  the  tube  up  to  the  mark  loi .     On  shaking  the  pipette  for  a  few  minutes, 
the  admixture  will  take  place,  aided  by  the  movements  of  the  glass  ball. 

Fig.  loi  shows  both  a  surface  view,  a,  and  a  section  view,  b,  of  the  counting 
chamber.  This  consists  of  an  oblong  glass  plate,  o,  on  which  are  cemented  two 
small  pieces  of  glass,  one  of  which,  WD,  has  in  the  center  a  circular  opening 
in  which  is  placed  the  other,  B,  a  circular  disc  or  stage.  Their  relation  is  such 
that  a  narrow  groove  or  moat  separates  the  one  from  the  other,  the  floor  of  which 
is  formed  by  the  glass  plate.  The  surface  of  the  circular  stage  is  exactly  o.i  mm. 
lower  than  that  of  the  cover-glass,  r.  On  the  surface  of  the  glass  stage  a  series 
of  small  squares  is  engraved,  C,  each  one  of  which  has  a  side  length  of  -j*,,  mm. 
and  an  area  of  ^^^y  square  mm.  To  facilitate  counting,  a  group  of  16  such 
squares  is  surrounded  by  a  thick  line.  Fig.  iC2. 
adjoining  groups,  also  enclosed  by  thick  lines,  by 
an  intermediate  fine  line,  which  serves  as  a  guide 
in  passing  from  one  group  to  another.  When  a 
cover-glass  is  accurately  applied  to  the  glass,  b, 
each  one  of  the  small  squares  will  have  a  cubic 
capacity  of  ^^qXo.i,  or  4,700  cubic  millimeter. 

Before  placing  the  diluted  blood  on  the  count- 
ing stage,  the  fluid  in  the  tube  of  the  pipette 
should  be  blown  out  and  discarded,  as  it  contains 
no  portion  of  the  blood.  A  small  drop  is  then 
placed  on  the  glass  stage  and  covered  with  the  cover- 
glass.  After  a  few  minutes  the  corpuscles  settle 
upon  the  ruled  spaces  and  are  ready  for  counting. 
The  number  of  corpuscles  in  at  least  five  series 
of  sixteen  small  squares  is  then  counted;  this 
number  is  then  multiplied  by  the  degree  of  dilu- 
tion (100  or  200  as  the  case  may  be)  and  this  divided  by  the  cubic  contents 
of  each  small  square  (^o^o  0) '  the  product  is  then  divided  by  the  number  of  squares 
counted  (80  in  the  instance  given  above):  e.g.,  five  series  of  sixteen  small  squares 
contain  500  corpuscles 

soo  X200  X4000  .1  i 

~^-^  =5,000,000  erythrocytes  per  c.mm.. 

The  accuracy  of  the  counting  is  proportional  to  the  number  of  squares  counted. 
If  200  squares  are  counted  with  each  of  two  different  drops,  and  the  average  taken 
the  probable  limit  of  error  will  be  less  than  2  per  cent. 

The  Effects  of  Reagents  on  the  Red  Corpuscles. — Within  the  blood- 
vessels the  physical  conditions  and  chemic  composition  of  the  plasma  are 
such  that  both  the  form,  and  the  composition  of  the  corpuscle  or  the  relation 
of  the  hemoglobin  to  the  stroma,  are  maintained  in  the  normal  or  physiologic 
condition.  The  plasma  is  preservative  of  the  structure  and  function  of  the 
corpuscle.  The  reason  assigned  for  this  is  that  the  osmotic  pressure  of  the 
salts  in  the  plasma  and  of  the  salts  in  the  corpuscle  exactly  balance  one 
another  so  that  there  is  neither  an  absorption  of  water  from,  nor  a  yielding 
of  water  to,  the  plasma  on  the  part  of  the  corpuscle.  The  plasma  having 
an  osmotic  pressure  equal  to  that  within  the  corpuscle  is  said  to  be  isotonic 
with  it. 

When  blood  is  to  be  prepared  for  microscopic  examination  with  a  view  of 
determining  the  histologic  features  of  the  corpuscles  or  for  purposes  of 
enumeration,  it  must  be  diluted,  and  unless  special  precautions  are 
observed  the  condition  of  equal  osmotic  pressure  will  be  disturbed  by  the 


Fig.     102. — Microscopic     i^p- 

PEARANCE  OF  THE  SMALL  SQUARES 

AND    THE    Distribution   or    the 
Corpuscles. 


240  TEXT-BOOK  OF  PHYSIOLOGY. 

diluting  agent  and  the  corpuscles  will  lose  their  characteristic  form  and 
structure  from  either  an  absorption  or  loss  of  water. 

If  distilled  water  is  employed  for  this  purpose,  the  osmotic  pressure  of  the 
plasma  is  of  course  diminished,  and  in  consequence  the  osmotic  pressure  of 
the  inorganic  constituents  of  the  corpuscles  (particularly  potassium  phos- 
phate) causes  an  inflow  of  water.  The  corpuscle  therefore  swells  and 
assumes  a  more  or  less  spheric  form;  the  hemoglobin  is  dissociated  and 
discharged  into  the  surrounding  fluid  throughout  which  it  diffuses.  Such 
an  environment  having  an  osmotic  pressure  less  than  that  of  the  corpuscle  is 
said  to  be  hypotonic,  hypiso  tonic,  or  hypo-i  so  tonic  to  it. 

If  on  the  contrary,  water  containing  inorganic  salts  (particularly  sodium 
chlorid)  is  added  in  amounts  which  impart  to  the  plasma  an  osmotic  pressure 
greater  than  that  within  the  corpuscle,  there  will  be  an  outflow  of  water 
from  the  corpuscle,  a  shrinkage  of  the  volume  and  a  crenation  of  its  surface. 
Such  an  environment  having  an  osmotic  pressure  greater  than  that  of  the 
corpuscle  is  said  to  be  hypertonic,  or  hyperisotonic  to  it.  It  is  essential 
therefore  in  diluting  the  plasma  with  water,  that  the  latter  contains  inorganic 
salts  in  such  amounts  that  the  resulting  mixture  (plasma  and  water)  possesses 
an  osmotic  pressure  equal  to  that  of  the  original  plasma  or  to  that  of  the  cor- 
puscle. A  diluting  agent  well  adapted  for  this  purpose  is  the  well-known 
Ringer's  mixture.  Other  solutions  which  preserve  the  form  of  the  corpuscles 
during  the  time  required  for  their  enumeration  are  the  solutions  devised  by 
Hayem.Toisson,  and  Gowers  alluded  to  on  a  preceding  page.  Because  of  the 
fact  that  sodium  chlorid  is  the  chief  inorganic  constituent  of  the  plasma  it  is 
common  in  laboratory  work  to  dilute  the  plasma  of  mammalian  blood  and 
of  frog's  blood  with  solutions  of  sodium  chlorid  of  o.g  per  cent,  and  0.6  per 
cent,  respectively,  which  though  not  absolutely  are  sufficiently  isotonic  for 
the  purpose  desired. 

Many  other  saline  solutions  with  an  osmotic  pressure  greater  or  less  than 
normal  plasma,  dilute  solutions  of  acids  and  alkalies,  bile  salts,  chloroform, 
ether,  ammonium  sulphocyanid,  electricity,  etc.,  also  destroy  the  physical 
and  chemic  integrity  of  the  corpuscle  and  cause  the  hemoglobin  to  separate 
from  the  stroma  and  diffuse  into  the  plasma  without  itself  undergoing  any 
appreciable  change  in  composition.  With  the  escape  and  dii^usion  of  the 
hemoglobin  the  blood  becomes  transparent  and  changes  to  a  dark  red  color 
to  which  the  term  "laky"  has  been  given.  The  mechanism  by  which 
the  hemoglobin  becomes  dissociated  and  discharged  from  the  corpuscle  by 
these  agents  is  unknown.  The  disintegration  of  the  corpuscle  and  the 
diffusion  of  the  hemoglobin  into  and  its  solution  by  the  surrounding  medium 
is  termed  hemolysis  and  the  agents  by  which  it  is  produced  are  termed 
hemolytic  agents. 

The  Corpuscles  of  Other  Vertebrated  Animals. — In  all  mammals, 
with  the  exception  of  the  camel,  llama,  and  dromedary,  the  red  corpuscles 
present  the  same  shape  and  structure  as  the  corpuscles  of  man,  and  may  be 
described  as  circular,  flattened,  biconcave  disks.  In  the  animals  excepted 
the  corpuscles  are  oval.  The  size,  however,  varies  in  different  animals  from 
0.0092  mm.  (2T43-  inch)  in  the  elephant  to  0.0023  ^™-  (72^2^  inch)  in  the 
musk-deer,  while  in  most  animals  the  average  lies  between  0.0084  ^ini-  ^^^d 
0.0050  mm.     Inasmuch  as  the  question  may  arise  as  to  whether  the  corpus- 


THE  BLOOD. 


241 


cles  of  any  given  specimen  of  blood  are  those  of  a  human  being  or  of  some 
other  mammal,  a  knowledge  of  the  size  of  the  corpuscles  is  a  matter  of 
medicolegal  as  well  as  of  physiologic  interest.  Though  the  differences  in 
size  are  slight,  yet  it  is  possible  for  skilled  microscopists,  when  examining  fresh 
blood,  to  make  a  diagnosis  between  the  corpuscles  of  man  and  those  of  the 
domesticated  animals,  with  the  exception,  perhaps,  of  the  guinea-pig.  The 
diagnosis  of  the  corpuscles  of  dried  blood  which  have  been  altered  by  the 
action  of  various  external  agents,  even  though  capable  of  a  certain  degree 
of  restoration,  is  most  difficult,  and  should  not  be  attempted  in  criminal 
cases  without  large  experience  in  microscopy,  in  measurements  and  methods 
of  preparation  of  all  kinds  of  blood-corpuscles,  and  a  proper  conception  of 
corpuscular  forms  and  sizes.  In  the  following  table  the  average  results  of 
the  measurements  of  the  corpuscles  in  different  classes  of  animals  are  given 
(abstracted  from  Formad's  compilation) : 


Man 

Guinea  Pig. 
Dog. 


Gulliver 


Wormley 


C.  Schmidt 
Mallinin 


French  Medi- 
colegal Soc. 
Welcker 


Formad 


Inch 


Mm.      Inch      Mni.  ;    Inch 


Mm. 


Inch       Mm.      Inch 


Mm. 


1. 3200  0.0079  1. 3250  0.0078  I 

1.3538  0.0071  1.3223  0.0079  I 

•3532  0.0071  I. 3561  0.0071  I 

Rabbit ii. 3607  0.0070  i. 3653  0.0070  i 

Ox !i. 42670. 0060  1. 42190. 0060  1 

Pig 1.42300.0060  1.42680.0059  I 

Horse i  .4600  0.0057  i  .4243  0.0059  i 

Cat ji.44oe  0.0058  1. 4372  0.0058  I 

Sheep 1 1 .5300  0.0048  1 .4912  0.0031  I 

Goat 1 .6366  0.0040  1 .6189  0.0041  I 


3300 
3300* 
3(136 
3968 

43S4 
4098 
4464 
4545 
5649 
6369 


.0077  I 
.0077  I 
.0070  I 
.0064  I 
.0058  I 
.0062  I 
.0057  I 
. 0056  I , 
.0045  I, 
. 004O  I , 


32S7  o 

32i3t  o 

3485  o 

3653  o 

4545  o 

4098  o 

454S  o 

3922  o 

5076  o 

5525  !o 


.0078  I 
.0079  I 
.0073  I 
,0069  I 
,0056  I 
.0062  I, 
.0056  I 
,0065  . 
.0059,1, 
.0046  I , 


3200  0.0079 
3400  0.0075 
3580  0.0071 
3662  0.0069 
4200  0.0060 
4250  0.0060 
4310  o. 0059 

5000  o .0051 
6100  0.0042 


In  birds,  reptiles,  and  amphibians  the  corpuscles  are  larger  than  in 
mammals,  are  oval  in  shape,  and  nucleated.  (See  Figs.  103  and  104.) 
As  the  scale  of  animal  life  is  descended  the  corpuscles  increase  in  size,  until 
in  Proteus  and  Amphiuma  the  long  diameter  at- 
tains an  average  length  of  0.058  mm.  and  0.077  ^im. 
respectively.  In  fish  the  corpuscles  are  smaller, 
oval,  and  nucleated,  with  the  exception  of  the 
lamprey  eels,  in  which  they  are  circular,  biconcave, 
and  nucleated,  though  the  nucleus  is  generally  con- 
cealed in  the  peripheral  portion  of  the  corpuscle. 
As  in  these  animals  the  corpuscles  are  almost  twice 
the  size  of  the  human  red  corpuscles,  they  can.  not- 
withstanding the  similarity  of  shape,  be  readily  dis- 
tinguished from  them. 

The  Function  of  the  Red  Corpuscles. — The 
red  corpuscles,  by  virtue  of  the  capacity  of  their 
contained  hemoglobin  for  oxygen  absorption,  maybe 
regarded  as  carriers  of  oxygen  from  the  lungs  to  the  tissues,  and  therefore 
important  factors  in  the  general  respiratory  process.  The  size  as  well  as  the 
number  of  the  corpuscles  in  diiJerent  classes  of  animals  appears  to  be  directly 
related  to  the  activity  of  the  respiratory  process.  In  those  animals  in  which 
the  corpuscles  are  small  and  numerous  and  the  total  superficial  area  large, 


Fig.  103.  Fig.  104. 
Amphibian  Colored 
Blood-corpuscles.  Fig. 
103,  on  the  flat;  Fig.  104, 
on  edge.  —  {Landois  and 
Stirling.) 


*  Masson. 
16 


t  Woodward. 


242  TEXT-BOOK  OF  PHYSIOLOGY. 

respiration  is  active,  the  quantity  of  oxygen  absorbed  is  large,  and  the  energy 
evolved  through  oxidation  great.  In  those  animals,  on  the  contrary,  in 
which  the  corpuscles  are  large  and  relatively  few  in  number,  the  reverse 
conditions  obtain.  This  is  in  accordance  with  the  fact  that  the  superficial 
area  of  any  given  volume  of  substance  is  increased  in  proportion  to  the 
extent  to  which  it  is  subdivided. 

The  superficial  area  of  a  single  human  red  corpuscle  has  been  estimated 
at  0.000128  sq.  mm.  If  the  number  of  corpuscles  in  i  cubic  millimeter  of 
blood  averages  5,000,000,  the  superficial  area  would  amount  to  640  s([uare 
millimeters;  and  if  the  amount  of  blood  in  the  body  of  a  man  weighing  70 
kilos  is  taken  as  one-nineteenth  of  this  weight — that  is,  3864  grams  (3659  c.c.) 
— the  total  area  of  the  corpuscular  surface  will  amount  to  2341  square  meters. 

Life-history  of  Red  Corpuscles. — In  the  performance  of  their  functions 
the  red  corpuscles  undergo  more  or  less  disintegration  and  finally  destruction; 
but  as  the  average  number  is  maintained  under  normal  physiologic  condi- 
tions, there  must  be  a  constant  renewal  of  corpuscles  from  day  to  day.  The 
evidence  of  destruction  of  red  corpuscles  is  furnished  by  the  presence  in  the 
blood,  in  various  situations  of  the  body,  of  a  pigment  containing  iron  and  the 
presence  of  pigments  in  the  bile  and  urine,  all  of  which  are  believed  to  be 
derivatives  of  effete  hemoglobin.  The  blood-pigment  (hematin),  which 
contains  the  iron  of  the  hemoglobin,  is  found  in  the  capillaries  of  the  liver, 
in  the  cells  of  the  splenic  pulp,  and  in  the  marrow  of  the  bones.  Whether 
the  presence  of  the  pigment  in  these  organs  is  a  proof  that  the  corpuscles  are 
destroyed  here,  or  whether  they  are  to  be  regarded  merely  as  agents  con- 
cerned in  the  further  reduction  and  elimination  of  the  hematin,  is  uncertain. 
The  genetic  relationship  between  bile-pigment  and  hemoglobin  is  shown  by 
the  fact  that  any  artificial  destruction  of  hemoglobin  or  its  injection  into  the 
blood  is  attended  by  an  increase  in  the  quantity  of  bile-pigment  eliminated. 
It  appears  also  from  chemic  considerations  that  the  hemoglobin  will  undergo 
cleavage  into  a  globulin  body  and  hematin,  which  by  the  loss  of  its  iron  is 
readily  converted  into  the  bile-pigment,  bilirubin.  The  amount  of  this 
latter  pigment  may  therefore  be  taken  as  an  index  of  the  extent  of  corpuscular 
destruction. 

This  gradual  decay  of  corpuscles  as  well  as  the  losses  occasioned  by 
hemorrhages  necessitate  a  continuous  formation  of  new  corpuscles,  so  that 
the  normal  number  may  be  maintained.  The  rapidity  with  which  corpuscles 
may  be  renewed,  in  the  woman  at  least,  is  shown  by  a  computation  of  Mr. 
Charles  L.  Mix.  A  woman  loses  during  a  menstral  period  150  c.c.  of 
blood.  At  the  end  of  twenty-eight  or  thirty  days  this  volume  is  restored,  so 
that  in  one  day  5  c.c,  or  5000  c.mm.,  of  blood  must  be  formed,  or  208 
c.mm.  per  hour  and  3^  c.mm.  per  minute.  That  is,  during  a  certain  num- 
ber of  years  15,750,000  corpuscles  must  be  formed  every  minute,  and  this 
independent  of  the  daily  loss  due  to  functional  activity. 

At  the  present  time  there  is  a  general  agreement  among  histologists  that 
in  adult  life  the  red  corpuscles  are  derived  from  embryonic  forms,  the  so- 
called  erythroblasts,  cells  of  a  large  size  with  distinctly  reticulated  nuclei, 
which  are  found  chiefly  in  the  red  marrow  of  the  long  bones.  ^     In  this 

'  For  an  admirable  resume  of  the  various  views  regarding  the  origin  and  formation  of  red 
corpuscles  see  the  paper  of  Mr.  Charles  L.  Mix,  Boston  Med.  and  Surg.  Journal,  1892,  Nos. 
II  and  12;  also  paper  by  Prof.  W.  H.  Howell,  Journal  of  Morphology,  vol.  iv,  1892. 


THE  BLOOD. 


243 


situation  both  arterial  and  venous  capillaries  are  relatively  large  and  the 
blood  is  separated  from  the  surrounding  marrow  by  extremely  thin  walls. 
In  the  passages  of  this  capillary  network  the  erythroblasts  make  their  appear- 
ance most  probably  by  a  transformation  of  pre-existing  marrow  cells.  At 
first  they  are  large,  homogeneous,  colorless,  perhaps  slightly  tinged  with 
hemoglobin  and  distinctly  nucleated.  They  increase  in  number  by  kary- 
okinesis  and  at  the  same  time  increase  in  their  hemoglobin  content.  In  the 
course  of  their  development  the  nucleus  becomes  smaller  and  denser,  when 
the  cells  are  known  as  normoblasts.  Subsec[uently  the  nucleus  is  extruded, 
carrying  with  it  a  portion  of  the  perinuclear  cytoplasm,  after  which  the 
remainder  of  the  corpuscle  assumes  the  shape  and  size  of  the  adult  corpuscle 
and  is  carried  out  into  the  general  circulation.  After  severe  hemorrhage 
the  formative  processes  in  the  marrow  may  become  so  active  that  erythro- 
blasts and  normoblasts  make  their  appearance  in  the  blood-stream  before 
the  extrusion  of  the  nucleus  has  taken  place. 

CHEMIC  COMPOSITION  OF  RED  CORPUSCLES. 


When  analyzed  chemically  the  red  corpuscles  are  found  to  consist  of 
water  65  per  cent,  and  solid  matter  35  per  cent.  The  solids,  moreover,  have 
been  found  to  consist  of  a  pigment  hemo- 
globin 7,2,,  protein  0.9,  cholesterin  and 
lecithin  0.46,  and  inorganic  salts  (chiefly 
potassium  phosphate  and  chlorid  and 
sodium  chlorid)  1.4  per  cent,  respectively. 
Of  the  total  solids  the  hemoglobin  con- 
stitutes about  94  per  cent. 

Hemoglobin. — In  the  normal  condi- 
tion of  the  corpuscle  the  hemoglobin  is  in 
an   amorphous    condition   and  is    com 
bined   in   some  unknown  way  with  the 
stroma. 

When  hemoglobin  is  decomposed  in 
the  absence  of  oxygen  it  undergoes  a 
cleavage  into  a  protein,  globin,  and  an 
iron-holding  pigment  hemochromogen 
w^hich  constitutes  about  4  per  cent,  of 
the  molecule.  If  a  solution  of  hemo- 
chromogen be  exposed  to  air  it  absorbs 
oxygen  and  is  converted  into  hematin. 
This  latter  compound  can  also  be  de- 
rived directly  from  hemoglobin  by  the 
action  of  acids  and  alkalies.  It  is  to  the 
presence  of  hemochromogen  in  combina- 
tion with  the  protein  globin  that  the 
hemoglobin  is  indebted  for  its  power  of 
absorbing  and  carrying  oxygen. 

If  blood  which  has  been  rendered  laky,  by  water  or  any  other  of  the 
known  agencies,  be  allowed  to  evaporate  slowly,  the  dissolved  hemoglobin 


Fig.  105. — Crystallized  Hemo- 
globin, a,  b.  Cnstals  from  venous 
blood  of  man.  c.  From  blood  of  cat. 
d.  Of  guinea-pig.  e.  Of  marmot.  /. 
Of  squirrel. — {Gaitllcr). 


244 


TEXT-BOOK  OF  PHYSIOLOGY. 


undergoes  crystallization.  The  rapidity  with  which  the  crystals  form  varies 
in  the  blood  of  different  animals  under  similar  conditions.  According  to  the 
ease  with  which  crystallization  takes  place,  Preyer  has  classified  various 
animals  as  follows:  (i)  Very  difficult— calf,  pigeon,  pig,  frog;  (2)  difficult- 
man,  monkey,  rabbit,  sheep;  (3)  easy— cat,  dog,  mouse,  horse;  (4)  very  easy 
— guinea-pig,  rat. 

The  hemoglobin  crystals  vary  in  shape  according  to  the  blood  from 
which  they  are  obtained  (Fig.  105).  Those  obtained  from  the  guinea-pig 
are  tetrahedral;  those  from  man  and  most  mammals  are  prismatic  rhombs; 
those  from  the  squirrel  are  in  the  form  of  hexagonal  plates.  Notwith- 
standing these  slight  differences,  all  forms  belong  to  the  same  crystal  system, 
with  the  exception  of  those  from  the  squirrel. 

A  simple  but  very  effective  method  of  obtaining  blood-crystals  suggested 
by  Reichert  is  to  lake  defibrinated  blood,  especially  that  of  the  dog,  rat, 
guinea-pig,  and  horse,  with  acetic  or  ethylic  ether  and  then  add  a  solution, 
I  to  5  per  cent.,  of  ammonium  oxalate.  A  drop  of  this  mixture  placed  under 
the  microscope  will  show  crystal  formation  in  a  very  few  minutes. 

Chemic  Composition  of  Hemoglobin. — By  appropriate  methods 
hemoglobin  can  be  obtained  in  a  practically  pure  form,  and  when  subjected 
to  a  temperature  of  100°  C.  its  water  of  crystallization  is  driven  off,  after 
which  it  can  be  analyzed.  In  the  subjoined  table  the  results  of  several 
analyses  are  given  for  100  parts  of  hemoglobin. 


Dog 


C. 
O.. 
H.. 

N.. 
S.. 
Fe. 


53-91 

22  .62 

6.62 

15.98 

0.54 

0-33 

Jaquet. 


Horse 


51 

15 

23 

43 

6 

76 

17 

94 

0 

39 

0 

33 

Zinoffskv 

Dog 


Guinea-pig 


53-85 

54-12 

21.84 

20.68 

7-32 

7-36 

16.17 

16.78 

0-39 

0.58 

043 

0.48 

Hoppe- 

Sey 

ler. 

The  elementary  composition  of  hemoglobin  is  thus  seen  to  vary  slightly 
in  different  animals,  suggesting  that  there  may  be  different  kinds  of  hemo- 
globin. The  rational  molecular  formula  is  not  known.  On  the  assumption 
that  each  molecule  contains  one  atom  of  iron,  Preyer  suggested  the  following 
empirical  formula:  C6ooH98oNi5^0i79S3Fe,  with  a  molecular  weight  of 
^3^33^)  Jaquet  has  suggested  a  different  formula:  C75gHj203Nj95O2i8S3Fe, 
with  a  molecular  weight  of  16,669.  I^  ^^  very  evident  from  this  that  the 
molecule  is  of  enormous  size  and  exceedingly  complex. 

Quantity  of  Hemoglobin. — ^The  quantity  of  hemoglobin  in  blood  as 
determined  by  chemic,  chromometric,  and  spectro-photometric  methods 
amounts  to  about  14  per  cent,  in  man  and  13  per  cent,  in  woman.  Of  the 
chemic  methods,  that  based  on  the  amount  of  iron  is  the  one  generally 
employed.  Chemic  analysis  has  shown  that  hemoglobin  contains  0.43  per 
cent,  and  blood  0.056  per  cent,  of  iron;  with  these  two  factors  the  quantity 
of  hemoglobin  can  be  determined  by  the  following  formula:  x  =  ^^-^^=- 
13.33  P^r  cent.     The  total  quantity  of  hemoglobin  in  the  blood,  assuming 


THE  BLOOD.  245 

the  latter  to  be  about  3684  grams  (one-nineteenth  of  the  body-weight,  70 
kilos)  will  therefore  amount  to  491  grams;  e.g.,  x  =  ^^^^^^^  =4gj.  The 
total  amount  of  iron  in  the  blood  is  obtained  by  the  following  formula: 
viz.,  .v  =  ^^^-^^°°s6=  2.06  grams. 

'  lOO  " 

Clinic  Methods  for  the  Determination  of  the  Percentage  of  Hemo- 
globin.— Under  normal  physiologic  conditions  the  percentage  of  hemo- 
globin undergoes  but  slight  variation.  In  pathologic  states  there  is  fre- 
quently a  great  diminution  in  the  amount,  especially  in  chlorosis,  splenic 
leukemia,  and  pernicious  anemia,  diseases  in  which  it  diminishes  to  a  con- 
siderable per  cent,  in  many  instances.  For  clinic  purposes  it  becomes  a 
matter  of  importance  to  have  some  method  by  which  the  diminution  of 
hemoglobin  can  be  determined.  In  the  various  methods  employed  the 
normal  amount  of  hemoglobin  is  considered  as  100  per  cent,  and  the  normal 
number  of  red  corpuscles,  5,000.000  per  cubic  millimeter,  is  also  considered 
as  ICO  per  cent.  Under  such  conditions  the  corpuscles  have  a  normal  color 
known  as  the  color  inde.x.  This  is  expressed  by  a  fraction  of  which  the 
percentage  of  hemoglobin  is  the  numerator  and  the  percentage  of  corpuscles 
the  denominator.  The  normal  color  index  is  therefore  i  or  unity.  In 
some  pathologic  states  the  hemoglobin  alone  diminishes,  the  number  of  the 
corpuscles  remaining  the  same;  in  this  instance  the  color  index  is  less  than 
unity,  e.g.,  if  the  hemoglobin  be  reduced  to  80  per  cent.,  as  determined  by  the 
method  to  be  described,  then  the  color  index  will  be  y^'^^  =0.8  which  indicates 
that  each  corpuscle  retains  but  eight-tenths  of  the  normal  amoimt  of  hemo- 
globin, or  stated  in  the  reverse  way,  each  corpuscle  has  lost  two-tenths  of  the 
normal  amount  of  its  hemoglobin.  In  other  pathologic  states  there  is  both 
a  diminution  in  the  percentage  of  the  hemoglobin  and  in  the  percentage  of 
the  corpuscles  and  the  diminution  may  be  equal  or  unequal  in  degree.  If 
the  diminution  be  equal  the  color  index  is  unity;  if  it  be  unequal  the  color 
index  is  less  or  greater  than  unity;  e.g.,  if  the  percentage  of  hemoglobin  be 
but  60  and  the  percentage  of  red  corpuscles,  as  determined  by  the  method 
of  counting  be  but  80  (4,000,000  per  cubic  millimeter)  then  the  color  index 
is  11  =  0.75  which  indicates  that  each  corpuscle  retains  but  three-fourths 
of  the  normal  amount  of  hemoglobin;  if  on  the  contrary  the  percentage  of 
hemoglobin  be  but  60,  and  the  percentage  of  red  corpuscles  be  but  50  then 
the  color  index  is  1.2  which  indicates  that  each  corpuscle  contains  a  larger 
percentage  of  hemoglobin  than  normally.  This  condition  is  sometimes 
observed  in  pernicious  anemia. 

For  the  determination  of  these  variations  in  the  hemoglobin  for  clinical 
purposes  two  chromometric  methods  are  at  present  largely  employed,  that 
of  Gowers  and  v.  Fleischl.  All  chromometric  methods  are  based  on  the 
principle  that  if  two  equally  thick  and  equally  well-illuminated  solutions 
present  the  same  intensity  of  color,  their  richness  in  coloring-matter  is  the 
same.  There  are  two  methods  by  which  this  can  be  done:  (i)  By  diluting 
the  blood  to  be  examined  with  water  until  the  shade  of  color  corresponds 
to  that  of  a  solution  of  hemoglobin  of  known  strength  (Gowers).  (2) 
Diluting  a  given  quantity  of  blood  with  a  given  quantity  of  water  and  then 
finding  an  identical  color  which  represents  a  previously  determined  quantity 
of  hemoglobin  (v.  Fleischl). 


246 


TEXT-BOOK  OF  PHYSIOLOGY 


Gowers'  hemoglobinometer  consists  of  two  glass  tubes  of  exactly  the 
same  size  and  similar  to  those  shown  in  Fig.  106.  One,  A,  contains  glycerin 
jelly  colored  with  picro-carmine  the  shade  of  which  corresponds  to  that  of 
normal  blood  diluted  100  times  (20  c.mm.  in  2000  c.mm.  of  water),  repre- 
senting a  I  per  cent,  solution.  The  other  tube,  B,  is  ascendingly  graduated 
with  120  divisions,  each  one  of  which  corresponds  to  20  c.mm.  With  a 
graduated  pipette,  D,  20  cubic  millimeters  of  blood  are  accurately  measured 
and  dropped  into  the  bottom  of  the  tube  B,  in  which  a  few  drops  of  distilled 
water  have  been  placed  so  as  to  prevent  coagulation.  Water  is  then  added 
drop  by  drop  until  the  color  of  the  diluted  blood  is  exactly  that  of  the  stand- 
ard. The  division  of  the  scale  reached  by  the  dilution  will  represent  the 
relative  percentage  of  hemoglobin.  If  this  tint  is  not  obtained  until  the 
dilution  reaches  100  divisions,  the  (luantitv  of  hemoglobin  is  normal.     If 


Fig.  io5. — Haldane's  Modification  of  Gowers'  App.aratus. 

more  water  must  be  added,  it  is  in  excess;  if  less,  it  is  diminished.  If,  for 
example,  the  20  cubic  millimeters  of  blood  from  an  anemic  patient  gave  the 
standard  tint  at  60  divisions,  the  blood  contained  but  60  per  cent,  of  the 
normal  amount  of  hemoglobin. 

Haldane's  Modification  of  Gowers'  Method. — Haldane's  hemoglobin- 
ometer. Fig.  106,  is  a  modification  of  that  of  Gowers.  The  tube  A  contains 
also  a  I  per  cent,  solution  of  blood  having  the  normal  percentage  of  hemo- 
globin saturated  with  carbon  monoxid.  With  the  graduated  capillary 
pipette,  D,  20  cubic  millimeters  of  blood  are  then  obtained  from  a  slight 
wound  in  the  finger  or  elsewhere  and  then  dropped  into  the  tube  B,  in  which 
a  small  quantity  of  distilled  water  from  E  was  previously  placed  to  prevent 
coagulation.  The  cap  of  G  is  then  attached  to  a  gas  burner,  through  which 
flows  either  pure  CO  or  a  gas  containing  CO  and  the  rubber  tube  inserted 
into  D  to  the  level  of  the  water.     After  the  gas  has  been  flowing  for  a  few 


THE  BLOOD. 


H7 


seconds  the  rubber  tube  is  withdrawn,  and  while  the  glass  tube  is  yet  full  of 
the  gas,  it  is  closed  wnth  the  thumb  and  gently  shaken  so  as  to  convert  all  the 
hemoglobin  into  carbon-monoxid  hemoglobin.  This  is  then  diluted  very 
gradually  as  in  the  employment  of  the  Gowers  apparatus  until  the  tint  of 
the  solution  in  B  corresponds  to  that  in  A.  The  level  at  which  this  is 
observed  indicates  the  percentage  of  hemoglobin  in  the  blood  used.  The 
error  in  this  method  scarcely  exceeds  i  per  cent. 


Fig.  107. — ^\'^ON  Fleischl's  Hemometer.  K.  Red  colored  wedge  of  glass  moved  by  R. 
G.  Mixing  vessel  with  two  compartments,  a  and  a'.  M.  Table  with  hole  to  read  off  the  percentage 
of  hemoglobin  on  the  scale  P.     T.  Pinion  to  move  K.     S.  Mirror  of  plaster-of-Paris. 


Von  Fleischl's  hemometer  consists  of  a  metallic  cell  divided  into  two 
compartments,  a  and  a',  by  a  vertical  partition  (Fig.  107).  In  the  former 
a  definite  quantity  of  blood  is  placed  and  diluted  with  a  known  quantity  of 
water.  Beneath  the  compartment  a'  is  placed  a  glass  wedge  stained  with 
the  golden  purple  of  Cassius  or  similar  pigment,  the  color  of  which  passes 
from  a  deep  red  at  one  end  to  clear  glass  at  the  other  (Fig.  108).  To  the 
side  of  this  wedge  is  placed  a  scale 
ranging  from  o  to  120.  By  means  of 
the  screw,  R  T,  the  glass  wedge  is 
moved  until  the  color  of  the  glass  and 
diluted  blood  is  identical.  The  illum- 
ination of  the  blood  and  glass  wedge  is 


108. — Tinted    Glass   Wedge  of  the 
VON  Fleischl  Hemometer. 


accomplished  by  lamp-light  reflected     ^^^ 

from    the    white    reflecting    surface 

beneath.     The  depth  of  color  of  the 

glass  opposite  100  on  the  scale  corresponds  to  that  of  normal  blood.     If, 

therefore,  the  colors  are  identical  at  75  divisions,  the  blood  contains  but  75 

per  cent,  of  the  normal  amount  of  hemoglobin. 

Absorption  Spectra. — Both  oxyhemoglobin  and  reduced  hemoglobin, 
like  other  soluble  pigments,  have  an  absprbing  influence  on  certain  waves 
of  light,  and  hence  give  rise  to  absorption  bands  which  can  be  studied 


248 


TEXT-BOOK  OF  PHYSIOLOGY. 


with  the  spectroscope,  and  which  are  so  characteristic  as  to  serve  for  their 
identitication. 

In  principle  a  spectroscope  consists  of  a  prism  which  decomposes  the 
light  from  a  narrow  sUt  into  a  band  of  all  the  spectral  colors.     A  form  of 


Fig.  109. — The  Spectroscope.  A.  Telescope.  B.  Tube  for  the  admission  of  light  and 
carrying  the  collimator.  C.  Tube  containing  a  scale,  the  image  of  which  when  illuminated  is 
reflected  above  the  spectrum.     D.  The  fluid  examined. — {Landosi  and  Stirling.) 


Yellow 


Green. 


Cyan  Blue. 


Fig.  1 10. — Spectra  of  Hkmoiwobin  and  Some  of  its  Compounds. 

— {Landois  and  Stirling.) 

spectroscope  in  common  use  is  that  shown  in  Fig.  109.  It  consists  of  a 
tube,  B,  which  has  at  one  end  a  slit  that  can  be  narrowed  or  widened  by 
means  of  a  screw.  The  Hght,  having  passed  through  it,  falls  on  an  achro- 
matic convex  lens  (called  the  collimator)  at  the  opposite  end  of  the  tube 


THE  BLOOD. 


249 


which  renders  the  divergent  rays  of  light  parallel.  These  parallel  rays  sub- 
sequently fall  on  the  prism,  by  which  they  are  dispersed  and  directed  into  the 
tube,  A,  which  is  nothing  more  than  a  small  telescope.  On  looking  into  it 
at  the  ocular  end  the  spectral  colors  are  seen.  If  the  light  has  been  derived 
from  the  sun,  the  spectrum  will  present  vertical  dark  lines,  the  so-called 
Fraunhofer's  lines.  They  are  given  from  A  to  F  in  Fig.  no.  If  a  colored 
medium  be  held  in  front  of  the  sUt  so  that  the  light  has  to  pass  through  it  first, 
certain  dark  bands  will  appear  in  the  spectrum,  owing  to  the  absorption  of 
certain  rays. 

Dilute  solutions  of  arterial  blood  show  absorption  bands  between  the 
Fraunhofer  lines,  D  and  E,  in  the  green  and  yellow  portion  of  the  spectrum. 
(See  Fig.  no.)  The  band  nearest  D,  frequently  designated  as  alpha,  is  dark 
in  the  center  and  sharply  defined.  The  band  which  lies  toward  E,  desig- 
nated as  beta,  is  broader  and  less  sharply  defined. 


Fig.  III. — Graphic  Representa- 
tion OF  THE  Absorption  of  Light  in 
A  Spectrum  by  Solutions  of  Oxy- 
hemoglobin OF  Different  Strengths. 
The  shading  indicates  the  amount  of 
absorption  of  the  spectrum,  and  the 
numbers  at  the  side  the  strength  of  the 
solution. 


£b    F 


Fig.  1X2. — Graphic  Representation 
OF  the  Absorption  of  Light  in  a 
Spectrum  by  Solutions  of  Hemoglobin 
of  Different  Strengths.  The  shad- 
ing indicates  the  amount  of  absorption 
of  the  spectrum,  and  the  numbers  at 
the  side  the  strength  of  the  solution. 


As  the  amount  of  light  absorbed  varies  with  the  concentration  of  the 
solution  as  well  as  its  thickness,  and  gives  rise  to  absorption  bands  of  different 
widths  and  intensities,  it  becomes  necessary,  in  order  to  obtain  the  character- 
istic bands,  to  employ  only  dilute  solutions. 

The  absorption  spectra,  as  seen  with  different  strengths  of  solution  one 
centimeter  thick,  are  shown  graphically  in  Fig.  in.  It  will  be  observed 
that  solutions  varying  in  strength  from  o.i  per  cent,  to  0.6  per  cent,  give  rise 
to  the  two  characteristic  bands,  but  with  gradually  increasing  breadths. 
With  a  percentage  greater  than  0.65  per  cent,  the  light  between  D  and  E, 
the  yellow-green,  becomes  extinguished  and  the  two  bands  fuse  together, 
forming  a  single  band  overlapping  slightly  the  lines  D  and  E.  At  the  same 
time  there-  is  a  progressive  darkening  of  the  violet  end  of  the  spectrum.  At 
0.85  per  cent.,  all  the  light  is  absorbed  with  the  exception  of  a  small  amount 
of  the  red.  Solutions  less  than  o.oi  per  cent,  to  0.003  P^^  cent,  show  but  a 
single  absorption  band— that  nearest  D. 

A  solution  of  venous  blood  or  of  reduced  hemoglobin  shows  but  a  single 


250  TEXT-BOOK  OF  PHYSIOLOGY. 

absorption  band  (see  Fig.  no),  frequently  designated  as  gamma,  broader 
and  less  marked  between  the  lines  D  and  E,  but  extending  slightly  beyond  D. 
Fig.  112  shows  in  the  same  graphic  manner  the  increasing  breadth  of  the 
absorption  band  with  increasing  strengths  of  solution,  as  well  as  the  simul- 
taneous absorption  of  light  at  both  the  red  and  violet  ends  of  the  spectrum. 

Compounds  of  Hemoglobin. — The  coloring-matter  of  the  blood  is 
characterized  by  the  property  of  combining  with  and  of  again  yielding  up 
oxygen.  The  union  is  a  chemic  one,  taking  place  under  certain  pressure 
conditions.  It  therefore  may  exist  in  two  states  of  oxidation,  distinguished 
by  a  difference  in  color  and  their  absorption  spectra.  If  hemoglobin  either 
in  blood  or  in  solution  be  shaken  with  air,  it  at  once  combines  with  oxygen 
and  is  converted  into  oxyhemoglobin,  which  imparts  to  the  blood  or  solution 
a  bright  red  or  scarlet  color.  If  the  blood  or  solution  be  now  deprived  of 
oxygen,  the  oxyhemoglobin  is  converted  into  reduced  hemoglobin,  which 
imparts  to  the  blood  or  solution  a  dark  bluish  or  purple  color. 

The  quantity  of  oxygen  absorbed  by  i  gram  of  hemoglobin  is  estimated 
at  1.56  c.c.  measured  at  0°  C.  and  760  mm.  of  mercury.  The  compound 
formed  by  the  union  of  oxygen  and  hemoglobin  is  a  very  feeble  one;  for 
when  the  pressure  is  lowered  the  union  becomes  less  stable,  and  as  the  zero 
point  is  approached,  as  in  the  TorricelHan  vacuum,  a  rapid  dissociation  of 
the  oxygen  takes  place.  This,  however,  is  not  due  entirely  to  a  fall  of 
pressure  but  partly  to  the  dissociation  force  of  heat,  which  increases  in  power 
as  the  pressure  falls.  The  same  dissociation  of  oxygen  can  be  brought  about 
by  passing  through  blood  indifferent  gases,  such  as  hydrogen,  nitrogen, 
carbon  dioxid,  which  lower  oxygen  pressure,  or  by  the  addition  of  reducing 
agents,  such  as  ammonium  sulphid  or  Stokes'  fluid. 

These  experimental  determinations  of  the  relation  of  oxygen  to  hemo- 
globin partly  explain  the  oxidation  and  deoxidation  of  the  hemoglobin  in  the 
lungs  and  tissues.  As  the  blood  passes  through  the  lungs  and  is  subjected 
to  the  oxygen  pressure  there,  the  hemoglobin  combines  with  a  definite 
quantity  of  oxygen,  and  on  emerging  from  the  lungs  exhibits  a  bright  red  or 
scarlet  color;  as  the  blood  passes  through  the  systemic  capillaries  where  the 
oxygen  pressure  in  the  surrounding  tissues  is  low,  the  oxyhemoglobin  yields 
up  a  portion  of  its  oxygen,  becoming  deoxidized  or  reduced,  and  the  blood 
on  emerging  from  the  tissues  exhibits  a  dark  bluish  color.  The  portion  of 
oxygen  given  up  to  the  tissues  is  termed  respiratory  oxygen.  In  100  parts  of 
arterial  blood  the  coloring-matter  presents  itself  almost  exclusively  in  the 
form  of  oxyhemoglobin.  In  passing  through  the  capillaries  about  5  per  cent, 
only  gives  up  its  oxygen  and  becomes  reduced,  so  that  both  kinds  are  present 
in  venous  blood.  In  asphyxiated  blood  only  reduced  hemoglobin  is  present. 
It  is  this  capability  of  combining  with  and  of  again  yielding  up  oxygen,  that 
enables  hemoglobin  to  become  the  carrier  of  oxygen  from  the  lungs  to  the 
tissues. 

Carbon  Monoxid  Hemoglobin.^Carbon  monoxid  is  a  constituent  of 
coal-gas  and  more  largely  of  water-gas.  From  either  source  it  is  likely  to 
accumulate  in  the  air,  and  if  inspired  for  any  length  of  time  produces  a  series 
of  effects  which  may  eventuate  in  death.  If  blood  be  brought  into  contact 
with  this  gas,  it  assumes  a  bright  cherry-red  color,  which  is  quite  persistent 
and  due  to  the  displacement  of  the  loosely  combined  oxygen  and  the  union 


THE  BLOOD.  251 

of  the  carbon  monoxid  with  the  hemoglobin.  The  compound  thus  formed 
is  very  stable  and  resists  the  action  of  various  reducing  agents.  The  passage 
of  air  or  of  some  neutral  gas  through  the  blood  for  a  long  period  of  time  will 
gradually  displace  the  carbon  monoxid  and  enable  the  hemoglobin  again  to 
absorb  oxygen.  It  is  for  this  reason  that  partial  poisoning  with  the  gas  is 
not  fatal.  It  is  to  its  power  of  displacing  oxygen  and  forming  a  stable  com- 
pound with  hemoglobin  and  thus  interfering  with  its  respiratory  function 
that  carbon  monoxid  owes  its  poisonous  properties.  Examined  spectro- 
scopically,  solutions  of  carbon  monoxid  hemoglobin  exhibit  two  absorption 
bands  closely  resembling  in  position  and  extent  those  of  oxyhemoglobin;  but 
careful  examination  shows  that  they  are  slightly  nearer  the  violet  end  of  the 
spectrum  and  closer  together.  (See  Fig.  no.)  A  useful  test  for  CO  blood 
is  the  addition  of  caustic  soda,  which  produces  a  cinnabar  red  precipitate. 

Methemoglobin. — This  is  a  pigment,  closely  related  to  oxyhemoglobin, 
found  in  the  blood  after  the  administration  of  various  drugs,  in  cysts  and  in 
the  urine  in  hematuria  and  hemoglobinuria.  It  is  also  produced  when  a 
solution  of  hemoglobin  is  exposed  to  the  air  and  becomes  acid  in  reaction 
and  brown  in  color.  The  spectrum  shows  two  absorption  bands  similar  to 
oxyhemoglobin,  but  in  addition  a  new  and  quite  distinct  band  near  the  line 
C,  in  the  red.  If  the  acid  solution  be  rendered  alkaline  by  the  addition  of 
ammonia,  this  band  disappears  and  another  makes  its  appearance  near  the 
line  D.  The  addition  of  ammonium  sulphid  develops  reduced  hemoglobin, 
which,  on  the  absorption  of  oxygen,  produces  again  oxyhemoglobin,  as 
shown  by  the  spectroscope. 

Hematin. — Boiling  hemoglobin  or  adding  to  it  acids  or  alkalies  decom- 
poses it  and  develops  one  or  more  protein  bodies  to  which  the  general  term 
globulin  has  been  given,  and  an  iron-holding  pigment  termed  hematin. 
This  is  regarded  as  an  oxidation  product  of  hemoglobin  and  constitutes 
about  4  per  cent,  of  its  composition.  When  obtained  in  a  pure  state,  it  is  a 
non-cr}'stallizable  blue-black  powder  with  a  metallic  luster.  According  as 
it  is  treated  with  acids  or  alkalies,  two  combinations  of  hematin  can  be 
obtained  (acid  and  alkaline),  each  of  which  has  special  properties,  giving 
rise  to  different  absorption  bands. 

Hemin. — This  pigment  is  a  derivative  of  hematin,  presenting  itself  in 
the  form  of  microscopic  rhombic  plates  or  rods  (Teichmann's  crystals), 
which  are  so  characteristic  as  to  serve  as  tests  for  blood-stains  in  medicolegal 
inquiries.  These  crystals  are  readily  obtained  by  adding  to  a  small  cpantity 
of  dried  blood  on  a  glass  slide  a  few  drops  of  glacial  acetic  acid  and  a  crystal 
of  sodium  chlorid;  after  heating  gently  for  a  few  minutes  over  a  spirit  lamp 
and  then  allowing  the  mixture  to  cool,  crystallization  of  the  hemin  soon  takes 
place. 

Hematoidin. — This  term  has  been  applied  to  a  pigment  which  occurs 
in  the  form  of  yellow  crystals  in  old  blood-clots  or  in  blood  which  has  been 
extra vasated  into  the  tissues.  In  its  chemic  composition  and  in  its  reactions 
it  closely  resembles  bilirubin,  the  pigment  of  the  bile,  exhibiting  the  same 
characteristic  play  of  colors  on  the  addition  of  nitric  acid. 

The  Stroma. — The  stroma  of  the  red  corpuscles  obtained  by  the  methods 
which  dissolve  out  the  hemoglobin  has  been  shown  by  analysis  to  consist  of 
from  60  to  70  per  cent,  of  water  and  40  to  30  per  cent,  of  solid  material, 


252  TEXT-BOOK  OF  PHYSIOLOGY. 

containing   a   protcid   resembling   cell-globulin,   lecithin,   cholesterin,   and 
inorganic  salts,  among  which  potassium  phosphate  is  especially  abundant. 

HISTOLOGY  OF  THE  WHITE  CORPUSCLES  OR  LEUKOCYTES. 

The  presence  of  white  corpuscles  in  the  blood  can  be  readily  observed 
under  the  same  conditions  as  the  red  corpuscles  are  observed.  Thus  when 
the  mesentery  of  the  frog  or  the  guinea-pig  is  examined  with  the  microscope 
the  white  corpuscles  are  seen  adhering  to  the  walls  of  the  blood-vessels;  in 
a  drop  of  freshly  drawn  blood  they  are  found  in  the  spaces  between  red 
corpuscles  (Fig.  97.)  A  careful  examination  of  the  blood  by  the  employ- 
ment of  appropriate  methods  has  revealed  the  presence  of  several  varieties  of 
white  corpuscles,  to  which  reference  will  be  made  in  a  subsequent  paragraph. 

Shape  and  Size. — In  the  resting  condition,  whether  seen  in  the  vessel 
or  on  the  stage  of  the  microscope,  the  white  corpuscle,  as  its  name  implies,  is 
grayish  in  color,  round  or  globular  in  form,  though  often  presenting  a  more 
or  less  irregular  surface.  Its  diameter  varies  from  0.004  to  0.013  rnni-) 
though  the  average  is  about  o.oii  mm.  or  about  -^-^Vo"  irich. 

Structure. — A  typical  white  corpuscle  consists  of  a  ground-substance 
uniformly  transparent  and  apparently  homogeneous,  in  which  are  embedded 
a  number  of  granules  of  varying  size,  some  of  which  are  very  fine,  while 
others  are  large.  By  various  reagents  it  has  been  demonstrated  that  the 
granules  are  fatty,  protein,  and  carbohydrate  (glycogen)  in  character.  In 
the  fresh  cells  the  existence  of  a  nucleus  is  difficult  of  detection,  though  its 
presence  can  be  demonstrated  by  the  addition  of  acetic  acid,  which  renders 
the  perinuclear  cytoplasm  more  transparent  and  makes  the  nucleus  con- 
spicuous and  sharply  defined.  From  its  structure  it  is  apparent  that  the 
white  corpuscle  belongs  to  the  group  of  undifferentiated  tissues  and  resembles 
the  cells  of  the  embryo  in  its  earliest  stages  as  well  as  the  unicellular  organism, 
the  amoeba. 

Chemic  Composition. — The  chemic  composition  of  the  white  corpuscles 
has  been  inferred  from  an  analysis  of  pus-corpuscles,  with  which  they  are 
practically  identical,  and  of  lymph-corpuscles  from  the  lymph-glands.  Of 
the  corpuscle  about  90  per  cent,  is  water  and  the  remainder  solid  matter 
consisting  mainly  of  proteins,  of  which  nuclein,  nucleo-albumin,  and  cell 
globulin  are  the  most  abundant.  The  two  former  are  characterized  by  the 
presence  of  a  considerable  c[uantity  of  phosphorus,  amounting  to  as  much  as 
10  per  cent.  Lecithin,  fat,  glycogen,  and  earthy  and  alkaline  phosphates 
are  also  present. 

Number  of  White  Corpuscles. — The  number  of  white  corpuscles  per 
cubic  millimeter  of  blood  is  much  less  than  the  number  of  red  corpuscles,  the 
ratio  being  in  the  neighborhood  of  i  white  to  700  red.  This  ratio,  however, 
varies  within  wide  limits  in  different  portions  of  the  body  and  under  normal 
variations  in  physiologic  conditions.  In  the  blood  of  the  splenic  artery  there 
is  but  I  white  to  2260  red,  while  in  the  splenic  vein  there  is  i  white  to  every 
60  red;  or  about  thirty-eight  times  as  many  as  in  the  artery.  In  the  portal 
vein  there  is  i  white  to  740  red,  while  in  the  hepatic  vein  there  is  i  white  to 
170  red. 

The  total  number  of  white   corpuscles  per  cubic  millimeter  has  been 


THE  BLOOD. 


253 


estimated  at  from  5000  to  10,000,  though  the  average  is  about  7500.  The 
number,  however,  is  intiuenced  by  a  variety  of  physiologic  conditions.  The 
ingestion  of  food  rich  in  protein  material  raises  the  count  from  30  to  40  per 
cent.,  as  compared  with  the  count  before  the  meal.  In  the  new-born  the 
number  is  greater  than  in  adults — 17,000  to  20,000  per  cubic  millimeter. 
Cabot  states  that  30,000  is  never  a  high  count  after  a  meal  in  infants  under 
two  vears.  In  the  later  months  of  pregnancy,  especially  in  primiparae,  the 
number  increases  to  16,000  to  18,000.  Many  pathologic  conditions  of  the 
body  also  influence  the  count  very  considerably. 

Fasting  for  a  few  days  always  lowers  the  count,  and  in  a  case  of  total 
abstinence  of  food  for  a  week,  reported  by  Luciani,  the  count  fell  to  861  per 
cubic  millimeter,  after  which  it  rose  to  1530,  where  it  practically  remained 
for  the  succeeding  three  weeks  of  the  fasting  period. 

When  the  number  of  leukocytes  present  in  the  peripheral  blood  exceeds 
the  normal,  i.e.,  10,000  per  cubic  millimeter  the  condition  is  termed  leuko- 


•  • 


Fig.  113. — Amceboid  Mo\'ements  of  a  White  Corpuscle  from  the  Frog.  The  form 
changes  occurred  within  ten  minutes.  The  black  particles  are  Chinese  ink  which  had  been 
injected  twenty-four  hours  before  into  the  dorsal  lymph  sac. — {Ranher-Kopsch ) 

cytosis;  when  the  number  falls  below  the  normal  the  condition  is  termed 
leukopenia.  Both  conditions,  however,  may  be  only  temporary  and  therefore 
physiologic,  or  they  may  be  permanent,  associated  with  certain  diseased  states 
of  the  body  and  therefore  pathologic.  It  is  therefore  permissible  to  speak 
of  a  physiologic  and  a  pathologic  leukocytosis  and  leukopenia. 

The  method  for  counting  the  white  corpuscles  is  similar  to  that  used  in 
counting  the  red  corpuscles.  The  given  volume  of  blood  should,  however, 
be  diluted  with  10  or  20  volumes  of  a  one  per  cent,  solution  of  acetic  acid, 
which  disintegrates  the  red  corpuscles  and  thus  facilitates  the  counting  of 
the  white.  The  pipette  should  have  a  larger  bore  than  that  used  for  the 
red,  and  a  much  greater  number  of  squares  in  the  counting  chamber  should 
be  counted,  so  as  to  diminish  the  percentage  of  error. 

Physiologic  Properties. — The  white  corpuscles  and  especially  the 
leukocytes  possess  the  characteristic  property  of  exhibiting  movements 
similar  to  those  observed  in  the  amoeba,  and  are  therefore  termed  amoeboid. 
These  movements  consist  in  alternate  protrusions  and  retractions  of  portions 
of  the  cell  body,  as  a  result  of  which  they  exhibit  a  great  variety  of  forms. 


2  54 


TEXT-BOOK  OF  PHYSIOLOGY. 


(See  Fig.  113.)  The  protruded  process,  the  pseudopod,  can  also  attach  itself 
to  some  point  of  the  surface  on  which  it  rests,  and  then  draw  the  body  of 
the  corpuscle  after  it.  By  a  repetition  of  this  process  the  corpuscle  can 
slowly  creep  about  and  change  its  position  in  reference  to  its  environment. 
By  virtue  of  these  amoeboid  movements  the  corpuscle  can  appropriate  small 
particles  of  pigment,  such  as  indigo  or  carmine,  and  after  a  short  time  elimi- 
nate them  from  various  parts  of  the  surface.  It  is  also  capable  of  thrusting 
a  process  into  and  through  the  wall  of  the  capillary  vessel,  after  which  the 

remainder  of  the  corpuscle  follows  (Fig.  114). 
This  continues  until  the  corpuscle  is  outside  the 
vessel  and  in  the  lymph-space,  where  it  resumes  its 
original  shape  and  movement.  This  process  is  best 
observed  in  inflammatory  conditions,  when  the 
blood  has  come  to  rest  and  the  vessels  are  occluded 
with  both  red  and  white  corpuscles.  To  this  pas- 
sage of  the  white  blood-corpuscles  through  the 
capillary  wall  the  term  diapedesis  is  given.  The 
movements  of  the  white  corpuscles  are  increased  by 
a  rise  in  temperature  up  to  40°  C,  beyond  which 
they  cease,  owing  to  the  coagulation  of  the  cell- 
substance.  A  low  temperature  also  arrests  the 
movements.  Induced  electric  currents  also  cause 
contraction  and  death  of  the  cell.  Moisture  and 
oxygen  are  necessary  to  their  activity.  From  their 
similarity  to  lower  organisms  the  white  corpuscles 
may  be  regarded  as  independent  organisms  living 
in  the  animal  fluids,  just  as  the  amceba  lives  in  its 
natural  liquid  medium. 

Varieties  of  Leukocytes. — A  detailed  study  of 
the  blood  with  the  aid  of  the  triacid  staining  fluid 
of  Ehrlich  or  any  of  the  various  eosin  and  methy- 
lene-blue  stains,  reveals  the  presence  of  five  distinct 
varieties  of  leukocytes  and  transitional  forms  which 
may  be  classified  as  follows: 

I.  Small  lymphocytes,  so  called  from  their  resem- 
blance to  the  corpuscles  of  the  lymph-glands, 
consisting  of  a  deeply  staining  and  relatively 
large  round  nucleus,  encircled  by  a  narrow  rim 
of  cytoplasm.     Found  in  from  20  to  25    per   cent,  of  all  leukocytes. 
They  vary  in  size  from  0.004  to  0.007  ^n^^- 
Large  lymphocytes  or  hyaline  cells,  which  are  believed  by  some  to  represent 
the  preceding  type  at  a  later  stage  of  development,  by  others  to  have  an 
independent  origin,  are  distinguished  by  a  round  or  ovoid  nucleus 
staining  faintly  and  surrounded  by  a  relatively  larger  layer  of  cytoplasm 
than  is  seen  in  the  small  lymphocyte.     The  large  lymphocyte  is  present 
to  the  extent  of  from  4  to  8  per  cent.     Transitional  forms,  usually  pre- 
sent from   I  to  2  per  cent,  are  very  much  like  the  large  lymphocyte  in 
appearance  and  size,  with  the  exception,  however,  that  they  possess  a 
cresentic  or  indented  nucleus  and  have  a  somewhat  greater  affinity  for 


Fig.  114. — Small  Ves- 
sel SHOWING  Various 
Stages  in  the  Diapedesis 
OF  Leukocytes.  {G.Bach- 
man.) 


{Triacid  Stain.) 

I,  2,  3,  4.  Small  Lymphocytes. 

Contrast  the  faintly  colored  protoplasm  of  these  cells  in  the  triple  stained  specimen,  with 
their  intensely  basic  protoplasm  in  the  film  stained  with  eosin  and  methylene-blue,  17  and  18. 
The  cell  body  of  i  is  invisible.     Note  the  kidney-shaped  nucleus  in  4. 

5,  6.  Large  Lymphocytes. 

With  this  stain  the  nucleus  reacts  more  strongly  than  the  protoplasm;  with  eosin  and  meth- 
ylene-blue (ig,  20),  on  the  contrary,  the  protoplasm  is  so  deeply  stained  that  the  nucleus 
appears  pale  by  contrast.  This  peculiarity  is  also  observed  in  the  smaller  forms  of 
lymphocytes. 

7,  8.  Transitional  Forms. 

Note  the  moderately  basic  and  indented  nucleus,  and  the  almost  hyaline  non-granular 
protoplasm.  Compare  8  wdth  the  myelocyte,  7,  Plate  I,  these  cells  differing  chielly  in  that 
the  myelocyte  contains  neutrophile  granules. 

9,  10,  II.  Polynuclear  Neutrophiles. 

These  cells  are  characterized  by  a  polymorj)hous  or  polynuclear  nucleus,  surrounded  by 
a  cell-body  tilled  with  fine  neutrophile  granules.  In  11  the  nuclear  structure  is  obviously 
separated  into  four  parts;  in  9  it  is  moderately,  and  in  10  markedly,  polymorphous. 

12,  13.  Eosinophiles. 

The  nuclei  are  not  unlike  those  of  the  polynuclear  neutrophile,  except  that  they  are  some- 
what less  convoluted,  and  poorer  in  chromatin,  staining  less  intensely.  The  protoplasm  is 
filled  with  coarse  eosinophile  granules,  the  characteristics  of  which  are  clearly  illustrated 
by  13,  a  "fractured"  eosinophile. 

14.  Eosinophile  Myelocyte. 

Compare  wdth  15. 

15,  16.  Myelocytes.     {Neutrophilic) 

These  cells  are  morphologically  similar  to  14,  except  that  they  contain  neutrophile  instead 
of  eosinophile  granules.  Note  that  the  granules  of  the  myelocyte  are  identical  with  those 
of  the  polynuclear  neutrophile.     A  dwarf  form  of  myelocyte  is  represented  by  16. 

{Eosin  and  Methylene-blue.) 

17,  18.  Small  Lymphocytes. 

Note  the  narrow  rim  of  pseudo-granular  basic  protoplasm  surrounding  the  nucleus,  and 

the  pale  appearance  of  the  latter. 
19,  20.  Large  L3rmphocytes. 

Budding  of  the  basic  zone  of  protoplasm  is  represented  by  20.     Both  of  these  cells  belong 

to  the  same  type  as  5  and  6. 
21,  22.  Large  Mononuclear  Leukocytes. 

Compared  with  19  and  20,  these  cells  have  a  decidedly  less  basic  protoplasm,  but  a  somewhat 

more  basic  nucleus.     In  the  triple  stained  film  these  differences  cannot  be  detected,  so 

that  they  must  be  classed  as  large  lymphocytes. 

23.  Transitional  Form. 

The  distinction  between  this  cell  and  24  is  not  marked;  the  nucleus  of  the  latter  simply 
being  somewhat  more  basic  and  convoluted. 

24,  25,  26,  27.  Polynuclear  Neutrophiles. 

With  this  stain  these  cells  show  a  feebly  acid  protoplasm,  and  lack  granules.     Note  that 
the  more  twisted  the  nucleus  the  deeper  it  is  stained.     Compare  with  9,  10  and  11. 
28,  29.  Eosinophiles. 

Compare  with  12  and  13. 

30.  Eosinophilic  Myelocyte. 

Compare  with  14. 

31.  Basophile.     {Finely  granular.) 

This  cell  is  characterized  by  the  presence  of  exceedingly  fine  ^-granules,  staining  the  pure 
color  of  the  basic  dye.  The  nucleus  is  markedly  convoluted  and  deficient  in  chromatin. 
The  cell  here  shown  was  found  in  normal  blood. 

32.  33.  34,  35-  36.  Mast  Cells. 

The  granules  take  a  modified  basic  color,  as  shown  by  their  royal-purple  tint  in  this  illus- 
tration. Note  their  unusually  large  size  and  ovoid  shape  in  35,  their  peculiar  distribution 
in  35  and  36,  and  their  irregularity  in  size  in  32  and  36.  With  the  triacid  mixture  these 
granules,  as  well  as  those  of  the  finely  granular  basophile,  31,  remain  unstained,  showing 
as  dull-white  stippled  areas  in  the  cell-bocly.  The  nuclear  chromatin  of  the  mast  cell  is  so  deli- 
cate and  so  feebly  stained  that  it  is  barely  visible.  These  cells  were  found  in  the  blood  of  a 
case  of  splenomedullary  leukemia. 


PLATE  I. 


o 


^7: 


'At^mfH,. 


•1  .  A.> »  .42'  v.. 


/       %0 


%P 


"o 


^ 


9 


H'-^pJii. 


The  Leukocytes. 

I2-16,  Triacid  Stain;  17-36,  £osin  and  Methylene -blue.) 

(E.  F.  Faber,  /ec.) 

(From  DaCosta's  "Clinical  Hematology.") 


THE  BLOOD.  255 

basic  dyes.     They  are  usually  counted  with  the  large  lymphocytes. 
Both  varieties   of  lymphocytes  are   characterized  by   a   cytoplasm 
which   is   devoid   of   granules.     Rarely,   basophilic   granules   may  be 
present. 

3.  Polymorphonuclear  neutrophiles.     The  nucleus  of  this  cell  is  irregular 

and  assumes  a  great  variety  of  shapes  in  different  cells,  a  feature  which 
has  suggested  the  name  given  to  the  cell.  The  perinuclear  cystoplasm 
contains  a  large  number  of  fine  granules  which  are  neutrophilic  or 
faintly  acidophilic  in  their  staining  reaction.  They  make  up  about  60 
to  70  per  cent,  of  the  whole  number  of  the  white  blood-cells.  They 
vary  in  size  from  0.007  to  o.oio  of  a  mm. 

4.  Eosinophile  cells.     The  nucleus  resembles  in  many  respects  that  of  the 

preceding  variety;  it  is,  however,  less  apt  to  stain  so  deeply.  It  is  also 
very  irregular  in  shape  and  many  cells  possess  several  apparently  dis- 
tinct nuclei.  The  cytoplasm  is  ill-defined  but  its  presence  is  easily 
revealed  through  the  large,  intensely  acidophilic  granules  which  it 
possesses. 

It  is  present  to  the  extent  of  0.5  to  2  per  cent. 

5.  Basophile  cells,  the  nucleus  of  which  is*round  or  slightly  irregular.     The 

granules,  which  may  be  large  or  small,  are  basophilic  and  stain  more 
deeply  than  the  nucleus,  though  they  have  the  same  color.  It  is  rare 
for  this  cell  to  be  present  above  0.5  per  cent,  of  all  leukocytes. 

In  abnormal  states  of  the  blood  other  forms  of  leukocytes  are  fre- 
quently present,  e.g.,  myelocytes,  leukoblasts,  myeloplaxes,  etc.,  the 
significance  of  which  is  not  always  apparent. 

Origin  of  the  White  Corpuscles. — Of  the  various  theories  advanced  to 
explain  the  origin  of  leukocytes,  that  formulated  by  Ehrlich  has  found  the 
most  credence.  According  to  this  theory  the  leukocytes  may  genetically  be 
classed  into  two  groups.  In  the  first  group  are  the  large  and  small  lympho- 
cytes which  take  their  origin  entirely  from  the  lymph-adenoid  tissues  of  the 
body,  e.g.,  the  lymph-glands,  solitary  and  agminated  follicles  of  the  intes- 
tines, etc.  As  the  lymph  flows  through  these  structures  the  lymph-corpus- 
cles, as  the  future  lymphocytes  of  the  blood  are  called  in  these  situations,  are 
washed  out  and  carried  by  way  of  the  lymph-stream  into  the  general 
circulation. 

In  the  second  group  are  the  transitional  forms,  the  polymorphonuclear, 
eosinophile  and  basophile  leukocytes  which  originate  from  the  bone-marrow 
only.  The  immediate  ancestors  of  these  cells  are  known  as  myelocytes  and 
are  normally  found  in  the  red  bone-marrow.  These  cells,  through  transi- 
tional stages,  assume  the  characteristics  of  the  leukocytes  just  mentioned  and 
pass  directly  into  the  capillaries  of  the  marrow  w^hence  they  are  distributed 
throughout  the  body. 

Several  attempts  have  been  made  by  different  investigators  to  trace  all 
varieties  of  leukocytes  to  a  common  mother  cell.  While  this  is  believed  to 
take  place  during  embryonal  life,  the  proofs  of  such  an  origin  of  leukocytes  in 
the  normal  adult  are  insufificient  and  unconvincing. 

After  an  unknown  period  of  life  the  leukocytes  undergo  dissolution  and 
disappear. 


256  TEXT-BOOK  OF  PHYSIOLOGY. 

Functions.— The  functions  of  the  white  corpuscles  are  but  imperfectly 
known,  and  at  present  no  positive  statements  can  be  made.  It  has  been 
suggested  that  wherever  found  in  the  body,  whether  in  blood  or  tissues,  they 
are  engaged  in  the  removal  of  more  or  less  insoluble  particles  of  disintegrated 
tissues,  in  attacking  and  destroying  more  or  less  effectively  various  forms  of 
invading  bacteria  and  thus  protecting  the  body  against  their  deleterious 
activity.  This  they  do  by  surrounding,  enveloping,  and  incorporating  either 
the  tissue  particle  or  bacterium  and  digesting  it.  On  account  of  this  swallow- 
ing action  these  cells  were  termed  by  MetchnikofT  phagocytes  and  the  process 
phagocytosis.  The  cells  engaged  in  this  process  are  the  polymorphonuclear 
leukocytes  and  the  large  and  the  small  lymphocytes.  He  regards  them  as 
the  general  scavengers  of  the  body.  It  has  been  suggested  that  they  are 
also  engaged  in  the  absorption  of  fat  from  the  lymphoid  tissue  of  the  intestine. 
In  their  dissolution  they  contribute  to  the  blood-plasma  certain  protein 
materials  which  assist  under  favorable  circumstances  in  the  coagulation 
of  the  blood. 

HISTOLOGY  OF  THE  BLOOD-PLATELETS. 

The  blood-platelets  or  plac[ues  are  small  histologic  elements  circulating 
in  the  blood-plasma.  They  were  discovered  and  described  in  1845  by  Arnold. 
Hayem,  later  applied  to  them  the  term  hematoblasts,  on  the  supposition 
that  they  were  the  early  stages  in  the  development  of  the  red  corpuscles. 
This  is  now  known  to  be  erroneous.  On  account  of  their  specific,  distinct 
characters,  and  their  constant  presence  in  the  blood  of  living  animals  (guinea- 
pig  and  bat),  they  are  now  regarded  as  normal  constituents  of  the  blood  and 
designated  sometimes  as  the  third  corpuscle.  When  blood  is  freshly  drawn 
from  the  body,  the  plaques  rapidly  undergo  disintegration  and  disappear; 
but  by  treating  the  blood  with  osmic  acid,  the  form  and  structure  of  the 
plaque  may  be  retained.  They  may  also  be  preserved  by  preparing  and 
staining  the  tissues  with  Wright's  blood  stain. 

The  blood-platelet  may  be  defined  as  a  colorless,  grayish- white,  homo- 
geneous or  finely  granular  protoplasmic  disk,  varying  in  diameter  from  1.5 
to  3.5  micro-millimeters.  The  edges  are  rounded  and  well  defined,  but  it 
is  not  certain  whether  they  are  only  flattened  or  are  slightly  biconcave. 
There  is,  however,  no  nucleus,  though  the  central  portion  is  granular  and 
the  peripheral  portion  clear.  The  ratio  of  the  plaques  to  the  red  corpuscles 
is  I  to  18  or  20,  and  the  total  number  per  cubic  millimeter  has  been  estimated 
to  be  250,000  to  300,000  or  more. 

When  blood  is  shed  they  tend  to  adhere  to  each  other  and  form  irregular 
masses  known  as  Schultze's  "granular  masses.  If  threads  are  suspended  in 
blood,  the  plaques  accumulate  in  enormous  numbers  upon  them  and  appear 
to  form  a  center  from  which  fibrin  filaments  radiate  as  coagulation  proceeds. 
The  white  thrombi  which  form  in  blood-vessels  in  consequence  of  diseased 
states — e.g.,  endocarditis,  atheromatous  ulceration,  etc. — are  composed  very 
largely  of  blood-plac[ues  and  fibrin  threads. 

The  blood-plaques  can  be  seen  with  high  powers  of  the  microscope  in  the 
blood-vessels  of  the  omentum  of  the  guinea-pig  and  rat,  especially  when  the 
blood-stream  begins  to  slow.  They  are  also  readily  seen  in  the  blood-vessels 
of  subcutaneous  connective  tissue  of  various  animals,  and  especially  in  that 


THE  BLOOD.  257 

of  the  new-born  rat.  A  small  quantity  of  this  tissue  moistened  with  normal 
saline  and  examined  microscopically  with  suitable  powers  will  show  large 
numbers  of  plaques  within  the  blood-vessels. 

As  to  the  origin  of  the  blood  platelets  there  has  been  much  difference  of 
opinion.  Many  theories  have  been  proposed,  none  of  which  have  been 
accepted.  As  a  result  of  long  continued  observations  Wright  has  recently 
published  results  which  make  it  probable  that  they  are  fragments  or  detached 
portions  of  the  cytoplasm  of  giant  cells,  megakaryocytes,  found  in  the 
marrow  of  the  bones.  The  cytoplasm  is  prolonged  into  pseudopod-like 
processes  which  become  detached,  and  as  they  are  in  close  relation  to 
the  blood  channels  they  are  soon  taken  up  and  carried  into  the  blood 
of  the  general  circulation  when  they  are  known  as  blood  platelets  or 
plaques. 

The  function  of  the  blood-plaques  is  unknown,  but  it  has  been  surmised 
that  in  some  way  they  are,  like  the  leukocytes,  concerned  in  the  coagula- 
tion of  the  blood.  Whenever  they  are  diminished  in  number,  as  in  purpura 
and  hemophilia,  coagulation  takes  place  very  slowly. 

THE  TOTAL  QUANTITY  OF  THE  BLOOD;  ITS  GENERAL 
COMPOSITION. 

The  determination  of  the  total  quantity  of  the  blood  in  an  animal  is  best 
made  by  the  chromometric  method,  somewhat  modified  at  present,  of 
Welcker.  This  consists,  first,  in  bleeding  an  animal,  collecting  ail  the  blood 
it  yields,  and  weighing  it;  second,  in  washing  out  the  vessels  with  a  normal 
saline  solution  until  the  fluid  comes  from  the  veins  clear  and  free  from  blood; 
third,  in  mincing  the  tissues  of  the  body,  after  removal  of  the  contents  of  the 
alimentary  canal,  soaking  them  in  water  for  twenty-four  hours,  and  then 
expressing  them.  All  the  washings  are  collected  and  weighed.  A  given 
volume  of  the  normal  defibrinated  blood,  treated  with  carbon  monoxid  so  as 
to  give  it  uniform  color,  is  then  diluted  with  water  until  its  tint  is  identical 
with  that  of  the  washings  similarly  treated  with  carbon  monoxid.  From  the 
quantity  of  water  necessary  to  dilute  the  blood  the  quantity  of  blood  in  the 
washings  is  readily  determined.  The  animal  having  been  previously 
weighed  and  the  weight  of  the  contents  of  the  alimentary  canal  deducted,  the 
ratio  of  the  total  weight  of  the  blood  to  the  weight  of  the  body  at  once  be- 
comes apparent.  By  this  method  it  has  been  shown  that  the  ratio  of  blood 
to  body-weight  in  a  human  adult  is  1:13;  in  an  infant,  1:19;  in  a  dog,  1:13; 
in  a  cat,  1:21. 

The  more  recent  investigations  of  Haldane  and  Smith  and  of  Plesch 
with  the  employment  of  a  dift'erent  method  make  it  probable  that  the  ratio 
is  approximately  1:19.  Thus  a  man  weighing  70  kilos  would  have  3684 
grams  of  blood. 

The  amount  of  blood  in  the  different  organs  has  been  determined  by 
ligating  the  blood-vessels  in  the  living  animal,  removing  the  organ,  and  after 
allowing  the  blood  to  escape  subjecting  the  tissues  to  the  chromometric 
methods  described  above.  According  to  Ranke,  the  volume  of  the  blood  is 
distributed  as  follows:  Heart,  lungs, arteries,  and  veins,  \;  liver,  |;  muscles,  J; 
other  organs,  J. 
17 


2  58  TEXT-BOOK  OF  PHYSIOLOGY. 

General  Composition. — The  results  of  the  analyses  of  the  blood  will 
vary  with  the  animal  and  the  methods  employed.  The  following  table, 
taken  from  Gad,  shows  the  average  composition,  expressed  in  whole  numbers, 
of  horse's  blood.  In  essential  respects  the  ratio  of  the  constituents  in  human 
blood  would  not  be  materially  different. 

One  thousand  parts  of  blood  contain: 

f  Water 200 200 

Cells 328 -j                                         r  Hemoglobin 116 

[  Solids 128  \  Other  organic  matter 10 

[  Salts 2 

f  Water 604 604 

p'^^"^^ ^^72 1  [A^^min.;;.:::::: :::::::::::::  s^ 

,  Fat I 

]  Other  organic  matter 3 

Potassium  and  sodium  salts 4 

[  Calcium  and  magnesium  salts i 


Solids 68 


CHEMISTRY  OF  COAGULATION. 

The  changes  which  eventuate  in  the  formation  of  fibrin,  and  hence  all 
the  subsequent  phenomena  of  coagulation,  are  chemic  in  character;  but  as 
these  changes  take  place  in  organic  compounds  the  composition  of  which  is 
but  imperfectly  known,  the  intimate  nature  of  the  process  is  quite  obscure. 
All  the  theories  which  have  been  advanced  in  explanation,  though  approxi- 
mating the  truth,  are  more  or  less  incomplete  and  in  some  respects  contra- 
dictory. Since  the  coagulation  is  coincident  with  the  appearance  of  the 
fibrin,  the  antecedents  of  this  substance,  the  physical  and  chemic  conditions 
which  condition  its  development,  and  the  succession  of  chemic  changes 
involved  must  be  determined,  before  any  consistent  theory  can  be  established. 

Extra-vascular  Coagulation. — At  present  it  is  generally  believed 
that  the  immediate  factors  concerned  in  extra-vascular  coagulation  are 
fibrinogen,  a  calcium  salt,  and  an  agent  thrombin.  As  to  the  manner  in 
which  these  three  bodies  react  one  with  another  there  is  a  diversity  of  opinion. 

As  an  outcome  of  a  long  series  of  experiments  that  have  been  performed 
to  determine  the  nature  and  the  succession  of  the  chemic  phenomena 
underlying  the  coagulation  of  the  blood,  the  following  facts  seem  to  be  well 
established,  viz:  the  immediate  cause  of  the  coagulation  is  the  appearance 
of  fibrin,  a  derivative  of  an  antecedent  substance  always  present  in  the  blood 
termed  fibrinogen;  the  cause  of  the  conversion  of  the  soluble  fibrinogen  into 
the  insoluble  fibrin  is  the  presence  and  activity,  under  the  circumstances,  of 
an  agent  termed  thrombin,  the  chemic  nature  of  which  is  a  subject  of  discus- 
sion. By  some  chemists  it  is  regarded  as  a  ferment  which  causes  a  mo- 
lecular rearrangement  of  the  fibrinogen;  by  others  it  is  regarded  as  a  definite 
organic  colloidal  body  which  unites  in  some  physico-chemic  manner  with 
the  fibrinogen  to  form  fibrin. 

The  crux  of  the  problem  is  the  source  and  the  conditions  necessary  for 
the  production  of  the  thrombin.  It  is  generally  conceded  that  thrombin  is  a 
derivative  of  an  antecedent  substance  prothrombin  or  thrombogen,  a  substance 
always  present  in  the  blood  plasma,  a  product  of  the  decomposition  of  blood- 
platelets  and  leukocytes.  With  prothrombin  there  is  physiologically  associ- 
ated a  calcium  salt,  the  presence  of  which  is  absolutely  essential  for  coagula- 


THE  BLOOD. 


259 


tion  or  the  conversion  of  prothrombin  into  thrombin  as  was  conclusively 
shown  by  Arthus  and  Pages:  For  if  it  is  precipitated  by  the  addition  of 
oxalate  of  potassium,  coagulation  will  not  take  place.  At  all  times  then, 
there  are  present  in  the  blood,  prothrombin,  a  calcium  salt  and  fibrinogen. 
Given  the  two  former  factors,  the  question  arises  why  do  they  not  react  to 
form  thrombin  in  the  circulating  blood,  and  why  do  they  so  react  in  shed 
blood?  The  answer  of  Morawitz  is,  that  prothrombin  requires  an  activating 
agent,  a  kinase  which  is  wanting  in  circulating  blood  but  is  present  in  shed 
blood.  It  is  supposed  to  develop  in  the  disintegration  of  the  cell  elements  of 
the  blood,  leukocytes  and  blood  platelets,  and  perhaps  from  the  cell  elements 
of  the  injured  tissues  as  the  blood  flows  over  them.  Shortly  after  its  appear- 
ance the  kinase,  with  the  aid  of  the  calcium  salt  converts  the  prothrombin  into 
thrombin,  after  which  it  unites  with  fibrinogen  to  form  fibrin.  For  this 
reason  the  kinase  has  been  termed  thromho-kinase. 

The  answer  of  Howell  to  the  foregoing  question  is  somewhat  different  and 
based  on  a  long  series  of  experiments  recently  published.  From  the  results 
of  these  experiments  the  answer  given  is  that  prothrombin  is  prevented  from 
reacting  with  the  calcium  salt  to  form  thrombin  in  the  circulating  blood,  by 
reason  of  the  presence  and  union  with  prothrombin  of  an  agent  termed 
anti-thrombin.  So  long  as  this  combination  is  not  disturbed  the  blood 
remains  fluid.  When  blood  is  shed  there  is  supposed  to  develop  from  the  cell 
elements  of  the  blood,  the  leukocytes  and  blood  platelets,  and  perhaps  from 
the  cell  elements  of  the  injured  tissues  as  well,  a  plastin,  the  specific  action  of 
which  is  to  combine  with  the  anti-thrombin  and  thus  set  free  the  prothrombin. 
This  having  been  accomplished  the  calcium  salt  activates  the  prothrombin, 
and  converts  it  into  thrombin,  after  which  it  combines  with  the  fibrinogen. 
For  this  reason  the  plastic  agent  has  been  termed  thromho-plastin. 

Intra-vascular  Coagulation. — So  long  as  the  relations  of  the  blood 
and  the  vascular  apparatus  remain  physiologic,  no  coagulation  occurs  in  the 
vessels.  The  reasons  assigned  for  this  are:  (i)  the  absence  of  thrombo- 
kinase  in  sufficient  amounts;  (2)  the  presence  of  an  anti-thrombin.  On  either 
assumption  the  reaction  between  prothrombin  and  calcium  with  the  forma- 
tion of  thrombin  does  not  take  place.  If  the  vessels  are  injured  as  they  are 
when  ligated  or  torn  or  in  any  way  impaired,  coagulation  promptly  takes 
place  with  the  subsequent  occlusion  of  the  vessel.  As  to  whether  the  injured 
tissues  or  the  blood  cells  now  generate  an  agent,  thrombo-kinase,  which 
activates  the  prothrombin  and  calcium,  or  whether  they  generate  an 
agent  thrombo-plastin,  which  neutralizes  an  anti-thrombin,  is  a  subject  of 
discussion. 

Under  pathologic  conditions  of  the  circulatory  apparatus,  especially  of 
the  internal  lining,  intra-vascular  coagulation  frequently  arises,  though  the 
process  cannot  be  considered  as  identical  with  extra-vascular  coagulation. 
Many  pathologists  assert  that  in  its  origin,  mode  of  formation,  and  structure 
the  intra-vascular  coagulum  or  thrombus  is  not  a  true  coagulum  as  ordinarily 
understood,  but  rather  a  conglutination  of  blood-plaques  and  leukocytes. 
Whenever  the  integrity  of  the  internal  wall  of  the  vessel  is  impaired  by 
disease  or  by  the  introduction  of  foreign  bodies,  there  is  primarily  a  deposition 
and  accumulation  of  blood-plaques  at  the  injured  area  or  on  the  foreign  body 
which  constitutes  to  a  large  extent  the  mass  of  the  thrombus  which  at  once 


26o  TEXT-BOOK  OF  PHYSIOLOGY. 

forms.  The  thrombi  which  form  on  the  surface  of  atheromatous  ulcers,  on 
the  valves  of  the  heart,  and  in  the  veins  in  consequence  of  diseased  states,  on 
threads  or  needles  passed  through  the  vessels,  at  the  orifices  of  torn  blood- 
vessels, consist  largely  of  blood-plaques.  A  thrombus  so  formed  may  con- 
tain a  number  of  delicate  fibrin  threads,  which,  however,  present  a  different 
appearance  from  the  fibrin  of  the  extra-vascular  clot.  In  the  thrombi  which 
form  around  foreign  bodies  there  is  a  larger  quantity  of  fibrin  than  in  those 
originating  from  causes  wholly  within  the  vessel. 


CHAPTER  XIII. 
THE  CIRCULATION  OF  THE  BLOOD. 

Each  organ  and  tissue  of  the  body  is  the  seat  of  a  more  or  less  active 
metaboHsm,  the  maintenance  of  which  is  essential  to  its  physiologic  activity. 
This  metabolism  is  characterized  by  the  assimilation  of  food  materials  and 
the  production  of  waste  products;  that  it  may  be  maintained  it  is  imperative 
that  there  shall  be  a  continuous  supply  of  the  former  and  a  continuous 
removal  of  the  latter.  Both  conditions  are  subsen^ed  by  the  blood.  In 
order,  however,  that  this  fluid  may  fulfil  these  functions  it  must  be  kept  in 
continuous  movement,  must  flow  into  and  out  of  the  tissues  in  volumes  vary- 
ing with  their  activity,  with  a  certain  velocity  and  under  a  given  pressure. 

The  apparatus  by  which  these  results  are  attained  is  termed  the  circula- 
tory apparatus.  This  consists  of  a  central  organ,  the  heart;  a  series  of 
branching  diverging  tubes,  the  arteries;  a  network  of  minute  passageways 
with  extremely  delicate  walls,  the  capillaries;  a  series  of  converging  tubes, 
the  veins.  These  structures  are  so  arranged  as  to  form  a  closed  system  of 
vessels  within  which  the  blood  is  kept  in  continuous  movement  mainly  by  the 
pressure  produced  by  the  pumping  action  of  the  heart,  though  aided  by  other 
forces.     (See  Fig.  115.) 

In  this  system  a  particle  of  blood  which  passes  any  given  point  w^ill 
eventually  return  to  the  same  point,  no  matter  how  intricate  or  tortuous  the 
route  may  be  through  which  it  in  the  meanwhile  travels;  for  this  reason  the 
blood  is  said  to  move  in  a  circle,  and  the  movement  itself  is  termed  the 
circulation. 

In  order  to  understand  the  reasons  for  the  movement  of  the  blood  in  one 
direction  only,  as  well  as  for  many  other  phenomena  connected  with  the 
circulation,  a  knowledge  of  the  structure  of  the  heart  and  its  internal  mechan- 
ism is  of  primary  importance. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  HEART. 

The  heart  is  a  conic  or  pyramid-shaped  hollow  muscle  situated 
in  the  thorax  just  behind  the  sternum.  The  base  is  directed  upward  and  to 
the  right  side;  the  apex  downward  and  to  the  left  side,  extending  as  far  as  the 
space  between  the  cartilages  of  the  fifth  and  sixth  ribs.  In  this  situation 
the  heart  is  enclosed  and  suspended  in  a  iibro-serous  sac,  the  pericardium, 
attached  to  the  great  vessels  at  its  base. 

The  heart  is  a  hollow,  double  muscle  organ,  consisting  of  a  right  and  a 
left  half,  separated  by  a  musculo-membranous  septum.  The  general 
cavity  of  each  side  is  subdivided  by  an  incomplete  transverse  fibrous  septum 
into  two  sm^aller  cavities,  an  upper  and  a  lower,  known  respectively  as  the 
auricle  and  the  ventricle.  The  heart  may  therefore  be  said  to  consist  of  four 
cavities,  the  walls  of  which  are  composed  of  muscle-tissue.     Of  these  four 

261 


262 


TEXT-BOOK  OF  PHYSIOLOGY. 


cavities,  the  right  auricle  and  the  right  ventricle  constitute  the  venous  heart; 
the  left  auricle  and  the  left  ventricle,  the  arterial  heart. 

The  right  auricle  is  quadrangular  in  shape  and  presents  on  its  posterior 

aspect  two  large  openings,  the  terminations 
of  the  two  final  trunks  of  the  venous  system, 
the  superior  and  inferior  vence  cavce  (Fig.  116). 
Below  the  auricle  communicates  with  the  ven- 
tricle by  a  large  opening  which,  from  its  posi- 
tion, is  termed  the  auricula-ventricular  open- 
ing. The  walls  of  the  auricle  are  extremely 
thin,  not  measuring  more  than  two  millimeters 
in  thickness. 

The  right  ventricle,  as  shown  on  cross- 
section,  is  crescentic  in  shape  owing  to  the 
projection  of  the  ventricular  septum.  It  pre- 
sents at  its  upper  left  angle  a  cone-shaped 
prolongation,  the  conus  arteriosus.  From  this 
prolongation,  and  continuous  w^ith  it,  arises 
the  pulmonary  artery.  The  wall  of  the  ven- 
tricle measures  in  the  middle  about  four  milli- 
meters in  thickness.  The  inner  surfaces  of 
the  ventricle  show:  (i)  a  complicated  system 
of  muscle  ridges  and  bands,  the  columnce  car- 
necE  (fleshy  columns),  and  (2)  a  set  of  muscle 
projections,  the  muscuH  papillares  (papillary 
muscles),  which  arise  by  a  broad  base  from 
the  walls  of  the  ventricle  and  project  upward 
toward  the  auriculo-ventricular  opening. 
From  the  apex  of  each  papillary  muscle  there 
are  given  off  fine  tendinous  cords,  the  chordce 
tendinecB,  which  become  attached  above  to  the 
under  surface  of  the  auriculo-ventricular  valve. 
The  left  auricle,  similar  in  general  shape  to 
the  right,  presents  posteriorly  four  openings, 
the  terminations  of  the  four  final  trunks  of  the 
venous  system  of  the  lungs,  the  pulmonary 
veins.  Below  is  found  the  corresponding 
auriculo-ventricular  opening.  The  wall  of  the 
auricle  measures  about  3  mm.  in  thickness. 
The  left  ventricle  (Fig.  117)  is  conic  in  shape 
from  above  downward  and  oval  or  circular  in 
shape  on  cross-section.  At  its  upper  inner 
angle  it  presents  a  circular  orifice,  the  mar- 
gins of  which  give  attachment  to  the  walls  of 
the  aorta,  the  main  arterial  trunk  of  the  sys- 
temic circulation.  The  inner  surfaces  of  the 
ventricle  show  a  similar  though  better  developed  system  of  columnae  car- 
neas,  musculi  papillares,  chordae  tendineae,  etc.  The  wall  of  the  left  ven- 
tricle measures  about  ii.^  mm.  in  thickness  in  the  middle. 


Fig.  115. — Diagram  of  the 
Circulation,  i.  Heart.  2. 
Lungs.  3.  Head  and  upper  ex- 
tremities. 4.  Spleen.  5.  Intestines. 
6.  Kidney.  7.  Lower  extremities. 
8.  Liver. — {After  Daltoti.) 


THE  CIRCULATION  OF  THE  BLOOD. 


263 


The  Endocardium. — The  cavities  of  both  the  right  and  left  sides  of  the 
heart  are  lined  by  a  thin,  firm  connective-tissue  membrane,  closely  adherent 
to  the  muscle-tissue,  termed  the  endocardium.  It  contains  also  elastic  tibers 
and  smooth  muscle-fibers.  Its  entire  surface  is  covered  with  a  layer  of 
polygonal  endothelial  cells.     This  membrane  serves  partially  to  resist  undue 


Fig.  116  Fig.  117. 

Fig.  116. — Interior  of  Right  Auricle  and  Ventricle,  Exposed  by  the  Removal  of 
A  P.-\RT  OF  Their  W.^lls.  1,  Superior  vena  cava;  2,  inferior  vena  cava;  2',  hepatic  veins;  3,  3', 3", 
inner  wall  of  right  auricle;  4,  4,  cavity  of  right  ventricle;  4',  jfapillar}'  muscle;  5,  5',  5",  flaps  of 
tricuspid  valve;  6,  pulmonary-  artery,  in  the  wall  of  which  a  window  has  been  cut;  7,  on  aorta 
near  the  ductus  arteriosus;  8,  9,  aorta  and  its  branches;  10,  11,  left  auricle  and  ventricle. — {Allen 
Thomson.) 

Fig.  117. — Left  Auricle  and  Ventricle,  Opened  .\nd  Part  of  Their  W.^lls  Remov'ed 
TO  Show  Their  Cavities,  i,  Pulmonary  vein  cut  short;  i',  cavity  of  left  auricle;  t,,t,",  thick 
wall  of  left  ventricle;  4,  portion  of  the  same  with  papillan*  muscle  attached;  5,  the  other  papillary 
muscles;  5',  wall  of  the  ventricle;  6,  6',  the  segments  of  the  mitral  valve;  7,  the  figure  in  aorta  is 
placed  over  the  semilunar  valves;  7',  aorta;  8,  pulmonary  artery;  10,  branches  of  aorta. — {Allen 
Thomson.) 

distention  of  the  heart  during  contraction  and  to  prevent  separation  of  the 
muscle-fibers.  The  endocardium  is  continuous  with  the  lining  membrane 
of  the  blood-vessels. 

The  inter-auricular  septum  is  quite  thin  and  composed  of  the  two  layers 
of  the  endocardium,  between  which  is  a  layer  of  muscle-fibers.  It  presents 
at  its  low^er  portion  an  oval  depression,  t)\e  fossa  ovalis. 

The  inter-ventricular  septum  is  quite  thick  and  well  developed,  and  com- 


264  TEXT-BOOK  OF  PHYSIOLOGY. 

posed  of  the  two  layers  of  the  endocardium  enclosing  the  muscle-iibers.  In 
the  upper  and  central  portion  of  the  septum,  there  is,  however,  a  small  region 
which  is  thin  owing  to  the  absence  of  muscle-tissue  and  composed  of  endo- 
cardium only.     This  region  is  known  as  the  pars  membranacea  septi. 

The  Cardio-pulmonary  Vessels. — Though  the  two  sides  of  the  heart 
are  separated  from  each  other  by  the  auriculo-ventricular  septum,  they  are 
anatomically  and  physiologically  connected  by  the  intermediation  of  the 
•pulmonary  system  of  vessels:  viz.,  the  pulmonary  artery,  capillaries,  and 
veins  (Fig.  115). 

The  pulmmiary  artery  arises  from  the  conus  arteriosus  of  the  right  ven- 
tricle. After  a  short  upward  course  it  divides  into  a  right  and  a  left  branch, 
which  enter  the  corresponding  lungs.  The  vessel  at  once  divides  and  sub- 
divides into  a  number  of  branches,  which,  after  following  the  bronchial  tubes 
to  their  termination,  give  origin  to  capillaries  that  surround  the  air-cells  of 
the  pulmonary  lobules. 

The  capillaries  in  this  situation  are  extremely  abundant  and  well  developed. 
They  lie  close  to  the  inner  surfaces  of  the  air-cells.  The  blood  is  thus 
brought  into  intimate  relationship  with  the  intra-pulmonary  air,  and  the 
exchange  of  gases, — the  excretion  of  carbon  dioxid  and  the  absorption  of 
oxygen — for  which  the  cardio-pulmonary  vessels  exist,  is  readily  accom- 
plished. 

The  pulmonary  veins  which  return  the  blood  to  the  heart  are  formed 
by  the  convergence  and  union  of  the  small  veins  which  emei'ge  from 
the  capillary  system.  The  final  trunks  thus  formed,  the  four  pulmonary 
veins — two  from  each  lung — enter  the  posterior  wall  of  the  left  auricle. 

The  Course  of  the  Blood  through  the  Heart. — There  is  thus  estab- 
lished a  pathway  between  the  venae  cavae  on  the  right  side  and  the  aorta  on 
the  left  side,  by  way  of  the  right  side  of  the  heart,  the  cardio-pulmonary 
vessels,  and  the  left  side  of  the  heart. 

The  venous  blood  flowing  toward  the  heart  is  emptied  by  the  superior 
and  inferior  venae  cavae  into  the  right  auricle,  from  which  it  passes  through 
the  auriculo-ventricular  opening  into  the  right  ventricle  (Fig.  115);  thence 
into  and  through  the  pulmonary  artery  and  its  branches  to  the  pulmonary 
capillaries,  where  it  is  arterialized  by  the  exchange  of  gases — the  giving  up  of 
a  portion  of  carbon  dioxid  to  the  lungs  and  the  absorption  of  oxygen — and 
changed  in  color  from  bluish-red  to  scarlet-red.  The  arterialized  blood, 
flowing  toward  the  heart,  is  emptied  by  the  pulmonary  veins  into  the  left 
auricle,  from  which  it  passes  through  the  auriculo-ventricular  opening  into 
the  left  ventricle;  thence  into  the  aorta  and  its  branches  to  the  systemic 
capillaries,  where  it  is  de-arterialized  by  a  second  but  opposite  exchange  of 
gases — the  giving  up  of  a  portion  of  its  oxygen  to  the  tissues  and  the  absorp- 
tion of  carbon  dioxid  from  the  tissues — and  changed  in  color  from  scarlet  to 
bluish-red.  The  venous  blood  is  again  returned  by  the  systemic  veins  to  the 
venae  cavae.  Though  the  blood  is  thus  described  as  flowing  first  through  the 
right  side  and  then  through  the  left  side,  it  must  be  kept  in  mind  that  the  two 
sides  fill  synchronously;  that  while  the  blood  is  flowing  into  the  right  side 
from  the  venae  cavae,  it  is  also  flowing  from  the  pulmonary  veins  into  the  left 
side  in  equal  quantities  and  velocities. 

Though  there  is  but  one  set  of  capillaries,  as  a  rule,  between  arteries  and 


THE  CIRCULATION  OF  THE  BLOOD. 


26: 


veins,  there  is  an  exception  in  the  case  of  the  arteries  and  veins  of  some  of  the 
abdominal  viscera.  Thus  the  veins  emerging  from  the  capillaries  of  the 
stomach,  intestines,  pancreas,  and  spleen,  instead  of  passing  directly  to  the 
inferior  vena  cava,  unite  to  form  a  large  vein — the  portal  vein — which  enters 
the  liver.  In  this  organ  the  portal  vein  divides  to  form  a  second  capillary 
system  which  is  in  close  relation  to  the  liver  cells  and  from  which  arise  the 
veins  which  unite  to  form  the  hepatic  veins.  These  latter  vessels  empty 
and  discharge  the  blood  into  the  inferior  vena  cava  just  below  the  diaphragm. 
From  the  foregoing  facts  physiologists  frequently  divide  the  general 
circulation  into: 
I.  The  pulmonary  circulation,  which  includes  the  course  of  the  blood  from 

the  right   side  of  the  heart  through  the  lungs  to  the  left  side  of  the 

heart. 


Fig.  118. — Right  Cavities  of  the 
Heart.  Auriculo-ventricular  valves  open, 
arterial  valves  closed. — {Dalton.) 


Fig.  119. — Right  Cavities  of  the  Heart. 
Auriculo-ventricular  valves  closed,  semilunar 
valves  open. — {Dalton.) 


2.  The  systemic  circulation,  which  includes  the  course  of  the  blood  from 

the  left  side  of  the  heart  through  the  aorta  and  its  branches,  through 
the  capillaries  and  veins,  to  the  right  side  of  the  heart. 

3.  The  portal  circulation,  which  includes  the  course  of  the  blood  from  the 

capillaries  of  the  stomach,  intestines,  pancreas,  and  spleen  through 

the  portal  vein,  to  the  liver. 
Orifices  and  Valves. — The  movement  of  the  blood  along  the  path  of 
the  circle  above  outlined  is  accomplished  by  the  alternate  contraction  and 
relaxation  of  the  muscle  walls  of  the  heart.  That  the  movement  may  be  a 
progressive  one,  that  there  shall  be  no  regurgitation  during  either  th6  con- 
traction or  the  relaxation,  it  is  essential  that  some  of  the  orifices  of  the 
heart  be  closed  during  each  of  these  periods.  This  is  accomplished  by  the 
heart  valves. 

The  right  auriculo-ventricular  opening  is  surrounded  and  strengthened 
by  a  ring  of  fibrous  tissue  to  which  is  attached  a  membrane  partially  sub- 


266 


TEXT-BOOK  OF  PHYSIOLOGY. 


divided  into  three  portions  or  cusps,  which  during  the  period  of  relaxation 
are  directed  into  the  ventricle  (Fig.  ii8);  during  the  period  of  contraction 
they  are  raised  and  pkiced  in  complete  apposition,  when  they  act  as  a  valve 
preventing  a  backward  flow  into  the  auricle  (Fig.  119).  In  the  former 
position  the  valve  is  open;  in  the  latter,  shut.  For  these  reasons  this  struc- 
ture is  known  as  the  tricuspid  valve.  This  valve  is  formed  of  fibrous  tissue 
derived  from  the  librous  ring,  and  some  muscle-fibers,  and  covered  over  by  a 
reduplication  of  the  endocardium.  To  the  under  surface  and  to  the  edges 
of  this  valve  the  tendinous  cords  of  the  papillary  muscles  are  firmly  and 
intricately  attached.  These  cords  are  just  sufficiently  long  to  permit  closure 
of  the  valve  and  to  prevent  its  being  floated  into  the  auricle. 

The  orifice  of  the  pulmonary  artery  is  also  surrounded  by  a  ring  of 
fibrous  tissue  to  which  are  attached  three  semilunar  or  pocket-shaped  mem- 
branes, the  semilunar  valves.  Each  valve  is  formed  by  a  reduplication  of 
the  endocardium  strengthened  by  fibrous  tissue.  In  the  center  of  the  free 
edge  of  the  valve  there  is  a  small  nodule  of  fibro-cartilage   (the  corpus 

Aurantii).  The  outer  edge  of  the 
valve  is  strengthened  by  a  delicate 
fibrous  band.  A  similar  band 
strengthens  the  convex  attached 
portion  of  the  valve  just  where  it  is 
joined  to  the  fibrous  ring.  A  third 
set  of  fibers  pass  toward  the  nod- 
ule, interlacing  in  all  directions. 
Two  narrow  crescentic-shaped  areas 
(the  lunulae)  near  the  free  edge  are 
devoid  of  these  fibers.  During  the 
period  of  relaxation  of  the  heart  the 
edges  of  the  valves  are  in  close  ap- 
position and  prevent  a  return  of 
the  blood  into  the  ventricle  (Fig. 
118);  during  the  contraction  they 
are  directed  into  the  artery  (Fig. 
119).  In  the  former  position  they 
are  shut;  in  the  latter,  they  are  open. 
The  left  auriculo-ventricular  opening  is  provided  with  a  similar  though 
better  developed  fibrous  ring  and  membranous  valve.  It  is,  however, 
subdivided  into  but  two  portions  or  cusps,  and  is  therefore  termed  the 
bicuspid  valve,  or,  from  its  fancied  resemblance  to  a  bishop's  mitre,  the 
mitral  valve.  The  general  arrangement,  connections,  and  mode  of  action 
of  .this  valve  are  similar  in  all  respects  to  those  of  the  tricuspid  valve.  The 
orifice  of  the  aorta  is  also  surrounded  by  a  ring  of  fibrous  tissue  to  which 
are  attached  three  semilunar  or  pocket-shaped  valves  (Fig.  117),  which  in 
their  arrangement,  connections,  and  mode  of  action  are  similar  in  all  respects 
to  those  at  the  orifice  of  the  pulmonary  artery.  The  anatomic  relations  of 
the  cardiac  orifices  one  to  the  other  and  the  appearance  presented  by  the 
valves  when  closed  are  represented  in  Fig.  120. 

The  Heart  Muscle-fibers  and  Their  Arrangement. — The  muscle- 
fibers  of  the  heart  represent  in  their  structure  a  type  between  the  ordinary 


Fig.  120.— Valves  of  the  Heart,  i.  Right 
auriculo-ventricular  orifice,  closed  by  the  tri- 
cuspid valve.  2.  Fibrous  ring.  3.  Left  auric- 
ulo-ventricular orifice,  closed  by  the  mitral 
valve.  4.  Fibrous  ring.  5.  Aortic  orifice  and 
valves.  6.  Pulmonic  orifice  and  valves.  7,8,9. 
Muscular  fibers.' — {Bonamy  and  Bean.) 


THE  CIRCULATION  OF  THE  BLOOD. 


267 


lateral  union 


capil- 
lary 


Striated  muscle  and  the  smooth  muscle.  A  longitudinal  section  of  the 
heart-muscle  shows  a  reticulated  arrangement  of  the  fibers,  the  outcome  of 
a  similar  reticulated  condition  of  the  mesodermic  material  in  which  they 
develop.  The  mesodermic  reticulum  containing  numerous  nuclei  is 
termed  a  syncytium.  As  the  heart  develops  the  muscle-fibers  make  their 
appearance  in  the  protoplasm  and  assume  an  arrangement  which  corre- 
sponds to  that  of  the  trabeculee  composing  the  reticulum  (Fig.  121).  In 
the  adult  heart  the  intermediary  spaces  are  reduced  to  narrow  clefts  in 
consequence  of  the  multiplication  of  the  muscle-fibers.  The  clefts  are 
occupied  with  connective  tissue,  blood-vessels,  lymphatics,  etc.  The  indi- 
vidual fiber  consists  of  alternate  dim 
and  light  bands  similar  to  the  corre- 
sponding bands  of  the  ordinary  skeletal 
muscles,  though  it  is  devoid  of  a  sar- 
colemma.  Among  the  fibers  large  oval 
nuclei  are  distributed.  At  varying  in- 
tervals the  fibers  are  interrupted  by  in- 
tercalated. When  the  heart  muscle  is 
treated  with  caustic  potash  the  trabec- 
ulas  separate  at  the  level  of  these  disks, 
forming  what  has  hitherto  been  termed 
the  muscle  cell  or  fiber. 

The  arrangement  of  the  muscle- 
fibers  is  quite  complicated  and  in  ac- 
cordance with  the  functions  of  the  in- 
dividual portions  of  the  heart.  In  the 
auricles  the  fibers  are  arranged  in  two 
sets:  an  outer  transverse  set,  which 
pass  from  auricle  to  auricle,  and  an 
inner  longitudinal  set,  which  pass  over 
the  auricles  and  are  attached  anteriorly 
and  posteriorly  to  the  connective  tissue 
of  the  transverse  auriculo-ventricular 
septum.  The  longitudinal  fibers  of  the 
auricles  are  practically  independent  of 
ecah  other.  Circularly  arranged  fibers 
are  present  near  the  terminations  of 
the  venae  cavae  and  pulmonary  veins. 

In  the  ventricles  the  muscle-fibers  are  also  arranged  in  two  sets,  a 
superficial  longitudinal  and  a  deep  transverse,  though  their  arrangement  is 
somewhat  more  complicated  than  that  observed  in  the  auricles.  In  a 
general  way  it  may  be  said  that  the  superficial  longitudinal  fibers  on  both 
the  anterior  and  posterior  surfaces  take  their  origin  in  the  connective  tissue 
of  the  auriculo-ventricular  septum.  The  superficial  fibers  on  the  anterior 
surface  of  the  heart  pass  obliquely  downward  and  forward  from  right  to  left 
toward  the  apex,  where  they  turn  backward  and  inward  in  a  vertical  manner 
after  which  they  ascend  to  terminate  in  the  wall  of  the  septum,  the  columnae 
carneas  and  muscuH  papillares.  The  superficial  fibers  of  the  posterior  sur- 
face of  the  heart  pass  obhquely  downward  from  left  to  right,  wind  around 


Nucleus  of 

a  muscle 

fiber. 


Intercalated 
disc. 


Nucleus  of 
a  connective 
tissue  cell. 

Fig.  121. — From  a  Longitudinal  Sec- 
tion OF  A  P.APILLARY  MuSCLE  OF  THE  HUM.AN 

Heart,      x  360.     {S'chr.) 


268 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  apex,  turn  upward  and  end  in  the  same  structures  as  do  the  fibers  from 
the  anterior  surface.  The  fibers  from  the  base  of  the  right  ventricle  termi- 
nate in  the  structures  of  the  left  ventricle,  while  those  from  the  left  ventricle 
terminate  in  the  structures  of  the  right  ventricle.  Longitudinal  fibers  are 
also  found  on  the  inner  surface.  The  transverse  fibers  are  very  abundant 
and  surround  each  ventricle  separately  though  they  are  continuous  with 
each  other  across  the  septum.  Between  the  superficial  longitudinal  and  deep 
transverse  fibers  there  are  several  layers  of  fibers  which  possess  varying 


Fig.  12  2. — Arr.\ngement  of'Ventricul.ar  Muscle-fibers.  (After  MacCallum.)  I  and 
II,  Superficial  fibers  of  the  left  ventricle  and  conus  arteriosus;  III,  deep  layers  of  the  left  ven- 
tricle; LAV,  mitral  orifice;  RAV,  tricuspid  orifice;  PA,  pulmonarj^  artery. — {Hirschf elder.) 

degrees  of  obliquity.  The  general  arrangement  of  the  fibers  is  such  as  to 
ensure  a  complete  and  simultaneous  discharge  of  blood  from  both  auricles 
as  well  as  from  both  ventricles  (Fig.  122). 


THE  MUSCLE  CONNECTION  BETWEEN  THE  AURICLES  AND 

VENTRICLES. 

The  Muscle  Band  of  His,  or  the  Auriculo-ventricular  Bundle. — In 

the  mammalian  heart  there  is  no  continuity  of  the  muscle-fibers  across  the 
auriculo-ventricular  groove,  uniting  auricles  and  ventricles,  such  as  exists  in 
the  frog  or  turtle  heart.  The  muscle-fibers  of  the  auricles  and  ventricles  are 
completely  separated  from  each  other  by  the  transverse  fibrous  septum  to 
which  they  are  attached.  This  fact  has  for  a  long  time  made  it  difficult  to 
understand  how  the  contraction  process  which  begins  in  the  auricles  (and  to 
which  there  wall  be  occasion  to  refer  in  subsecjuent  paragraphs)  is  conducted 
to  the  ventricles.  The  physiologic  necessity  for  the  existence  of  a  muscle 
connection  between  the  auricles  and  ventricles  led  to  a  series  of  investigations 
which  have  resulted  in  the  discovery  of  an  elaborate  system  of  muscle-fibers 
by  which  they  are  united  both  anatomically  and  physiologically. 

In  1893  Wilhelm  His,  Jr.,  discovered  the  existence  of  a  band  or  bundle 
of  muscle-fibers  which  apparently  took  its  origin  from  the  posterior  part  of 
the  right  side  of  the  auricular  septum,  from  which  point  it  passed  forward 
just  above  the  auriculo-ventricular  septum  to  a  point  near  the  aortic  opening, 
where  it  divided  into  two  portions,  a  right  and  a  left,  of  which  the  latter 
apparently  ended  in  the  basis  of  the  aortic  leaflet  of  the  mitral  valve.  This 
bundle  has  been  termed  "the  muscle-bundle  of  His."  In  1904  Retzer  and 
Braunig,  working  independently,  corroborated  the  existence  of  this  bundle 
and  described  its  anatomic  course  more  completely.  The  investigations  of 
Braunig  led  to  the  conclusion  that  this  bundle  of  muscle-fibers  which  was 


THE  CIRCULATION  OF  THE  BLOOD.  269 

constantly  present  in  all  animals  examined,  including  man,  began  on  the 
right  side  of  the  auricular  wall  below  the  fossa  ovalis.  from  which  point  it 
passed  forward,  and  anteriorly  penetrated  the  auriculo-ventricular  septum 
to  become  connected  with  the  musculature  of  the  ventricular  septum  just 
below  the  pars  memhranacea  septi.  Though  both  these  obser^'ers  state  that 
the  bundle  divides  into  a  right  and  left  limb  as  it  enters  the  ventricular 
septum,  the  ultimate  distribution  and  termination  of  these  limbs  was  not 
clearly  determined.  Retzer  estimated  that  this  bundle  was  18  mm.  long, 
2.5  mm.  broad,  and  1.5  mm.  thick.  By  these  investigators  this  bundle  was 
termed  the  "auriculo-ventricular  bundle." 

In  1906  Tawara  published  the  results  of  an  extended  series  of  investiga- 
tions made  on  the  embryonic  and  adult  hearts  of  many  mammals  including 
man,  which  resulted  in  a  further  increase  of  knowledge  concerning  the 
development,  anatomic  course,  and  histologic  features  of  this  bundle,  and 
established  beyond  doubt  that  it  is  the  pathway  along  which  the  contraction 
process  is  conducted  from  the  auricles  to  the  ventricles. 

A  brief  summary  of  Tawara's  account  of  this  bundle  is  as  follows:  It 
arises  near  the  opening  of  the  coronary  sinus  where  it  is  connected  with  the 
true  auricular  fibers.  From  their  origin  the  fibers  converge  to  form  a  dis- 
tinct bundle  which  then  passes  forward  on  the  right  side  of  the  auricular 
septum  between  the  lower  edge  of  the  fossa  ovalis  and  the  auriculo-ventric- 
ular septum;  just  above  the  insertion  of  the  median  cusp  of  the  tricuspid 
valve  the  bundle  presents  a  very  complicated  network  of  muscle-fibers  which 
has  been  designated  as  a  knot  or  the  auriculo-ventricular  node  or  the  node 
of  Tawara;  from  the  anterior  portion  of  the  node  a  bundle  of  fibers  turns 
downward  and  penetrates  the  auriculo-ventricular  septum,  beyond  which  it 
passes  below  the  pars  membranacea  septi  to  the  upper  limit  of  the  muscle 
portion  of  the  ventricular  septum.  It  then  divides  into  two  limbs  or 
branches  which  descend  on  either  side  of  the  septum  under  the  endocar- 
dium, the  right  limb  lying  somewhat  deeper  than  the  left.  Each  of  these 
limbs  is  enclosed  by  a  layer  of  connective  tissue  which  isolates  it  from  the 
musculature  of  the  ventricular  septum  as  far  as  the  lower  third  of  the  ven- 
tricular cavities.  In  this  region  they  divide  into  a  number  of  bundles,  some 
of  which  enter  the  papillary  muscles,  while  others,  forming  tendon-like 
strands,  branch  freely  beneath  the  endocardium  and  spread  in  all  direc- 
tions over  the  entire  inner  surface  of  the  ventricle  and  enter  into  histologic 
connection  with  the  true  cardiac  muscle-fibers. 

The  fibers  composing  this  system,  and  termed  by  Tawara  from  its  sup- 
posed function  the  "  conduction  system  "  are  histologically  different  from  the 
cardiac  fibers,  in  so  far  as  they  are  poorer  in  sarcoplasm  and  similar  in  their 
appearance  to  embryonic  muscle-fibers.  In  the  auricular  portion  of  the 
bundle  the  fibers  exhibit  a  more  or  less  reticular  arrangement;  in  the  ven- 
tricular portion,  the  fibers  are  more  regularly  arranged,  are  richer  in  sarco- 
plasm and  present  a  number  of  fibrillar  near  their  periphery.  In  associa- 
tion with  the  muscle  fibers  composing  the  auriculo-ventricular  bundle  there 
is  a  special  collection  of  nerve  cells  and  nerve  fibers.  Their  function  is 
unknown. 

The  ultimate  termination  of  the  system,  beneath  the  endocardium,  con- 
stitutes the  so-called  Purkinje  fiber  layer.     In  the  sheep,  calf,  and  in  other 


270  TEXT-BOOK  OF  PHYSIOLOGY. 

animals  these  fibers  are  abundant  and  readily  recognized;  though  they  are 
not  so  well  developed,  they  are  nevertheless  present  and  extensively  dis- 
tributed in  the  human  heart. 

The  Keith-Flack  Node  or  the  Sino-Auricular  Node. — This  is  a 
small  body,  discovered  by  the  investigators  whose  names  it  bears,  situated 
in  the  sulcus  terminalis  "just  below  the  fork  formed  by  the  junction  of  the 
upper  surface  of  the  auricular  appendix  with  the  superior  vena  cava."  It 
appears  to  be  a  remnant  of  primitive  muscle  tissue  at  what  was  formerly  the 
junction  of  the  sinus  venosus  and  the  auricle.  In  its  structure  it  resembles 
the  auriculo-ventricular  (Taw^ara's)  node,  in  that  it  consists  of  peculiar 
muscle-fibers,  nerve-cells,  and  nerve-fibers  enclosed  by  connective  tissue.  It 
is  also  provided  with  an  abundant  blood  supply.  In  the  human  heart,  the 
muscle-fibers  of  this  remnant  are  striated,  possess  w^ell  marked  and  elongated 
nuclei  and  are  plexiform  in  arrangement.  From  the  node  the  muscle-fibers 
extend  downward  along  the  sulcus  terminalis  for  about  two  centimeters. 
The  thickness  of  the  bundle  is  about  two  millimeters.  Superiorly  the  node 
appears  to  be  connected  with  or  continuous  with  fibers  in  the  superior  vena 
cava;  interiorly  it  is  connected  with  the  true  auricular  fibers.  The  dissection 
of  this  node  shows  that  the  terminal  branches  of  the  vagi  and  sympathetic 
nerves  are  in  histologic  relation  with  the  nerve-cells.  The  situation,  struc- 
ture and  relations  of  this  neuro-muscle  node  appear  to  justify  the  assump- 
tion that  it  is  directly  concerned  in  the  initiation  of  the  heart-beat. 

THE  MECHANICS  OF  THE  HEART. 

Methods  of  Observation. — The  movements  of  the  heart,  as  well  as 
many  phenomena  connected  with  the  flow  of  blood  through  its  cavities,  have 
been  determined  by  observation  of,  and  experimentation  on,  the  exposed  heart 
of  a  mammal — e.g.,  dog,  cat,  rabbit — supplemented  and  corrected  by  experi- 
ments on  the  heart  in  its  normal  relations.  Valuable  information  as  to  the 
heart-beat  and  the  influences  which  modify  it  has  been  obtained  from  experi- 
ments made  on  the  isolated  heart  of  the  turtle,  frog,  and  allied  animals. 

If  the  thorax  of  a  dog,  completely  anesthetized,  is  opened  and  artificial 
respiration  established,  the  heart  will  be  observed  in  active  movement  inside 
the  pericardium.  If  this  sac  is  divided  and  turned  aside,  the  heart  will  be 
fully  exposed  to  view.  At  the  normal  rate  of  movement  of  the  heart 
characteristic  of  the  dog  it  will  be  almost  impossible  to  determine  either  the 
succession  of  events  or  their  duration.  But  by  observing  the  heart  under 
different  conditions  at  different  rates  of  movement  and  with  instrumental 
aids,  physiologists  have  succeeded  not  only  in  analyzing  the  movements,  but 
in  describing  their  sequence  and  in  estimating  their  time  duration. 

Phenomena  Observed. — From  many  observations  and  experiments  it 
has  been  determined  that  the  heart  at  each  beat  presents  two  distinct  move- 
ments which  alternate  with  each  other  in  quick  succession.  One  is  the 
movement  of  contraction,  or  the  systole,  by  which  the  blood  contained  within 
its  cavities  is  ejected  into  the  arteries — pulmonary  artery  and  aorta;  the 
other  is  the  movement  of  relaxation,  or  the  diastole,  followed  by  a  pause 
during  which  the  cavities  again  fill  up  with  the  blood  from  the  venge  cavse  and 
pulmonary  veins. 


THE  CIRCULATION  OF  THE  BLOOD.  271 

The  contraction  of  any  part  of  the  heart  is  termed  the  systole;  the  relaxa- 
tion, the  diastole.  As  each  side  of  the  heart  has  two  cavities  the  walls  of 
which  contract  and  relax  in  succession,  it  is  customary  to  speak  of  an  auricu- 
lar systole  and  diastole,  and  a  ventricular  systole  and  diastole.  As  the  two 
sides  of  the  heart  are  in  the  same  anatomic  relation  to  each  other,  they 
contract  and  relax  in  the  same  periods  of  time. 

It  has  also  been  ascertained  that  the  contraction  of  the  auricles  and 
ventricles  as  well  as  their  subsequent  relaxations,  though  occurring  with 
extreme  rapidity,  do  not  take  place  simultaneously  but  successively;  that 
the  contraction  process  passes  over  the  heart  in  the  form  of  a  wave;  that  it 
begins,  indeed,  at  the  terminations  of  the  great  veins,  viz.,  the  vence  cava,  then 
passes  to  and  over  the  auricles,  thence  to  and  over  the  ventricles  from  base 
to  apex  with  great  rapidity,  but  occupying  in  these  different  regions  unequal 
periods  of  time;  that  the  relaxation  immediately  succeeds  the  contraction,  in 
the  same  order,  and  that  at  the  close  of  the  ventricular  relaxation  there  is  a 
period  during  which  the  whole  heart  is  in  repose,  passively  filling  with  blood. 

The  immediate  cause  of  the  movement  of  the  blood  through  the  vessels 
is  the  contraction  and  relaxation  of  the  muscle-walls  of  the  heart,  and  more 
particularly  of  the  walls  of  the  ventricles,  each  of  which  plays  alternately 
the  part  of  a  force-pump,  and  perhaps  to  a  slight  extent  of  a  suction-pump. 
The  motive  power  is  furnished  by  the  heart  itself,  by  the  transformation 
of  potential  energy,  stored  up  during  the  period  of  rest,  into  kinetic 
energy — i.e.,  heat  and  mechanic  motion. 

Changes  in  Position  and  Form. — It  is  also  apparent  under  the  condition 
of  the  foregoing  observation  that  the  heart  during  each  pulsation  undergoes 
changes  of  both  position  and  form.  In  the  diastolic  condition,  during  which 
the  heart  is  in  repose,  the  apex  is  directed  obliquely  downward  and  to  the 
left;  the  body  of  the  heart  is  enlarged  and  its  walls  relaxed.  As  the 
systole  begins  and  reaches  its  maximum,  the  apex  is  tilted  upward,  the 
entire  heart  is  rotated  on  its  axis  from  left  to  right  and  forced  forward  by 
the  expansion  and  elongation  of  the  pulmonary  artery  and  aorta.  As  the 
diastole  begins  and  rapidly  passes  to  its  completion  a  reverse  series  of 
movements  is  presented,  viz. :  an  ascent  of  the  heart  due  to  the  recoil  and 
shortening  of  the  pulmonary  artery  and  aorta,  a  rotation  of  the  heart  on 
its  axis  from  right  to  left,  and  a  fall  of  the  apex.  With  the  completion  of 
this  latter  event,  the  heart  for  a  brief  period  is  in  repose. 

It  is  probable,  however,  that  these  movements  are  not  permitted  to 
the  same  extent  in  the  unopened  chest,  for  the  following  reasons:  the 
heart  is  enclosed  in  the  pericardium,  is  supported  posteriorly  by  the 
expanded  lungs,  and  both  posteriorly  and  inferiorly  by  the  diaphragm,  all 
of  which  cooperate  in  keeping  the  heart,  and  more  particularly  the  right 
ventricle,  in  close  contact  with  the  chest-wall  and  limiting  its  movements. 
By  means  of  needles  inserted  into  the  apex  of  the  heart,  through  the  chest- 
walls,  it  has  been  shown  by  their  slight  movement  that  the  apex  is 
practically  a  fixed  point. 

In  the  diastolic  condition  the  shape  of  the  heart  near  the  base  is  elliptic 
on  cross-section,  the  long  diameter  extending  from  side  to  side.  In  the 
completed  systolic  condition  the  shape  of  the  same  cross-section  approxi- 
mates that  of  a  circle.     In  passing  from  the  diastolic  to  the  systolic  condition 


272  TEXT-BOOK  OF  PHYSIOLOGY. 

the  transverse  diameter  diminishes  while  the  antero-posterior  diameter 
increases,  and  the  whole  heart  becomes  somewhat  more  conic  in  shape. 
It  is  questionable  if  the  vertical  diameter  perceptibly  shortens.  During  the 
systole  the  heart  hardens,  increases  in  convexity,  and  is  more  forcibly  pressed 
against  the  chest  wall.  As  this  takes  place  suddenly,  it  gives  rise  to  a 
marked  vibration  of  the  chest  wall,  known  as — 

The  Cardiac  Impulse. — This  impulse  is  principally  observed  in  the 
space  between  the  fifth  and  sixth  ribs  about  an  inch  internal  to  a  line  drawn 
vertically  from  the  middle  of  the  clavicle.  The  cardiac  impulse  is  synchron- 
ous with  the  cardiac  systole. 

The  cardiac  impulse  may  be  recorded  with  an  appropriate  apparatus 
known  as  a  cardiograph;  the  record  obtained  with  it  is  known  as  a  cardiogram. 
A  cardiograph  consists  of  a  tambour  covered  with  a  thin  rubber  membrane 
provided  with  a  button.  The  tambour  is  supported  by  a  metallic  frame 
which  permits  of  an  easy  and  accurate  adjustment  of  the  button  over  the 
seat  of  the  cardiac  impulse.  A  rubber  tube  connects  the  cardiographic 
tambour  with  a  second  tambour  provided  with  a  recording  lever  and  thus 
transmits  all  variations  in  the  pressure  of  the  air  in  the  former  to  the  latter. 
When  all  adjustments  are  carefully  made  a  tracing  similar  to  that  shown 
in  Fig.  123  will  be  obtained,  in  which  the  slight  elevation   a   represents  the 

contraction  of  the  auricle  which,  completing 
the  fining  of  the  ventricle,  causes  the  apex  of 
the  heart  to  press  more  vigorously  against  the 
chest  wall;  b-c  represents  the  contraction  of 
the  ventricles,  at  which  moment  the  apex  is 
suddenly  and  forcibly  driven  against  the  chest 
wall;  c-d  represents  the  systolic  plateau,  the 
time  during  which  the  ventricle  is  discharging 
blood  into  the  aorta;  d-e  represents  the  relaxa- 
tion of  the  ventricle;  while  e-/ represents  the 
time  of  the  diastole,  during  which  the  heart 
Fig   123.— a  Cardiogram.  cavities  are  enlarging  with  the  incoming  of  a 

(After  Pachon.)  new  volume  of  blood  in  consequence  of  which 

the  heart  is  pressing  against  the  chest  walls.  The  systolic  plateau  is  charac- 
terized by  one  or  more  elevations  and  depressions,  the  true  cause  of  which  is 
unknown. 

'  The  Cardiac  Cycle. — The  term  cardiac  cycle  is  employed  to  express  the 
sequence  of  events  from  the  beginning  of  one  auricular  systole  and  the 
beginning  of  the  auricular  systole  which  immediately  follows  it.  An  examina- 
tion of  the  heart  shows  that  each  pulsation  may  be  divided  into  three  phases, 
viz. : 

1.  The  auricular  systole. 

2.  The  ventricular  systole. 

3.  The  pause  or  period  of  repose  during  which  both  auricles  and  ven- 
tricles are  at  rest. 

For  the  purpose  of  obtaining  accurate  information  as  to  the  sequence  of  events,  their  time 
relations,  as  well  as  of  the  pressure  %vithin  the  heart  cavities  during  each  phase  of  its  activity,  it 
is  necessary  to  obtain  graphic  records  of  the  entire  cardiac  cycle. 

This  was  first  successfully  accomplished  by  Chauveau  and  Marey,  by  means  of  sounds  or 
tambours  (Fig.  24)  introduced  through  the  jugular  vein  into  the  cavities  of  the  right  heart.     Each 


THE  CIRCULATION  OF  THE  BLOOD. 


273 


tambouir  consists  of  a  metallic  frame  covered  by  a  thin  rub- 
ber membrane.  By  means  of  flexible  tubes,  a.  v.,  the  in- 
terior of  each  tambour  can  be  placed  in  communication 
with  the  interior  of  a  second  tambour  provided  with  a  re- 
cording lever.  Pressure  appHed  to  the  cardiac  tambour  w^ill 
be  followed  by  a  movement  of  the  enclosed  air  toward  the 
recording  tambour  indicated  by  an  outward  movement  of 
its  membrane  and  a  rise  of  the  lever;  removal  of  the  pres- 
sure will  be  followed  by  a  movement  of  the  enclosed  air 
toward  the  cardiac  tambour  indicated  by  an  inward  move- 
ment of  the  membrane  and  a  fall  of  the  lever. 

When  the  tambours  are  introduced  into,  and  carefully 
adjusted  to  the  interior  of  the  right  heart,  the  auricular  and 
ventricular  contractions  \\'ill  exert  pressure  on  their  enclosed 
tambours  as  indicated  by  the  rise  of  the  levers  of  the  re- 
cording tambours,  which  continues  so  long  as  the  pressure 
lasts.  With  the  relaxation  of  the  auricular  and  ventricular 
walls  the  pressure  is  remo\ed  and  the  levers  fall  to  their 
former  position.  When  the  levers  are  applied  to  the  surface 
of  a  recording  cylinder  a  record  of  auricular  and  ventricular 
contractions  is  obtained  such  as  that  shown  in  Fig.  125. 

A  similar  record  would  be  obtained  if  the  tambours  were 
placed  in  the  cavities  of  the  left  side  of  the  heart. 

In  this  record  the  upper  and  lower  hnes  rep- 
resent respectively  the  contraction  and  relaxa- 
tion of  the  auricle  and  ventricle  as  well  as  the 
variations  of  pressure  occurring  within  them. 
A  study  of  this  record  shows  that  during  the 
period  of  repose  there  is  a  gradual  ascent  of  the 
tips  of  the  recording  levers,  the  result  of  a 
gradual  increase  of  pressure  due  to  the  accumu- 
lation of  blood  within  the  heart  cavities.  When 
this  reaches  a  certain  level  the  auricular  con-  ,  ■,■  , 

,  ,,      ,       ...  ,,  membrane    surroundmg    metal 

traction  occurs  rather  suddenly,  followed  by  an  frame-work;  a,  v,  ends  of  tubes  in 
equally    sudden    relaxation,    after  which  the    connection  with  tambours.— 
auricular  walls  remain  at  rest  for  a  relatively    ^^'^"-^^y-' 
long  period,  though  the  pressure  within  the  auricle  undergoes  variations  both  in 
the  way  of  increase  and  decrease  as  shown  by  small  undulations  on  the  curv^e. 


Fig.  124. — Cardiac  Sounds. 
V,  Tambours  to  be  inserted  into 
the  ventricle;  a,  tambour  to  be  in- 
serted into  the  auricle;  m,  rubber 


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Fig.  125. — Tracings  of  (i)  the  Intra-auricular  Pressure;  and  (2)  the  Intra-ventricular 
Pressure  of  the  Horse. — {Chauveau  and  Marey.) 


With  the  close  of  the  auricular  systole,  the  ventricular  systole  occurs 
quickly  and  energetically  and  endures  for  some  time,  after  which  the  ventricu- 


18 


274  TEXT-BOOK  OF  PHYSIOLOGY. 

lar  walls  quickly  relax  and  remain  at  rest  until  the  close  of  the  next  auricular 
contraction.  The  summit  of  the  ventricular  tracing  generally  spoken  of  as 
the  plateau  presents  a  series  of  elevations  and  depressions  as  stated  in  a 
foregoing  paragraph. 

A  comparison  of  the  two  traces  shows  that  between  the  close  of  the 
auricular  and  the  beginning  of  the  ventricular  systole  there  is  a  slight  pause 
known  as  the  inter  systolic  pause  (Chauveau).  The  tracings  also  show  that 
between  the  close  of  the  ventricular  contraction  and  the  beginning  of  the 
succeeding  auricular  contraction  there  is  a  period  during  which  the  whole 
heart  is  at  rest  and  the  cavities  filling  with  blood. 

For  the  purpose  of  obtaining  the  time  of  all  these  events,  the  recording 
surface  was  divided  into  ecjual  spaces  by  vertically  drawn  lines.  The  rate 
of  movement  of  the  surface  was  such  that  each  division  corresponded  to 
one-tenth  of  a  second.  The  record  thus  indicates  that  the  auricular  con- 
traction lasted  approximately  0.2  second,  the  ventricular  contraction  0.4 
second,  and  the  pause  0.4  to  0.6  second. 

From  similar  experiments  made  on  other  animals,  e.g.,  the  dog,  similar 
results  have  been  obtained;  but  by  reason  of  the  employment  of  more 
sensitive  and  more  quickly  responsive  tambours,  the  curve  of  the  auricular 
contraction  exhibits  variations  not  recorded  by  the  forms  of  tambour  used 
in  earlier  experiments.  Reference  to  these  variations  will  be  alluded  to  in 
subsequent  paragraphs.  The  results  obtained  by  recent  observers  now 
generally  accepted  are  in  accord  with  the  results  obtained  by  Chauveau  and 
Marey  by  means  of  their  cardiac  tambours  as  shown  in  Fig.  125. 

From  the  foregoing  facts  it  is  apparent  that  with  the  relaxation  of  the 
auricular  walls,  blood  at  once  flows  from  the  vense  cavae  and  the  pulmonary 
veins  into  the  auricular  cavities  and  continues  so  to  do  throughout  the  entire 
auricular  diastole.  With  the  relaxation  of  the  ventricular  walls  the  blood 
that  has  accumulated  in  the  auricles  up  to  this  time,  or  its  equivalent  coming 
from  the  venae  cavae  and  pulmonary  veins,  now  flows  into  the  ventricles  until 
they  are  nearly  filled.  Before  they  are  filled,  however,  the  auricular  diastole 
comes  to  an  end,  the  auricular  walls  again  contract  and  force  their  contained 
blood  into  the  ventricles  and  thus  rapidly  complete  the  filling.  The  ven- 
tricular systole  immediately  follows,  during  which  the  blood  is  driven  into  the 
pulmonary  artery  and  aorta.  This  having  been  accomplished,  the  ven- 
tricles relax,  and  the  blood  that  has  been  accumulating  in  the  auricles  begins 
to  flow  into  the  ventricles,  after  which  the  same  series  of  events  follows  as  in 
the  previous  cycle. 

The  Action  of  the  Valves  During  the  Cycle. — As  previously  stated, 
the  forward  movement  of  the  blood  is  permitted  and  regurgitation  prevented 
by  the  alternate  action  of  the  semilunar  and  the  auriculo-ventricular  valves. 
As  a  point  of  departure  for  a  consideration  of  the  action  of  the  valves  and 
their  relation  to  the  systole  and  diastole  of  the  heart,  the  close  of  the  ventricu- 
lar systole  may  be  conveniently  selected. 

At  this  moment,  if  the  blood  is  not  to  be  returned  to  the  ventricles,  the 
semilunar  valves  must  be  instantly  and  completely  closed.  This  is  accom- 
plished in  the  following  manner:  During  the  outflow  of  blood  from  the 
ventricles  the  valves  are  pushed  outward  toward  the  w^alls  of  the  vessels, 
though  not  coming  into  contact  with  them,  for  behind  them  are  the  pouches 


THE  CIRCULATION  OF  THE  BLOOD.  275 

of  Valsalva,  containing  blood,  continuous  with  and  under  the  same  pressure 
as  that  in  the  vessels  themselves.  With  the  cessation  of  the  outflow  and  the 
beginning  of  the  relaxation  the  pressure  of  the  blood  behind  the  valves 
suddenly  forces  them  inward  until  their  free  edges,  including  the  lunulae, 
come  into  complete  apposition.  By  this  means  the  orifices  of  the  pulmonary 
artery  and  aorta  are  securely  closed  and  a  return  flow  prevented.  Reversal 
of  the  valves  is  prevented  by  their  mode  of  attachment  to  the  fibrous  rings  of 
the  orifices. 

During  the  ventricular  systole  the  relaxed  auricles  have  been  filling  with 
blood.  With  the  ventricular  relaxation  this  volume,  or  its  equivalent,  flows 
readily  into  the  empty  and  easily  distensible  ventricles,  its  place  being  taken 
by  an  additional  volume  of  blood  flowing  from  the  venje  cavae  and  pulmonary 
veins.  Whether  the  ventricles  exert  a  suction  power  at  the  moment  of 
their  relaxation  is  an  undecided  question.  A  steady  stream  of  blood  into 
the  auricles  and  ventricles  continues  throughout  the  entire  period  of  rest  until 
both  cavities  are  filled.  The  tricuspid  and  bicuspid  valves  which  hang  down 
into  the  ventricular  cavities  are  now  floated  up  by  currents  of  blood  welling 
up  behind  them  until  they  are  nearly  closed.  The  auricles  now  contract, 
forcing  their  contained  volumes,  or  at  least  the  larger  portions  of  them,  into 
the  ventricles,  which  become  fully  distended. 

With  the  cessation  of  the  auricular  systole  the  ventricular  systole  begins. 
If  the  blood  is  not  to  be  returned  to  the  auricles  at  this  moment,  the  tricuspid 
and  mitral  valves  must  be  suddenly  and  completely  closed.  This  is  readily 
accomplished  by  reason  of  the  position  of  the  valves,  which  have  been 
floated  up  and  placed  almost  in  apposition  by  the  blood  itself.  With  the 
beginning  of  the  ventricular  pressure  the  blood  is  forced  upward  against  the 
valves  until  their  free  edges  are  brought  together  and  the  orifices  closed. 
Reversal  of  these  valves  into  the  auricles  is  prevented  by  their  attachment  to 
the  chordcB  tendinece,  and  the  latter  are  kept  from  moving  bodily  upward 
during  the  ventricular  contraction  by  the  compensatory  downward  pull  of 
the  papillary  muscles.  The  blood  now  confined  in  the  ventricle  between  the 
closed  auriculo-ventricular  and  semilunar  valves  is  subjected  to  pressure 
from  all  sides.  As  the  pressure  rises  proportionately  to  the  vigor  of  the 
contraction,  there  comes  a  moment  when  the  intra-ventricidar  pressure 
exceeds  that  in  the  aorta  and  pulmonary  artery.  Immediately  the  semilunar 
valves  of  both  vessels  are  thrown  open  and  the  blood  discharged.  The 
discharge  of  the  blood  by  the  contraction  of  the  ventricular  walls  is  probably 
aided  by  the  simultaneous  downward  displacement  of  the  more  central 
portion  of  the  auriculo-ventricular  septum,  due  to  the  contraction  of  the 
papillary  muscles.  Both  contraction  and  outflow  continue  until  the  ven- 
tricles are  practically  empty,  after  which  ventricular  relaxation  sets  in, 
attended  by  a  rapid  fall  of  pressure.  Lender  the  influence  of  the  positive 
pressure  of  the  blood  in  the  sinuses  of  Valsalva  the  semilunar  valves  are 
again  closed,  the  column  of  blood  supported,  and  regurgitation  is  prevented. 
In  the  meantime  and  while  the  ventricles  are  contracting,  blood  is  again 
flowing  into,  and  accumulating  in  the  auricles  and  thereby  distending  them 
preparatory  to  the  next  systole.  With  the  accumulation  of  blood  in  the 
auricles  the  cardiac  cycle  is  completed. 

The  approximate  changes  in  the  shape  of  the  heart,  the  variations  in  the 


276 


TEXT-BOOK  OF  PHYSIOLOGY 


size  of  its  cavities  and  in  that  of  the  blood-vessels  arising  from  them,  and 
the  relative  position  of  the  valves  during  systole  and  diastole  are  shown  in 
Fig.  126. 

Relative  Functions  of  Auricles  and  Ventricles. — Though  both 
auricles  and  ventricles  are  essential  to  the  continuous  movement  of  blood, 
they  possess  unequal  values  in  this  respect.  The  passage  of  the  blood 
through  the  pulmonary  and  systemic  vessels  is  accomplished  by  the  driving 
power  of  the  right  and  left  ventricles  respectively,  aided,  however,  by  minor 
extra-cardiac  forces.     They  may  be  regarded  therefore  &?>  force-pumps. 

If  the  heart  consisted  of  ventricles  only,  the  flow  of  blood  from  the  venae 
cavae  and  pulmonary  veins  would  be  temporarily  arrested  during  their  systole 


S.a.-D.v. 


D.a.-S.v. 


Fig.  126. — Diagrammatic  Representation  of  the  Auricular  Systole,  S.a.,  with  the 

VENTiaCULAR  DiASTOLE,  D.  v.,  AND  OF  THE  AURICULAR  DiASTOLE,  D.  a.,  WITH  THE  VENTRICULAR 

Systole,  S.v.  C.s.  and  C.i.  Superior  and  inferior  cavae;  A.d.  (atrium  dextrum)  right  auricle; 
A.s.  (atrium  sinistrum)  left  auricle;  V.d.  (ventriculus  dexter)  right  ventricle;  V.s.  (ventriculus 
sinister)  left  ventricle;  P.  pulmonary  artery;  A.  aorta;  P.P.  papillary  muscles. — {Landois.) 


and  their  subsequent  refilling  delayed.  This  is  obviated,  however,  by  the 
addition  of  the  auricles;  for  during  the  ventricular  systole  the  blood  continues 
to  flow  into  the  auricles,  in  w^hich  it  is  temporarily  stored  until  the  ventricular 
relaxation  sets  in.  With  this  event  the  accumulated  blood  passes  into  the 
ventricles,  which  are  thus  practically  filled  before  the  auricular  systole  occurs 
by  which  the  filling  is  completed.  By  this  means  there  is  no  delay  in  the 
filling  of  the  ventricles,  and  hence  their  effective  working  as  force-pumps  is 
more  readily  secured.  The  auricles  may  therefore  be  regarded  as  feed- 
pumps. For  this  reason  it  is  probable,  notwithstanding  the  contraction  of 
the  circular  muscle-fibers  at  the  terminations  of  the  venous  system,  that  the 
flow  of  blood  into  the  auricles  is  never  entirely  arrested.     Regurgitation  in 


THE  CIRCULATION  OF  THE  BLOOD. 


277 


max  valve 


to  manometer 


mm  valve 


these  vessels  does  not  occur  for  the  reason  that  the  pressure  in  the  auricles 
is  not  higher  than,  if  as  high  as,  in  the  great  veins. 

Synchronism  of  the  Two  Sides  of  the  Heart. — If  the  balance  of  the 
circulation  is  to  be  maintained,  the  two  sides  of  the  heart  must  act  synchron- 
ously. That  they  do  so  can  be  shown  by  attaching  levers  to  their  walls,  and 
thus  recording  their  activities.  The  synchronism  is  so  perfect  that  until 
recently  it  was  generally  believed  to  be  dependent  on  nerve  connections;  but 
Porter  has  shown  that  if  the  ventricles  are  cut  away  from  the  auricles,  in 
which  the  nerve  mechanism  seems  to  lie,  the  synchronism  of  the  former  is 
not  interfered  with;  that  the  apical  halves  of  the  ventricles  will  beat  syn- 
chronously if  perfused  with  blood  through  an  artery;  that  a  very  small  bridge 
of  muscle-tissue  will  carry  the  wave  of  excitation  from  one  part  to  neighboring 
parts  of  the  ventricle.  It  is  therefore  probable  that  the  synchronism  is 
accomplished  through  muscle  connections  only.  The  left  ventricle,  in 
keeping  with  the  greater  work  it  has  to  do, 
has  a  greater  development  than  the  right, 
and  therefore  contracts  more  energetically. 
The  ratio  between  the  energy  of  the  left  and 
right  sides  is  approximately  3  to  i. 

Intra-ventricular  Pressure. — It  has 
been  stated  that  during  the  pause  of  the 
heart  when  its  cavities  are  filling  with  blood 
the  semilunar  valves  are  kept  closed  by  the 
pressure  of  the  blood  in  the  pulmonary 
artery  and  aorta,  a  pressure  due  to  the  resis- 
tance, as  will  be  explained  later,  offered  to 
the  flow  of  the  blood  mainly  by  the  smaller 
arteries  and  capillaries;  that  they  are  opened 
only  when  the  pressure  of  the  blood  within 
the  ventricle  exceeds  that  in  the  arteries.  It 
becomes,  therefore,  a  matter  of  importance 
to  determine  the  extent  of  this  pressure  as 
well  as  its  variations  during  the  course  of  a 
cardiac  cycle.  This  can  be  done  by  inserting  a  long  catheter  into  either  the 
right  or  left  ventricle,  through  the  jugular  vein  or  the  carotid  artery  respec- 
tively, and  connecting  its  free  extremity  with  a  mercurial  manometer.  By 
the  interposition  of  a  double  valve  such  as  represented  in  Fig.  135,  it  becomes 
possible,  according  to  the  direction  in  which  the  blood  is  permitted  to  flow, 
to  obtain  either  the  maximal  or  the  minimal  pressure  that  occurs  in  the  heart 
during  a  series  of  cycles.  Thus  Goltz  found  in  the  left  ventricle  of  the  dog  a 
maximal  pressure  of  114  to  135  mm.;  in  the  right  ventricle,  a  pressure  of  35 
to  62  mm.  Minimal  pressures  of  —23  to  —52  mm.  for  the  left  ventricle  have 
also  been  obtained. 

The  maximal  pressure  in  the  ventricles  during  the  systole,  though  always 
higher  than  that  in  the  arteries,  is  neither  a  fixed  nor  an  invariable  pressure, 
as  it  rises  and  falls  with  the  latter  from  moment  to  moment.  Within  limits 
the  cardiac  power,  and  therefore  the  intra-ventricular  pressure,  is  capable  of 
considerable  increase.  The  function  of  the  heart  is  to  drive  the  blood 
through  the  vessels  with  a  given  velocity.     This  is  possible  only  by  first  over- 


FlG.     127. — V. 


to  heart 


Tliis  is  placed  in  the  course  of  the 
tube  between  heart  and  manometer, 
so  that  the  latter  may  be  used  as  a 
maximum,  minimum,  or  ordinary 
manometer  according  to  the  tap  which 
is  left  open. — (Starling.) 


278  TEXT-BOOK  OF  PHYSIOLOGY. 

coming  the  rtsistance  to  the  flow  offered  by  the  vessels,  as  indicated  by  the 
arterial  pressure.  As  this  is  a  variable  factor,  rising  and  falling  very  con- 
siderably at  times,  the  heart  must  meet  and  exceed  each  rise,  within  limits  if 
the  circulation  is  to  be  maintained.  This  it  does  by  calling  on  the  reserve 
power  with  which  it  is  endowed.  The  power  put  forth  by  the  heart  is 
proportional  to  the  work  it  has  to  perform.  If  the  arterial  pressure  continues 
higher  than  the  average  for  any  length  of  time,  the  heart  meets  the  condition 
bv  an  hypertrophy  of  its  walls,  but  in  so  doing  it  encroaches  on  the  reserve 
power  proportionally  and  when  the  latter  has  become  exhausted  the  heart 
may,  on  some  sudden  rise  of  pressure  in  the  aorta,  be  unequal  to  the  discharge 
of  blood  from  its  cavities  and  hence  become  paralyzed. 

The  Intra-ventricular  Pressure  Curve  of  the  Dog. — It  was  stated  in 
a  previous  paragraph  that  the  contraction  of  the  auricles  and  ventricles  of 
animals  other  than  the  horse  have  been  graphically  recorded.  This  is 
especially  true  of  the  heart  of  the  dog.  A  graphic  record  of  the  intra- 
ventricular pressure,  its  course,  its  variations,  and  time  relations  is  necessary 
for  the  interpretation  of  the  heart  mechanisms.  With  such  a  record  may  be 
compared  the  records  of  the  pressures  in  the  venae  cavae  and  auricles  on  the 
one  hand,  and  in  the  aorta,  on  the  other  hand,  and  their  relations  one  to 
another  accurately  defined. 

The  intra-ventricular  pressure  has  been  obtained  by  specially  de\ased 
manometers  or  tonometers  or  tono graphs,  as  they  are  variously  termed,  the 


Fig.  128. — ^\'.  Curve  of  the  Pressure  in  the  Ventricle  of  the  Dog.  A.  Curve  of 
THE  Pressure  in  the  Aorta.  The  curves  were  taken  simultaneously,  s.  Tuning-fork  vibrations 
each  corresponding  to  i/ioo  of  a  second.  The  ordinates  0-5  correspond  in  the  two  records, 
o,  Closure  of  the  auriculo- ventricular  valve;  i,  opening  of  the  semi-lunar  valves;  2,  point  of 
maximum  pressure;  3,  beginning  of  the  ventricular  relaxation;  4,  closure  of  the  semi-lunar  valves; 
5,  opening  of  the  auriclo-ventricular  valve.    {Hilrthle.) 


construction  of  which  is  such  as  to  enable  them  to  respond  instantly  to  the 
very  rapid  variations  of  the  pressure  which  occur  during  the  brief  cardiac 
cycle.  One  of  the  best  is  that  of  Hiirthle.  This  consists  of  a  small  metallic 
tambour  5  or  6  millimeters  in  diameter,  covered  by  a  thin  rubber  membrane. 
A  small  button  resting  on  the  membrane  plays  against  an  elastic  steel  spring, 
by  the  tension  of  which  the  pressure  of  the  blood  is  counterbalanced.  The 
movements  of  the  membrane  are  taken  up,  magnified,  and  recorded  by  a 
suitable  lever.     A  long  cannula  is  inserted  into  the  right  ventricle  through 


THE  CIRCULATION  OF  THE  BLOOD.  279 

the  jugular  vein  or  into  the  left  ventricle  through  the  carotid  artery.  Both 
cannula  and  tambour  are  filled  with  an  alkaline  solution  to  prevent  coagula- 
tion of  the  blood,  and  then  made  air-tight.  The  pressure  of  the  blood  in  the 
ventricle  is  thus  transmitted  by  a  liquid  column  to  the  tambour  and  to  its 
attached  lever.  With  such  a  manometer  a  curve  is  registered  similar  to  that 
shown  in  Fig.  128.  To  obtain  the  absolute  value  of  this  curve  in  millimeters 
of  mercury  it  is  necessary  to  graduate  the  instrument  previously.  i\n 
examination  of  the  cur^T  shows  that  previous  to  the  ventricular  contraction 
there  is  a  very  slight  rise  of  pressure  above  that  of  the  atmosphere,  repre- 
sented by  the  Hne  a — b.  This  may  be  due  to  the  inflow  of  blood  from  the 
auricle  during  the  diastole.  At  o  the  pressure  suddenly  rises,  passes  quickly 
to  its  maximum  value,  (2),  which  is  maintained  with  slight  variations  for 
some  time,  and  then  suddenly  (3)  begins  to  fall,  and  rapidly  reaches  the 
line  of  atmospheric  pressure,  or  even  passes  below  it,  becoming  negative  in 
fact  for  a  short  period.  The  curve  may  also  be  taken  as  a  record  of  the 
ventricular  contraction,  for  there  are  reasons  to  believe  that  the  two  closely 
coincide  throughout  their  entire  course.  A  characteristic  feature  of  this 
curve  is  the  more  or  less  horizontal  portion  comprised  between  the  points 
2  and  3,  marked  by  several  elevations  and  depressions,  which  has  been 
termed  the  systolic  plateau. 

With  other  forms  of  elastic  manometers,  especially  those  in  which  the 
transmission  of  the  intra-ventricular  pressure  is  effected  by  air  or  by  a  com- 
bination of  air  and  liquid,  this  portion  of  the  curve  is  represented  by  a  single 
peak,  which  is  taken  as  an  indication  that  the  maximum  pressure  once  reached 
is  not  maintained,  but  immediately  begins  to  fall  to  its  original  level,  not- 
withstanding the  continued  contraction  of  the  ventricle.  Those  who  adhere 
to  this  view  attribute  the  plateau  to  the  closure  of  the  orifice  of  the  catheter 
by  the  contracting  and  approximating  walls  of  the  ventricle.  There  are 
reasons  for  believing,  however,  that  the  former  curve  is  the  more  correct  repre- 
sentation of  the  course  of  the  intra-ventricular  pressure.  Bayliss  and  Star- 
ling photographed  on  a  moving  surface  the  oscillations  of  a  fluid,  a  solution 
of  sodium  sulphate,  in  a  capillary  glass  tube  one  end  of  which  was  closed, 
the  other  end  placed  in  connection  with  an  intra-cardiac  catheter,  the  oscil- 
lations representing  the  variations  in  pressure.  The  photogram  thus 
obtained  resembles  the  curve  obtained  by  Hiirthle's  membrane  manometer. 

The  Relation  of  the  Intra-ventricular  Pressure  Curve  to  the  Intra- 
cardiac Mechanisms. — By  itself  the  curve  of  the  intra-ventricular  pressure 
affords  no  indication  as  to  events  occurring  within  the  heart:  i.e.,  as  to  the 
times  during  the  systole,  of  the  closure  of  the  auriculo-ventricular  valves  and 
the  opening  of  the  semilunar  valves,  or  the  times  during  the  diastole,  of  the 
closure  of  the  semilunar  valves  and  the  opening  of  the  auriculo-ventricular 
valves. 

By  registering  the  curve  of  pressure  in  the  aorta  simultaneously  with  the 
pressure  in  the  left  ventricle  (Fig.  128),  and  by  comparing  these  with  the 
curv'e  of  the  successive  differences  of  pressure  in  these  two  cavities  as  deter- 
mined by  the  "  dift'erential  manometer,"  it  becomes  possible  to  mark  on  the 
ventricular  pressure  cun'e  the  points  at  which  the  foregoing  events  take 
place.  As  the  outcome  of  many  observ^ations  and  determinations,  the 
following  statements  may  be  made:  As  a  point  of  departure  for  a  considera- 


28o  TEXT-BOOK  OF  PHYSIOLOGY. 

tion  of  the  relation  of  the  intra-vcntricular  pressure  to  the  time  of  action 
of  the  valves,  the  close  of  the  ventricular  systole  may  be  conveniently 
selected. 

During  the  systolic  plateau  the  blood  is  passing  from  the  ventricle  into  the 
aorta.  Independent  of  the  slight  elevations  and  depressions  there  is  an 
absolute  fall  of  pressure  between  the  beginning  and  the  end  of  the  plateau. 
There  is  also  a  corresponding  fall  in  the  aortic  pressure,  corresponding  to 
these  two  points.  The  cur\'e  of  the  difference  of  pressure  shows,  however, 
that  the  ventricular  pressure  is  slightly  higher  than  the  aortic.  This  fall  in 
both  ventricular  and  aortic  pressures  is  due  to  the  escape  of  blood  from  the 
arterial  into  and  through  the  capillary  system.  At  3  (see  Fig.  128),  however, 
whether  completely  emptied  or  not,  the  ventricle  suddenly  relaxes,  and  its 
pressure  soon  falls  below  that  in  the  aorta.  As  soon  as  this  takes  place 
the  semilunar  valves  must  close,  if  regurgitation  into  the  ventricular  cavity 
is  to  be  prevented.  A  comparison  of  the  aortic  pressure  curve  shows  a 
slight  notch,  the  "dicrotic  notch,"  just  preceding  a  slight  elevation,  the 
"dicrotic"  wave.  This  notch  occurs  at  the  moment  when  the  semilunar 
valves  close.  The  corresponding  point  on  the  ventricular  pressure  curve 
has  been  placed  just  where  the  ordinate  4  cuts  the  descending  portion. 
As  yet,  however,  the  pressure  is  higher  in  the  ventricle  than  in  the  auricle, 
and  continues  so  until  near  the  line  of  atmospheric  pressure.  At  this 
point  the  pressure  in  the  auricle,  due  to  the  accumulation  of  blood 
during  the  ventricular  systole,  now  forces  open  the  mitral  valve  and  the 
blood  flows  into  the  ventricle.  The  opening  of  the  mitral  valve  occurs  about 
the  point  where  the  ordinate  5  cuts  the  curve. 

The  ventricular  pressure  curve  affords  but  slight,  if  any,  indication  of  the 
auricular  systole.  It  apparently  does  not  give  rise  to  any  noticeable  increase 
in  the  ventricular  pressure.  The  slight  rise  in  the  pressure  curve,  which 
just  precedes  the  abrupt  rise  due  to  the  ventricular  systole,  may  be  taken  as 
an  indication  of  an  increasing  pressure  due  to  the  inflow  of  blood  from  the 
auricle,  as  a  result  of  the  auricular  systole.  Immediately  following  this 
event  the  ventricular  systole  begins  and  as  soon  as  the  pressure  in  the  ven- 
tricle exceeds  that  in  the  auricle  the  mitral  valve  closes.  This  is  marked 
on  the  curve  where  the  ordinate  cuts  it,  at  o.  With  the  closure  of  the  mitral 
valve  the  blood  becomes  imprisoned  within  a  closed  cavity,  closed  at  one 
orifice  by  the  mitral  valve  and  at  the  other  orifice  by  the  semilunar  valves. 
As  the  blood  is  incompressible  the  intra-ventricular  pressure  under  the  force 
of  the  ventricular  contraction  rapidly  rises  and  continues  so  to  do  until  the 
pressure  in  the  ventricle  exceeds  that  in  the  aorta,  at  which  moment  the  semi- 
lunar valves  are  suddenly  opened  and  the  blood  discharged.  A  comparison 
of  the  aortic  curve  shows  that  for  a  short  time  during  the  ventricular  systole 
the  pressure  is  falling,  but  at  one  point  the  curve  turns  at  a  sharp  angle  and 
rapidly  rises.  This  is  an  indication  that  the  semilunar  valves  are  suddenly 
thrown  open  and  the  blood  begins  to  pass  into  the  aorta.  This  event  occurs  at 
a  moment  marked  on  the  ventricular  curve  by  the  ordinate  i.  Beyond  this 
point  the  pressure  continues  to  rise,  for  the  aortic  pressure  must  not  only  be 
exceeded,  but  a  certain  velocity  must  be  imparted  to  the  blood.  Between 
the  ordinates  i  and  4,  the  semilunar  valves  remain  open  and  the  blood 
passes  into  the  aorta. 


THE  CIRCULATION  OF  THE  BLOOD.  281 

In  accordance  with  the  foregoing:  the  ventricular  systole  may  be  sub- 
divided into  two  periods: 

1.  The  period  of  rising  tension,  from  the  beginning  of  the  systole  and  the 

closure  of  the  auriculo-ventricular  valves  to  the  opening  of  the  semi- 
lunar valves,  the  pre-sphygmic  period,  occupying  from  c.02  to  0.04 
second. 

2.  The  period  of  ejection,  the  sphygmic-period,  from  the  opening  of  the 

semilunar  valves  to  the  end  of  the  systole,  occupying  about  0.2  second. 
The  ventricular  diastole  may  also  be  divided  into  two  periods : 

1.  The  period  of  falling  tension  or  relaxation,  the  post-sphygmic  period, 

from  the  end  of  the  systole  and  the  closure  of  the  semilunar  valves  to 
the  opening  of  the  auriculo-ventricular  valves,  occupying  about  0.05 
second. 

2.  The  period  of  filling,  from  the  opening  of  the  auriculo-ventricular  valves 

to  the  beginning  of  the  succeeding  auricular  systole. 

Negative  Pressure. — As  shown  by  the  ventricular  pressure  curve  there 
is  a  moment  when  the  pressure  falls  below  atmospheric  pressure,  becoming 
negative  to  it.  The  extent  to  which  this  takes  place,  its  duration  and  fre- 
quency, have  never  been  satisfactorily  determined.  The  cause  of  the 
negative  pressure,  its  influence  on  the  opening  of  the  auriculo-ventricular 
valves,  and  on  the  entrance  of  blood  into  the  ventricles  are  equally  unknown. 
A  probable  cause  is  an  expansion  of  the  base  of  the  ventricles  due  to  the 
enlargement  of  the  aorta  and  pulmonary  artery.  That  it  is  not  due  to  the 
expansion  of  the  thorax  is  evident  from  the  fact  that  it  occurs  when  the 
thorax  is  open  and  the  heart  exposed. 

The  Pulse  Volume. — The  pulse  volume  or  the  systolic  output  or  the 
amount  of  blood  discharged  by  the  ventricle  at  each  systole  has  long  been  a 
subject  of  investigation,  but  by  reason  of  the  inherent  difficulties  of  the 
problem  the  results  that  have  been  obtained  have  varied  within  wide  limits, 
viz.:  from  180  c.c.  to  50  c.c.  The  methods  that  .have  been  employed  for 
the  determination  of  this  volume  are  complicated  and  need  not  be  detailed 
here.  Suffice  it  to  say  that  the  results  of  the  more  recent  experiments 
would  indicate  that  the  volume  varies  from  80  c.c.  to  100  c.c.  If  the  pulse 
volume  be  assumed  to  weigh  ico  grams  and  the  total  volume  of  blood  in  a 
man  weighing  70  kilograms  to  weigh  3864  grams  then  the  pulse  volume  will 
be  about  one-thirty-eighth  of  the  total  amount  of  blood.  In  38  heart  beats 
therefore  the  entire  amount  of  blood  will  have  passed  through  the  heart. 

The  Intra-auricular  Pressure. — During  the  auricular  systole  the 
pressure  within  tlie  auricle  undergoes  variations  as  shown  by  direct  examina- 
tion by  means  of  a  cannula  inserted  into  the  auricular  cavity  and  connected 
externally  with  a  recording  tambour,  or  by  indirect  examination  by  means 
of  an  exploratory  tambour  placed  over  the  right  jugular  vein  in  close  relation 
to  the  clavicle.  The  pressure  variations  in  the  jugular  vein  which  are  thus 
recorded  by  means  of  a  tambour  provided  with  a  writing  lever  are  believed 
to  be  caused  by,  closely  follow  and  reproduce  the  pressure  variations  in  the 
auricle. 

Among  the  most  important  of  the  direct  examinations  of  the  auricular 
pressure  are  those  of  Porter,  carried  out  by  the  insertion  of  a  large  cannula  in 
the  auricular  appendix,  or  in  a  pulmonary  vein  close  to  the  auricle  and  con- 


282 


TEXT-BOOK  OF  PHYSIOLOGY. 


nected  by  its  free  extremity  with  a  Hiirthle  tambour.  The  curve  of  pressure 
thus  obtained,  shown  in  Fig.  129/  is  characterized  by  three  positive  and  three 
negative  waves.  Among  the  more  important  of  the  indirect  determinations 
of  the  auricular  pressure  variations  are  those  of  Bachmann,  carried  out  with 
highly  sensitive  recording  tambours.  The  curve  of  pressure  variations  in 
the  jugular  vein  thus  obtained,  by  Bachmann,  Fig.  130,  is  placed  in  juxta- 
position for  purposes  of  comparison. 

The  first  positive  wave,  a,  is  caused  by  the  systole  of  the  auricle  and 
amounts  to  about  9  millimeters  of  mercury.  The/n7  negative  wave  is  due 
to  the  relaxation  of  the  auricle. 

^  s 


Fig.  129. — CxiRVE  OF  Pressure  Variations      Fig.  130. — Curve  of  Pressure  Variations  in 
IN  THE  Auricle.      (Enlarged). — {Porter.)  the  Jugular  Vein.    (Enlarged.) — {Bachmann.) 

The  second  positive  wave,"  s,  is  not  of  auricular  origin,  but  is  due  to 
the  systole  of  the  ventricle  in  its  early  stage  corresponding  to  the  period 
between  the  closure  of  the  auriculo-ventricular  valve,  and  the  opening  of 
the  semilunar  valves,  the  period  of  rising  tension,  and  amounts  to  about 
5  mm.  of  Hg.  It  is  probably  due  to  the  bulging  of  the  auriculo-ventricular 
valve  into  the  auricular  cavity,  by  the  still  higher  ventricular  pressure,  thus 
diminishing  its  size  and  raising  the  pressure. 

The  second  negative  wave,  af,  begins  with  the  opening  of  the  semi- 
lunar valves,  determined  by  comparison  with  a  simultaneously  recorded 
curve  of  intra-ventricular  pressure,  and  is  due  in  part  to  the  relaxation  of 
the  auricular  walls,  but  more  especially  to  a  descent  of  the  more  central 
portions  of  the  auriculo-ventricular  septum,  into  the  ventricular  cavity, 
due  to  the  contraction  of  the  papillary  muscles  during  the  ventricular 
systole.  The  hollow  cone  thus  formed  enlarges  the  auricular  cavity, 
withdraws  some  of  its  contained  blood,  and  hence  lowers  the  pressure, 
thus  contributing  materially  to  the  filling  of  the  auricle.  This  negative 
pressure  amounts  to  about  —10  mm.  of  Hg. 

The  third  positive  wave,  v,  occurs  toward  the  end  of  the  ventricular 
systole  and   is   probably  caused  by  an  inflow  of  blood  from  the  veins  as 

'  The  original  tracing  obtained  by  Porter  is  shown  in  the  ac- 
companying Fig.  131.  The  letters  designating  the  waves  have 
the  following  significance.  A,  systolic  rise;  AB,  first  diastolic 
fall;  BC,  first  diastolic  rise;  CD,  second  diastolic  fall;  E,  second 
diastohc  rise;  F,  third  diastolic  fall;  G,  pause.  In  Fig.  129 
^  the  tracing  has  been  enlarged  and  the  waves  relettered  and  named 

riG.     131.     L,UR  E       F       ^j^  accordance  with  the  terminoloery  in  vogue  in  the  Hterature  of 
Pressure    Variations    in      .Unicai  medicine. 
THE  Auricle. — {Porter.) 

-The  corresponding  wave  on  the  curve  of  the  pressure  variations  in  the  jugular  vein  is 
believed  by  ISIackenzie  to  be  due  to  the  impact  of  the  expanding  carotid  artery,  and  hence  calls  it 
the  carotid,  c,  wave;  inasmuch  as  it  occurs  in  point  of  time  with  the  beginning  of  the  ventric- 
ular systole,  it  is  also  called  the  systolic,  s,  wave. 


THE  CIRCULATION  OF  THE  BLOOD.  283 

well  as  by  a  return  of  the  auriculo-ventricular  septum  to  its  normal  position, 
the  result  of  a  relaxation  of  the  papillary  muscles  at  a  time  when  the  intra- 
ventricular pressure  is  still  higher  than  the  intra-auricular  pressure.  It 
amounts  to  about  5  mm.  of  Hg. 

The  Ihird  negative  wave,  I'f,  appears  very  shortly  after  the  relaxation  of 
the  ventricle  and  though  there  is  at  this  moment  a  rapid  fall  of  intra- 
ventricular pressure,  on  opening  of  the  auriculo-ventricular  valves  and  a 
descent  of  blood  into  the  ventricle,  the  fall  of  auricular  pressure  seldom 
amounts  to  more  than  0.5  mm.  of  Hg. 

A  Graphic  Record  of  the  Auricular  and  Ventricular  Contractions 
of  the  Human  Heart. — From  the  similarity  of  the  anatomic  arrangement 
of  the  human  heart  to  that  of  mammals  in  general  it  is  permissible  to  assume 
that  a  graphic  record  of  the  auricular  and  ventricular  contractions  of  the 
human  heart  would  resemble  in  its  general 
features  that  of  the  hearts  of  mammals  hereto- 
fore experimented  on,  and  that  the  same  series 
of  events  present  themselves  in  the  human  heart 
during  each  cycle,  though  by  reason  of  the 
difiference  in  the  rate  of  the  beat,  the  duration 
of  each  event  in  the  cycle  is  somewhat  different. 

The  nearest  approach  to  obtaining  a  graphic 
record'  of  the  auricular  and  ventricular  con- 
tractions of  the  human  heart  by  the  direct  ap- 
plication of  exploratory  tambours  was  made  by 
Franfois  Frank  on  a  woman  whose  heart  was  Fig.  132.— Tr.\cl\gs  of  the 
congenitally  displaced  into  the  abdominal  cavity.  ^^r^^Soxs"'"?.oM'r  Wo'SS 
An  investigation  revealed  the  fact  that  this  with  Ectopla.  of  the  Heart,  a, 
woman  had  a  large  opening  in  the  anterior  Auricular;  v,  ventricular.— (Fran- 
portion  of  the  diaphragm  through  which  the  ^^"" 

ventricle  had  passed  and  formed  a  large  protrusion  in  the  epigastric 
region.  Through  thin  and  relaxed  abdominal  walls  the  ventricular  pul- 
sations could  be  distinctly  felt  as  well  as  the  pulsation  of  what  ap- 
peared to  be  the  inferior  portion  of  the  right  auricle.  A  fibrous  ring 
around  the  edge  of  the  opening  in  the  diaphragm  supported  the  heart 
at  the  auriculo-ventricular  groove.  On  the  application  of  exploratory 
tambours  in  connection  with  recording  tambours  one  to  the  right 
ventricle,  the  other  to  the  right  auricle,  the  record  shown  in  Fig.  132  was 
obtained  of  which  the  upper  line  represents  the  contraction  of  the  auricle 
and  the  lower  line  the  contraction  of  the  ventricle.  A  comparison  of  the 
record  with  that  obtained  from  the  horse.  Fig.  125,  p.  273,  shows  that  the 
relation  of  the  auricular  to  the  ventricular  systole  is  the  same  in  the  former 
as  in  the  latter  and  that  in  their  general  features  the  two  records  correspond, 
from  which  it  may  be  inferred  that  in  the  human  heart  the  events  occurring 
during  the  cycle  are  practically  identical  with  those  occurring  in  the  hearts 
of  other  mammals.  The  small  size  of  the  auricular  curve  and  the  absence  of 
undulations  are  probably  due  to  the  fact  that  the  tambour  was  placed  on 
only  a  portion  of  the  auricle. 

A  Schematic  Representation  of  the  Events  of  a  Cardiac  Cycle  in 
Man. — From  graphic  studies  of  the  cardiac  impulse,  of  the  pressure  changes 


284 


TEXT-BOOK  OF  PHYSIOLOGY. 


In  the  auricle  and  ventricle  as  indicated  by  pressure  changes  in  the  jugular 
vein  and  carotid  artery  respectively  it  has  become  possible  to  construct  a 
diagram  of  the  cardiac  cycle  of  the  human  heart,  to  designate  on  the  ventricu- 
lar curve  the  time  of  the  opening  and  closing  of  the  valves,  as  well  as  the 
time  relations  of  the  entire  series  of  events.  A  scheme  of  this  character  is 
shown  in  Fig.  133,  based  on  that  constructed  by  Fredericq. 


4.SYST<        AUfilCVLAfi      DIASTOLE 


rause  of  entire  Mean 


ML 


"Ope/diiff  of 
Semilunar  Dalvcs. 

Closure  of 
f\uiicijlo  vcntiicular 
valve. 


'Closure  of Se?ni  lunar 
>,  valves. 

T...Openincf   of 
jurictdoventricui 
valve. 


Opening   of 
Se/ni lunar  valves. 

Closure   of 
lu.ricido-vcntriadar 

valve. 


\iMRICtJLARSySTOlE 


MENIRICULAR  DIASTOLL 


I'-i  Sound. 


2 -Sound. 


Seconds 


I         I         I 


0       .1       .Z       .3       .4-      .J      .6       .7       .8      .1      .2      .3       .-i      .S 

Fig.  133.— a  Schematic  Representation  of  the  Events  of  a  Cardiac  Cycle. 


Though  the  foregoing  numerical  values  are  given  for  the  duration  of  the 
auricular  and  ventricular  systoles  and  of  the  common  pause,  it  must  be 
borne  in  mind  that  they  are  true  only  for  the  heart  beating  approximately 
70  times  per  minute.  If  the  number  of  beats  increases,  not  only  does  the 
entire  cycle  diminish  in  duration,  but  its  different  subdivisions,  auricular 
systole,  ventricular  systole,  and  diastole  also  diminish  in  duration,  though 
in  unequal  degrees.  Thus  it  has  been  determined  that  with  each  increase 
of  10  beats,  the  ventricular  systole  shortens  by  about  0.02  second  and  the 
ventricular  diastole  by  about  o.io  second.  The  opposite  holds  true  if  the 
number  of  beats  decreases  below  70  per  minute. 

The  Relation  of  the  Cardiogram  to  the  Events  of  the  Cardiac  Cycle. 
— A  comparison  of  a  typical  cardiogram,  such  as  is  seen  in  Figs.  123  and 
134,  with  the  curve  of  intra-ventricular  pressure,  shows  that  they  correspond 
in  essential  features.  The  slight  elevation  (a)  on  the  cardiogram  represents 
the. contraction  of  the  auricle,  which  completing  the  filling  of  the  ventricle 
causes  it  to  press  more  vigorously  against  the  chest  wall;  ?>-c  represents  the 
contraction  of  the  ventricles,  at  which  moment  the  apex  is  suddenly  and 
forcibly  driven  against  the  chest  wall;  c-d  represents  the  systolic  plateau, 
the  time  during  which  the  ventricle  is  discharging  blood  into  the  aorta;  d-e 
represents  the  relaxation  of  the  ventricle,  while  e-f  represents  the  time  of  the 
diastole  during  which  the  heart  cavities  are  enlarging  with  the  incoming  of 


THE  CIRCULATION  OF  THE  BLOOD. 


285 


a  new  volume  of  blood,  in  consequence  of  which  the  heart  is  pressing 
against  the  chest  walls.  The  systolic  plateau  is  characterized  by  one  or 
more  elevations  and  depressions,  the  true  cause  of  which  is  unknown. 

From  the  correspondence  of  the  curve  of  cardiac  pressure  against  the 
chest  wall  with  the  cun'e  of  intra-ventricular  pressure  it  becomes  possible 
to  indicate  with  approximate  accuracy  the  time  of  the  opening  and  closing 
of  the  auriculo-ventricular  valves  and  the 
semilunar  valves  and  hence  the  time  of  oc- 
currence of  the  heart  sounds  and  other  fea- 
tures of  the  cardiac  cycle.  Such  a  construc- 
tion is  shown  in  Fig.  134. 

Heart-sounds. — Two  sounds  accompany 
each  pulsation  of  the  heart,  both  of  which 
may  be  heard   by  applying  the  ear  or  the 

stethoscope  to  the  chest-walls,  especially  over  c  o'  c'o 

the  region  of  the  heart.     One  of  these  sounds       Fig.  134.— Cardiogram,    a,  Au- 

islow^in  pitch,  dull  and  prolonged;  the  other  ricularstysole;  J,C(f,  ventricular  sys- 

,  .   ,      i  .     1         ,  11  rxii  tole;   a,  e,  ventricular  diastole;  C,  O, 

is    high    in    pitch,    clear    and    short.       T  hese  closing  and  opening  of  the  auriculo- 

SOUnds  can  be  approximatelv  teproduced  by  ventricularvalves;0',C',  opening  and 

pronouncing    the  syllables  -liM-dup,^  luhb-  ^SS^ SS^^^;;^ ^:^:i 

dup.  The  long  dull  sound  occurs  with  the  ventriculardischarge;CC',  timeof  the 
systole,  the  first  phase  of  a  new  cardiac  cycle,    occurrence    of_  the   first   and    second 


and  is  therefore  termed  the  first  sound;  the 


sounds  respectively. 


short  clear  sound  occurs  at  the  beginning  of  the  diastole,  with  the  second 
phase  of  the  cardiac  cycle,  and  is  therefore  termed  the  second  sound.  The 
first  sound  is  the  systolic,  the  second  the  diastolic.  With  the  ear  it  can 
readily  be  determined  that  there  is  a  brief  pause  between  the  first  and  second 
sounds,  and  a  longer  pause  between  the  second  and  the  first  sounds.  The 
duration  of  the  first  sound  is  almost  equal  to  the  duration  of  the  systole — viz., 
0.3  second;  the  duration  of  the  second  sound  is  not  more  than  o.i  second. 
The  systolic  sound  is  heard  most  distinctly  over  the  body  of  the  heart;  the 
diastolic  sound  is  heard  most  distinctly  in  the  neighborhood  of  the  third  rib 
to  the  right  of  the  sternum. 

The  causes  of  the  heart-sounds  have  enlisted  the  attention  of  clinicians 
and  physiologists  for  years,  and  many  factors  have  been  assigned  for  their 
production.  At  present  it  is  generally  believed  that  the  first  sound  is  the 
product  of  at  least  two,  possibly  three,  factors:  viz.,  the  contraction  of  the 
muscle  walls  of  the  ventricles,  the  simultaneous  closure  and  subsequent 
vibration  of  the  tricuspid  and  mitral  valves,  and  the  sudden  increase  of 
pressure  of  the  apex  of  the  heart  against  the  chest-wall. 

That  the  contraction  of  the  ventricular  muscle  gives  rise  to  a  sound  is 
certain  from  the  fact  that  it  is  perceptible  in  an  excised  heart  when  the 
cavities  are  free  from  blood  and  when  the  valves  are  prevented  from  closing. 
The  explanation  of  this  sound  is  extremely  difficult,  as  the  contraction, 
though  prolonged,  is  not  of  the  nature  of  a  tetanus  and  therefore  not  charac- 
terized by  rapid  variations  of  tension.  The  apex  element  may  be  eliminated 
by  placing  the  individual  in  the  recumbent  position. 

The  second  sound  is  the  product  of  the  simultaneous  closure  and  subse- 
quent  vibration  of   the  aortic  and  pulmonary  valves  which  occur  at  the 


286  TEXT-BOOK  OF  PHYSIOLOGY. 

beginning  of  the  ventricular  diastole  as  the  blood  surges  back  against  the 
closed  vah^es.  This  has  been  definitely  proved  by  the  fact  that  the  sound 
disappears  when  the  valves  are  destroyed  or  held  back  by  hooks  introduced 
into  the  aorta  and  pulmonary  artery.  It  is  also  possible  that  the  vibration 
of  the  column  of  blood  produces  an  additional  tone  which  adds  itself  to  that 
produced  by  the  valves. 

The  Frequency  of  the  Heart-beat. — The  frequency  of  the  heart-beat 
varies  with  a  variety  of  conditions:  e.g.,  age,  sex,  posture,  exercise,  etc. 

Age. — The  most  important  normal  condition  which  modifies  the  activity 
of  the  heart  is  age.     Thus: 

Before  birth,  the  number  of  beats  a  minute  averages 140 

During  the  first  year  it  diminishes  to 128 

During  the  third  year  it  diminishes  to 95 

From  the  eighth  to  the  fourteenth  year  it  averages 84 

In  adult  males  it  averages 72 

Sex. — The  heart-beat  is  more  rapid  in  females  than  in  males.  Thus 
while  the  average  beat  in  males  is  72,  in  females  it  is  usually  8  or  10  beats 
more. 

Posture. — Independent  of  muscle  efforts  the  rate  of  the  beat  is  influenced 
by  posture.  It  has  been  found  that  when  the  body  is  changed  from  the  lying 
to  the  sitting  and  to  the  standing  position,  the  beat  will  vary  as  follows — 
from  66  to  71  to  81  on  the  average. 

Exercise  and  digestion  also  temporarily  increase  the  number  of  beats. 

A  rise  in  blood-pressure  from  any  cause  whatever  is  usually  attended  by 
a  decrease,  while  a  fall  in  blood-pressure  is  attended  by  an  increase  in  the 
rate. 

The  Blood-supply  to  the  Heart. — The  nutrition  of  the  heart,  its 
irritability  and  contractility,  the  force  and  frequency  of  the  beat,  are  depend- 
ent on  and  maintained  by  the  introduction  of  arterialized  blood  into  and 
the  removal  of  waste  products  from  its  tissue. 

In  frogs  and  allied  animals  the  heart  muscle  is  nourished  by  the  blood 
flowing  through  its  cavities.  During  the  diastole  the  blood,  under  the 
influence  of  the  slight  pressure  developed,  passes  from  the  interior 
of  the  heart  into  a  system  of  irregular  passage-ways  or  channels,  which 
penetrate  the  heart-wall  in  all  directions  and  thus  comes  into  direct  contact 
with  the  heart-cells.  With  the  beginning  of  the  systole  the  blood  is  forced 
out  of  these  channels  into  the  interior  of  the  ventricle,  bringing  with  it  the 
products  of  tissue  metabolism.  In  mammals  the  entire  inner  surface  of  the 
heart,  as  shown  by  the  investigations  of  Pratt,  also  presents  a  series  of  openings, 
the  foramina  of  Thebesius,  which  lead  into  a  similar  series  of  passage-ways 
penetrating  in  various  directions  the  heart-walls,  and  there  are  reasons  for 
believing  that  the  heart  of  the  mammal  may  be  to  some  extent  nourished  in 
a  manner  similar  to  the  manner  by  which  the  frog  heart  is  nourished.  Thus, 
if  a  glass  tube  be  inserted  and  fastened  into  the  aortic  opening  of  the  excised 
heart  of  a  cat  and  the  interior  of  the  ventricle  filled  with  warm  defibrinated 
blood  of  the  same  animal,  under  a  pressure  of  about  75  mm.,  the  heart  will 
recommence  and  continue  to  beat  for  a  period  varying  from  one  to  several 
hours,  thus  showing  that  the  mammalian  heart  may  to  some  extent  so 
receive  nutritive  material.  By  reason  of  the  fact  that  the  metabolism  of  the 
heart  of  the  mammal  is  so  much  more  active  than  that  of  the  heart  of  the 


THE  CIRCULATION  OF  THE  BLOOD.  287 

frog,  this  method  is  far  from  being  sufficient  for  nutritive  purposes  and  hence 
a  more  perfect  and  active  blood  supply  is  necessitated  for  furnishing 
nutritive  material  and  the  removal  of  the  waste  products.  These  results  are 
accomplished  by  the  coronary  arteries,  on  the  one  hand,  and  the  coronary 
veins,  on  the  other.  The  arteries,  two  in  number,  the  right  and  left,  arise 
from  the  aorta  in  the  pouches  of  Valsalva  just  above  the  right  and  left  semi- 
lunar valves.  Turning  in  opposite  directions,  they  ultimately  anastomose, 
forming  a  circle  around  the  base  of  the  ventricles.  From  both  the  right  and 
left  artery  branches  are  given  off  which  run  over  the  walls  of  both  auricles 
and  ventricles,  the  most  important  of  which  in  man  are  the  anterior  and 
posterior  inter-ventricular.  These  main  vessels  He  in  grooves  on  the  surface 
of  the  heart  beneath  the  visceral  pericardium,  surrounded  by  connective 
tissue  and  fat.  From  their  relation  to  the  outer  surface  of  the  heart  they 
may  be  designated  extra-mural  vessels.  From  these  vessels  small  branches 
are  given  off  which  penetrate  the  walls  of  the  heart,  in  which  they  divide 
into  many  branches  before  terminating  in  a  capillary  system.  Because  of 
their  relation  to  the  heart-muscle  they  may  be  designated  intra-mural  vessels. 
From  the  capillary  areas  small  veins  arise  which,  passing  backward,  con- 
verge to  form  the  coronary  veins.  These  follow  the  course  of  the  arteries 
and  finally  terminate  in  the  coronary  sinus,  located  in  the  auriculo-ventricu- 
lar  groove  on  the  posterior  surface  of  the  heart.  This  sinus  opens  into  the 
right  auricle  between  the  opening  of  the  inferior  vena  cava  and  the  auriculo- 
ventricular  opening.  Its  orifice  is  guarded  by  a  valve,  which  is  usually  single, 
though  sometimes  double. 

While  by  far  the  larger  portion  of  the  blood  is  returned  by  the  coronary 
veins,  it  is  also  certain  that  some  of  it  is  returned  by  small  veins  which  open 
into  little  pits  or  depressions  on  the  inner  surface  of  the  heart-walls,  known 
as  the  foramina  Thebesii.  It  has,  however,  been  shown  by  Pratt  that  these 
foramina  are  present  not  only  in  the  auricular  walls,  as  generally  stated,  but 
in  the  walls  of  the  ventricular  cavities  as  well.  They  communicate  through 
a  capillary  plexus  with  both  arteries  and  veins,  and  by  special  large  passages 
with  the  veins  alone. 

The  Filling  of  the  Coronary  Arteries. — The  period  of  time  in  the 
cardiac  cycle  during  which  the  coronary  (the  extra-mural)  arteries  are  filled 
with  blood,  whether  during  the  systole  or  the  diastole,  has  been  a  subject  of 
•much  discussion.  Thus  it  was  asserted  and  maintained  by  Briicke  that 
this  event  must  occur  during  the  diastole,  because  of  the  supposed  fact  that 
the  semilunar  valves  during  the  systole  are  so  closely  pressed  against  the 
walls  of  the  aorta  and  over  the  openings  of  the  coronary  arteries  as  to  prevent 
the  entrance  of  blood  into  them;  but  with  the  diastole  and  the  return  of  the 
valves  to  their  former  position  the  blood  flows  freely  into  them.  It  was 
further  assumed  that  the  coronary  arteries  empty  themselves  into  the  capil- 
laries during  the  time  of  the  systole.  According  to  Briicke  the  emptying  of 
the  coronary  arteries  and  the  consequent  fall  of  pressure  within  them  pro- 
moted the  contraction  of  the  ventricle,  w^hile  the  filling  of  the  vessels  and  the 
consequent  rise  of  pressure  facilitated  the  diastole.  This  anatomic  mechan- 
ism and  its  associated  functional  activity  constituted  according  to  Briicke  an 
apparatus  by  which  the  activity  of  the  heart  could  be  self-regulated.  This 
theory,  however,  has  been  disproved  and  is  no  longer  entertained. 


288  TEXT-BOOK  OF  PHYSIOLOGY. 

At  the  present  time  it  is  generally  believed  as  the  result  of  many  forms 
of  experimentation  that  the  extra-mural  coronary  arteries  are  filled  during 
the  time  of  the  systole.  For  it  has  been  shown  that  the  semilunar  valves  do 
not  close  the  openings  of  the  coronary  arteries  by  reason  of  the  presence  of 
blood  behind  them  under  a  high  pressure;  that  a  division  of  one  of  the 
branches  of  these  arteries  is  followed  by  a  spurt  of  blood  synchronous  with 
the  systole.  Moreover,  if  a  kymographic  trace  of  the  pressures  within  the 
coronary  artery  be  compared  with  the  trace  of  the  pressures  within  the 
carotid  artery,  it  wall  be  found  that  there  is  a  complete  agreement  between 
them  as  the  pressures  in  the  two  vessels  rise  and  fall  simultaneously  and 
as  a  corollary  are  filled  during  the  systole.  Because  of  the  pressure  which 
the  heart-muscle  must  exert  upon  the  smaller  arteries  and  veins  within  its 
own  substance  during  systole,  it  is  probable  that  there  is  a  temporary  retarda- 
tion of  the  flow  of  the  blood  during  the  systole  in  the  coronary  (the  extra- 
mural) vessels,  followed  by  a  return  of  the  velocity  during  the  period  of 
diastolic  repose. 

During  the  diastole  the  blood  flows  freely  from  the  extra-mural  vessels 
into  the  intra-mural  arteries  and  capillaries.  It  is  at  this  time  too  that  the 
heart-muscle  receives  from  the  capillary  blood-vessels  its  nutritive  material 
and  returns  to  the  blood  the  products  of  its  metabolism.  During  the  systole 
the  intra-mural  capillaries  and  veins  are  compressed  and  the  blood  driven 
into  the  extra-mural  veins.  The  greater  the  force  and  frequency  of  the  beat, 
the  larger  the  volume  of  blood  passing  through  the  coronary  system. 

Vaso-motor  Fibers  for  the  Coronary  Arteries. — The  presence  in  the 
vagus  and  sympathetic  ner\'es,  of  vaso-motor  fibers  for  the  coronary  arteries 
has  been  a  subject  of  much  investigation  and  discussion.  By  reason  of  the 
fact  that  stimulation  of  these  nerves  modifies  the  rate  and  the  force  of  the 
heart-beat,  and  these  in  turn  modify  the  flow  of  blood  through  the  vessels,  it 
is  difficult  to  state  whether  the  observed  effects  are  the  result  of  changes  in 
the  caliber  of  the  arteries  or  to  a  change  in  the  character  of  the  heart-beat. 
Moreover  owing  to  the  anatomic  relation  which  the  arteries  bear  to  the  heart 
muscle,  the  rapidity  of  the  flow  through  them  must  vary  with  each  contrac- 
tion and  relaxation  and  thereby  the  difficulty  of  interpretation  is  inceased. 
The  results  of  direct  experimental  investigations  of  Porter,  however,  lead  to 
the  conclusion  that  the  existence  of  vaso-motor  (constrictor)  fibers  for  the 
coronary  arteries  is  c[uite  probable. 

The  Effects  of  Ligation  of  the  Coronary  Arteries. — As  stated  in  a 
foregoing  paragraph  the  nutrition  of  the  heart-muscle,  its  irritability  and 
contractility,  depend  on  the  blood-supply  derived  from  the  coronary  vessels. 
This  is  shown  by  the  effects  which  follow  its  withdrawal.  Ligation  of  both 
coronary  arteries  in  the  dog  is  followed  by  a  diminution  in  the  force  and 
frequency  of  the  heart-beat,  and  in  a  few  minutes  by  complete  cessation. 
Ligation  of  even  a  single  branch  of  a  coronary  artery  of  the  dog  heart,  pro- 
vided it  supply  a  sufficiently  large  territory — e.g.,  the  arteria  circumflexa — 
is  sufficient  to  cause  arrest  in  at  least  80  per  cent,  of  animals  (Porter).  With 
the  ligation  of  this  vessel  there  occurs  a  gradual  diminution  in  the  force  and 
frequency  of  the  systole.  As  the  power  of  coordinate  contraction  declines 
the  heart-muscle  frequently  exhibits  a  series  of  independent  contractions  of 
individual  fibers  and  cells  known  as  fibrillary  contraction.     All  the  results 


THE  CIRCULATION  OF  THE  BLOOD.  289 

which  follow  ligation  are  to  be  attributed  in  the  light  of  experiment  to  the 
sudden  anemia  which  is  thus  established.  The  removal  of  the  ligature  and 
the  return  of  the  blood  will  restore  the  nutrition  and  re-establish  coordinate 
contractions.  The  excised  heart  of  the  mammal  which  has  passed  into  the 
condition  of  fibrillary  contraction  may  be  again  made  to  beat  rhythmically 
and  vigorously  by  first  cooling  it  with  normal  saline,  and  then  perfusing  it 
with  warm  defibrinated  blood  through  the  coronary  vessels  under  a  suitable 
pressure.  The  same  result  can  be  brought  about  by  first  perfusing  it  with 
a  I  per  cent,  solution  of  potassium  chlorid  until  the  heart  comes  to  rest  and 
then  perfusing  it  with  Ringer's  solution. 

The  Beat  of  the  Excised  Heart. — The.beat  of  the  heart,  its  frequency 
and  regularity,  its  continuance  from  the  early  stages  of  fetal  development  till 
death,  has  long  been  an  interesting  subject  for  physiologic  investigation. 
Though  related  to  the  functional  activities  of  the  body  at  large,  the  activity 
of  the  heart  is  in  a  sense  independent  of  them,  for  it  will  continue  for  a 
variable  length  of  time  after  they  have  ceased.  The  heart  of  the  frog  or 
the  turtle  will  continue  to  beat  under  appropriate  conditions  for  hours  after 
separation  of  all  anatomic  connections  and  removal  from  the  body.  The 
heart  of  the  dog  or  cat  will,  however,  beat  but  for  a  few  minutes.  The 
human  heart  would  in  all  probability  act  in  the  same  way.  Nevertheless 
there  are  good  reasons  for  believing  that  though  the  spontaneous  beat  has 
ceased,  the  irritability  yet  endures  though  perhaps  in  lessened  degree.  For 
if,  after  the  heart  has  ceased  to  beat  for  some  time,  warm  defibrinated  and 
oxygenated  blood  or  Locke's  modification  of  Ringer's  solution  be  passed 
through  the  coronary  vessels  the  beat  will  reappear  and  continue  at  its  usual 
rate  for  some  hours.  (See  paragraph  relating  to  the  action  of  inorganic 
salts  on  the  mammalian  heart,  page  299.) 

The  reason  for  the  longer  continuance  of  the  beat  of  the  excised  heart 
of  the  cold-blooded  animal  beyond  that  of  the  warm-blooded  animal 
lies  probably  in  the  difference  in  the  rate  of  their  respective  metabolisms. 
There  is  reason  to  believe  that  each  cell  of  the  heart-muscle,  in  common  with 
other  tissue-cells,  during  life  stores  up  and  holds  in  reserv'e  a  larger  quantity 
of  nutritive  material  than  is  necessary  for  its  immediate  needs.  When  sepa- 
rated from  the  general  blood-supply,  the  cells  begin  to  utilize  this  reserved 
material.  With  its  consumption  the  irritability  declines  and  after  a  variable 
period  of  time  the  contraction  ceases.  As  the  metabolism  is  far  more  rapid 
in  the  warm-blooded  than  in  the  cold-blooded  animal,  it  is  probable  that  the 
reserved  nutritive  material  is  utilized  more  quickly  in  the  former  than  in  the 
latter  other  conditions  being  equal.  So  long  as  it  lasts  in  either  class,  the 
irritability  and  contractility  persist. 

Whatever  the  immediate  or  exciting  cause  of  the  heart  contraction  may  be, 
the  fundamental  condition  for  its  manifestation  is  the  maintenance  of  the 
irritability.  So  long  as  this  persists  at  a  sufiiciently  high  level  the  heart- 
muscle  will  contract  in  response  to  the  appropriate  stimulus. 

THE  PHYSIOLOGIC  PROPERTIES  OF  THE  HEART-MUSCLE. 

The  physiologic  properties  of  the  heart-muscle  on  which  its  efficiency  as 
a   pumping    organ    depends,    viz.,    irritability,    conductivity,    rhythmicity, 
tonicity,  automaticity,  have  been  largely  determined  by  a  study  of  the  heart 
19 


290  TEXT-BOOK  OF  PHYSIOLOGY. 

of  the  frog.    As  some  of  the  facts  to  be  stated  in  subsequent  paragraphs  have 

reference  to  this  heart,  it  will  be  found  conducive  to  clearness  if  its  anatomic 

structure  and  physiologic  action  be  understood.     P'or  this  reason  a  brief 

account  of  the  frog  heart  will  be  found  in  the  appendix. 

I.  Irritability. — The  heart-muscle  in  common  with  other  muscles  possesses 

irritability,  by  A-irtue  of  which  it  responds  by  a  change  of  form  to  the 

action  of  a   stimulus.     Whatever  the   stimulus,   here,   as   elsewhere, 

there  is  a  conversion  of  potential  into  kinetic  energy — heat,  electricity, 

and  mechanic  motion.     The  normal  physiologic  stimulus  has  not  been 

positively  determined.     In  common  with  other  forms  of  muscle-tissue, 

the  heart-muscle  may  be  jnade  to  contract  by  artificial  stimuli — e.g., 

mechanic,  thermic,  chemic,  and  electric. 

For  the  demonstration  of  this  fact  it  is  necessary  to  eliminate  the 
action  of  the  physiologic  stimulus  and  to  bring  the  heart  to  rest  in  the 
condition  of  diastole.  This  can  be  done  with  the  frog's  heart,  by 
ligating  the  tissues  at  the  sino-auricular  junction,  a  procedure  which 
prevents  the  passage  of  the  contraction  wave  which  originates  in  the 
sinus,  over  the  auricles  and  ventricles  (a  fact  that  will  be  more  fully 
alluded  to  in  a  subsequent  paragraph).  With  the  heart  thus  prepared 
and  while  still  in  situ,  the  apex  may  be  connected  with  a  recording 
lever  and  its  evoked  contractions  registered  on  a  recording  surface.  In 
this  condition  it  will  respond  by  a  contraction  to  any  form  of  an  ade- 
quate stimulus,  such  as  the  induced  electric  current. 

In  its  irritability,  contractility,  and  manner  of  response  to  stimuli, 
the  heart  of  the  mammal  corresponds  in  all  essential  respects  to  the 
heart  of  the  frog  or  turtle. 

The  irritability  of  the  heart-muscle  depends  primarily  on  the  blood- 
supply  and  secondarily  on  the  maintenance  of  a  normal  temperature, 
and  so  long  as  both  conditions  are  maintained  the  muscle  will  respond 
by  a  contraction  to  any  adequate  stimulus,  physiologic  or  artificial. 

a.  The  Blood-supply.— The  supply  of  blood  to  the  mammalian  heart  is 
derived  from  the  coronary  arteries  which,  though  filled  during  the 
systole,  deliver  the  blood  to  the  intra-mural  arterioles  and  capillaries 
during  the  diastole.  The  facts  relating  to  the  blood-supply  have  been 
presented  fully  in  a  foregoing  paragraph  (page  286). 

b.  The  Injiiience  oj  Temperature. — For  the  manifestation  of  the  irrita- 
bility and  contractility  it  is  essential  that  the  heart-muscle  be  kept  at  asuf- 
ficiently  high  temperature  in  order  that  the  physiologic  or  a  given  arti- 
ficial stimulus  may  evoke  a  maximal  contraction.  This  is  accomplished 
by  immersing  the  suspended  heart  in  a  bath  of  Ringer's  solution  the  tem- 
perature of  which  can  be  readily  decreased  or  increased  by  appropriate 
means.  The  optimum  temperature  for  the  frog  heart  is  about  25°  C. 
As  the  temperature  is  lowered  both  rate  and  force  decrease  until  at 
about  from  4°  C.  to  0°  C.  both  cease.  Beyond  35°  C.  it  also  ceases  to 
contract,  because  of  a  coagulation  of  the  muscle  substance.  The 
mammalian  heart  attains  its  maximum  activity  at  a  temperature  of 
37°  C.  It  ceases  to  beat  at  about  47°  C.  on  the  one  hand  and  at  about 
17°  C.  on  the  other  hand. 


THE  CIRCULATION  OF  THE  BLOOD.  ,291 

2.  Conductivity. — Conductivity  of  living  material  may  be  defined  as  the 
ability  to  transmit  through  itself  a  condition  of  activity  due  to  the 
action  of  a  stimulus.  In  muscle  material  the  condition  of  acti\dty  is 
characterized  by  a  molecular  process  known  as  the  excitation  process, 
followed  almost  immediately  by  a  change  of  shape  known  as  the  con- 
traction wave. 

In  skeletal  muscle  conductivity  is  developed  to  a  high  degree.  Thus 
if  a  stimulus,  e.g.,  an  induced  electric  current,  be  sent  transversely 
through  one  end  of  a  muscle  an  excitation  process  is  developed,  followed 
by  a  contraction  wave,  both  of  which  are  conducted  through  the  muscle 
without  interruption  to  the  other  end  with  a  speed,  in  the  frog  muscle, 
of  about  10  meters  per  second.  In  the  cardiac  muscle  the  physiologic 
stimulus  acts  at  or  near  the  terminations  of  the  venae  cavae,  from  which 
point  an  excitation  process  and  a  subsequent  contraction  wave  are 
conducted  over  the  auricles,  thence  to  the  ventricles  from  base  to  apex 
with  extreme  rapidity.  It  is  evident  therefore  that  the  heart-muscle 
also  possesses  conductivity  to  a  high  degree.  It  is  now  generally  believed 
that  the  propagation  of  both  processes  is  accomplished  by  muscle-tissue 
alone,  independently  of  the  nerve  system.  The  conductivity,  however, 
is  not  ec^ually  well  developed  in  every  part  of  the  heart. 

In  the  frog  heart  this  is  especially  true 
of  the  tissue  at  both  the  sino-auricular  and 
the  auriculo-ventricular  junctions.  At 
these  points  the  contraction  wave  is  de- 
layed for  an  appreciable  period  (a  condi- 
tion attributed  to  the  embryonic  charac- 
ter of  the  muscle-tissue),  so  that  what 
would  otherwise  be  a  single  wave  becomes 
divided  into  three  smaller  waves,  so  that 
it  becomes  possible  to  observe  and  dis- 
tinguish the  contraction  of  the  different     „  Jj!::   'i^;"?^^"^^   %  ™,f 

o  r      1        1  T        1        r        )        Contraction     of     the     Frog  s 

chambers    of    the    heart.     In  the  frog  s     heart, 

heart  the  excitation  process  and  the  con- 
traction wave  begin  in  the  sinus  venosus,  from  which  they  are 
conducted  to  the  auricles,  thence  to  the  ventricles.  The  successive 
contractions  of  the  walls  of  the  subdi\'isions  oj  the  heart  can  be 
readily  recorded  with  suitable  apparatus.  In  Fig.  135,  which  is 
a  graphic  record  of  the  heart-beat,  the  two  elevations  of  the  lever  on  the 
up-stroke,  a  and  b,  represent  the  contraction  of  the  sinus  and  the  auricle 
respectively,  followed  by  the  vigorous  and  long  continued  contraction 
of  the  ventricle,  while  the  two  depressions,  c  and  d,  indicate  the  delay 
in  the  transmission  of  the  contraction  wave  at  the  two  junctions.  There 
is  here  an  anatomic  obstacle  to  the  conduction  of  the  contraction  wave. 
The  block  between  the  sinus  and  the  auricle  may  be  artificially 
increased  to  such  an  extent  as  to  prevent  absolutely  the  passage  of  the 
contraction  wave  by  ligation  of  the  tissue,  as  first  suggested  by  Stannius. 
Under  such  circumstances  the  auricles  and  ventricle  remain  at  rest 
while  the  sinus  continues  to  beat  at  its  usual  rate.  The  obstacle  between 
the  auricles  and  ventricle  may  be  increased  by  the  same  method  or 


292  TEXT-BOOK  OF  PHYSIOLOGY. 

better  by  means  of  a  suitable  and  adjustable  clamp.  By  carefully 
regulating  the  pressure  of  the  clamp  it  is  possible  to  so  block  the  wave 
that  three  or  four  auricular  contractions  may  occur  before  the  excitation 
process  forces  the  block  and  excites  a  ventricular  contraction,  (Fig. 
136.)  If  the  block  is  complete,  rather  than  partial,  the  ventricle  will 
come  to  rest  and  so  remain.  From  the  foregoing  facts  it  is  evident  that 
the  physiologic  stimulus  exerts  its  action  in  the  sinus  venosus  and  that 
the  auricular  and  ventricular  beats  are  in  turn  dependent  on  it. 

In  the  mammalian  heart  the  seat  of  the  stimulus  and  the  point  of 
origin  of  the  excitation  process  and  the  subsequent  contraction  wave 
have  been  a  subject  of  much  investigation  and  discussion.  For  some 
time  it  has  been  believed  that  these  processes  originate  at  the  termina- 
tions of,  or  between  the  terminations  of  the  venae  cavae  in  a  region 
corresponding  to  the  sinus  venosus  in  the  frog  heart\  from  which  they 


Fig.  136. — Record  of  the  Auricular  and  Ventricular  Contractions  defore 

AND  AFTER  THE  CLOSURE  OF  THE  ClAMP  AT  a. 

pass  over  the  auricles,  thence  to  the  ventricles.  On  the  basis  of  this  belief 
it  has  been  assumed  that  there  is  a  specialized  area  in  which  the  stimulus 
arises  and  which  determines  the  rate  and  rhythm  of  the  entire  heart. 
At  present  it  is  believed  that  this  area  is  identical  with  the  region  occu- 
pied by  the  sino-auricular  node,  the  lower  portion  of  the  sulcus  terminalis. 
With  the  view  of  determining  the  truth  of  this  assumption  Flack  per- 
formed a  number  of  experiments  on  the  hearts  of  dogs,  cats,  and  rabbits, 
some  of  the  reswlts  of  which,  abstracted  from  his  paper,  are  as  follows: 
The  application  of  cold  either  through  metallic  tubes  or  by  means  of  an 
ethyl  chlorid  spray,  the  remainder  of  the  heart  being  protected,  caused 
slowing  of  both  auricles  and  ventricles.  Weak  electric  stimulation 
caused  marked  inhibition  of  both  auricles  and  ventricles;  slightly 
stronger  stimulation  caused  a  mixed  effect  of  inhibition  and  acceleration, 
the  latter  usually  predominating;  still  stronger  stimulation  gave  rise  to 
marked  acceleration  of  the  whole  heart  rhythm  or  an  altered  rhythm  of 

'In  the  mammalian  heart  the  sinus  venosus  as  a  distinct  chamber  has  been  obliterated, 
but  it  is  represented  by  the  following  remnants:  (i)  The  termination  of  the  superior  vena 
cava  (the  right  duct  of  Cuvier) ;  (2)  the  coronary  sinus  (the  left  duct  of  Cuvier) ;  (3),  a 
stratum  submerged  beneath  auricular  tissue  at  the  taenia  terminalis;  (4)  the  remnants  of 
the  venous  valves,  i.e.,  the  Thebesian  and  Eustachian  valves  (Flack).  In  addition  there 
is  a  remnant  of  primitive  tissue  at  the  sino-auricular  junction,  that  is,  where  the  superior 
vena  cava  joins  the  toenia  terminalis  of  the  right  auricle,  and  knovni  as  the  sino-auricular 
node. 


THE  CIRCULATION  OF  THE  BLOOD.  293 

auricles  and  ventricles.  When  electric  stimuli  were  applied  to  other 
regions  of  the  superior  vena  cava  or  sulcus  no  effects  were  noticeable. 

Mechanic  stimulation  as  pinching  the  node  with  forceps  called  forth 
similar  results.  Destruction  of  the  node,  however,  had  no  effect  on 
the  rhythm.  The  application  of  a  weak  solution  of  atropin  abolishes 
the  customary  effects  of  both  vagus  and  sympathetic  nerve  stimulation. 
From  the  foregoing  facts  it  may  be  assumed  that  the  usual  seat  of  origin 
of  the  stimulus  to  the  cardiac  contraction  is  the  sino-auricular  node,  but 
as  the  heart  continues  to  contract  after  the  node  is  destroyed,  it  is 
evident  that  some  other  portion  or  portions  of  the  auricular  wall  are 
also  capable  of  developing  under  the  circumstances  an  adequate  stimulus. 

A  further  proof  that  the  sino-auricular  node  is  the  initiator  of  the  car- 
diac contraction  is  found  in  its  change  of  electric  potential.  It  has  long 
been  established  that  when  any  portion  of  living  material  enters  into  a 
state  of  activity  it  becomes  electro-negative  to  all  other  portions  which 
are  at  the  same  instant  electro-positive.  Lewis  with  special  electrodes 
in  connection  with  a  string  galvanometer  found  in  a  series  of  determina- 
tions that  with  the  beginning  of  a  cardiac  contraction,  the  sino-auricu- 
lar node  was  the  point  of  initial  electro-negativity,  a  fact  that  is  in  accord 
with  the  general  truth  that  the  region  of  greatest  activity  exhibits  the 
greatest  degree  of  negativity.  The  sino-auricular  node  may  therefore 
be  regarded  as  the  primary  seat  of  the  stimulus  or  excitation  process 
and  the  initiator  of  the  beat. 

From  the  sino-auricular  node  the  excitation  process  is  conducted  to 
the  auricles  and  ventricles  in  quick  succession,  though  between  the  end 
of  the  auricular  contraction  and  the  beginning  of  the  ventricular  con- 
traction there  is  also  a  perceptible  interval  similar  to  that  obserA'ed  in  the 
frog  heart.  For  a  long  time  it  was  assumed  that  the  excitation  process 
and  the  contraction  wave  passed  directly  from  auricles  to  ventricles 
across  the  auriculo-ventricular  junction  as  in  the  frog  and  that  the  interval 
between  the  auricular  and  ventricular  contractions  was  due  to  an  interfer- 
ence with  the  passage  of  the  contraction  wave  across  the  junction  because 
of  the  extreme  scarcity  of  the  muscle  fibers  in  this  region  or  to  their 
embryonic  character.  In  recent  years,  however,  this  view  has  been 
abandoned  because  the  real  bond  of  union  between  the  auricular  and 
ventricular  tissues,  across  which  the  excitation  process  passes,  has  been 
found,  as  stated  on  page  268,  in  the  system  of  muscle-fibers,  described 
in  part  by  His,  Retzer  and  Braunig,  and  Tawara  and  in  part  by  Keith 
and  Flack  and  known  as  the  conduction  system  of  the  heart.  This 
system  it  is  believed  constitutes  the  anatomic  and  physiologic  path 
across  which  the  excitation  process  passes  from  auricles  to  ventricles. 
The  excitation  process  originating  in  the  sino-auricular  node  passes 
first  to  the  auricular  walls,  exciting  them  to  contraction  and  then  into 
and  through  the  auriculo-ventricular  bundle  to  the  ventricular  walls, 
exciting  them  to  contraction.  The  supposition  that  this  was  the  case 
has  been  demonstrated  by  Hering  and  others  who  succeeded  in  dividing 
the  muscle-bundle  in  the  excised  .hearts  of  rabbits  and  dogs,  kept 
actively  beating  by  perfusion  with  Ringers'  solution.  On  division 
of  the  bundle  both  auricles  and  ventricles  continued  to  beat  though 


294 


TEXT-BOOK  OF  PHYSIOLOGY, 


with  different  rates  and  independently  of  each  other.  These  and 
other  experiments  of  a  similar  character  have  demonstrated  beyond 
question  that  the  auriculo-ventricular  bundle  with  its  widespread 
ramifications  is  the  true  conducting  system  between  auricles  and 
ventricles.  In  this  system  the  sino-auricular  node  is  regarded  as  the 
primary  dominating  "pace  maker"  of  the  rate  and  rhythm  of  the  heart. 
Inasmuch,  however,  as  the  heart  will  continue  to  beat,  after  the  destruc- 
tion of  the  sino-auricular  node  it  is  evident  that  it  is  not  the  only  region 
that  can  initiate  the  contraction.  Whether  the  contraction  under  such 
circumstances  is  due  to  an  excitation  arising  in  some  other  portion  of  the 
auricular  wall  or  in  the  subsidiary  auriculo-ventricular  node  is  a  subject 
of  discussion.  The  cause  assigned  by  Tawara,  for  the  interval  between 
the  auricular  and  ventricular  contraction  is  not  so  much  the  embryonic 
character  of  the  fibers  of  the  system,  as  it  is  the  length  of  the  system  as 

a  whole,  which  he  estimates  at  from  4  to  6 
V  /^^^^''''^^^T--^        centimeters.     This    time,  estimated  from  the 

-^*'—      //=  ^1         beginning    of    the    auricular   systole    to    the 

beginning  of  the  ventricular  systole  amounts 
to  from  0.1  to  0.2  second.  The  interval  be- 
tween these  two  events,  determined  from  the 
time  between  the  occurrence  of  the  a  and  c 
or  s  waves  on  the  jugular  pulse  tracing  is 
known  as  the  a-c  interval. 

With  the  mammalian  heart  as  with  the  frog 
heart  it  is  possible  to  increase  the  length  of 
the  interval  between  the  auricular  and  the 
ventricular  contraction,  the  inter-systohc  period, 
by  compression  of  a  portion  of  the  tissues  be- 
tween auricles  and  ventricles  including  pre- 
sumably the  central  part  of  the  conducting  sys- 
tem, the  muscle  bundle  of  His.  This  has  been 
accomplished  in  the  dog  by  Erlanger  by  means 
of  a  specially  devised  hook  clamp  (Fig.  137), 
which  consists  of  an  L-shaped  hook  of  steel  wire  the  arm  of  which 
can  be  made  to  approach  a  brass  block  by  means  of  a  bolt  and 
screw.  The  L-shaped  hook  is  inserted  into  the  right  wall  of  the 
aorta,  then  passed  downward  and  backward  into  the  left  ventricle, 
then  pushed  through  the  ventricular  septum  into  the  right  ventricle. 
In  this  position  it  lies  under  the  auriculo-ventricular  bundle.  Com- 
pression is  now  brought  about  by  approximating  the  hook  to  the 
brass  block  by  means  of  the  nut.  When  the  compression  is  brought 
about  suddenly  and  completely  the  ventricles  at  once  cease  beating, 
though  the  auricles  continue  to  beat  with  their  customary  rate  and 
regularity.  After  a  variable  period  of  time,  varying  from  a  few 
seconds  to  70  seconds,  during  which  the  ventricles  are  relaxed  and 
gradually  filling  with  blood  from  the  auricles,  the  ventricular  beat 
returns,  at  first  slowly  but  with  a  gradually  increasing  frequency  until 
a  definite  but  a  comparatively  slow  rate  is  attained.  The  rhythm  thus 
developed  is  termed  the  ideo-ventricular  rhythm. 


-The  Erlanger 
heart-block  clamp  compress- 
ing the  auriculo-ventricular 
bundle  (AVE).  SM,  Septum 
membranaceum;  MV,  mitral 
val  ve . — {H  irschf elder.) 


THE  CIRCULATION  OF  THE  BLOOD.  295 

In  experiments  on  the  dog  heart  performed  by  Erlanger  the  following 
results  were  obtained  when  the  auriculo-ventricular  bundle  was  com- 
pletely crushed. 

Aur.  rate  per  minute.  Ven.  rate  per  minute.         Ratio  of  Aur.  to  Ven. 

Max.  216  Max.  6q.8  3.09 

Min.  117.8  Min.   34.8  3.38 

Ave.   166.9  ^'^^-    52 -3  3   19 

The  reason  assigned  for  the  cessation  of  the  ventricular  contraction 
is  the  non-arrival  of  the  excitation  process  at  the  ventricular  end  of  the 
conducting  system,  because  of  the  blocking  or  compression.  Under 
physiologic  conditions  the  ventricular  beat  is  directly  dependent  on  the 
arrival  of  the  excitation  process  from  the  auricles  and  if  it  fails  to  arrive 
the  ventricle  does  not  contract  for  some  seconds.  The  return  of  the 
beat  during  complete  blocking  is  attributed  to  the  development  of  a 
hitherto  dormant  inherent  rhythmicity.  When  this  is  established  both 
auricles  and  ventricles  continue  to  beat  though  with  totally  different 
rhythms. 

The  effects  which  follow  gradual  compression  of  the  muscle-bundle 
are  somewhat  different  from  those  which  follow  sudden  compression. 
If  the  clamp  is  accurately  adjusted  and  the  compression  gradually 
applied,  the  first  perceptible  effect  is  a  lengthening  of  the  normal  pause, 
the  inter-systolic,  between  the  auricular  and  the  ventricular  contraction. 
With  an  increase  in  the  compression  there  will  come  a  moment  when 
one  of  the  auricular  contraction  waves  fails  to  reach  the  ventricle,  or  if  it 
does,  it  is  so  enfeebled  that  it  is  incapable  of  exciting  the  ventricle, 
which  in  consequence  fails  to  contract.  This  dropping  out  of  a  ven- 
tricular contraction  may  occur  once  in  every  10,  9,  8,  7,  6,  etc.,  auricular 
beats,  in  accordance  with  the  degree  of  compression.  With  a  further 
tightening  of  the  clamp,  the  blocking  of  the  excitation  process  may  be 
still  further  increased  so  that  only  every  second,  third,  or  fourth  auricular 
beat  is  capable  of  developing  a  ventricular  beat,  establishing  what  has 
been  termed  the  2:1,  3:1,  4:1,  rhythms  respectively;  and  finally 
when  the  blocking  is  complete  no  excitation  process  can  reach  the 
ventricle. 

Owing  to  the  capability  of  the  mammalian  ventricle  to  develop  an 
independent  rhythm  when  not  stimulated  by  the  auricles  for  a  few 
seconds  or  more,  it  is  not  always  possible  to  state  at  what  particular 
moment  in  the  successive  stages  of  compression  the  independent  ventric- 
ular rhythm  becomes  manifest.  Usually  when  the  rhythm  is  of  the  3  :  i 
type,  i.e.,  when  the  third  auricular  contraction  fails  to  reach  the  ventricle, 
it  will  begin  to  beat  of  itself.  Under  such  circumstances  the  auricles 
and  ventricles  become  dissociated  even  though  the  block  is  not  quite 
complete. 

These  experimental  facts  have  afforded  an  explanation  of  the  altered 
rhythm  between  auricles  and  ventricles  often  found  in  that  pathologic 
condition  known  as  Adams-Stokes  disease.  In  this  disease  the  rhythm 
may  be  any  one  of  the  rhythms  stated  in  the  foregoing  paragraph.  In 
two  instances  the  following  ratio  of  the  ventricle  to  the  auricle  was 
observed  by  Erlanger. 


296  TEXT-BOOK  OF  PHYSIOLOGY. 

Aur.  rate  per  minute.  Yen.  rate  per  minute.         Ratio  of  aur.  to  Van. 

79-6  22.4  3.5s 

84.6  31.0  2.73 

In  a  few  cases  of  death  from  this  disease  a  post-mortem  examination 
showed  a  lesion  of  the  auriculo-ventricular  bundle. 

3.  Rhythmicity. — Rhythmicity  may  be  defined  as  the  abihty  to  act  in 

regularly  recurring  cycles  or  the  property  of  anything  so  acting.  As 
the  heart-beat  recurs  in  regular  cycles  or  at  regular  intervals,  it  may 
therefore  be  said  that  the  heart-muscle  is  characterized  by  rhythmicity. 
The  beat  of  the  heart  as  well  as  each  phase  of  the  beat  occupies  a 
regular  measure  of  time  and  is  therefore  rhythmic  in  character.  Experi- 
mental procedures,  however,  show  that  the  rhythmic  power  or  at  least 
the  frequency  of  the  rhythm  varies  in  each  of  its  subdivisions  when  they 
are  separated  one  from  the  other.  Thus  if  the  tissue  between  the  sinus 
and  auricle  in  the  frog  or  turtle  heart  be  divided,  the  auriculo-ventricular 
portion  at  once  ceases  to  beat,  while  the  sinus  continues  to  beat  as  usual. 
In  a  short  time,  however,  the  auricles  and  ventricles  begin  again  to  beat, 
but  with  a  slower  rhythm.  Division  of  the  tissue  between  auricles  and 
ventricles  is  again  followed  by  rest.  In  a  short  time  the  auricles  begin 
to  beat,  while  the  ventricle  remains  quiescent.  If  the  ventricle  now  be 
stimulated  in  a  rhythmic  manner  it  may  resume  rhythmic  actiAdty. 
These  facts  are  taken  as  an  indication  that  the  rhythmic  power  is 
greatest  in  the  sinus,  less  in  the  auricles,  and  least  in  the  ventricles. 
In  the  warm-blooded  animal,  e.g.,  dog,  cat,  rabbit,  there  is  also  a 
difference  in  the  rhythmicity  of  the  auricles  and  ventricles.  This  is 
shown  by  the  effects  which  follow  division  of  the  auriculo-ventricular 
bundle,  or  sudden  and  complete  compression  of  that  portion  of  the 
auriculo-ventricular  tissue  containing  it.  In  either  case  the  ventricle 
for  a  short  time  remains  at  rest,  though  the  auricles  continue  to  beat  at 
their  usual  rate.  After  a  variable  number  of  seconds  the  ventricle 
develops  a  rhythm  of  its  own,  though  it  never  attains  that  of  the  auricle. 
From  these  facts  it  is  probable  that  in  each  division  of  the  heart  a  stimu- 
lus similar  to  that  acting  in  the  sinus  is  developed  when  the  heart 
chambers  are  separated  one  from  the  other. 

4.  Tonicity. — Tonicity  may  be  defined  as  a  condition  of  muscle  material 

characterized  by  a  slight  degree  of  contraction  which  varies  in  extent, 
however,  from  time  to  time  under  physiologic  conditions.  Whatever 
the  cause  of  the  tonicity  may  be  in  any  given  form  of  muscle,  the 
slight  degree  of  contraction  which  characterizes  it  not  only  resists  undue 
extension  but  permits  of  a  quicker  response  to  the  action  of  a  stimulus 
and  a  more  effective  performance  of  work.  The  heart-muscle,  like 
the  skeletal  muscle,  maintains  continuously  a  certain  degree  of  contrac- 
tion, which  not  only  prevents  undue  expansion  of  the  heart  during  the 
period  of  diastole,  but  increases  its  efficiency  as  a  pumping  organ  at  the 
beginning  and  during  the  systole.  This  tone  may,  however,  be  increased 
or  decreased  by  the  action  of  various  external  agents.  Thus  the  passage 
of  dilute  solutions  of  various  drugs — e.g.,  alkalies,  digitalis — through 
the  cavities  of  the  excised  heart  will  so  increase  the  tonicity,  or  the 


THE  CIRCULATION  OF  THE  BLOOD.  297 

contractile  power,  that  complete  relaxation  is  prevented,  until  finally 
the  heart  comes  to  a  standstill  in  the  condition  of  systole.  The  passage 
of  dilute  solutions  of  lactic  acid,  muscarine,  etc.,  through  the  heart  will, 
on  the  contrary,  so  decrease  the  tonicity  or  the  contractile  power  that 
the  normal  contraction  is  not  attained.  The  relaxation  therefore 
gradually  increases  until  the  heart  finally  comes  to  a  standstill  in  the 
condition  of  extreme  diastole.  In  the  first  instance  the  tonicity  is  said 
to  be  increased;  in  the  second  instance,  decreased. 

5.  Automaticity. — Automaticity  may  be  defined  as  the  power  of  maintain- 
ing activity  by  a  self-acting  cause  or  the  power  of  acting  independent 
of  external  causes.  Inasmuch  as  the  heart  continues  to  contract  in 
a  perfectly  rhythmic  manner  after  removal  from  the  body  and  apparently 
without  the  aid  of  an  external  stimulus,  it  is  said  that  the  heart-muscle 
is  automatic  or  spontaneous  in  action.  Strictly  speaking,  however,  this 
is  not  the  case,  for  the  reason  that  all  movement,  that  of  the  heart 
included,  is  the  resultant  of  the  action  of  natural  causes  though  their 
true  nature  may  be  beyond  the  reach  of  present  methods  of  investigation. 
The  Nature  of  the  Stimulus.^ — As  the  heart  continues  to  beat  after 

removal  from  the  body,  it  is  evident  that  the  stimulus  does  not  originate  in 

the  central  nerve  system  but  in  the  heart  itself.     Two  views  have  been  held 

as  to  its  origin  and  nature: 

1.  That  it  originates  in  the  nerve-cells  found  in  various  parts  of  the  heart- 

muscle;  that  it  is  a  nerve  impulse  rhythmically  and  automatically 
discharged  by  these  cells  and  transmitted  by  their  axons  to  the  heart- 
muscle  cells. 

2.  That  it  originates  in  the  muscle-cells  themselves;  that  it  is  chemic  in 

character  and  due  to  a  reaction  between  the  chemic   constituents, 
organic  and  inorganic,  of  the  muscle-cells  and  those  of  the  lymph  by 
which  they  are  surrounded. 
According  to  the  first  view  the  stimulus  is  neurogenic,  according  to  the 
second  view  myogenic. 

The  presence  of  nerve-cells;  their  relation  to  the  muscle-cells;  the  pro- 
nounced rhythmic  activity  of  the  sinus  and  auricles  in  which  the  nerve-cells 
are  abundant;  the  feeble  actiWty  of  the  apex,  in  which  they  are  wanting — 
these  and  other  facts  lend  support  to  the  view  that  the  stimulus  originates  in 
the  nerve-cells.  To  them  have  been  attributed  the  power  of  automatic 
activity. 

The  absence  of  nerve-cells  in  portions  of  the  heart-muscle,  which  never- 
theless exhibit  rhythmic  contractions  for  quite  a  long  period  of  time;  the 
rhythmic  beat  of  the  embryonic  heart  before  the  migration  of  nerve-cells  to  its 
walls  shows  that  the  stimulus  does  not  necessarily  originate  in  nerve-cells. 
Moreover,  Porter  has  conclusively  shown  that  the  apex  of  the  dog's  heart, 
which  is  generally  believed  to  be  totally  devoid  of  nerve-cells,  can  be  made 
to  beat  for  hours  by  feeding  it  through  its  nutrient  artery  with  warm  defibrin- 
ated  blood.  Unless  it  be  assumed  that  the  heart-muscle  contracts  auto- 
matically, without  a  cause,  it  is  a  fair  assumption  that  the  exciting  cause  of 
the  contraction  arises  within  the  muscle-cells  themselves,  and  that  it  is  in 
all  probability  the  outcome  of  a  reaction  between  the  chemic  constituents 
of  the  blood  or  lymph  on  the  one  hand,  and  the  chemic  constituents  of  the 


298  TEXT-BOOK  OF  PHYSIOLOGY. 

muscle-cells  on  the  other.  The  discovery  that  some  of  the  inorganic  salts  of 
the  blood  have  a  specific  physiologic  action  on  the  heart-muscle  was  made  in 
1882  by  Ringer.  Since  then,  many  attempts  have  been  made  to  isolate 
these  constituents,  to  determine  not  only  their  individual,  but  also  their 
collective  action,  when  combined  in  proportions  approximating  those  in 
which  they  exist  in  the  blood. 

The  Action  of  Inorganic  Salts. — i.  On  the  Frog  and  Terrapin  Heart. — 
The  inorganic  salts  which  are  most  directly  concerned  in  exciting  and  sustain- 
ing the  heart-beat  are  sodium  chlorid,  calcium  phosphate  or  chlorid,  and  potas- 
sium chlorid.  A  combination  of  these  salts  in  the  proportions  in  which  they 
exist  in  the  blood  was  first  suggested  by  Ringer  and  is  made  by  saturating  a 
0.65  per  cent,  solution  of  sodium  chlorid  with  calcium  phosphate,  and  then 
adding  to  each  100  c.c,  2  c.c.  of  a  i  per  cent,  solution  of  potassium  chlorid. 
A  frog's  heart  immersed  in  this  solution  will  continue  to  beat  for  some  hours. 
A  combination  of  the  chlorids  of  sodium,  calcium,  and  potassium  in  amounts 
which  will  vary  for  different  animals  is  equally  efficient  in  maintaining  the 
heart-beat. 

The  collective  as  well  as  the  individual  actions  of  these  salts  have  been 
strikingly  brought  out  by  the  experiments  of  Profs.  Howell  and  Greene,  from 
whose  published  results  the  following  statements  are  derived.  Instead  of 
employing  the  entire  heart,  they  used  for  various  reasons  strips  from  the 
terminations  of  the  venae  cavae  and  from  the  ventricle  of  the  terrapin  heart. 
The  proportion  of  the  inorganic  salts  most  favorable  for  the  contraction  of 
the  vena  cava  strips  is  the  following:  viz.,  sodium  chlorid,  0.7  per  cent.; 
calcium  chlorid,  0.026  per  cent.;  potassium  chlorid,  0.03  per  cent.  When 
vena  cava  strips  are  immersed  in  this  solution,  they  begin  in  a  short  time  to 
exhibit  rhythmic  contractions  which  may  continue  for  several  days.  In  the 
same  strength  of  solution  the  ventricular  strips  remain  inactive  but  if  the 
percentage  of  the  calcium  chlorid  be  raised  from  0.026  per  cent,  to  0.04,  or 
0.05  per  cent.,  spontaneous  contractions  soon  develop  and  continue  for 
several  days  or  more.  In  the  foregoing  solution  when  the  calcium  chlorid 
is  present  only  to  the  extent  of  0.026  per  cent.,  though  the  ventricular  strip 
does  not  contract,  it  is  kept  in  good  condition  for  contraction,  for  even  after 
many  hours  the  raising  of  the  percentage  of  calcium  chlorid  to  0.04  or  0.05 
per  cent,  will  call  forth  after  a  brief  latent  period,  rapid  and  energetic  con- 
tractions. From  this  fact  it  is  inferred  that  the  vena  cava  region  is  more 
sensitive  to  the  combined  action  of  the  salts  than  is  the  ventricle. 

The  action  of  the  individual  salts  is  also  best  shown  with  ventricular  strips. 
In  a  0.7  per  cent,  sodium  chlorid  solution  the  strip  beats  rhythmically  and 
energetically,  but  for  a  short  period  and  with  gradually  diminishing  force, 
until  it  entirely  ceases  to  beat.  A  reason  assigned  for  this  is  the  removal  of 
other  salts  necessary  to  the  excitation  of  the  contraction.  In  a  calcium 
chlorid  solution — 0.9  per  cent. — i.e.,  isotonic  with  the  sodium  chlorid — the 
heart  strip  is  thrown  into  strong  tone,  but  does  not  rhythmically  contract. 
If,  however,  the  strip  is  placed  in  normal  saline,  and  calcium  chlorid  added 
in  amounts  equal  to  that  present  in  the  blood,  it  will  after  a  very  short  latent 
period  begin  to  contract  rapidly  and  energetically  and  for  a  longer  time 
than  when  in  sodium  chlorid  solution  alone.  The  contractions  not  infre- 
quently occur  before  relaxation  is  completed,  so  that  the  strip  passes  into  the 
condition  of  contracture. 


THE  CIRCULATION  OF  THE  BLOOD.  299 

In  potassium  chlorid  solutions  isotonic — 0.9  per  cent. — with  sodium 
chlorid  solution  the  heart  strip  also  fails  to  contract.  This  is  the  case  also 
when  the  potassium  is  added  to  the  sodium  chlorid  in  amount  practically 
equal  to  that  found  in  the  blood. 

2.  On  the  Mammalian  Heart. — The  collective  action  of  the  inorganic 
salts  on  the  isolated  heart  of  all  members  of  this  class  of  animals  which  have 
been  made  the  subject  of  experimentation,  is  as  marked,  if  not  more  so,  than 
it  is  on  the  heart  of  the  frog  or  terrapin  especially  when  the  coronary  blood- 
vessels are  perfused  with  Ringer's  solution  or  the  modification  of  it  suggested 
by  Locke,  as  follows:  NaCl  0.90  per  cent.;  CaCU  0.024  per  cent.;  KCl  0.042 
per  cent.;  XaHCOg  0.02  per  cent.,  dextrose  o.i  per  cent.  The  re^'i^dng  and 
sustaining  power  of  this  solution  is  extraordinary.  Locke  and  Rosenheim 
were  able  to  re\dve  the  isolated  heart  of  a  rabbit  and  to  excite  it  to  active 
contraction,  for  several  hours  at  a  time,  on  four  consecutive  days  by  perfusing 
it  with  this  solution  saturated  with  oxygen  and  at  a  temperature  of  35°  C. 
No  special  precautions  were  observed  other  than  keeping  it  cool  (10°  C.)  and 
moist  during  the  intervals  of  experimentation.  The  duration  of  the  irrita- 
bility and  contractiUty  extended  over  a  period  of  95  hours.  Kuliabko 
revived  the  heart  of  a  rabbit  for  an  hour  nearly  three  days  after  removal  from 
the  body  of  the  animal.  It  was  then  placed  on  ice,  and  after  four  days  it 
was  again  revived  by  perfusing  it  with  Ringer's  solution.  Altogether  this 
heart  retained  its  irritability  for  seven  days.  Hering  re\dved  the  heart  of  a 
monkey  on  three  different  occasions,  the  first,  4J  hours,  the  second,  28 
hours,  and  the  third  54  hours  after  the  death  of  the  animal.  In  the  inter- 
vening periods  the  heart  was  also  kept  on  ice.  In  this  animal  it  was  even 
possible  to  increase  and  decrease  the  activity  of  the  heart  by  stimulation  of 
the  nerves  which  normally  control  the  rate  of  the  beat.  Kuliabko  was  also 
able  to  re\dve  the  isolated  heart  of  a  child  20  hours  after  death  from  a  double 
pneumonia.  It  was  made  to  beat  rhythmically  at  a  rate  varying  from  70  to  80 
per  minute  when  the  solution  had  a  temperature  of  39°  C,  and  at  a  rate  of 
98  to  102  per  minute  when  it  had  a  temperature  of  41°  C,  though  at  this  tem- 
perature the  beat  became  arrhythmic.  All  these  instances  demonstrate  the 
extreme  persistence  of  the  irritability  of  the  heart-muscle  under  appropriate 
conditions. 

The  action  of  individual  salts  has  been  shown  experimentally  on  the 
hearts  of  rabbits,  cats,  dogs,  monkeys,  by  Gross,  Howell  and  others.  Thus 
it  has  been  found  that  when  an  isolated  heart  is  rhythmically  beating  in 
response  to  the  perfusion  of  Ringer's  or  Locke's  solution,  the  addition  of 
potassium  chlorid  in  small  amounts  is  followed  by  a  decrease  in  the  rate  and 
force  of  the  contraction,  and  in  larger  amounts  by  a  complete  cessation  of  the 
contraction  and  a  standstill  in  diastole.  On  the  withdrawal  of  the  potassium, 
the  former  frequency  and  vigor  are  regained.  Potassium  exerts  a  depressor  or 
an  inhibitor  influence  on  the  irritability  and  contractility  of  the  heart- 
muscle. 

Under  the  same  conditions,  the  addition  of  calcium  chlorid  in  sufficient 
amounts  is  followed  by  an  increase  in  the  rate  and  in  the  \dgor  of  the  con- 
tractions; on  its  withdrawal  both  rate  and  force  return  to  the  previous  condi- 
tion. Calcium  exerts  an  accelerator  and  an  augmentor  influence  on  the 
irritability  and  contractility  of  the  heart. 


300  TEXT-BOOK  OF  PHYSIOLOGY. 

A  Theory  of  the  Heart-beat. — P'rom  the  foregoing  facts  it  seems 
probable  that  the  heart-beat  is  connected  with  and  dependent  on  the  presence 
and  interaction  of  the  inorganic  salts  present  in  the  lymph,  though  as  to  the 
manner  in  which  they  interact  to  initiate  the  beat,  there  is  some  obscurity. 
A  very  plausible  theory  as  to  the  part  played  by  the  inorganic  salts  in  initiat- 
ing the  contraction  and  one  in  accordance  with  the  facts  has  been  presented 
by  Howell  as  follows: 

The  heart-muscle,  it  is  assumed,  contains  a  stable  organic  energy-yielding 
compound  of  which  potassium  is  one  of  the  constituents  and  on  which  its 
stability  depends.  This  compound  must  be  present  in  relatively  large 
amounts  as  the  heart  will  continue  to  contract  and  expend  energy  for  many 
hours  after  the  blood-supply  has  been  withdrawn. 

During  the  diastole  a  reaction  takes  place  between  this  compound  and 
the  calcium  or  the  calcium  and  the  sodium  salts,  whereby  a  portion  of  the 
organic  compound  is  freed  from  potassium  and  is  then  combined  with  calcium 
or  with  calcium  and  sodium.  In  consequence,  this  portion  of  the  organic 
compound  in  combination  with  the  calcium  acquires  and  gradually  increases 
in  instability,  reaching  its  maximum  at  the  end  of  the  diastole,  when  it  under- 
goes a  dissociation  giving  rise  to  a  chain  of  events  that  culminate  in  a  con- 
traction. The  initial  step,  therefore,  is  a  dissociation  of  a  complex  unstable 
molecule  followed  by  an  oxidation  of  the  dissociated  products.  That  an 
active  dissociation  of  some  character  takes  place  is  evident  from  the  consump- 
tion of  oxygen,  the  production  of  carbon  dioxid,  the  liberation  of  heat, 
electricity,  and  mechanic  motion. 

Inasmuch  as  the  contraction  is  always  maximal  and  as  the  heart  is  refrac- 
tory to  a  stimulus  during  the  systole,  the  probabilities  are  that  all  of  the 
unstable  portion  of  the  energy-yielding  compound  is  dissociated  with  each 
contraction.  With  the  relaxation  there  is  a  renewal  of  the  unstable 
combination,  of  calcium  with  the  organic  molecules,  which  increases 
in  amount  until  the  maximum  is  again  .  attained  when  another  dis- 
sociation occurs  followed  by  another  contraction.  The  rhythmicity  of 
the  heart's  action,  the  appearance  of  a  refractory  condition  during  the 
systole  and  its  gradual  disappearance  during  the  diastole,  as  well  as  other 
phenomena,  are  readily  explained  by  the  foregoing  hypothesis. 

The  cause  of  the  dissociation  of  the  energy-yielding  material  is,  however, 
a  subject  of  discussion.  According  to  Howell  it  is  not  necessary  to  assume 
the  presence  of  any  cause  other  than  the  extreme  instability  of  the  organic 
compound  in  question.  According  to  Engelmann,  Langendorff  and  others, 
the  dissociation  is  not  spontaneous  but  is  the  result  of  the  action  of  a  specific 
stimulus,  an  "inner  stimulus,"  arising  within  the  muscle  elements  themselves 
through  metabolic  processes;  and  so  long  as  these  processes  are  chemically 
and  physically  conditioned  by  blood  or  tissue  fluids  containing  the  inorganic 
salts,  so  long  will  this  stimulus  be  produced.  As  to  the  nature  of  this  stimu- 
lus, whether  chemic,  electric  or  enzymic,  nothing  definite  can  be  stated  at 
present. 

The  Response  of  the  Heart  to  the  Action  of  an  Artificial  Stimulus. — 
The  heart  of  the  frog  as  well  as  of  some  other  animals  may  be  brought  to  a 
standstill  by  the  ligation  of  the  tissues  between  the  sinus  venosus  and  the 
auricle,  a  procedure  first  introduced  by  Stannius  and  now  known  as  the  first 


THE  CIRCULATION  OF  THE  BLOOD.  301 

Stannius  ligature.  Under  such  circumstances  the  heart  may  be  made  to 
contract  by  stimulating  it  with  the  single  induced  current.  With  each 
passage  of  the  current  the  heart  contracts.  Contrary  to  what  is  observed 
in  skeletal  muscles,  the  heart  contraction,  if  it  occurs  at  all,  at  once  reaches 
its  maximal  value.  Any  increase  in  the  strength  of  the  stimulus  above  the 
threshold  .value  has  no  greater  effect  on  the  extent  or  force  of  the  contraction 
than  the  minimal  stimulus.  A  conclusion  which  may  be  drawn  from  this 
fact,  according  to  Engclmann  is  as  follows:  By  reason  of  the  fact  that  the 
heart  contracts  at  its  maximum  value  to  the  action  of  any  strength  of  stimulus, 
under  given  conditions,  there  is  always  ensured  a  more  or  less  complete 
emptying  of  the  ventricular  contents  and  a  uniform  discharge  of  blood  into 
the  arteries,  which  would  not  be  the  case  if  the  exient  of  the  contraction 
varied  with  the  strength  of  the  stimulus;  and  there  are  reasons  for  believing 
that  the  normal  stimulus  for  the  contraction  varies  within  wide  limits  above 
the  threshold  value  both  in  normal  and  abnormal  conditions  of  the  heart. 
The  changes  in  the  extent  or  force  of  the  contraction  are  the  result,  not  of 
changes  in  the  intensity  of  the  stimulus,  but  of  changes  in  the  heart-muscle, 
caused  by  variations  in  mechanical  resistances. 

The  periodicity  of  the  heart's  action  or  its  rhythm  may  also  be  elucidated 
by  the  foregoing  fact.  There  are  reasons  for  believing  that  at  the  time  of 
the  contraction  practically  all  of  the  available  energy-yielding  material  is 
completely  utilized,  after  which  the  heart  relaxes  and  remains  at  rest  in  the 
diastolic  condition  for  a  given  period;  and  before  a  second  excitation  wave 
can  be  developed  and  pass  from  the  sinus  over  the  heart  there  must  be  a 
re-accumulation  of  energy-yielding  material,  and  a  restoration  of  the  irrita- 
bility. This  is  accomplished  during  the  diastole.  By  virtue  of  this  fact  the 
heart  cannot  act  otherwise  than  in  a  periodic  manner. 

Inasmuch  as  there  is  a  conversion  of  all  of  the  potential  energy  into 
kinetic  energy  during  the  systole,  there  is  of  necessity,  a  lowering  of  the 
irritability,  and  to  so  great  an  extent  is  this  the  case  that  the  heart  will  not 
respond  to  the  action  of  a  second  stimulus  either  physiologic  or  artificial 
during  the  systolic  period.  This  non-responsiveness  of  the  heart  may  be 
shown  by  throwing  into  it  a  second  stimulus  at  any  moment  during  the  systole. 
Whatever  the  moment  or  whatever  the  strength  of  the  stimulus  may  be  the 
extent  of  the  contraction  remains  the  same.  During  the  systolic  period  the 
heart  is  said,  therefore,  to  be  refractory  or  non-responsive  to  a  second 
stimulus.  If,  however,  a  second  stimulus  of  average  strength  be  thrown 
into  the  ventricle  at  any  moment  during  the  relaxation,  a  second  contraction 
will  be  developed,  which  is  known  as  the  extra  systole. 

The  Extra  Systole. — ^The  extent  of  this  extra  systole  will  be  propor- 
tional to  the  time  at  which  the  stimulus  is  thrown  into  the  ventricle  as  it 
passes  from  the  beginning  to  the  end  of  its  relaxation.  Whatever  the  extent  of 
the  extra  systole,  its  height  is  no  greater  than  that  of  the  first  systole  (Fig. 
139).  For  this  reason  it  is  believed  a  tetanic  contraction  cannot  be  developed. 
If  the  stimulus  be  thrown  into  the  heart  just  as  the  relaxation  is  completed, 
the  extra  systole  attains  the  same  height  as  the  preceding  systole.  In  passing 
from  the  beginning  to  the  end  of  the  relaxation  and  into  the  diastolic  or 
resting  period,  it  has  been  found  that  the  extra  systole  can  be  evoked  by  a 
stimulus  which  is  steadily  decreased  in  intensity.     It  is  evident  from  this 


302 


TEXT-BOOK  OF  PHYSIOLOGY. 


fact  that  the  restoration  of  the  energy-yielding  material  and  the  return  of 
the  irritability  gradually  increases  from  the  beginning  of  the  relaxation  to 
the  end  of  the  diastole  (Fig.  138).  For  this  reason  weak  stimuli  are  more 
effective  in  the  later  than  in  the  earlier  period  of  the  relaxation  and  the 
diastole. 

After  the  development  and  disappearance  of  the  extra  systole  a  consider- 
able pause  in  the  heart's  action  occurs  to  which  the  term  compensatory  pause 


Refraclori/ 


Period. 


Irritable  .>^ 


Fig.  138. 


-Diagram  Showing  the  Variations  of  Irritability  during  the  Systole  and  the 
Diastole. — {Modified  from  Waller.) 


has  been  given  (Fig.  139),  on  the  assumption  that  it  was  necessary  on  the 
part  of  the  heart  to  compensate  for  the  disturbance  of  the  rhythm  by  remain- 
ing at  rest  until  the  time  of  the  next  beat  and  thus  restore  the  rhythm.  This 
was  thought  to  be  a  special  property  of  the  heart-muscle.  This  view,  how- 
ever, is  no  longer  entertained.  For  if  an  isolated  ventricle  of  a  frog  heart  be 
employed  and  made  to  contract  rhythmically  by  an  artificial  stimulus,  or  if 
a  spontaneously  beating  portion  of  the  dog's  heart  be  employed  for  experi- 
mentation instead  of  the  whole  heart,  the  results  of  the  same  methods  of 
stimulation  are  different.  Though  an  extra  systole  is  called  forth  as  usual, 
there  is  no  compensatory  pause;  indeed,  if  anything  the  pause  is  shorter 
than  the  regular  pause.  The  theory  that  a  compensatory  pause  is  necessi- 
tated for  the  restoration  of  the  normal  rhythm  is  therefore  not  tenable. 

The  explanation  assigned  and  generally  ac- 
cepted at  present  for  the  production  of  a  com- 
pensatory pause  is  as  follows:  In  a  spontaneously 
beating  heart  the  ventricular  systole  is  evoked  by  the 
arrival  of  an  excitation  process  coming  from  the 
auricles.  When  the  extra  systole  is  induced  by  an 
artificial  stimulus,  the  next  succeeding  excitation 
from  the  auricle  falls  into  the  refractory  period  and 
hence  the  ventricle  is  not  stimulated.  It,  therefore, 
simply  waits  for  the  arrival  of  the  second  succeeding  excitation,  when  it 
responds  and  takes  up  the  regular  rhythm. 

This  fact  is  of  great  interest  clinically  for  it  frequently  happens  that 
extra  systoles  of  the  ventricle  arise  in  the  human  heart  in  conditions  of  the 
circulation  characterized  by  a  high  blood-pressure  and  especially  when  th^e 


Fig.  139. — The  Extra 
Systole  and  the  Compen- 
satory Pause.  The  break 
in  the  horizontal  line  indicates 
the  moment  the  electric  cur- 
rent passes  through  the  heart. 


THE  CIRCULATION  OF  THE  BLOOD.  303 

is  coincidently  an  impairment  in  the  irritability  and  contractility  of  the  heart- 
muscle.  Extra  systoles,  however,  may  have  their  origin  in  the  auricular 
walls  as  well. 

If  a  series  of  successive  stimuli  be  thrown  into  the  heart-muscle  the 
effect  will  vary  in  accordance  with  their  time  intervals.  Should  this  be  less 
than  about  three  seconds  there  will  be  a  gradual  increase  in  the  height  for 
some  half  dozen  contractions,  a  result  to  which  the  term  "staircase"  or 
'' treppe"  has  been  given.  This  increase  in  the  height  of  the  contraction 
is  attributed  to  an  increase  in  the  irritability  and  contractility  of  the  muscle 
the  result  of  the  primary  stimulating  action  of  fatigue  products. 

THE  NERVE  MECHANISM  OF  THE  HEART. 

By  this  term  is  meant  a  combination  of  nerves  and  nerve-centers  which 
cooperate  to  increase  or  decrease  either  the  rate  or  force — or  both — of  the 
heart's  contraction  in  accordance  with  the  needs  of  the  system.  That  the 
heart  is  normally  influenced  by  the  central  organs  of  the  nerve  system  in 
response  to  the  action  of  nerve  impulses  reflected  to  them  from  many  organs 
of  the  body  is  a  matter  of  personal  experience;  that  it  is  abnormally  influenced 
by  the  same  or  other  organs  in  response  to  nerv^e  impulses  reflected  to  them 
in  consequence  of  pathologic  and  traumatic  processes  occurring  in  different 
regions  of  the  body,  and  that  both  heart  and  nerves  are  modified  in  different 
ways  by  the  action  of  drugs  introduced  into  the  body,  are  matters  of  daily 
clinical  experience. 

The  nerves  comprising  this  mechanism  and  the  relation  they  bear  one  to 
another  are  represented  in  Fig.  140. 

It  was  stated  in  a  previous  paragraph,  page  289,  that  the  contraction  of 
the  heart-muscle  is  independent  of  its  connection  with  the  central  organs  of 
the  nerve  system,  and  that  it  will  continue  to  contract  in  a  rhythmic  manner 
for  a  variable  length  of  time  even  after  its  removal  from  the  body  of  the 
animal,  the  length  of  time  varying  with  the  animal  and  the  conditions  to 
which  it  is  subjected;  that  the  stimulus  is  myogenic  in  origin  and  chemic  in 
character,  the  result  of  a  reaction  between  the  chemic  constituents,  organic 
and  inorganic,  of  the  muscle-cells  and  those  in  the  lymph  by  which  they  are 
surrounded.  It  has  also  been  further  shown  that  even  in  the  living  animal 
the  heart  will  continue  to  beat  and  fulfil  its  functions  after  division  of  all 
nerves  in  connection  w^ith  it.  A  dog  thus  experimented  on  lived  for  eleven 
months,  and  beyond  the  fact  of  becoming  fatigued  more  readily  upon  exer- 
tion than  formerly,  exhibited  no  striking  disturbance  of  its  functions. 
Nevertheless  groups  of  nerve-cells  are  present  in  certain  portions  of  the  heart 
in  all  classes  of  vertebrate  animals,  which  bear  an  anatomic  and  physiologic 
relation  to  the  heart-cells  on  the  one  hand,  and  to  the  nerves  connecting 
them  with  the  central  organs  of  the  nerve  system  on  the  other  hand. 

Intra-cardiac  Nerve-cells. — In  the  frog  heart  a  group  of  nerve-cells 
is  found  in  the  sinus  at  its  junction  with  the  auricle,  known  as  the  crescent 
or  ganglion  of  Remak;  a  second  group  is  found  at  the  base  of  the  ventricle 
on  its  anterior  aspect,  and  known  as  the  ganglion  of  Bidder;  a  third  group 
is  found  in  the  auricular  septum,  known  as  the  septal  ganglion,  or  v.  Bezold's 
or  Ludwig's.     The  majority  of  the  cells  are  situated  on  the  surface  of  the 


304 


TEXT-BOOK  OF  PHYSIOLOGY. 


heart  just  beneath  the  pericardium.  From  the  cell-body  fine  non-medullated 
fibers  pass  into  the  substance  of  the  heart,  to  become  histologically  and 
physiologically  related  with  the  muscle-fiber. 

In  the  dog  heart  and  in  the  mammalian  heart  generally,  though  nerve- 
cells  are  present,  they  are  not  arranged  in  such  definite  groups,  but  are  more 


Emotional  Centers 

Exhilarating  (blue) 
Depressing  f^^oj 


Cardio-Inhlbitor  Center. 


Ganglion  Stellatum 


Inlra-CarciiacKeroe  Cells  t 


Canslio  Acceleratc^r  Center 


^l^gusNerue 

Iffmnt  unkibltor(ff£D) 


Sympathetic  Heroes- 

Accelerator  i^Auymentor 


Fir,.  140. — Diagram  of  the  Nerve  Mechanism  of  the  Heart. — (G.  Bachman.) 


widely'^distributed  in  the  terminations  of  the  venae  cavae,  pulmonary  veins, 
the  walls  of  the  auricles,  and  in  the  neighborhood  of  the  base  of  the  ventricles. 
Extra-cardiac  Nerves. — The  extra-cardiac  nerves  which  connect  the 
heart  with  the  central  nerve  system  and  through  which  the  activities  of  the 
heart  are  influenced  are  two:  viz.,  the  sympathetic  and  the  vagus  or  pneumo- 


THE  CIRCULATION  OF  THE  BLOOD.  305 

gastric.  Experimental  investigation  has  established  the  fact  that  the  sympa- 
thetic is  the  motor  nerve  to  the  heart,  the  nerv^e  which  accelerates  the  rate 
and  augments  the  force  of  the  natural  beat;  while  the  vagus  is  the  inhibitor 
nerve,  the  nerve  which  inhibits  or  controls  the  rate  and  the  force  of  the  beat 
in  accordance  with  the  necessities  of  blood  distribution.  For  this  reason 
these  two  nerves  will  be  considered  in  the  order  stated.  The  course  of 
the  fibers  composing  these  nen^s,  from  their  origin  to  their  termination,  and 
the  relation  they  bear  to  one  another  and  to  neighboring  structures,  vary 
somewhat  in  different  animals. 

The  Origin  and  Distribution  of  the  Sympathetic  Nerves  in  Mammals. 
— The  sympathetic  ner\'e-fibers  which  influence  the  action  of  the  heart,  are 
connected  on  the  one  hand  with  the  heart-muscle  itself  and  on  the  other 
hand  with  nerv-e-fibers  coming  from  the  central  nerve  system.  The  former, 
are  non-medullated  and  post-ganglionic,  the  latter  meduUated  and  pre- 
ganglionic. 

The  pre-ganglionic  fibers  have  their  origin  in  the  medulla  oblongata  and 
very  probably  from  nerve-cells  in  the  gray  matter  beneath  the  floor  of  the 
fourth  ventricle.  From  this  origin  they  descend  the  spinal  cord  as  far  as  the 
level  of  the  second  and  third  thoracic  nerves.  At  this  level  they  emerge  from 
the  cord  in  company  with  the  nerve-fibers  composing  the  anterior  roots  of 
the  second  and  third  thoracic  nerves.  After  a  short  course,  they  enter  the 
white  rami  communicantes,  enter  the  sympathetic  chain  and  pass  upward 
to  the  ganglion  stellatum,  and  by  way  of  the  annulus  of  Vieussens  to  the 
inferior  cervical  ganglion  as  well,  around  the  nerve-cells  of  which  their 
terminal  branches  arborize.  From  the  nerve-cells  of  both  the  stellate  and 
inferior  cervical  ganglia,  the  sympathetic  nerves  proper  arise,  which  after 
emerging  from  the  ganglia  pass  toward  the  heart  and  become  associated 
with  the  fibers  of  the  vagus  and  assist  in  the  formation  of  the  cardiac  plexuses. 
On  reaching  the  heart  they  may  terminate  directly  in  the  muscle-cell  or 
indirectly  through  the  intermediation  of  intra-cardiac  nerve-cells.  The 
former  mode  of  termination  is  the  more  probable.  Experiment  has  shown 
that  both  the  pre-  and  post-ganglionic  fibers  are  efferent  in  function. 

The  Origin  and  Distribution  of  the  Vagus  Nerve  in  Mammals.— 
The  vagus  nerve-fibers  which  influence  the  heart  are  connected  on  the  one 
hand  with  the  heart,  through  the  intermediation  of  the  intra-cardiac  cells,  and 
on  the  other  hand  with  the  central  nerve  system.  Histologic  investigation  has 
shown  that  the  vagus  nerve-trunk  of  man  and  mammals  generally,  contains 
medullated  fibers  of  large  and  small  size.  Experiment  has  shown  that  the 
large  fibers  are  afferent,  the  small  fibers  efferent  in  function. 

The  large  afferent  fibers  arise  in  the  ganglia  situated  on  the  trunk  of  the 
nerve.  From  their  contained  nerve-cells  a  short  axon  process  proceeds 
which  soon  divides  into  a  central  and  a  peripheral  branch.  The 
central  branch  passes  toward  and  into  the  gray  matter  beneath  the  floor  of 
the  fourth  ventricle,  where  its  end- tufts  arborize  around  nerve-cells;  the 
peripheral  branch  passes  toward  the  general  periphery  to  be  distributed  to 
the  mucous  membrane  of  the  lungs,  stomach,  intestine,  etc.  The  small 
efferent  fibers  are  the  peripherally  coursing  axons  of  nerve-cells  situated  in 
the  gray  matter  beneath  the  floor  of  the  fourth  ventricle  at  the  tip  of  the 
calamus  scriptorius.     The  exact  course  of  these  fibers  from  their  origin  into 


3o6  TEXT-BOOK  OF  PHYSIOLOGY. 

the  trunk  of  the  vagus  is  not  positively  known.  According  to  some  investi- 
gators, they  leave  the  medulla  by  way  of  the  spinal  accessory  nerve  and  enter 
the  trunk  of  the  vagus  through  its  internal  or  anastomotic  branch;  according 
to  recent  investigations  made  by  Schaternikoff  and  Friedenthal,  they  leave 
the  medulla  along  the  path  by  which  the  afferent  fibers  enter  and  never 
become  associated  with  the  spinal  accessory  nerve  at  its  origin. 

In  the  neighborhood  of  the  inferior  or  recurrent  laryngeal  nerves,  branches 
containing  efferent  libers  are  given  off,  which  pass  to  the  heart  by  way  of  the 
cardiac  plexus.  The  terminal  branches  of  these  fibers  are  not  distributed 
directly  to  the  heart-muscle,  but  to  the  intra-cardiac  nerve-cells,  around 
the  bodies  of  which  they  end  in  basket-like  formations.  The  fibers  in  the 
vagus  are  pre-ganglionic;  those  of  the  nerve-cells  post-ganglionic.  (See 
Fig.  147.) 

The  Origin  and  Distribution  of  the  Sympathetic  and  Vagus  Nerves 
in  the  Frog. — In  the  frog  and  allied  animals  the  relation  of  these  two  sets  of 
nerve-fibers,  viz.,  the  efferent  sympathetic  fibers  and  the  efferent  vagus 
libers,  is  somewhat  different;  and  because  of  the  fact  that  these  nerves  in  this 
animal  are  largely  employed  for  determining  experimentally  their  respective 
actions  on  the  heart,  this  relation  should  be  clearly  understood. 

The  sympathetic  nerve-fibers  in  this  animal  are  also  in  connection  with 
the  heart  on  the  one  hand  and  with  nerve-fibers  coming  from  the  central 
nerve  system  on  the  other  hand.  The  pre-ganglionic  fibers  take  their  origin 
very  probably  in  nerve-cells  in  the  medulla  oblongata.  From  this  origin 
they  descend  and  emerge  from  the  spinal  cord  in  the  anterior  roots  of  the 
third  spinal  nerve,  then  pass  through  the  white  rami  communicantes  to  the 
third  sympathetic  ganglion  around  the  nerve-cells  of  which  their  terminal 
fibers  arborize. 

From  the  nerve-cells  of  this  ganglion,  the  sympathetic  nerves  proper, 
the  post-ganglionic,  non-meduUated  fibers  arise.  From  this  origin  they 
ascend,  passing  successively  through  the  second  sympathetic  ganglion,  the 
annulus  of  Vieussens,  the  first  sympathetic  ganglion,  to  the  ganglion  on  the 
trunk  of  the  vagus,  at  which  point  they  enter  the  sheath  of  the  vagus  fibers 
and  in  company  with  them  pass  to  the  heart.  For  this  reason  the  common 
trunk  is  generally  spoken  of  as  the  vagosympathetic  nerve. 

The  vagus  nerve  is  connected  wdth  the  medulla  oblongata  by  a  series 
of  from  six  to  eight  roots.  A  short  distance  from  the  medulla,  the  nerve 
trunk  passes  through  a  large  opening  in  the  cranium  beyond  which  it  presents 
an  enlargement,  termed  the  vagus  ganglion.  The  peripheral  end  of  this 
ganglion  gives  off  two  trunks,  one  the  glossopharyngeal,  the  other  the  vagus 
proper. 

The  vagus  nen^e  proper  in  the  frog  also  consists  of  both  afferent  and 
efferent  fibers  which  have  practically  the  same  origin,  distribution  and 
termination  as  the  corresponding  fibers  in  the  mammal. 

After  the  union  of  the  sympathetic  fibers  with  the  vagus  fibers,  the  com- 
mon trunk  passes  forward  to  the  angle  of  the  jaw,  winds  around  the  pharynx 
just  beneath  the  border  of  the  petro-hyoid  muscle  and  in  close  relation  with 
the  carotid  artery.  As  the  nerve  approaches  the  heart  it  divides  into  two 
branches,  the  pulmonary  and  the  cardiac.  At  the  sinus  venosus  some  of 
the  fibers  become  related,  histologically  and  physiologically,  with  the  ganglion 


THE  CIRCULATION  OF  THE  BLOOD. 


307 


cells,  while  others  plunge  into  the  heart,  course  along  the  auricular  septum 
on  the  left  side  and  finally  terminate  at  or  near  the  ganglion  cells  of  the  base 
of  the  ventricle.  The  mode  of  termination  of  both  the  vagus  and  sympa- 
thetic fibers  is  similar  to  that  observed  in  the  mammals. 

The  Physiologic  Actions  of  the  Sympathetic  Nerves  in  the  Frog. — ■ 
The  information  now  possessed  regarding  the  influence  which  the  central 
nerve  system  exerts  on  the  heart  through  these  nerves,  has  been  derived 
largely  from  experiments  made  on  the  nerves  of  the  frog,  toad,  and  turtle. 
Inasmuch  as  the  sympathetic  and  vagus  nen-es  in  the  frog  and  related 
animals  are  bound  up  in  a  common  sheath,  it  is  necessary  in  order  to  demon- 
strate their  respective  functions  first  to  divide  the  nerves,  above  their  union 
at  the  vagus  ganglion,  and  then  stimulate  their  peripheral  ends.  The  heart 
should  be  exposed  and  attached  to  a  recording  lever  so  that  its  movements 
may  be  taken  up  and  recorded  on  a  moving  recording  surface. 


Fig.  141. — Tracings  Showing  the  Effects  on  the  Heart-beat  of  the  Frog  from  Stimu- 
lation OF  THE  Sympathetic  Nerves  Prior  to  Their  Union  with  the  Vagus  Nerve.  The 
upper  tracing  shows  an  increase  in  the  rate,  which  before  stimulation  was  15  per  minute  and 
during  stimulation  30  per  minute.  Before  stimulation  the  height  of  the  ventricular  beat  was 
9  mm.  and  during  the  stimulation  it  was  12  mm.  The  lowest  tracing  shows  a  similar  series  of 
effects,  the  differences  being  only  of  degree. — (Brodie.) 


Stimulation  of  the  sympathetic  fibers  with  induced  electric  currents, 
prior  to  their  union  with  the  vagus,  is  followed  by  an  increase  in  the  rate, 
or  an  augmentation  in  the  force  of  the  heart-beat  or  both,  at  the  same  time. 
The  effects  of  such  a  stimulation  with  induced  currents  of  moderate  intensity 
are  graphically  shown  in  Fig.  141.  The  upper  tracing  shows  that  the  heart  was 
first  accelerated,  the  beats  increasing  from  15  per  minute  before  stimulation, 
to  30  per  minute  during  stimulation.  On  the  cessation  of  the  stimulation, 
the  heart  slowly  returned  to  its  former  rate.  Coincidently  with  this  acceler- 
ation of  the  rate  there  was  an  augmentation  of  the  force  of  the  ventricular 
contraction  as  shown  by  an  increase  in  the  height  of  the  ventricular  con- 
traction which  before  stimulation  was  9  mm.,  but  during  stimulation  12  mm. 


3o8  TEXT-BOOK  OF  PHYSIOLOGY. 

In  addition  to  the  foregoing  changes  in  the  heart-beat  there  is  an  altera- 
tion in  the  sequence  of  the  beat.  The  natural  delay  in  the  conduction  of  the 
excitation  process  from  the  auricles  to  the  ventricle  is  increased,  in  conse- 
quence of  which  the  auricle  completely  relaxes  before  the  ventricular  con- 
traction begins.  Moreover,  the  auricular  contraction  again  occurs  before 
the  ventricle  has  completely  relaxed.  After  the  effect  of  the  stimulation 
passes  away,  the  acceleration  diminishes,  the  augmentation  declines  and  a 
reverse  change  in  the  sequence  occurs.  The  lower  tracing  shows  a  similar 
series  of  effects.  If  the  stimulus  be  applied  to  the  pre-ganglionic  sympathetic 
nerves,  an  acceleration  or  augmentation  of  the  heart  follows,  similar  in  all 
respects  to  that  which  follows  stimulation  of  the  post-ganglionic  or  sympa- 
thetic fibers  proper;  and  the  inference  may  be  drawn  that  if  the  stimulus 
could  be  applied  directly  to  the  nerve-cells  in  the  medulla  oblongata  from 
which  the  fibers  take  their  origin,  the  same  acceleration  or  augmentation 
would  follow;  for  this  reason  this  collection  of  nerve-cells  is  known  as  the 
cardio-accelerator  or  augmentor  center.  Since  stimulation  of  the  nerve  in 
any  part  of  its  course,  which  in  all  probability  exaggerates  its  normal  function, 
is  followed  by  an  acceleration  or  an  augmentation,  the  sympathetic  is  said 
to  have  an  accelerator  or  an  augmentor  influence  on  the  heart-beat;  with 
the  cessation  of  the  stimulation,  and  very  frequently  before,  the  heart  returns 
to  its  normal  condition. 

The  Physiologic  Action  of  the  Vagus  Nerve  in  the  Frog. — Stimulation 
of  the  intra-cranial  roots  of  the  vagus  with  very  weak  induced  electric  cur- 
rents is  followed  by  a  gradual  diminution  in  the  rate  and  a  diminution  in  the 
force  of  the  heart-beat.  If  the  induced  currents  are  moderate  in  strength, 
the  heart  will  at  once  come  to  a  standstill  in  diastole.  (Fig.  142.)  If  the 
stimulus  be  applied  to  the  trunk  or  the  peripheral  portion  of  the  vagus,  for 
example  close  to  the  sinu-auricular  junction,  an  inhibition  occurs  similar  in 


Fig.  142  — Tracing  showing  the  Effect  on  the  Heart-beat  of  the  Toad  of  Long 
Stimulation  of  the  Intra-cranial  Roots  of  the  Vagus  with  Moderately  Strong  Electric 
Currents. — (GaskeU.) 

all  respects  to  that  which  follows  stimulation  of  the  intra-cranial  roots,  and 
judging  from  what  is  known  regarding  the  action  of  nerve-cells,  the  inference 
may  be  drawn  that  if  the  stimulus  could  be  applied  directly  to  the  group  of 
nerve-cells  from  which  the  efferent  fibers  arise,  the  same  inhibition  would 
follow;  for  this  reason  this  collection  of  nerve-cells  is  known  as  the  cardio- 
inhihitor  center.  Since  stimulation  of  the  nerve,  either  at  its  center,  in  its 
course,  or  at  its  periphery,  which  in  all  probability  exaggerates  its  normal 
function,  is  followed  by  a  period  of  rest  or  inactivity,  the  vagus  is  said  to 
have  a  retarding  or  an  inhibitor  influence  on  the  beat  of  the  heart. 

During  the  continuance  of  the  inhibition,  the  heart-muscle  is  relaxed, 
its  cavities  dilated  and  filled  with  blood.  The  dilatation  usually  exceeds 
that  .observed  prior  to  the  vagus  stimulation,  from  which  it  is  inferred 


THE  CIRCULATION  OF  THE  BLOOD. 


309 


that  some  fibers  of  the  vagus  at  least  diminish  the  tonicity  of  the  heart- 
muscle. 

After  cessation  of  the  stimulation,  the  heart  resumes  its  activity.  At 
first  the  beat  usually  is  slow  and  feeble,  but  with  each  succeeding  beat  both  the 
rate  and  force  increase,  until  they  attain  or  exceed  that  observed  prior  to  the 
stimulation.  In  some  cases,  however,  the  heart  begins  to  beat  with  as  much 
and  even  more  vigor  than  it  did  prior  to  the  stimulation.  The  duration  of 
the  inhibitor  effect  varies  with  the  duration  of  the  stimulation.  Thus  during 
and  after  a  stimulation  of  thirty-eight  seconds  the  heart  of  the  toad  remained 


Fig.  143- 


-Tracing  showing  the  Diminution  in  the  Rate  of  the  Heart-beat 
FOLLOWING  Weak  Tetanization  of  the  Vagus  Trunk. 


at  rest  for  292  seconds  (Gaskell);  the  heart  of  a  snake  for  from  one-half  to 
one  hour  (Meyer);  the  heart  of  a, turtle  for  four  and  a  half  hours  (Mills). 
The  period  of  inhibition  will  depend  on  the  strength  of  the  electric  current 
employed,  the  ners'e  stimulated,  the  season  of  the  year,  etc. 

The  effects  on  the  heart-beat  which  will  follow  stimulation  of  the  vago- 
sympathetic in  its  course  vary,  however,  because  of  the  antagonistic  action 
of  the  inhibitor  and  accelerator  nerve  impulses.  Thus  stimulation  of  the 
peripheral  end  of  the  divided  trunk  of  the  vagus  in  the  frog  or  the  toad  with 
weak  tetanizing  induced  electric  currents  is  followed  by  an  increase  in  the 


Fig.  144. — Tracing  showing  Complete  Inhibition  following  Strong  Tetan- 
ization of  the  Vagus  Trunk. 

rate  of  the  heart-beat  because  of  the  stimulation  of  the  accelerator  fibers, 
which  apparently  respond  before  the  inhibitor  fibers;  stimulation  with  some- 
what stronger  currents  is  followed  by  a  diminution  in  the  rate  of  the  beat 
because  of  the  greater  effect  on  the  inhibitor  nerve-fibers  (Fig.  143).  Stim- 
ulation with  strong  tetanizing  currents  is  followed  by  complete  inhibition 
(Fig.  144). 

The  foregoing  facts  lead  to  the  inference  that  the  cardio-accelerator 
and  the  cardio-inhibitor  centers  have  as  their  function  the  discharge  of  nerve 
impulses  which  are  conducted  by  their  related  nerves,  the  efferent  sympathetic 


3IO  TEXT-BOOK  OF  PHYSIOLOGY. 

and  vagal  fibers,  to  the  heart,  and  which,  in  a  manner,  as  yet  unexplained 
accelerate  or  augment  or  inhibit,  the  action  of  the  heart.  The  relation  which 
these  two  centers  bear  one  to  the  other  and  the  manner  in  which  they  are 
influenced  in  their  activities  both  directly  and  reflcxly  and  thus  regulate  the 
action  of  the  heart  from  moment  to  moment  will  be  considered  in  a  subse- 
quent paragraph. 

Changes  in  the  Conductivity  of  the  Heart. — In  addition  to  the 
changes  in  the  rate  and  force  of  the  heart  caused  by  stimulation  of  the  inhib- 
itor and  the  augmentor  nerves,  it  is  stated  by  Gaskell  that  there  is  also  during 
the  inhibition  a  decrease  in  the  conductivity  of  the  heart  at  both  the  sino- 
auricular  and  auriculo-ventricular  junctions,  and  an  increase  in  the  con- 
ductivity during  acceleration  of  the  beat.  The  decrease  in  conductivity 
may  be  so  pronounced  that  only  every  second  or  third  contraction  of  the 
auricle  will  be  followed  by  a  contraction  of  the  ventricle.  In  other  instances 
both  auricles  and  ventricles  remain  at  rest  while  the  sinus  maintains  its  usual 
rate.  The  increase  in  conductivity  is  shown  by  first  artificially  blocking  the 
contraction  wave  at  the  auriculo-ventricular  junction  with  the  clamp,  until 
only  every  second  or  third  auricular  contraction  is  conducted  to  the  ventricle, 
and  then  stimulating  the  sympathetic.  At  once  the  auricular  contraction 
forces  the  block,  and  passes  to  the  ventricle,  calling  forth  a  normal  contraction 

The  Physiologic  Actions  of  the  Sympathetic  Nerves  in  Mammals. — 
In  the  mammal,  stimulation  of  the  sympathetic  nerves  in  any  part  of  their 
course,  either  through  the  rami  communicantes,  the  ventral  portion  of  the 
annulus  of  Vieussens,  or  after  their  emergence  from  the  stellate  or  inferior 


Fig.  145. — Acceleration  of  the  Heart  following  Stimulation  of  the  Cardiac 
Branches  which  come  from  the  Annltlus  of  Vieussens. 

cervical  ganglia  is  followed  by  effects  similar  to  those  observed  in  the  frog: 
viz.,  an  acceleration  or  augmentation,  or  both,  of  the  heart-beat.  The 
percentage  increase  in  the  acceleration  varies  in  different  animals.  In  some 
instances  the  increase  varies  from  58  per  cent,  to  100  per  cent.  (Hunt).  If 
the  heart  is  beating  slowly  before  stimulation,  the  acceleration  is  more  marked 
than  if  it  is  beating  rapidly. 

The  effect  of  the  accelerator  impulses  is  apparently  a  change  in  the  inner 
mechanism  of  the  heart-muscle  itself  and  not  a  change  in  the  peripheral 
portion  of  the  inhibitor  apparatus.  This  is  indicated  by  the  fact  that 
acceleration  occurs  after  the  full  physiologic  action  of  atropin,  which  acts 
upon,  and  impairs  the  conductivity  of,  the  intra-cardiac  nerve-cell  terminals. 

A  peculiarity  of  the  sympathetic  nerve  is  that  it  does  not  respond  to 
stimulation  as  rapidly  as  do  many  nerves,  so  that  a  rather  long  latent  period 
intervenes  between  the  moment  of  stimulation  and  the  appearance  of  the 
acceleration  as  shown  in  Fig.  145.  A  further  peculiarity  is  that  the  accelera- 
tion sometimes  continues  after  the  stimulus  is  withdrawn,  and  sometimes 
ceases  before  it  is  withdrawn. 

Though  an  increase  in  both  the  rate  and  force  frequently  occur  simul- 


THE  CIRCULATION  OF  THE  BLOOD.  311 

taneously,  there  is  no  necessary  relation  or  connection  between  the  two  as 
they  can  and  do  occur  independently  of  each  other.  For  this  reason  it  is 
generally  assumed  that  the  sympathetic  nerves  contain  two  groups  of  fibers, 
viz.,  accelerators  and  augmentors,  the  functions  of  which  are  respectively 
to  accelerate  the  rate  and  augment  the  force  of  the  heart-beat.  From  the  fact 
that  both  auricles  and  ventricles  exhibit  these  changes  it  is  assumed  that 
the  nerve  impulses  stimulate  both  chambers.  This  is  rendered  probable 
also  from  the  experihients  of  Erlanger,  who  found  that  after  complete  heart- 
block,  stimulation  of  the  sympathetic  caused  independent  acceleration  of 
both  auricles  and  ventricles. 

The  Physiologic  Action  of  the  Vagus  Nerve  in  Mammals. — In  the 
mammal  the  same  or  similar  effects  result  from  stimulation  of  the  vagus  as 
in  the  frog.  If  the  thorax  of  the  dog  is  opened  and  artificial  respiration 
maintained  the  heart  wall  continue  to  beat  in  a  practically  normal  manner  for 
a  long  time.  Under  such  conditions  if  the  vagus  nerve  on  one  side  be  divided 
and  its  peripheral  end  stimulated  with  induced  electric  currents  of  moderate 
strength,  the  heart  will  be  seen  to  come  to  a  standstill  almost  immediately  in 
the  condition  of  diastole,  and  may  be  so  kept  for  a  variable  period,  from 
fifteen  to  thirty  seconds  or  more,  during  which  its  w^alls  are  re'axed  and  its 
cavities  filled  with  blood.  On  cessation  of  the  stimulation  the  contractions 
return  and  in  a  very  short  time  the  former  rate  and  force  of  the  beat  are 
regained.  If  the  electric  currents  are  of  feeble  strength,  the  heart  will 
come  to  rest  gradually,  through  a  gradual  diminution  in  the  rate  and  force 
of  the  contraction.  During  the  period  of  the  inhibition  the  heart  presents 
an  appearance  similar  to  that  presented  by  the  heart  of  the  cold-blooded 
animal.  When  the  heart  of  an  animal  is  thus  exposed,  the  auricle  and  the 
ventricle  of  one  side  may  be  attached  by  threads  to  writing  levers  and  their 
contractions  registered  on  a  moving  recording  surface.  The  effects  on  both 
auricles  and  ventricles  which  follow  vagus  stimulation  will  then  become 
more  apparent.  Fig.  146  is  a  tracing  thus  obtained.  The  animal  employed 
was  a  rabbit. 

The  inhibitor  effect  of  the  vagus  varies  in  degree  and  duration  in 
different  animals.  In  the  dog  the  effect  of  vagus  stimulation  is  usually 
pronounced,  lasting  from  15  to  30  seconds;  in  the  rabbit  it  is  perhaps  equally 
well  pronounced  but  somewhat  less  in  duration;  in  the  cat  it  is  almost  want- 
ing. In  this  latter  animal  a  complete  standstill,  even  for  a  few  seconds,  is 
very  rarely  seen ;  usually  there  is  produced  merely  a  slight  diminution  in  the  rate 
of  the  beat  even  though  the  stimulus  employed  is  quite  strong.  In  all  these 
animals,  however,  after  a  very  short  time  the  nerve  impulses  lose  their 
inhibitor  influence  on  the  heart-muscle,  and  notwithstanding  continued 
stimulation  of  the  vagus,  the  heart  returns  to  its  former  rate  and  vigor. 
This  result  is  in  marked  contrast  to  that  observed  during  stimulation  of  the 
vagus  in  the  cold-blooded  animals,  in  which  the  heart  may  be  kept  at  rest 
for  relatively  very  long  periods  of  time.  No  satisfactory  explanation  for 
this  loss  of  vagus  control  or  escape  of  the  heart  from  the  vagus  control  has 
as  yet  been  offered. 

Seat  of  Action  of  the  Vagus  Impulses. — In  a  foregoing  experiment  of 
w'hich  Fig.  146  is  a  graphic  result,  stimulation  of  the  left  vagus  with  a  fairly 
strong  current  was  followed  by  a  diminution  in  both  the  rate  and  force  of  the 


312  TEXT-BOOK  OF  PHYSIOLOGY. 

contraction  of  both  auricles  and  ventricles,  though  the  effect  was  most 
marked  in  the  auricles.  From  this  and  similar  facts  it  has  come  to  be  the 
general  belief  that  the  inhibitor  nerve  impulses  exert  their  influence  mainly, 
if  not  exclusively,  on  the  auricle,  and  that  the  cessation  of  ventricular  action 
is  a  secondary  effect  due  to  the  non-arrival  across  the  conducting  apparatus 


Fig.  146. — Result  of  the  Stimulation  of  the  Peripheral  End  of  the 
Divided  Left  Vagus  in  the  Rabbit. — (Brodie.) 

of  the  normal  excitation  process  from  the  auricle.  This  is  the  case  un- 
doubtedly in  the  cold-blooded  animals,  and  the  experiments  of  Erlanger  on 
the  heart  of  the  dog  indicate  that  the  same  holds  true  for  the  mammals. 
This  investigator  has  found  that  when  the  auriculo-ventricular  tissues  are 
suddenly  clamped,  ificluding  presumably  the  muscle  band  of  His,  there  is  for  a 
time  a  complete  cessation  of  ventricular  activity,  but  after  a  variable  period  of 
time,  fifty  seconds  or  more,  the  ventricle  develops  an  independent  rhythm 
which  gradually  increases  in  frequency,  but  seldom,  if  ever,  attains  that  of 


THE  CIRCULATION  OF  THE  BLOOD.  313 

the  auricles.  Under  such  circumstances  tetanic  stimulation  of  the  auriculo- 
ventricular  tissues  by  means  of  the  clamp  now  transformed  into  stimulating 
electrodes,  failed  to  bring  about  a  stoppage  of  the  ventricles.  Moreover, 
if  during  the  time  the  clamp  is  applied  and  after  the  ventricle  has  developed 
a  rhythm  of  its  own,  the  vagus  is  stimulated,  the  auricles  will  cease  to  beat 
as  usual,  but  the  ventricles  will  continue  to  beat  at  their  usual  rate.  These 
and  similar  facts  lead  to  the  conclusion  that  vagal  inhibitor  action  is 
limited  to  the  auricles. 

From  the  foregoing  facts  it  is  apparent  that  the  accelerator  and  augmentor 
effects  of  the  sympathetic  nen-e  impulses,  and  the  inhibitor  effects  of  the 
vagus  nerve  impulses,  closely  resemble  on  the  one  hand,  the  accelerator  and 
augmentor  effects  of  increasing  amounts  of  diffusible  calcium  salts,  and  on 
the  other  hand,  the  inhibitor  effects  of  increasing  amounts  of  diffusible 
potassium  salts  in  the  blood  or  other  circulating  fluid;  and  so  closely  do  these 
two  sets  of  phenomena  resemble  each  other,  that  they  are  by  some  observers 
regarded  as  identical. 

Some  additional  facts  in  this  connection  have  been  presented  by  Howell, 
viz.,  that  an  increase  (within  limits)  and  a  decrease  in  the  percentage  of 
diffusible  calcium  salts  in  a  circulating  fluid  passing  through  the  cavities  of 
the  mammalian  (cat)  heart,  increases  on  the  one  hand,  and  decreases  on 
the  other  hand,  the  sensitiveness  of  the  heart  to  sympathetic  acceleration 
and  augmentation.  From  this  the  inference  is  deduced  that  the  acceleration 
and  augmentation  of  the  heart-beat  which  follow  stimulation  of  the  sympa- 
thetic nerves  are  due  to  the  presence  in  the  heart  tissue  of  a  certain  percentage 
of  diffusible  calcium  salts,  which  have  been  freed  from  combination  with 
organic  matter  by  the  action  of  the  sympathetic  nerve  impulses.  Again, 
that  an  increase  (within  limits)  and  a  gradual  decrease  in  the  percentage 
of  diffusible  potassium  salts  in  a  circulating  fluid  passing  through  the  cavities 
of  the  frog  and  the  cat  heart,  increases  on  the  one  hand  and  decreases  and 
finally  abolishes  on  the  other  hand  the  sensitiveness  of  the  heart  to  vagus 
inhibition.  From  this  the  inference  is  deduced  that  the  inhibition  of  the 
heart-beat  which  follows  stimulation  of  the  vagus  nerve  is  due  to  the  presence 
in  the  heart  tissue  of  a  certain  percentage  of  diffusible  potassium  salts, 
which  have  been  freed  from  combination  with  organic  matter  by  the  action 
of  the  vagus  nerve  impulses. 

The  foregoing  effects  of  the  sympathetic  and  vagus  nerves  on  the  heart 
muscle,  viz.;  changes  in  its  irritability,  conductivity,  rapidity,  and  the 
energy  of  the  beat,  have  been  termed  by  Engelmann  bathmotropic,  dromo- 
tropic,  chronotropic,  and  inotropic.  Any  one  of  these  effects,  e.g.,  the  chrono- 
tropic, may  be  modified  in  a  positive  direction  by  the  sympathetic,  or  in  a 
negative  direction  by  the  vagus. 

The  Cardio-Accelerator  Center.— The  collection  of  nerve-cells  from 
which  the  pre-ganglionic  fibers  of  the  sympathetic  system  arise  is  known  as 
the  cardio-accelerator  or  augmentor  center.  The  exact  location  of  this 
center  in  the  central  nerve  system  has  not  been  as  yet  accurately  determined. 
It  is  probably  located  in  the  medulla  oblongata. 

From  experiments  which  have  been  made  on  the  sympathetic  nerve 
apparatus  in  its  entirety,  it  is  believed  that  the  function  of  this  center  is  the 
discharge  of  nerve  impulses  which,  conducted  to  the  heart  by  the  pre- 


314  tp:xt-book  of  physiology. 

ganglionic  and  post-ganglionic  sympathetic  fibers,  cause  an  acceleration  in 
the  rate  or  an  augmentation  in  the  force,  or  both,  of  the  heart-beat.  It  is 
also  generally  believed  since  the  publication  of  Hunt's  investigations  that 
this  center  is  in  a  state  of  tonic  activity.  This  is  shown  by  the  fact  that  after 
the  division  of  the  vagus  nerves  and  the  removal  of  all  possible  inhibitor 
influences,  division  of  the  sympathetic  nerves  or  extirpation  of  the  stellate 
or  inferior  cervical  ganglion,  is  yet  followed  by  a  decrease  in  the  rate  of  the 
heart-beat.  After  division  of  the  sympathetic  nerves  and  the  removal  of 
accelerator  influences  it  is  also  easier  to  bring  about  inhibition  through  vagus 
stimulation. 

The  Factors  which  Determine  the  Activity  of  the  Cardio-Accelera- 
tor  Center. — The  question  has  been  raised  as  to  whether  the  tonic  activity 
of  this  center  is  maintained  by  central  or  peripheral  stimuli,  i.e.,  whether  it 
is  maintained  by  causes  within  itself,  the  result  of  an  interaction  between  the 
constituents  of  the  cell  substance  and  those  of  the  surrounding  lymph,  or 
whether  it  is  maintained  by  nerve  impulses  reflected  to  it  through  various 
afferent  or  sensor  nerves.  Inasmuch  as  there  is  no  way  of  determining 
whether  the  causes  are  central,  except  by  dividing  all  afferent  nerves,  it  is 
impossible  to  state  how  much  influence  is  to  be  attributed  to  this  factor. 
On  the  contrary,  though  it  is  readily  demonstrable  that  stimulation  of  many 
afferent  nerves  will  cause  an  acceleration  of  the  heart  it  cannot  be  stated 
positively  that  this  is  the  result  of  a  reflex  stimulation  of  the  accelerator  center. 
Though  earlier  investigators  believed  this  to  be  the  correct  interpretation, 
the  more  recent  experiments  of  Hunt  apparently  disprove  it;  for  this  investi- 
gator has  shown  that  if  the  vagus  nerves  are  divided  it  is  impossible  to  pro- 
duce reflex  acceleration  of  the  heart.  His  conclusion,  confirming  that  of 
others,  is  that  cardiac  acceleration  is  the  result  of  an  inhibition  of  the  cardio- 
inhibitor  center.  A  freer  play  to  the  tonic  activity  of  the  accelerator  center 
would  thus  be  made  possible. 

The  Cardio-Inhibitor  Center. — The  collection  of  nerve-cells  from 
which  the  small  efferent  fibers  of  the  vagus  nerve  arise  is  known  as  the 
cardio-inhibitor  center.  It  is  situated  in  the  medulla  oblongata  or  more 
exactly  in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle  near 
the  tip  of  the  calamus  scriptorius.  It  is  in  all  probability  a  part  of  the 
nucleus  ambiguus. 

From  the  experiments  which  have  been  made  on  the  vagus  inhibitor 
apparatus  in  its  entirety  it  is  believed  that  the  function  of  this  center  is  the 
discharge  of  nerve  impulses  which  conducted  to  the  heart  by  the  vagus 
fibers  cause  an  inhibition  of  its  beat  of  greater  or  less  extent.  In  the  dog, 
and  probably  in  many  other  mammals,  this  center  exerts  a  more  or  less 
constant  inhibitor  or  restraining  influence  on  the  heart's  activity.  This  is 
indicated  by  the  fact  that  the  rate  of  the  beat  is  very  much  increased  by 
simultaneous  division  of  both  vagi.  The  degree  of  the  inhibition  which  this 
center  exerts  varies  greatly,  however,  in  different  animals.  In  the  cat  and 
in  the  rabbit  the  inhibitor  control  is  normally  so  slight  that  there  is  but  a 
relatively  slight  increase  in  the  rate  of  the  beat  after  division  of  the  vagi. 
The  tone  of  the  vagus  in  these  animals  is,  therefore,  said  to  be  slight  or 
feeble.  In  human  beings  the  tone  of  the  inhibitor  apparatus  is  poorly 
developed  in  early  childhood,  as  shown  by  the  fact  that  the  administration 


THE  CIRCULATION  OF  THE  BLOOD.  315 

of  atropin,  which  removes  temporarily  inhibitor  control,  is  not  followed  by  an 
increase  in  the  rate  of  the  beat.  It  develops  steadily  and  reaches  a  maximum 
at  from  the  twenty-fifth  to  the  thirtieth  year.  In  advanced  years  the  tone 
again  declines.  For  these  and  other  reasons  it  is  believed  that  this  center 
is  in  a  state  of  tonic  activity  in  many  if  not  all  mammals,  discharging  nen'e 
impulses  which  exert  a  regulative  influence  on  the  cardiac  mechanism  in  ac- 
cordance with  its  needs  and  especially  in  reference  to  the  variable,  resistances 
offered  to  the  flow  of  blood  which  the  heart  must  overcome. 

The  Factors  which  Determine  the  Activity  of  the  Cardio-Inhibitor 
Center. — -The  question  has  also  been  raised  as  to  whether  the  tonic  activity 
of  this  center  is  maintained  by  central  or  peripheral  stimuli,  i.e.,  whether  it  is 
maintained  by  causes  within  itself  the  result  of  an  interaction  between  the 
constituents  of  the  cell  substance  and  those  of  the  surrounding  lymph,  or 
whether  it  is  maintained  by  nerve  impulses  reflected  to  it  through  various 
afferent  or  sensor  nerves.  Though  both  factors  play  an  important  part  in 
the  maintenance  of  its  activity,  the  trend  of  evidence  points  to  the  conclusion 
that  the  reflected  impulses  are  by  far  the  more  important  of  the  two.  This 
latter  supposition  is  supported  by  the  results  of  direct  experimentation  upon 
sensor  nerves  in  almost  any  region  of  the  body.  Thus  stimulation  of  the 
dorsal  roots  of  the  spinal  nerves,  the  trunks  of  the  cranial  sensor  nerves,  the 
splanchnic  nerves,  the  pulmonary  branches  of  the  vagus,  etc.,  gives  rise  to  a 
more  or  less  pronounced  inhibition  of  the  heart.  As  a  rule,  stimulation  of 
the  peripheral  terminations  of  these  nerves  is  more  effective  than  stimulation 
of  their  trunks,  hence  an  explanation  is  at  hand  for  the  cardiac  inhibition 
which  results  from  sudden  distention  of  the  stomach  and  intestines,  or 
operative  procedures  in  the  nose,  mouth,  and  larynx. 

Reflex  inhibition  of  the  heart,  even  to  the  stage  of  absolute  and  permanent 
standstfll,  eventuating  in  the  death  of  the  individual  is  a  not  infrequent 
result  of  peripherally  acting  causes  of  a  pathologic  or  operative  character. 
From  the  results  of  experimental  procedures  the  inference  is  drawn  that 
normally,  nerve  impulses,  developed  by  the  action  of  physiologic  causes, 
are  reflected  continuously  from  many  peripheral  regions  of  the  body,  and 
falling  into  this  center  gently  stimulate  and  maintain  it  in  a  condition  of 
necessary  tonicity  or  activity. 

The  Causes  of  the  Variations  in  the  Heart-beat. — It  has  been  stated 
elsewhere  in  the  text  (page  286),  that  the  rate  of  the  heart-beat  is  influenced 
by  age,  muscle  activity,  the  position  of  the  body,  meals,  variations  in  blood 
pressure,  etc.  The  manner  in  which  these  changes  are  brought  about  is 
not,  however,  always  apparent.  In  addition  to  variations  that  are  strictly 
physiological  in  character  there  is  abundant  evidence  that  other  factors,  e.g., 
the  action  of  peripheral  stimuli  of  a  physiologic  or  pathologic  character  in 
various  regions  of  the  body,  can  and  do  cause  reflexly  at  one  time  or  in  one 
individual  an  acceleration  of  a  marked  character,  and  at  another  time  or  in 
another  or  the  same  individual  an  inhibition  which  may  be  so  pronounced 
as  to  lead  to  a  complete  standstill  in  diastole.  The  records  of  clinical  medi- 
cine contain  many  instances  which  show  that  gastric,  intestinal,  uterine  and 
other  organic  disorders  as  well  as  various  operative  procedures  in  dift'erent 
regions  of  the  body  cause  now  an  acceleration,  now  an  inhibition  of  the 
heart. 


3i6  TEXT-BOOK  OF  PHYSIOLOGY. 

The  first  explanation,  that  acceleration  of  the  heart,  the  result  of  a 
peripherally  acting  stimulus,  is  due  to  a  stimulation  of  the  cardio-accelerator 
center  by  the  arrival  of  nerve  impulses  coming  through  afferent  nerves, 
having  been  made  questionable  and  improbable  by  the  results  of  Hunt's 
experiments,  the  alternative  explanation  must  be  that  the  acceleration  is  due 
to  an  inhibition  of  the  normal  activity  of  the  cardio-inhibitor  center,  and 
that  inhibition  is  due  to  an  excitation  of  the  normal  activity  of  the  cardio- 
inhibitor  center,  and  hence  there  follows  the  corollary  that  afferent  nerves 
contain  two  sets  of  nerve-fibers  which  are  in  physiologic  relation  with  the 
cardio-inhibitor  center,  one  of  which  when  stimulated  peripherally  inhibits  its 
activity,  the  other  of  which  when  stimulated  excites  or  augments  its  activity. 

The  extent  to  which  both  sets  of  fibers  are  present  in  any  one  afferent 
nerve  is  unknown.  In  the  trigeminus  it  is  believed  the  excitator  fibers 
preponderate  for  the  reason  that  peripheral  stimulation  of  this  nerve  is 
followed  by  inhibition  of  the  heart;  in  the  sciatic,  it  is  believed  the  inhibitor 
nerves  preponderate,  for  the  reason  that  stimulation  of  the  central  end  of  the 
divided  nerve  is  followed  generally  by  acceleration  of  the  heart. 

It  is  probable  from  the  effects  which  follow  gastro-intestinal  disorders, 
that  the  vagus  nerve  contains  both  classes  of  fibers  as  represented  in  Fig.  141, 
inasmuch  as  stimuli  of  a  pathologic  character  in  one  individual  may  reflexly 
excite  or  increase  the  activity  of  the  cardio-inhibitor  center,  to  be  followed 
by  an  inhibition  of  the  heart;  and  in  another  individual,  may  reflexly  inhibit 
the  activity  of  the  same  center  and  to  such  an  extent  that  the  cardio-accelerator 
center  may  be  enabled  to  increase  either  the  rate  or  the  force  or  both,  of  the 
heart  movements.  Palpitation  of  the  heart  from  gastric  irritation  might 
thus  be  explained. 

The  Influence  of  Psychic  States. — The  cardio-inhibitor  and  the  cardio- 
accelerator  centers  may  be  increased  in  activity  also  by  nerve  impulses 
descending  from  the  cerebrum,  the  result  of  emotional  states;  thus  depressing 
emotions  according  to  their  intensity  may  so  increase  the  activity  of  the 
cardio-inhibitor  center  that  the  heart's  action  may  not  only  be  retarded  but 
even  completely  inhibited;  joyous  emotions,  on  the  contrary,  may  so  increase 
the  activity  of  the  cardio-accelerator  center  or  what  is  more  probable  inhibit 
the  activity  of  the  cardio-inhibitor  center  that  the  heart's  action  will  be 
increased  in  both  its  rate  and  force. 

From  the  results  of  stimulation  of  the  sympathetic  (accelerator)  and 
vagus  (inhibitor)  nerves  under  a  great  variety  of  conditions  it  has  been 
established  that  their  respective  centers  are  mutually  antagonistic;  that  the 
activity  of  the  accelerator  center  at  one  moment  limits  the  activity  of  the 
inhibitor  and  at  another  moment  is  limited  in  turn  by  it;  that  the  rate  of  the 
heart-beat  at  each  moment  is  the  resultant  of  the  relative  degree  of  activity 
of  the  two  centers. 

The  Depressor  Nerve.— The  vagus  trunk  also  contains  afferent  fibers 
stimulation  of  which  not  only  brings  about  a  reflex  inhibition  of  the  heart, 
but  also  a  dilatation  of  the  peripheral  arteries  and  a  fall  of  blood-pressure 
through  a  depressive  influence  on  the  vaso-motor  centers.  To  this  nerve 
the  term  depressor  has  been  given.  A  consideration  of  the  physiologic 
action  of  this  nerve  will  be  found  in  the  section  devoted  to  the  nerve  mechan- 
isms concerned  in  the  maintenance  of  the  blood-pressure. 


THE  CIRCULATION  OF  THE  BLOOD. 


317 


Seat  of  action 
of  J\ficottn 


Sympathetic 
neuro/i 


-■?m< 


Modifications  of  the  Nerve  Mechanism  of  the  Heart  due  to  the 
Physiologic  Action  of  Drugs. — The  functions  of  different  parts  of  the 
nerve  mechanism  of  the  heart  may  be  demonstrated  by  an  analysis  of  the 
•effects  which  follow  the  administration  of  slightly  toxic  doses  of  the  alkaloids 
of  various  drugs.  The  effects  can  be  shown  to  be  due  to  a  stimulation  or  to 
a  depression  of  the  normal  activity  of  one  or  more  portions  of  the 
mechanism.  The  alkaloid  may  exert  its  specific  action  on  the  central 
portions  in  the  medulla,  or  on  the  peripheral  portions  in  the  heart,  or  on 
both  simultaneously.  The  heart-muscle  may  at  the  same  time  be  stimu- 
lated or  depressed  in  its  action  either  in  the  same  or  in  the  opposite 
direction  to  that  of  the  nerve  mechanism.  As  a  result  the  heart-beat  may 
be  increased  or  decreased  both  in  rate  and  force. 

The  following  examples  will  illustrate  the  action  of  alkaloids  in  general. 

^^tropin. — After  the  administration  of  atropin  in  sufficient  amounts  the 
heart-beat  increases  in  frequency  in  all  animals  in  which  the  cardio-inhibitor 
centers  exert  a  steady  inhibitor  influence  over 
the  heart.  This  is  especially  true  in  man  and 
the  dog.  In  animals  in  which  the  inhibitor 
control  is  slight,  as  the  rabbit  and  frog,  the  in- 
crease in  frequency  is  not  very  marked.  In  all 
animals  thus  far  experimented  on  after  the  ad- 
ministration of  atropin,  neither  stimulation  of 
the  trunk  of  the  vagus  nor  stimulation  of  the 
intracardiac  ganglia  will  arrest  or  even  retard 
the  heart-beat.  The  inference,  therefore,  is 
that  the  alkaloid  exerts  its  action  upon  the  gan- 
glion cells  and  their  terminal  branches,  impair- 
ing their  chemic  integrity  and  abolishing  their 
normal  function,  that  of  conducting  nerve  im- 
pulses from  the  vagus  nerve  proper  to  the  heart- 
muscle.  Fig.  147.  In  consequence  of  this,  the 
influence  of  the  cardio-inhibitor  center  is  cut  ofl" 
and  the  cardio-accelerator  being  unopposed  in 
its  activaty,  the  rate  of  the  beat  is  increased;  After  a  variable  period 
the  heart  returns  to  its  normal  rate.  Stimulation  of  the  vagus  is  again 
followed  by  the  usual  inhibition.  As  atropin  is  partly  oxidized,  and 
partly  excreted,  it  is  assumed  that  the  nerve  terminals  have  been  restored 
by  nutritive  forces  to  their  normal  condition  and  their  conductivity  regained. 
This  ha\'ing  been  accomplished  the  vagus  nerve  impulses  can  again  reach 
the  heart-muscle  and  the  cardio-inhibitor  center  is  therefore  enabled  to 
re-establish  inhibitor  control  and  antagonize  the  activity  of  the  cardio- 
accelerator  center. 

Nicotin. — ^After  the  administration  of  nicotin  in  sufficient  amounts 
the  heart-beat  is  primarily  decreased  in  frequency  even  to  the  point  of  stand- 
still in  diastole  for  a  few  seconds,  and  secondarily  increased  both  in  fre- 
quency and  force  beyond  the  normal.  If  the  vagus  nen^es  be  first  divided 
this  primary  decrease  is  not  so  marked  and  the  inference  is  that  the  alkaloid 
primarily  stimulates  the  cardio-inhibitor  center  and  increases  its  normal 
function  and  perhaps  the  terminal  branches  of  the  vagus  fibers,  the  pre-gangli- 


iJa^vs  ner^e. 


Seat  of  action 
of  Atropin. 


*^t 


A/iiscie  fiire 

Fig.  147. — Diagram  showing 
THE  Relation  of  the  Vagus  to 
THE  Heart  Muscle-cell. 


3i8  TEXT-BOOK  OF  PHYSIOLOGY. 

onic,  as  well.  After  the  secondary  increase  in  the  rate  is  established  stimulation 
of  the  vagus  trunk  fails  to  inhibit  the  heart,  though  stimulation  of  the  intra- 
cardiac ganglia  is  at  once  followed  by  the  usual  inhibitor  phenomenon,  arrest 
of  the  heart  in  diastole.  For  this  reason  it  is  believed  that  nicotin  acts  on  the 
peripheral  terminations  of  the  pre-ganglionic  libers  of  the  vagus  as  they 
arborize  around  the  intra-cardiac  ganglia,  depressing  them  and  suspending 
their  normal  function,  that  of  conducting  nerve  impulses  from  the  vagus  to 
the  ganglion  cells.  Since  stimulation  of  the  pre-ganglionic  fibers  of  the 
accelerator  apparatus  fails  to  accelerate  the  rate  of  the  heart-beat,  though 
stimulation  of  the  post-ganglionic  fibers  has  the  usual  accelerating  effect, 
the  inference  is  that  nicotin  acts  upon  and  suspends  the  conductivity  of  their 
terminal  branches  in  the  ganglia.  The  acceleration  of  the  heart  must 
therefore  be  attributed  either  to  a  stimulation  of  the  post-ganglionic  fibers 
or  of  the  cardiac  muscle  itself  (Cushny). 

Pilocarpin  and  Muscarin. — These  alkaloids,  whether  administered  in- 
ternally or  applied  locally  to  the  heart,  diminish  the  frequency  and  the 
force  of  the  beat  to  such  an  extent  that  it  very  shortly  comes  to  rest  in  dias- 
tole. For  the  reason  that  the  internal  administration  or  the  local  applica- 
tion of  atropin  in  proper  doses,  which  has  a  depressive  action  on  the  intra- 
cardiac cell  terminations,  removes  the  inhibition  and  restores  the  normal 
rhythm,  the  inference  is  drawn  that  both  these  alkaloids  either  increase  the 
irritability  of  the  nerve-cells  or  heighten  the  conductivity  of  their  terminal 
fibers.  The  return  of  the  heart-beat  is  attributed  to  a  decline  in  irritability 
to  the  normal  level  in  consequence  of  the  antagonistic  action  of  the  atropin. 
Digitalin. — The  administration  of  digitalin  gives  rise  to  effects  the 
character  and  extent  of  which  vary  in  different  animals.  In  the  frog,  as  a  rule, 
the  only  effect  produced  is  a  gradual  increase  in  the  duration  and  force  of  the 
ventricular  systole,  with  a  corresponding  decrease  in  the  duration  of  the  dias- 
tole, until  the  heart  comes  to  rest  in  the  systolic  state.  As  this  effect  is 
observed  after  division  of  the  vagus  trunk  and  also  after  the  suspension  of 
the  activity  of  the  intra-cardiac  cell-fibers  by  atropin,  it  is  evidently  due  to  a 
direct  stimulation  of  the  heart-muscle.  In  some  instances,  however,  the 
opposite  effect  is  produced,  viz. ,  a  gradual  increase  in  the  length  of  the  diastole, 
a  decrease  in  the  duration  of  the  systole,  until  the  heart  comes  to  rest  in  the 
diastolic  state.  As  this  effect  arises  only  when  the  vagus  nerve  is  intact  it  is 
very  probably  due  to  a  stimulation  of  the  cardio-inhibitor  center  and  a  con- 
sequent increase  of  its  functional  activity.  Though  either  effect  may  be 
produced  in  the  frog  the  predominant  effect  is  the  increase  in  the  contrac- 
tion of  the  heart-muscle  rather  than  an  inhibition  of  the  beat. 

In  mammals  both  effects  are  observed,  viz.,  a  diminution  in  the  rate  of 
the  beat,  a  lengthening  of  the  diastole  and  an  increase  in  the  vigor  of  the 
systole,  which  are  evidently  due  to  a  simultaneous  stimulation  of  the  cardio- 
inhibitor  center  and  of  the  cardiac  muscle.  Digitalin  thus  expends  itself  on 
two  opposing  mechanisms;  as  to  which  gains  the  ascendency  will  depend  on 
the  dosage  and  the  character  of  the  animal. 


CHAPTER  XIV. 

THE  CIRCULATION  OF  THE  BLOOD  (Continued). 
THE  VASCULAR  APPARATUS  :  ITS  STRUCTURE  AND  FUNCTIONS. 

The  systemic  vascular  apparatus  consists  of  a  closed  system  of  vessels 
extending  from  the  left  ventricle  to  the  right  auricle,  and  includes  the  arteries, 
capillaries,  and  veins.  Though  serving  as  a  whole  to  transmit  blood  from 
the  one  side  of  the  heart  to  the  other,  each  one  of  these  three  divisions  has 
separate  but  related  functions,  which  are  dependent  partly  on  differences  in 
structure  and  physiologic  properties,  and  partly  on  their  relation  to  the  heart 
and  its  physiologic  activities. 

The  Structure,  Properties  and  Functions  of  the  Arteries. — The 
arteries  serve  to  transmit  the  blood  ejected  from  the  heart  to  the  capillaries; 
that  this  may  be  accomplished  they  divide  and  subdivide  and  ultimately 
penetrate  each  and  every  area  of  the  body.  Their  repeated  division  is 
attended  by  a  diminution  in  size,  a  decrease  in  the  thickness  and  a  change  in 
the  structure  of  their  walls. 

A  typical  artery  consists  of  three  coats:  an  internal,  the  tunica  intima;  a 
middle,  the  tunica  media;  an  external,  the  tunica  adventitia. 

The  internal  coat  consists  of  a  structureless  elastic  basement  membrane, 
on  the  inner  surface  of  which  rests  a  layer  of  elongated  spindle-shaped  en- 
dothelial cells.  The  middle  coat  consists  of  several  layers  of  circularly 
disposed,  non-striated  muscle-fibres,  between  which  are  networks  of  elastic 
fibers.  The  external  coat  consists  of  bundles  of  connective  tissue  of  the 
white  fibrous  and  yellow  elastic  varieties.  Between  the  external  and  middle 
coats  there  is  an  additional  elastic  membrane.  In  the  small  arteries  there  is 
but  a  single  layer  of  muscle-fibers.  In  the  large  arteries  the  elastic  tissue 
is  very  abundant,  exceeding  largely  in  amount  the  muscle-tissue.  It  is  also 
more  closely  and  compactly  arranged.  The  external  coat  is  well  developed 
in  the  large  arteries  (Fig.  148). 

In  virtue  of  the  presence  in  their  walls  of  both  elastic  and  contrac- 
tile elements,  the  arteries  possess  the  two  properties  of  elasticity  and 
contractility. 

The  elasticity  is  especially  well  developed  in  the  large  arteries,  which  are 
capable,  therefore,  of  both  distention  and  elongation,  and,  when  the  distend- 
ing force  is  withdrawn,  of  returning  to  their  previous  condition.  The 
elasticity  permits  of  a  wide  variation  in  the  amount  of  blood  the  arterial 
system  can  hold  between  its  minimum  and  maximum  distention.  Thus 
the  capacity  of  the  aorta  and  carotid  artery  of  the  rabbit  can  be  increased 
four  times  and  six  times  respectively  by  raising  the  intra-arterial  pressure 
from  o  to  200  mm.  of  mercury.  The  elasticity  also  converts  the  intermittent 
movement  of  the  blood  imparted  to  it  by  the  heart  as  it  is  ejected  from  the 
ventricle,  into  a  remittent  movement  in  the  arteries  and  finally  into  the  con- 

319 


320 


TEXT-BOOK  OF  PHYSIOLOGY. 


tinuous  and  equable  movement  observed  in  the  capillaries.  This  is  accom- 
plished in  the  following  manner:  With  each  contraction  of  the  left  ventricle 
more  blood  is  ejected  into  the  aorta  than  the  arteries  can  discharge  into  the 
capillaries  and  veins  during  the  time  of  the  contraction.  The  portion  not  so 
discharged  exerts  a  lateral  pressure  against  the  walls  of  the  arteries  which  at 
once  dilate  until  a  condition  of  equilibrium  is  established  between  the  pres- 
sure from  within  and  the  elastic  reaction  of  the  arterial  walls  from  without. 

With  the  cessation  of  the  contraction  the  elas- 
tic walls  recoil  and  propel  the  blood  toward 
the  capillaries.  The  intermittent  action  of 
the  heart  is  thus  succeeded  by  the  continuous 
reaction  of  the  arterial  wall. 

As  the  blood  advances  toward  the  periph- 
ery of  the  arterial  system  and  larger  amounts 
pass  into  the  capillaries,  both  the  distention 
and  the  elastic  recoil  diminish,  and  by  the  time 
the  blood  reaches  the  capillaries  its  intermit- 
tency  of  movement  has  been  so  far  obliterated 
by  the  elastic  recoil  that  as  it  enters  the  capil- 
laries the  movement  becomes  equable  and  con- 
tinuous. The  elasticity  thus  serves  the  pur- 
pose of  equalizing  the  movement  of  the  blood 
throughout  the  arterial  system. 

In  youth  the  arterial  walls  are  highly  dis- 
tensible and  elastic;  in  advanced  years  they 
are  frequently  relatively  rigid  and  inelastic, 
and  in  consequence  the  flow  of  blood  toward 
and  into  the  capillaries  approximates  in  its 
characteristics  the  flow  of  a  fluid  through  a 
rigid  tube  under  the  intermittent  action  of  a 
pump;  that  is,  the  intermittent  movement  im- 
parted by  the  heart  is  not  so  completely  con- 
verted into  a  continuous  movement,  and  hence 
the  blood  flows  through  the  capillaries  during 
the  systole  with  greater  velocity,  and  during  the  diastole  with  less  velocity, 
than  is  the  case  when  the  vessel  is  normally  elastic.  For  these  and  other 
reasons  the  tissues  are  not  so  well  nourished  and  hence  their  nutrition 
and  functional  activities  decline. 

The  contractility  permits  of  a  variation  in  the  amount  of  blood  passing 
into  a  given  capillary  area  in  a  unit  of  time.  Normally  each  artery  has  a 
certain  average  caliber  due  to  a  given  contraction  of  the  muscle  coat.  Be- 
yond this  average  condition  the  artery  can  pass  in  one  direction  or  the  other 
by  either  a  relaxation  or  increased  contraction  of  the  muscle  coat.  During 
the  functional  activity  of  any  organ  or  tissue  there  is  need  for  an  increase  in 
the  amount  of  blood  beyond  that  supplied  during  functional  inactivity  or 
rest.  This  is  accomplished  by  a  relaxation  of  the  muscle-fibers.  With 
the  cessation  of  activity  the  muscle-fibers  again  contract  and  reduce  the 
amount  of  blood  to  that  required  for  nutritive  purposes  only.  An  increased 
contraction  of  the  muscle-fibers  beyond  the  average,  diminishes  the  outflow 


Fig.  148. — Coats  of  a  Small 
Artery,  a.  Endoth  elium.  b. 
Internal  elastic  lamina,  c.  Cir- 
cular muscular  fibers  of  the  middle 
coat.  d.  The  outer  coat. — {Lan- 
dois  and  Stirling.) 


THE  CIRCULATION  OF  THE  BLOOD. 


321 


of  blood,  and  if  sufficiently  great  may  give  rise  to  anemia  and  pallor.  The 
contractile  elements  at  the  periphery  of  the  arterial  system,  in  the  so-called 
arteriole  region,  therefore  regulate  the  supply  of  blood  to  the  tissues  in 
accordance  with  their  functional  needs. 

Moreover,  as  will  be  stated  in  subsequent  paragraphs  the  degree  of 
contraction  of  the  arteriole  muscle  mtiuences  very  markedly  the  degree  of 
friction  which  the  blood  has  to  overcome  in  passing  from  the  arteries  into 
the  capillaries.  If  the  muscle  contracts  vigorously  the  caliber  of  the  arteriole 
is  diminished  and  the  friction  increases;  if  the  muscle  relaxes,  the  caliber 
of  the  arteriole  is  augmented  and  the  friction  decreases.  By  virtue  of  its 
tonic  activity,  the  arteriole  muscle  at  the  periphery  of  the  arterial  system 
offers  considerable  resistance  to  the  outflow  of  the  blood  and  this  is  there- 
fore spoken  of  generally  as  the  peripheral  resistance,  though  there  is  included 
under  this  term  the  resistance 
offered  by  the  small  caliber  of 
the  capillary  blood-vessel  as  well. 
This  latter  factor  is  constant, 
the  former  variable. 

The  Structure,  Properties, 
and  Functions  of  the  Capilla- 
ries.— The  capillaries  are  small 
vessels  that  connect  the  arteries 
with  the  veins.  Though  different 
in  structure  from  a  small  artery 
or  vein,  there  is  no  sharp  bound- 
ary between  them,  as  their  struc- 
tures pass  imperceptibly  one  into 
the  other.  A  true  capillary, 
however,  is  of  uniform  size  in 
any  given  tissue  and  does  not 
undergo  any  noticeable  decrease 
in  size  from  repeated  branchings. 
The  diameter  varies  in  different 
tissues  from  0.0045  mrn-  to  0.0075  J^m-j  just  sufficiently  large  to  permit  the 
easy  passage  of  a  single  red  corpuscle.  The  length  varies  from  0.5  mm.  to 
I  mm.  The  wall  of  the  capillary  (Fig.  149)  is  composed  of  a  single  layer  of 
nucleated  endothelial  cells  with  serrated  edges  united  by  a  cement  material. 
Though  extremely  short,  the  capillaries  divide  and  subdivide  a  number  of 
times,  forming  meshes  or  networks,  the  closeness  and  general  arrangement 
of  which  vary  in  different  localities. 

As  the  endothelial  cells  are  living  structures  and  characterized  by  irrita- 
bility, contractility  and  tonicity,  it  may  be  assumed  that  the  capillary  wall  as 
a  whole  is  characterized  by  the  same  properties.  Upon  the  possession  of 
these  properties  the  functions  of  the  capillary  depend. 

The  function  of  the  capillary  wall  is  to  permit  of  a  passage  of  the  nutritive 
materials  of  the  blood  into  the  surrounding  tissue  spaces  and  of  waste 
products  from  the  tissue  spaces  into  the  blood.  The  structure  of  the  capil- 
lary wall  is  well  adapted  for  this  purpose.  Composed  as  it  is  of  but  a  single 
layer  of  endotheUal  cells,  the  thickness  of  which  defies  accurate  measurement, 


Fig.  149. — Capillaries.  The  Outlines  of 
THE  Nucleated  Endothelial  Cells  with  the 
Cement  Blackened  by  the  Action  of  Silver 
Nitrate. — {Landois  and  Stirling.) 


322  TEXT-BOOK  OF  PHYSIOLOGY. 

it  readily  permits,  under  certain  conditions,  of  the  necessary  exchange  of 
materials  between  the  blood  and  the  tissues.  The  forces  which  are  con- 
cerned in  the  passage  of  materials  across  the  capillary  wall  are  embraced 
under  the  terms  dijfusion,  osmosis,  and  filtration..  As  a  result  of  the  inter- 
change of  materials  the  tissues  are  provided  with  nourishment  and  relieved 
of  the  presence  of  waste  products.  The  blood  at  the  same  time  changes  to  a 
variable  extent  in  chemic  composition;  because  of  the  loss  of  oxygen  and  the 
gain  of  carbon  dioxid  it  also  changes  in  color  from  red  to  bluish-red. 

In  order  that  the  nutritive  materials  may  pass  through  the  capillary 
wall  in  amounts  sufficient  to  maintain  the  necessary  supply  of  lymph  in  the 
lymph  or  tissue  spaces,  it  is  essential  that  the  blood  shall  flow  into  and  out 
of  the  capillary  vessels  constantly  and  equably,  in  volumes  varying  with  the 
activities  of  the  tissues,  under  a  given  pressure  and  with  a  definite  velocity. 
These  conditions  are  made  possible  by  the  cooperation  of  the  physical 
properties  and  physiologic  functions  of  the  heart  and  vascular  apparatus, 
the  nature  of  which  will  be  explained  in  subsequent  pages. 

The  Structure,  Properties,  and  Functions  of  the  Veins. — The  veins 
serve  to  collect  the  blood  from  the  capillary  areas  and  return  it  to  the  right  side 
of  the  heart.  As  they  emerge  from  the  capillary  areas  the  veins,  which  in 
these  regions  are  termed  venules,  are  quite  small.  By  their  convergence 
and  union  the  veins  gradually  increase  in  size  in  passing  from  the  periphery 
toward  the  heart.  Their  walls  at  the  same  time  correspondingly  increase 
in  thickness.  The  veins  from  the  lower  extremities,  the  trunk,  and  abdom- 
inal organs  finally  terminate  in  the  inferior  vena  cava.  The  veins  from  the 
head  and  upper  extremities  terminate  in  the  superior  vena  cava.  Both 
venae  cavae  empty  into  the  right  auricle. 

A  typical  vein  consists  of  the  same  three  coats  as  the  artery:  viz.,  the 
tunica  intima,  the  tunica  media,  and  the  tunica  adventitia.  The  media, 
however,  does  not  possess  as  much  of  either  the  elastic  or  muscle  tissue  as 
the  artery,  but  a  larger  amount  of  the  fibrous  tissue.  Hence  they  readily 
collapse  when  empty.  In  virtue  of  their  structure  the  veins  also  possess 
both  elasticity  and  contractility,  though  in  a  far  less  degree  than  the  arteries. 
These  properties  come  into  play  and  are  of  value  in  furthering  the  movement 
of  the  blood  toward  the  heart,  especially  after  a  temporary  obstruction. 

Veins  are  distinguished  by  the  presence  of  valves  throughout  their  course. 
These  are  arranged  in  pairs  and  formed  by  a  reduplication  of  the  internal 
coat,  strengthened  by  fibrous  tissue.  They  are  always  directed  toward  the 
heart  and  in  close  relation  to  the  walls  of  the  veins,  so  long  as  the  blood  is 
flowing  forward.  An  obstruction  to  the  flow  causes  the  valves  to  turn 
backward  until  they  meet  in  the  middle  line,  when  they  act  as  a  barrier 
to  regurgitation.  Under  these  circumstances  the  elastic  tissue  permits  the 
veins  to  distend  and  accommodate  the  blood.  With  the  removal  of  the 
obstruction  the  recoil  of  the  elastic  tissue,  and  perhaps  the  contraction  of  the 
muscle-tissue,  forces  the  blood  quickly  onward. 

HYDRODYNAMIC  CONSIDERATIONS. 

The  blood  flows  through  the  arteries,  capillaries  and  veins  in  accordance 
with  definite  laws.     During  its  transit  certain  phenomena  are  presented  by 


THE  CIRCULATION  OF  THE  BLOOD.  323 

each  of  these  three  divisions  of  the  vascular  apparatus.  Since  these  phenom- 
ena, as  well  as  the  laws  which  govern  them  are  similar  to,  though  more  com- 
plex than  the  phenomena  presented  by  relatively  simple  tubes  with  rigid  or 
elastic  walls  while  liquids  are  flowing  through  them  under  a  steadily  acting 
or  an  intermittently  acting  pressure,  it  will  be  conducive  to  clearness  of 
conception  of  the  mechanics  of  the  vascular  apparatus,  if  there  be 
considered : 

1.  The  flow  of  a  liquid  through  a  horizontal  tube  with  rigid  walls  and  of 
uniform  or  variable  diameter  under  a  steadily  acting  pressure. 

2.  The  flow  of  a  liquid  through  a  series  of  branching  and  again  uniting 
tubes  with  rigid  walls  under  a  steadily  acting  pressure. 

3.  The  flow  of  a  liquid  through  a  tube  with  elastic  walls  under  an  inter- 
mittently acting  pressure. 

THE  FLOW  OF  A  LIQUID  THROUGH  A  HORIZONTAL  TUBE  WITH 

RIGID  WALLS. 

The  phenomena  and  the  laws  which  govern  them,  that  attend  the  flow  of  a 
liquid  through  a  rigid  tube  of  uniform  diameter  under  a  steadily  acting  pressure 
may  be  readily  observed  in  an  apparatus  similar  to  that  represented  in  Fig.  150, 
which  consists  of  a  reservoir  or  pressure  vessel,  P,  provided  with  a  horizontal  tube 
into  which  is  inserted  at  equal  distances  a  series  of  vertical  tubes.  If  the  reservoir 
be  filled  with  a  liquid,  water  for  example,  the  latter  under  certain  conditions  will 
exert  a  downward  pressure  and  act  as  a  propelling  or  driving  power,  the  degree  of 
which  will  depend  on  the  height  of  the  column  and  may  be  represented  by  H.  If 
the  stopcock  at  O  be  opened  the  column  of  water,  which  has  heretofore  been  exert- 
ing an  equal  pressure  in  all  directions,  will  now  exert  a  downward  pressure  only, 
and  in  consequence  it  will  be  driven  into  and  through  the  horizontal  tube  and 
discharged  from  its  free  extremity  with  a  definite  velocity.  At  the  same  time  the 
fluid  will  rise  in  each  vertical  tube  to  a  height  directly  proportional  to  the  distance 
of  each  tube  from  the  free  extremity.  The  velocity  with  which  the  fluid  is  dis- 
charged can  be  determined  by  measuring  the  quantity,  q,  discharged  in  a  unit  of  time, 

(i  second)  and  dividing  it  by  the  area  of  the  tube,  -r^;  viz.,  v=  _^.    Inasmuch  as 

the  tube  is  of  uniform  diameter  the  velocity  through  each  cross-section  will  be  the 
same. 

As  the  water  flows  through  the  horizontal  tube  it  meets  with  resistance,  namely, 
the  cohesion  and  friction  of  its  molecules,  and  the  adhesion  between  the  walls  of 
the  tube  and  the  water  which  must  be  overcome  if  the  flow  is  to  continue.  Because 
of  the  fact  that  water  will  moisten  most  surfaces  with  which  it  comes  in  contact 
there  wfll  be  an  adhesion  between  the  walls  of  the  tube  and  the  outer  laver  of  the 
column  of  water,  in  consequence  of  which  it  will  become  more  or  less  stationary. 
Between  the  outer  stationary  layer  and  the  axis  of  the  stream,  there  is  an  infinite 
number  of  layers  of  molecules,  the  cohesion  of  which  one  for  the  other  is  more  and 
more  overcome  by  the  pressure  in  the  vessel,  P.  The  force  of  adhesion  between 
wall  and  fluid  together  with  the  force  of  cohesion  between  the  molecules  of  the  fluid 
give  rise  to  the  resistance  of  the  fluid  to  the  flow. 

As  a  result  of  the  resistance  the  forward  movement  of  the  water  under  the 
pressure  in  P,  is  somewhat  retarded,  and  as  a  consequence  it  will  exert  a  lateral  or 
radial  pressure  against  the  walls  of  the  tube.  That  such  a  pressure  exists  is  shown 
by  the  rise  of  the  fluid  in  each  of  the  vertical  tubes,  and  the  height  to  which  it  rises 
in  each  tube  is  a  measure  of  the  pressure  at  its  base.     In  the  tube  /,  the  fluid  rises 


324 


TEXT-BOOK  OF  PHYSIOLOGY. 


to  but  a  slight  extent  for  the  reason  that  the  resistance  yet  to  be  overcome  is  slight 
in  amount.  It  is,  however,  a  measure  of  the  resistance  or  friction  between  the  Dase 
of  the  tube  and  the  orifice  of  outflow.  In  the  tube  e  the  fluid  rises  twice  as  high 
as  in  /  because  of  the  additional  friction  between  the  bases  of  the  tubes  e  and/. 
What  is  true  of  these  two  points  is  equally  true  of  the  points  at  the  base  of  the  tubes 
d,  r,  h,  a.  Lines  drawn  to  the  pressure  vessel  from  the  lop  of  the  fluid  in  each  tube 
and  parallel  to  the  horizontal  tube  will  show  how  much  of  the  pressure  force  is 
utilized  in  overcoming  the  friction  in  each  section  of  the  horizontal  tube.  The 
amount  of  the  lateral  pressure  at  any  given  point  is  therefore  indicated  and  measured 
by  the  height  to  which  the  water  rises  in  the  tubes.  For  this  reason  these  tubes  are 
termed  pressure  tubes  or  piezometers. 

Since  the  resistance  in  a  tube  of  uniform  diameter  is  proportional  to  its  length 
the  lateral  pressure  will  gradually  but  progressively  decrease  from  the  reservoir  to 
tbe  outlet.     Therefore  the  pressure  at  any  given  point  is  proportional  to  the  resist- 


FlG. 


-A  Pressure  Vessel,  P,  with  a  Horizoxtal  Outflow  Tube,  O-n,  into  which 
Vertical  Tubes  or  IVIanometers  are  Inserted. 


ance  yet  to  be  overcome  and  conversely  the  resistance  to  be  overcome  is  indicated 
by  the  amount  of  the  pressure.  (In  the  conduct  of  an  experiment  the  propelling 
power  should  be  kept  constant  by  permitting  fluid  to  flow  into  the  reservoir  as 
rapidly  as  it  flows  out  of  the  horizontal  tube.) 

The  power  or  force  which  overcomes  the  resistance  in  the  horizontal  tube  and 
imparts  velocity  to  the  fluid  is  the  downward  pressure  of  the  water  in  the  reservoir, 
represented  by  H.  The  amount  of  this  power  utilized  in  overcoming  the  resistance 
is  approximately  indicated  by  the  height  of  the  fluid,  y,  at  which  point  the  line 
uniting  the  upper  limits  of  the  water  in  the  vertical  tubes  intersects  it.  The  height 
of  the  fluid  at  this  point  is  a  measure,  therefore,  not  only  of  the  resistance  but  also 
an  indication  of  the  relative  amount  of  the  pressure  used  in  overcoming  it  and  is 
therefore  known  as  the  pressure  heigh th. 

The  amount  of  the  pressure  consumed  in  imparting  the  observed  velocity  is 
determined  by  ascertaining  the  height  from  which  a  particle  must  fall  in  empty 
space  to  acquire  this  velocity.     This  is  obtained  by  dividing  the  square  of  the  veloc- 

v^ 

ity  by  twice  the  accelerating  force  of  gravity  as  expressed  in  the  formula,  — ;  the 

2g 

quotient  is  the  height  and  is  known  as  the  velocity  height.  Conversely  if  the  mov- 
ing fluid  were  discharged  into  empty  space  through  an  opening  in  the  tube  at  n, 


THE  CIRCULATION  OF  THE  BLOOD.  325 

it  would  ascend  an  equal  distance.  If  now  this  height  is  represented  by  F,  and  a 
line  be  drawn  from  it,  parallel  to  the  line  of  pressure  until  it  meets  the  reservoir  at 
X,  it  will  be  seen  what  percentage,  x  y,  or  h'  of  the  primary  propelling  power  is  con- 
sumed in  imparting  the  observed  velocity. 

Of  the  total  pressure  a  small  portion  is  left  over  which  is  utilized  in  forcing  into, 
and  overcoming  the  resistance  offered  by,  the  orifice  of  the  horizontal  tube.  The 
initial  pressure  in  P  therefore  divides  itself  mainly  into  two  portions;  one,  the  larger 
by  far,  //,  is  utilized  in  overcoming  the  resistance  to  the  flow  of  the  water;  the  other, 
the  smaller,  h'  in  imparting  velocity. 

Thus  the  two  phenomena  presented  by  the  flow  of  a  liquid  through  a  tube  with 
rigid  walls  and  of  uniform  diameter  are  velocity  and  pressure,  of  which  the  former 
is  the  same  for  each  cross-section,  and  the  latter  at  any  point  directly  proportional 
to  the  resistance  to  be  overcome. 

If,  instead  of  a  horizontal  tube  of  uniform  diameter,  there  be  substituted  a 
tube  the  middle  third  of  which  is  enlarged,  the  conditions  will  be  similar  to 
the  previous  case  until  the  fluid  flows  into  the  enlarged  portion,  when  the  velocity 
will  diminish,  being  inversely  proportional  to  the  area  of  the  cross-section.  The 
resistance  will  be  also  diminished  and  therefore  less  of  the  pressure  force  or 
driving  power  will  be  consumed  than  in  the  first  section  of  the  tube,  and  as  a  result, 
the  lateral  pressure  will  fall  less  rapidly  than  in  the  first  section.  When  the  liquid 
flows  into  the  narrow  or  third  section,  the  primary  velocity  returns.  Though  the 
resistance  again  increases  the  amount  to  be  overcome  is  small,  and  hence  there  is 
a  rapid  and  steady  fall  of  pressure. 

On  the  contrary,  if  a  tube  be  substituted  the  middle  third  of  which  is  narrowed, 
the  conditions  will  be  similar  to  the  previous  cases  until  the  liquid  flows  into 
the  narrowed  section,  when  at  once  the  velocity  increases  and  becomes  inversely 
proportional  to  the  area  of  the  cross-section;  the  resistance  being  increased  at  the 
same  time,  there  will  be  a  rapid  consumption  of  the  pressure  force  and  a  steep  fall 
of  lateral  pressure.  On  flowing  into  the  third  section,  the  velocity  again  diminishes 
and  the  pressure  falls  though  more  slowly  to  the  end  of  the  tube. 

THE  FLOW  OF  A  LIQUID  THROUGH  A  SERIES  OF  BRANCHING  AND 
AGAIN  UNITING  TUBES  WITH  RIGID  WALLS. 

In  a  system  of  this  character,  such  as  represented  in  Fig.  151,  there  must  follow 
as  a  result  of  the  repeated  branchings,  a  progressive  increase  in  the  total  sectional 
area  of  the  collective  tubes  coincident  with  a  progressive  decrease  in  the  sectional 
area  of  individual  tubes  in  the  section  b  c,  while  in  the  section  c  D,  there  must 
follow  a  progressive  decrease  in  the  total  sectional  area  of  the  collective  tubes  coin- 
cident with  a  progressive  increase  in  the  sectional  area  of  individual  tubes,  conse- 
quently there  will  be  a  combination  of  the  two  conditions  alluded  to  in  the  two 
preceding  paragraphs,  namely,  an  enlargement  of  the  stream  bed  coincident  with 
a  diminution  in  size  of  the  individual  tubes  composing  it,  in  the  middle  section. 
Moreover,  for  the  purpose  here  intended  it  may  be  assumed  that  the  tubes  com- 
posing the  middle  section  c  are  microscopic  in  size  and  that  their  total  sectional 
area  bears  to  the  sectional  area  of  tube  A  the  ratio  of  600  to  i . 

If  the  system  is  connected  with  a  pressure  vessel,  as  in  the  preceding  instance, 
and  the  stop  cock  is  suddenly  opened,  the  column  of  water  will  exert  a  downward 
pressure,  and  in  consequence  the  water  will  be  driven  into  and  through  the  system 
with  a  definite  velocity  and  pressure. 

The  velocity  of  the  fluid  will  gradually  decrease  from  b  to  c  in  a  ratio  inversely 
proportional  to  the  total  area  of  each  cross-section  until  at  c,  it  will  attain  its  mini- 
mal value;  the  velocity  will  again  increase  from  c  to  d  in  a  ratio  inversely  pro- 


326 


TEXT-BOOK  OF  PHYSIOLOGY. 


portional  to  the  total  area  of  each  cross-section  until  at  e,  when  it  will  attain  the 
value  it  had  in  A  if  the  entrance  and  exit  tubes  have  the  same  area. 

The  lateral  pressure  will  gradually  fall  from  the  beginning  to  the  end  of  the  sys- 
tem, though  the  fall  must  be  more  rapid  in  b-c  than  in  a-b  as  will  be  clear  from 
the  following  considerations. 

In  the  section  B-c  the  two  factors — viz.,  the  widening  of  the  stream  bed  which 
decreases  the  resistance,  and  the  narrowing  of  the  individual  tubes  which  increases 
the  resistance — exert  an  opposing  influence  on  the  pressure;  hence  the  fall  of 
pressure  will  be  proportional  to  the  ratio  between  these  two  factors.  As  the 
increase  in  the  resistance  due  to  the  progressive  decrease  in  the  size  of  the  individual 
tubes  preponderates  considerably  over  the  decrease  in  the  resistance  due  to  the 
widening  of  the  stream  bed,  there  must  be  an  increase  in  resistance  in  the  area 
B-c  and  therefore  a  more  rapid  fall  of  pressure  than  in  a-b.  This  fall,  however, 
will  not  be  as  steep  as  it  might  be  for  the  reason  that  the  decrease  in  the  velocity 
is  attended  by  a  decrease  in  the  resistance  and  hence  a  lessened  consumption  of 
the  propelling  power.  In  the  section  c-D  the  two  factors,  viz.,  the  narrowing  of 
the  stream  bed  which  increases  the  resistance,  and  the  enlarging  of  the  individual 


Fig.  151. — Pressure  Vessel  with  a  Series  of  Progressively  Branching  and  Reuniting 

Tubes. 

tubes  which  decreases  the  resistance,  exert  an  opposing  influence  on  the  pressure, 
hence  the  fall  of  pressure  will  be  proportional  to  the  ratio  between  these  two  factors. 
As  the  decrease  in  the  resistance  due  to  the  progressive  enlargement  of  the  individual 
tubes  preponderates  considerably  over  the  increase  in  the  resistance  due  to  the 
narrowing  of  the  stream  bed,  there  should  theoretically  be  a  rapid  fall  of  pressure 
from  c  to  E.  This  rapid  fall,  however,  will  be  to  some  extent  prevented  for  the 
reason  that  the  increase  in  velocity  due  to  the  narrowing  of  the  stream  bed  in- 
creases the  resistance  to  a  high  value  and  hence  the  pressure  falls  less  rapidly  than 
it  otherwise  would. 


THE  CIRCULATION  OF  THE  BLOOD.  327 

The  pressure  throughout  the  system  is  the  result  of  the  resistance  to  the  flow  of 
the  water  and  its  extent  in  any  one  section  will  be  proportional  to  the  resistance  yet 
to  be  overcome.  It  will  naturally  be  higher  in  the  section  a-b  than  in  the  section 
D-E,  though  the  difference  in  the  level  of  the  pressure  between  these  two  points  will 
not  be  as  great  as  might  theoretically  be  supposed  from  the  small  size  of  the  tubes 
in  c  for  the  decrease  in  velocity  counterbalances  in  part  tlie  resistance  which  they 
offer. 

The  general  curve  of  the  fall  of  pressure  in  this  system  is  indicted  by  the 
curved  line  extending  from  the  pressure  vessel  to  the  outlet  of  the  horizontal  tube. 

The  value  of  the  pressures  in  these  two  sections  and  their  relation  to  each  other 
could  be  varied  either  temporarily  or  permanently  by  the  insertion  of  a  series  of  stop 
cocks  a,  X,  along  the  course  of  the  tubes  between  b  and  c  in  the  neighborhood  of 
their  ultimate  branchings  by  which  an  additional  resistance  could  be  superposed 
on  the  system  from  a  to  the  stopcocks.  If  the  lumen  of  each  stopcock  has  a  certain 
average  value,  so  as  to  permit  of  a  certain  outflow  of  water,  the  pressure  will 
have  a  certain  value  in  both  a-b  and  d-e.  But  if  the  lumen  of  each  stopcock  is 
decreased,  there  will  be  an  increase  in  the  resistance  and  hence  a  rise  of  pressure  in 
A-B  and  a  fall  of  pressure  in  d-e.  If,  on  the  contrary,  the  lumen  of  each  stopcock  is 
increased,  there  will  be  a  decrease  in  the  resistance  and  hence  a  fall  of  pressure 
in  A-B  and  a  rise  of  pressure  in  d-e.  The  stopcocks  may  be  spoken  of  as  a 
variable  peripJieral  resistance. 

In  the  foregoing  exposition  it  has  been  assumed  that  in  all  instances  the  pressure 
in  the  pressure  vessel  was  steadily  acting.  If,  however,  the  pressure  be  made  to 
act  intermittently  as  it  can  be  by  alternately  opening  and  closing  the  stopcock,  at  A 
both  the  velocity  and  the  pressure  will  be  alternately  increased  and  decreased. 
The  outflow  of  the  fluid  during  the  moment  the  pressure  is  acting  will  be  rapid, 
and  during  the  moment  the  pressure  is  not  acting  the  outflow  will  cease.  It 
becomes  therefore  intermittent.  Coincidently  there  is  an  alternate  temporary 
increase  and  decrease  of  the  lateral  pressure. 

THE   FLOW  OF  A  LIQUID   THROUGH  A  TUBE  WITH   ELASTIC  WALLS 
UNDER  AN  INTERMITTENTLY  ACTING  PRESSURE. 

When  a  tube  with  elastic  walls  is  connected  with  a  pressure  vessel,  the  con- 
ditions which  are  established  on  opening  the  stopcock  and  the  consequent  flow  of 
water,  will  soon  approximate  those  observed  in  a  tube  with  rigid  walls.  As  the 
water  moves  forward,  it  encounters  friction,  exerts  a  lateral  pressure  and  causes  a 
distention  of  the  tube.  This  latter  effect  continues  until  the  elastic  recoil  of  the 
walls  of  the  tube  exactly  counterbalances  the  pressure  of  the  water  from  within. 
When  this  condition  is  established  the  tube  becomes  practically  a  tube  with  rigid 
walls,  and  hence  so  long  as  the  primary  pressure  is  uniform,  the  velocity  and  lateral 
pressure  will  obey  the  laws  which  hold  true  for  rigid  tubes. 

If,  however,  the  primary  pressure  be  intermittently  applied  or  alternately 
increased  or  decreased,  and  the  water  forced  into  the  tube,  previously  filled  with 
water  but  under  no  particular  pressure,  it  will  be  forced  out  of  the  peripheral  end 
of  the  tube  more  rapidly  during  the  period  of  the  increase  of  pressure  and 
less  rapidly  during  the  period  of  the  decrease  of  pressure  or  it  may  cease  entirely. 
The  extent  to  which  the  outflow  becomes  merely  remittent,  or  entirely  intermittent, 
will  depend  on  the  amount  of  resistance,  whether  this  be  due  to  length  of  tube  or 
a  narrowed  outlet,  and  the  degree  of  elasticity. 

When  these  factors  are  of  such  a  nature  that  the  resistance  is  very  high  and  the 
elasticity  slight,  the  outflow  will  be  intermittent.  But  if  they  are  made  to  change 
gradually,  and  this  is  especially  the  case  with  the  resistance,  from  a  slight  to  a 
greater  value,  the  outflow  gradually  changes  from  an  intermittent  to  a  remittent 
and  finally  to  a  continuous  outflow  and  for  the  following  reasons: 


328  TEXT-BOOK  OF  PHYSIOLOGY. 

With  a  given  resistance  and  elasticity,  the  t^uid  which  is  driven  into  the  tube  by 
the  action  of  the  primary  pressure  exerts  more  or  less  lateral  pressure,  gives  rise  to 
a  distention  of  the  tube,  and  acquires  a  certain  velocity  of  outflow.  In  consequence 
of  the  distention,  a  portion  of  the  fluid  accumulates.  With  the  cessation  in  the 
action  of  the  primary  pressure,  the  elastic  walls  recoil  and  force  the  accumulated 
fluid  forward  and  so  maintain  more  or  less  effectively  the  same  velocity  of  outflow 
until  there  is  a  return  of  the  pressure.  If  the  resistance  be  great  and  the  elasticity 
slight,  this  is  impossible  and  the  outflow  wfll  be  entirely  intermittent.  But  if  they 
are  made  to  increase  in  value,  the  proportionate  amount  of  the  fluid  which  accumu- 
lates during  the  action  of  the  primary  pressure  will  also  increase  in  amount  and 
hence  there  will  be  an  increase  in  the  distention  of  the  tube.  The  elastic  recoil 
will  therefore  be  greater  in  amount  and  longer  in  duration,  and  hence  the  outflow 
will  change  to  a  remittent  and  finally  to  a  continuous  outflow. 

.  Coincident  with  the  action  and  cessation  of  action  of  the  primary  pressure 
there  is  a  corresponding  increase  and  decrease  of  the  lateral  pressure  and  when  the 
intermittency  in  their  action  is  sufficiently  rapid,  the  excess  of  fluid  entering  the 
tube  over  that  discharged  becomes  sufficiently  great  to  maintain  a  certain  average 
or  mean  pressure,  which,  however,  undergoes  an  alternate  increase  and  decrease 
with  each  variation  in  the  primary  pressure. 

The  temporary  increase  and  decrease  of  the  pressure  and  the  consequent 
expansion  and  recoil  of  the  tube  in  the  neighborhood  of  the  pressure  vessel,  give 
rise  to  a  wave  on  the  surface  of  the  tube  which  is  propagated  with  more  or  less 
rapidity — though  with  decreasing  amplitude,  from  the  beginning  to  the  end  of 
the  tube  and  causing  in  each  section  a  corresponding  expansion  and  recoil,  and 
known  as  the  expansion  wave. 

THE  APPLICATION  OF  THE  FOREGOING  FACTS  TO  THE 
VASCULAR  APPARATUS. 

The  systemic  vascular  apparatus  may  be  conceived  of  as  a  system  of  tubes 
which  have  symmetrically  divided  and  subdivided  and  afterwards  again 
united  and  reunited  in  a  corresponding  manner.  The  arteries,  arterioles, 
capillaries,  venules,  and  veins  may  therefore  be  schematically  arranged 
(Fig.  152)  in  a  manner  identical  with  the  schematic  arrangement  of  tubes 
represented  on  page  325.  The  heart,  with  which  they  are  in  connection, 
when  filled  with  blood  may  be  compared  with  the  reservoir  filled  with  water, 
and  the  intra-ventricular  pressure  developed,  during  the  contraction,  to 
the  downward  pressure  of  the  water  when  the  stopcock  at  is  A  opened. 

The  Stream-bed. — The  stream-bed,  the  path  along  which  the  blood 
flows,  varies  widely  in  its  total  sectional  area  in  different  parts  of  its  course, 
being  least  in  the  aorta  and  venas  cavae,  and  greatest  in  the  capillaries.  In 
passing  from  the  base  of  the  aorta  toward  the  capillaries  the  sectional  area  of 
individual  arteries,  in  consequence  of  repeated  branching,  diminishes, 
though  their  total  sectional  area  increases  and  in  direct  proportion  to  their 
distance  from  the  heart.  In  the  capillary  system  the  sectional  area  of  an 
individual  capillary  attains  its  minimal  value,  though  the  total  sectional 
area  attains  its  maximal  value.  Comparing  one  with  the  other,  it  has  been 
estimated  that  the  total  sectional  area  of  the  aortic  bed  is  to  the  total  sectional 
area  of  the  capillary  bed  as  1  is  to  600  or  800.  In  passing  from  the  capillary 
into  the  venous  system  the  sectional  area  of  individual  veins  increases, 
though  the  total  sectional  area  decreases  and  in  direct  proportion  to  their 
distance  from  the  capillaries. 


THE  CIRCULATION  OF  THE  BLOOD. 


329 


The  stream-bed  in  the  aorta  is  relatively  narrow,  but  widens  gradually  as 
it  approaches  the  capillaries,  where  it  attains  its  maximum  width;  it  again 
narrows  gradually  as  it  passes  into  the  veins,  until  in  the  venae  cavae  it  be- 
comes almost  as  narrow  as  in  the  aorta.  As  the  combined  sectional  areas  of 
the  venae  cavae  are  greater  than  the  sectional  area  of  the  aorta,  the  stream- 
bed  of  the  former  never  becomes  as  narrow  as  that  of  the  latter. 

The  gradual  increase  in  the  width  of  the  stream-bed  from  the  beginning 
of  the  aorta  to  the  middle  of  the  capillary  system,  and  the  gradual  decrease 
in  the  width  of  the  stream-bed  from  the  middle  of  the  capillary  system  to  the 


Fig,  152. — Schematic  Arrangement  of  the  \'ascular  Appar.'\tus. 

terminations  of  the  venae  cavae,  which  result  from  the  repeated  branching 
and  subsequent  reuniting,  as  well  as  its  relative  width  in  the  arteries,  capil- 
laries, and  veins,  are  shown  graphically  in  Fig.  153. 

When  the  heart  contracts  and  the  intra-ventricular  pressure  rises  above 
the  pressure  in  the  aorta,  the  aortic  valves  are  suddenly  forced  open  and  the 
blood  is  driven  into  and  through  the  arteries,  capillaries,  and  veins  to  the 
right  side  of  the  heart  with  a  definite  velocity  and  pressure. 

The  velocity  of  the  blood  in  the  systemic  vascular  apparatus  will  gradu- 
ally decrease  in  accordance  with  foregoing  considerations  from  the  aorta 
to  the  middle  of  the  capillary  system  in  a  ratio  inversely  proportional  to  the 
total  area  of  any  given  cross-section  of  the  stream-bed,  until  in  the  capillaries 
it  will  attain  its  minimal  value,  which  is  especially  small  because  the 
resistance  to  the  flow  of  blood  in  the  capillaries  increases  inversely  as  the 
square  of  their  diameters,  while  in  the  larger  blood-vessels  the  increase  is 
inversely  proportional  to  the  simple  diameter;  the  velocity  will  again  in- 
crease  from  the  middle  of  the  capillary  system  to  the  ends  of  the  venae  cavae 


330 


TEXT-BOOK  OF  PHYSIOLOGY. 


in  a  ratio  again  proportional  to  the  total  area  of  each  cross-section  of  the 
stream-bed  until  in  the  venae  cavae  it  will  attain  its  maximal  value,  though 
it  will  not  attain  its  initial  value  in  these  vessels  because  their  combined 
sectional  area  is  greater  than  that  of  the  aorta. 

The  lateral  pressure  will  also  gradually  fall  from  the  beginning  of  the  aorta 
to  the  ends  of  the  venae  cavae,  though  the  fall  will  be  most  rapid  at  the  pe- 
riphery of  the  arteries.  In  the  arterial  system  the  fall  of  pressure  will  be 
proportional  to  the  ratio  between  the  increase  in  resistance  due  to  the  narrow- 
ing of  individual  vessels,  and  the  decrease  in  resistance  due  to  the  widening 
of  the  stream-bed;  as  the  former  preponderates  over  the  latter  there  must  be 
an  increase  in  resistance  from  the  aorta  to  the  capillaries  and  hence  a  sharper 
fall  of  pressure  toward  the  termination  of  the  arterioles,  which  is  very  steep 
for  reasons  to  be  stated  later.  In  the  venous  system  the  fall  of  pressure  will 
continue  and  its  rate  will  be  proportional  to  the  ratio  between  the  increase  in 


Fig.  153. — Diagram  Designed  to  Give  an  Idea  of  the  Aggregate  Sectional  Area  of 
THE  Different  Parts  of  the  Vascular  System.  A.  Aorta.  C.  Capillaries.  V.  Veins.  The 
transverse  measurement  of  the  shaded  part  may  be  taken  as  the  width  of  the  various  kinds  of 
vessels,  supposing  them  fused  together. — (Yeo.) 


resistance  due  to  the  narrowing  of  the  stream-bed,  and  the  decrease  of  resist- 
ance due  to  the  enlarging  of  individual  vessels;  as  the  latter  preponderates 
over  the  former  there  should  be  a  rapid  fall  of  pressure  from  the  capillary 
system  to  the  ends  of  the  venae  cavae.  This,  however,  is  to  some  extent 
prevented  for  the  reason  that  the  increase  in  velocity  due  to  the  narrowing  of 
the  stream-bed  increases  the  resistance  to  a  relatively  high  value  and  hence 
the  pressure  falls  less  rapidly  than  it  otherwise  would. 

The  high  pressure  characteristic  of  the  arterial  system  contrasted 
with  the  low  pressure  characteristic  of  the  venous  system  determined  by 
experiment  cannot  be  accounted  for  alone  by  the  resistance  offered  by  the 
small  diameter  of  the  vessels  of  the  capillary  system.  This  in  itself  would  be 
insufficient  to  maintain  the  observed  differences  in  pressure  in  the  different 


THE  CIRCULATION  OF  THE  BLOOD.  331 

sections  of  the  vascular  apparatus  necessary  for  physiologic  purposes.  To 
meet  this  necessity  there  has  been  developed  at  the  periphery  of  the  arterial 
system,  in  the  arteriole  wall,  a  special  muscle,  a,  x.  Fig.  152  which  by  con- 
tracting can  add  a  physiologic  resistance  to  what  might  be  termed  the  phy- 
sical resistance  of  the  system.  According  to  the  degree  of  its  contraction 
will  the  resistance  to  the  flow  of  blood  from  the  arteries  to  the  veins  at  the 
periphery  of  the  arterial  system  be  increased  and  the  arterial  pressure  be 
raised  and  the  venous  pressure  be  lowered.  According  to  the  degree  of  its 
relaxation  will  the  resistance  to  the  flow  of  blood  from  the  arteries  into  the 
veins  be  decreased  at  the  periphery  of  the  arterial  system  and  the  arterial 
pressure  be  lowered  and  the  venous  pressure  raised.  By  this  means  the 
extent  and  the  relation  of  the  pressure  in  the  two  main  sections  of  the 
systemic  vascular  apparatus  can  be  temporarily  or  permanently  changed  in 
one  direction  or  the  other.  The  effect  of  the  diminution  in  the  caliber  of 
the  arteriole  due  to  the  contraction  of  the  muscle  is  spoken  of  as  the 
peripheral  resistance. 

That  the  high  pressure  in  the  arteries  is  largely  due  to  this  physiologic  factor 
is  shown  by  the  rapid  and  pronounced  fall  of  pressure  that  occurs  when  this 
muscle  suddenly  relaxes  as  it  does  when  the  spinal  cord  is  transversely 
divided  in  the  cervical  region,  thus  cutting  oflf  from  the  arteriole  muscle 
those  nerve  influences  that  largely  determine  its  contraction.  Under 
such  circumstances  the  pressure  in  the  dog  may  fall  from  approximately 
140  mm.  to  40  mm.  of  mercury  or  even  less.  Stimulation  of  the  distal 
extremity  of  the  spinal  cord  will  be  followed  by  the  temporary  contraction  of 
the  muscle  and  a  rise  of  pressure  to  its  former  value. 

The  Distribution  of  the  Intra-ventricular  Pressure. — The  pressure 
developed  during  the  ventricular  contraction  is  thus  expended  in  imparting 
velocity  to  the  blood  and  overcoming  the  cohesion  and  friction  of  its  mole- 
cules. The  percentage  of  the  pressure  utilized  in  overcoming  the  resistance 
could  be  approximately  determined  from  the  pressure  in  the  aorta  if  this  were 
accurately  known;   the  percentage  of  the  pressure  utilized  in  imparting 

velocitv  could  be  determined  with  the  formula      ;  if  the  actual  velocitv  of  the 

.  2g  _  ^ 

blood  in  the  aorta  could  be  experimentally  determined.     On  account  of 

the  difficulty  in  obtaining  this  latter  factor  at  least,  the  results  must  be  only 

approximative. 

An  idea  of  the  ratio  between  the  velocity  pressure  and  the  resistance  pressure, 

however,  may  be  obtained  from  the  distribution  of  the  aortic  pressure  in  the 

dog  in  reference  to  the  carotid  artery.     Thus,  if  it  be  assumed  that  the  aver- 

age  velocity  of  the  blood  is  35  cm.,  the  velocity  pressure  is  ec^ual  to      ^,    or 

0.62  centimeters  of  blood  or  0.046  centimeters  of  mercury,  and  if  the  average 
aortic  pressure  is  150  mm.  of  mercury  ,  the  ratio  of  the  velocity  pressure  to 
the  resistance  pressure  is  as  i  to  326. 

The  phenomena  which  for  the  most  part  characterize  the  flow  of  blood 
through  the  blood-vessels  are  velocity  and  pressure,  combined  with  an 
alternate  expansion  and  recoil  of  the  arterial  vessels  due  to  the  intermittent 
character  of  the  heart-beat.  For  special  reasons  it  is  convenient  to  consider 
the  pressure  first. 


332  TEXT-BOOK  OF  PHYSIOLOGY. 

BLOOD-PRESSURE. 

From  theoretic  considerations  alone  it  may  be  inferred  that  the  blood, 
as  it  flows  through  the  vascular  apparatus,  exerts  a  pressure  against  the 
walls  of  the  vessels,  and  that  this  pressure  is  greatest  at  the  beginning  of  the 
aorta,  and  least  at  the  ends  of  the  venae  cavae.  The  fact  tjiat  the  blood 
flows  from  the  aorta  to  the  venae  cavae  indicates  that  there  is  a  higher  pres- 
sure in  the  former  than  in  the  latter.  The  same  holds  true  for  the  pulmonary 
artery  and  veins.  So  long  as  these  conditions  are  maintained,  the  blood 
must  flow  from  the  point  of  high  to  the  point  of  low  pressure. 

To  this  pressure  the  term  blood-pressure  is  given,  and  may  be  defined 
as  the  pressure  exerted  radially  or  laterally  by  the  moving  blood-stream 
against  the  sides  of  the  vessels.  That  there  is  such  a  pressure  within  the 
arteries,  capillaries,  and  veins,  different  in  amount  in  each  of  these  three 
divisions  of  the  vascular  apparatus,  is  evident  from  the  results  which  follow 
division  of  an  artery  or  a  vein  of  corresponding  size.  When  an  artery  is 
divided,  the  blood  spurts  from  the  opening  for  a  considerable  distance  and 
with  a  certain  velocity.  The  reason  for  this  lies  in  the  fact  that  the  vessel 
has  been  distended  by  the  pressure  from  within  and  its  walls  thrown  into  a 
condition  of  elastic  tension,  so  that  at  the  moment  there  is  an  outlet,  the 
vessel  suddenly  recoils  and  forces  the  blood  out  with  a  velocity  proportional 
to  the  distention.  When  a  vein  is  divided,  the  blood  as  a  rule  merely  wells 
out  of  the  opening  with  but  slight  momentum,  and  for  the  reason  that  the 
vessel  has  been  but  slightly,  if  at  all  distended  by  the  pressure.  These  re- 
sults indicate  that  the  blood  in  the  arteries  stands  under  a  pressure  con- 
siderably higher  than  that  of  the  atmosphere,  while  that  in  the  veins  stands 
under  a  pressure  perhaps  but  slightly  above  that  of  the  atmosphere.  Especi- 
ally true  is  this  of  the  larger  veins. 

The  same  facts  may  be  demonstrated  in  another  and  more  striking  way. 
A  dog  or  cat  is  anesthetized  and  securely  fastened  in  an  appropriate  holder. 
The  carotid  artery  on  the  right  side  and  the  jugular  vein  on  the  left  side  are 
freely  exposed  and  clamped.  Into  the  artery  there  is  inserted  on  the  distal 
side  of  the  clamp  and  in  the  direction  of  the  heart  a  cannula  to  which  is 
connected  a  tall  glass  tube,  200  cm.  high  and  of  about  4  mm.  internal  di- 
ameter. Into  the  vein  there  is  passed  on  the  proximal  side  of  the  clamp  and 
in  the  direction  of  the  capillaries  a  second  cannula,  to  which  is  connected  a 
similar  tube,  though  of  less  height.  If  the  two  clamps  are  removed  at  the 
same  time,  the  blood  will  mount  in  both  tubes  simultaneously.  In  the 
arterial  tube  the  blood  will  ascend  by  leaps  corresponding  to  the  heart-beats 
until  a  certain  height  is  reached,  when  the  column  becomes  relatively 
stationary,  being  kept  in  equilibrium  by  the  blood-pressure  within  the 
vessel  and  the  atmospheric  pressure  without.  Though  stationary  in  a 
general  sense,  nevertheless  the  blood-column  oscillates,  rising  and  falling 
with  each  contraction  and  relaxation  of  the  heart.  Not  infrequently  larger 
excursions  of  the  column  are  seen  which  correspond  in  a  general  way  to  the 
respiratory  movements.  This  experiment  was  originally  performed  on  the 
horse,  by  the  Rev.  Stephen  Hales  (1732). 

In  the  venous  tube  the  blood  also  rises  to  a  certain  height,  after  which  it 
remains  quite  stationary,  as  the  effect  of  the  cardiac  contraction  is  not 


THE  CIRCULATION  OF  THE  BLOOD. 


333 


propagated  under  normal  conditions  beyond  the  arterial  system.  The 
height  to  which  it  rises  is  but  slight  as  compared  with  that  in  the  arterial 
tube.  The  pressure  in  both  vessels  is  thus  recorded  in  millimeters  of  blood. 
Strictly  speaking  the  pressure  thus  obtained  does  not  represent  the  lateral 
pressure  in  the  carotid  artery  but  in  the  vessel  from  which  it  arises.  The 
central  end  of  the  carotid  is,  under  the  circumstances,  but  a  continuation 
of  the  cannula  and  the  pressure  thus  obtained  is  the  lateral  pressure  of  either 
the  innominate  artery  or  the  aorta  as  the  case  may  be.  In  order  to  obtain  the 
lateral  pressure  in  the  carotid  or  any  other  artery  it  is  only  necessary  to  take 
the  end  pressure  of  any  one  of  its  branches  or  what  amounts  to  the  same 
thing,  to  divide  the  vessel  and  insert  the  horizontal  portion  of  a  T-shaped 
tube  into  the  central  and  distal  ends  through  which  the  blood  can  con- 


B.  P.TRACING   y 


Fig.  154.  Dla.gr.\m  to  Show  the  Relation  of  the  Mercurl^l  >Iaxometer  to  the  Artery, 
ON  One  Hand,  and  to  the  Recording  Cylinder,  on  the  Other  H,a.nd,  when  Arr,a.nged  for 
Recording  Blood-pressure. 


tinue  to  flow,  and  to  connect  the  vertical  portion  with  a  vertical  pressure 
tube  or  with  a  mercurial  manometer.  The  absolute  pressure  on  any  given 
unit  of  vessel  surface — e.g.,  1  sq.  mm. — is  obtained  by  multiplying  the  height 
of  the  column,  expressed  in  millimeters,  by  the  unit  of  surface,  and  then 
determining  the  weight  of  this  mass  of  blood.  Thus  if  the  height  of  the 
column  of  blood  in  the  carotid  artery  tube  is  2000  mm.,  then  the  pressure  on 
I  sq.  mm.  is  2000  mm.  of  blood.  The  weight  of  2000  c.mm.  of  blood  is  equal 
to  2.1  grams. 

•The  Arterial  Blood-pressure. — For  accurate  and  long-continued  ob- 
servation the  arterial  blood-pressure  is  more  conveniently  studied  by  means 
of  a  U-shaped  tube  (a  manometer)  partially  filled  with  mercury.  One  limb 
of  the  manometer  is  connected  by  means  of  a  tube  and  a  cannula  with  an 
artery  (Fig.  154).  For  the  purpose  of  retarding  coagulation  of  the  blood  and 
for  preventing  the  escape  of  a  large  volume  of  blood  from  the  vessels,  the 
system  is  filled  with  a  solution  of  carbonate  of  soda  of  sp.  gr.  1060,  55.8 
grams  per  1000  c.c,  or  a  25  per  cent,  solution  of  magnesium  sulphate  of 


334 


TEXT-BOOK  OF  PHYSIOLOGY. 


sp.  gr.  1060,  and  under  a  pressure  approximately  equal  to  that  in  the  vessel 
of  the  animal  as  determined  in  previous  experiments.  When  communication 
is  established  between  the  vessel  and  the  cannula,  the  mercurial  column 
adjusts  itself  to  the  pressure  in  the  artery  and  at  once  exhibits  the  same 
cardiac  oscillations  and  respiratory  undulations  as  did  the  column  of  blood 
in  the  previous  experiment. 

The  height  of  the  mercurial  column  kept  in  equilibrium  by  the  pressure 
of  the  blood  within,  and  the  pressure  of  air  without  the  vessel  is  that 
between  the  lower  level  of  the  mercury  in  the  proximal,  and  the  higher  level 
in  the  distal  limb  of  the  manometer,  both  of  which  can  be  read  off  on  a  scale 
placed  between  the  two  Hmbs. 

The  height  of  the  mercury  as  well  as  its  oscillations  in  the  distal  limb 
may  be  recorded  by  placing  on  the  top  of  the  mercury  a  light  float,  the  upper 


r6o  ' 

A' 

g^.^^i'VVv^AA/VVv^v 

r^5 

r4o 

r35 

fso 

"25 

rzo            • 

( 

"10  TiTTie  Record  in  Seconds 

rsi  1  (  1  1  (  1  1  (  1  1  1  t  1  1  -1  1  1  1  1  1  1 

Line  of  AtmospKeric  Pressure 

Fig.  155. — A  Portion  of  a  Blood-pressure  Tracing  Obtained  from  the  Carotid  Artery 
OF  THE  Rabbit  with  a  Mercurial  Manometer.  The  small  oscillations  are  due  to  the  heart-beat; 
the  large  oscillations  are  due  to  the  respiratory  movements. 

end  of  which  carries  a  writing  point.  When  the  latter  is  placed  in  contact 
with  the  moving  blackened  surface  of  a  recording  cylinder  or  kymograph, 
the  height  and  the  oscillations  are  recorded  in  the  form  of  a  tracing  similar 
to  that  shown  in  Figs.  154  and  155,  in  which  the  smaller  oscillations  represent 
the  changes  in  pressure  due  to  the  systole  and  diastole  of  the  heart  and  the 
larger  oscillations  to  variations  in  the  average  pressure  due  to  the  respiratory 
movements.  The  height  of  the  mercurial  column  kept  in  equilibrium  at  any 
particular  moment  is  determined  by  measuring  the  distance  between  a  base- 
line or  abscissa,  which  represents  the  position  of  the  mercury  at  atmospheric 
pressure,  and  any  given  point  on  the  trace  above,  and  multiplying  it  by  2, 
for  the  reason  that  the  mercury  sinks  in  the  proximal  limb  as  high  as  it  rises 


THE  CIRCULATION  OF  THE  BLOOD. 


335 


max  valve 


mm  valve 


in  the  distal  limb  of  the  manometer  and  hence  the  column  of  mercury  sup- 
ported is  that  observed  between  the  upper  and  lower  levels  of  the  mercury 
in  the  distal  and  proximal  limbs  of  the  manometer. 

The  blood-pressure  as  revealed  by  the  tracing  may  be  resolved  into  two 
components:  viz.,  (i)  a  more  or  less  constant  element  represented  by  the 
pressure  in  the  arteries  during  the  period  of  the  cardiac  diastole,  which  is 
termed  the  diastolic  or  minimum  pressure;  and  (2)  a  variable  element 
represented  with  certain  limitations  by  that  additional  pressure  occurring 
at  the  time  of  the  cardiac  systole,  which  is  termed  the  systolic  or  maximum 
pressure.  Tht  diastolic  pressure  is  represented  by  the  distance  between  the 
base-line  and  the  points  of  the  curve  corresponding  to  the  diastolic  pause;  the 
systolic  pressure,  by  the  distance  between  the  base-line  and  the  apices  of 
the  curves  following  the  cardiac  systole.  The  relation  of  these  two  com- 
ponents varies  in  different  animals  and  in  the  same  animal  at  different  times. 
If  the  diastolic  pressure  is  low,  the  systolic  to  manometer 

increase  may  be  considerable;  if  the  former 
is  high,  the  latter  may  be  slight  in  extent. 

There  are  good  reasons  for  believing, 
however,  that  this  record  does  not  represent 
either  the  true  diastolic  or  the  true  systolic 
pressure  but  that  the  limits  between  the  two 
are  far  more  widely  apart  than  here  repre- 
sented. For,  owing  to  the  inertia  of  the 
mercury,  it  is  not  capable  of  following  the 
rapid  variations  of  the  pressure  throughout 
their  extent,  that  occur  with  each  heart-beat. 
The  employment  of  one  of  the  various  forms 
of  the  quickly  responsive  spring  manometers 
such  as  are  used  in  determining  the  rapid 
variations  of  intra-cardiac  pressure  will  show 
a  much  greater  difference  between  the  dia- 
stolic and  systolic  pressures,  often  amount- 
ing to  as  much  as  40  millimeters. 

For  the  purpose  of  obtaining  the  maximum  systolic  and  the  minimum 
diastolic  pressures,  it  is  best,  however,  to  insert  between  the  cannula  and  the 
manometer  a  maximum  and  a  minimum  valve  similar  in  principle  to  that 
shown  in  Fig.  156.  By  permitting  the  blood  to  exert  its  pressure  first  through 
the  maximum  valve  and  then  permitting  the  mercurial  column  to  exert  its 
pressure  through  the  minimum  valve  in  the  reverse  direction  for  a  certain 
length  of  time,  or  by  permitting  each  to  exert  its  pressure  with  alternate 
heart-beats,  the  maximum  systolic  and  the  minimum  diastolic  pressures 
will  be  recorded.  By  this  method  Dawson  found  an  average  maximum 
pressure  in  the  carotid  artery  of  the  dog  of  162,  and  a  minimum  pressure  of 
103  mm.  of  mercury,  a  difference  of  59  mm.  Hg.  The  difference  between 
these  two  pressures  is  known  as  the  pulse  pressure.  (A  diagram  showing 
the  relation  of  these  different  pressures  one  to  another  will  be  found  on 
page  338).  _ 

In  a  series  of  experiments  it  will  be  found  that  the  blood-pressure  in  the 
arteries,  recorded  with  the  mercurial  manometer,  though  rising  and  falling  a 


to  heart 

Fig.  156. — V.  Frank's  Valve. 
This  is  placed  in  the  course  of  the 
tube  between  heart  and  manometer, 
so  that  the  latter  may  be  used  as  a 
maximum,  minimum,  or  ordinary 
manometer  according  to  the  tap  which 
is  left  open. — (Starling.) 


336  TEXT-BOOK  OF  PHYSIOLOGY. 

certain  number  of  millimeters,  yet  retains  a  fairly  constant  general  average, 
the  result  of  an  adjustment  between  the  number  of  heart-beats  per  minute 
and  the  amount  of  the  resistance  offered  to  the  escape  of  blood  into  the  capil- 
laries and  veins.  Though  the  tracing  fails  to  record  accurately  the  diastolic 
and  systolic  pressures  it  approximates  a  certain  average  or  mean  of  the  pres- 
sure thus  recorded,  which  represents  the  power  driving  the  blood  through 
the  vessels.  It  is  frequently  stated  that  in  a  tracing  in  which  the  respiratory 
undulations  are  absent,  the  mean  pressure  is  the  arithmetic  mean  of 
the  systolic  and  diastolic  pressures.  This  is,  however,  not  strictly  correct, 
for  if  the  pressure  is  recorded  by  means  of  a  spring  manometer,or  a  sphygmo- 
graph  applied  over  the  artery  of  man  a  record  much  different  in  appearance 
and  similar  to  that  shown  in  Fig.  157  will  be  obtained.  In  such  a  record 
it  will  be  observed  that  the  return  of  the  pressure  from  the  systolic  to  the 
diastolic  level  not  only  occupies  a  longer  time  than  the  passage  from  the 
diastolic  to  the  systolic  level,  but  that  the  line  of  descent  is  interrupted  by  a 
secondary  rise  and  fall  of  pressure  before  the  original  diastolic  level  is 
reached.  It  is  evident,  therefore,  that  the  pressure  is  low  for  a  longer  period 
than  it  is  high  and  hence  the  mean  pressure  cannot  be  the  arithmetic  mean 
between  the  diastolic  and  systolic  pressures.  The  mean  pressure,  however, 
can  for  a  given  period  at  least  be  experimentally  determined.  Thus,  if  at 
some  one  point  between  the  artery  and  the  manometer,  the  lumen  of  the 
connecting  tube  be  largely  obliterated  by  a  constriction,  the  variations  in 
the  pressure  following  the  systole  and  diastole  of  the  heart  will  be  largely,  if 
not  entirely  excluded,  and  the  mercury,  instead  of  rising  rapidly  in  the  man- 
ometer and  fluctuating  with  each  heart-beat,  will  rise  slowly  to  a  certain  level 
and  then  remain  at  rest.  The  number  of  millimeters  of  mercury  thus 
supported  represents  the  mean  or  absolute  pressure.  The  same  result  can  be 
obtained  by  employing  the  compensatory  manometer  of  Marey  which 
presents  a  constriction  of  this  character.  From  many  experiments  made  by 
Dawson  it  has  been  learned  that  the  mean  pressure  lies  nearer  to  the  diastolic 
than  to  the  systolic  pressure  and  may  be  expressed  numerically  by  the  state- 
ment that  it  is  equal  in  millimeters  of  mercury  to  the  diastolic  pressure  plus 
one-third  of  the  pulse  pressure.  In  a  tracing  in  which  the  respiratory  undu- 
lations are  present  the  mean  pressure  can  be  calculated.  The  method  by 
which  this  is  done,  however,  is  rather  complicated  and  need  not  be  detailed 
here.  In  a  general  way  the  mean  pressure  In  such  a  tracing  may  be  repre- 
sented by  a  line  drawn  horizontally  across  the  tracing  midway  between  the 
apex  and  trough  of  the  undulation. 

Estimates  of  the  Mean  Arterial  Pressure. — Because  of  the  difficulty 
in  obtaining  the  pressure  in  small  arteries,  the  experimental  determinations 
have  for  the  most  part  been  confined  to  large  arteries  such  as  the  carotid, 
brachial,  and  femoral,  and  hence  the  results  which  have  been  obtained  have 
reference  to  the  lateral  pressure  in  the  aorta  or  in  the  large  vessels  which 
immediately  arise  from  it.  The  pressure  obtained  in  the  usual  way  at  the 
central  end  of  a  divided  carotid  is  generally  known  as  the  ''end  pressure" 
and  represents  the  mean  lateral  pressure  in  the  aorta  or  in  the  innominate 
artery.  Among  the  results  thus  obtained  in  different  experiments  from  the 
carotid  artery  of  different  animals  are  the  following:  In  the  horse,  from 
122  to  214  mm.  Hg.;  in  the  dog,  from  140  to  160  mm.;  in  the  cat,  150  mm.; 


THE  CIRCULATION  OF  THE  BLOOD.  337 

in  the  rabbit,  from  90  to  100  mm.;  in  the  sheep,  170  mm.;  in  the  calf,  from 
133  to  165  mm.  In  two  observations  made  on  human  beings  previous 
to  the  amputation  of  a  Hmb,  the  pressure  was  found  in  the  brachial 
artery  of  one  patient  to  vary  from  no  mm.  to  120  mm.  Hg.,  and  in  the 
anterior  tibial  artery  of  the  other  patient  from  no  mm.  to  160  mm.  Hg. 

The  investigations  made  in  different  parts  of  the  arterial  system  indicate 
that  the  mean  pressure  is  remarkably  constant  and  uniform  and  does  not 
show  any  noticeable  falling  off  until  near  the  arteriole  region  where  the  resist- 
ance suddenly  and  rapidly  increases.  Thus  Volkman  found  simultaneously 
in  the  carotid  artery  and  in  the  metatarsal  artery  of  the  sheep  a  mean  pressure 
of  165  and  146  mm.  Hg.  respectively  and  this  for  the  reason  that  the  resist- 
ance throughout  the  arterial  system  does  not  markedly  increase  until  the 
arteriole  region  is  reached.  The  careful  investigations  of  Dawson  show  that 
in  the  large  blood-vessels  of  the  dog  the  diastolic  pressure  is  as  constant 
as  the  mean  pressure  though  it  undergoes  slight  variations  in  different 
regions;  but  that  the  systolic  pressure,  as  shown  by  taking  the  end  pressure 
in  the  thyroid  and  similar  sized  arteries  in  different  parts  of  the  arterial  tree, 
undergoes  a  considerable  falling  oft',  though  it,  too,  remains  high  in  large 
arteries. 

The  numerical  expressions  of  these  various  pressures  in  different  parts 
of  the  arterial  system  are  shown  in  the  following  table  abstracted  from  the 
more  extensive  tables  of  Dawson.  The  results  were  obtained  from  experi- 
ments made  on  dogs.  The  figures  represent  in  millimeters  of  mercury  certain 
average  end  pressures  in  the  arteries  named. 


Artery  Systolic  Mean 


Brachio-cephalic 163  121 

Right  carotid 160  118 

Left  carotid 160  123 

Left  subclavian 168  123 

Left  brachial 160  118 

Left  retial 165  123 

Deep  femoral 152  118 

Thyroid 140  ,  118 


Diastolic 

Pulse  pressure 

103 

60 

no 

50 

lOI 

59 

105 

63 

no 

50 

103 

62 

102 

50 

97 

i               43 

The  Capillary  Pressure. — The  small  size  of  the  capillaries  precludes 
an  investigation  of  their  pressure  by  manometric  methods.  It  may  be  stated, 
however,  to  be  approximately  equal  to  the  pressure  required  to  obliterate 
their  lumina  and  to  whiten  the  skin.  The  apparatus  of  v.  Kries  is  based  on 
this  theory.  A  small  glass  plate,  from  2.5  to  5  sq.  mm.,  is  fastened  to  the 
under  surface  of  a  support  of  suitable  size  carrying  a  small  scale  pan.  The 
glass  plate  is  placed  on  the  skin  near  the  root  of  a  finger-nail  and  the  scale 
pan  gradually  weighted  until  the  vessels  are  obliterated,  as  shown  by  the 
blanching  of  the  skin.  From  results  obtained  wdth  this  apparatus  v.  Kries 
estimated  the  pressure  in  the  capillaries  of  the  hand  at  37  mm.  Hg.  and  in  the 
ear  at  20  mm. 

The  Venous  Pressure. — In  passing  from  the  capillaries  to  the  heart 
the  pressure  continues  to  fall.  The  increasing  size  of  the  veins  permits 
again  of  manometric  observations  in  different  regions.     In  the  crural  vein 


338 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  pressure  has  been  found  to  be  equal  to  14  mm.  Hg.,  and  in  the  brachial 
vein  9  mm.  of  Hg.  In  the  jugular  and  subclavian  and  other  vessels  near 
the  heart  it  is  zero  or  even  negative;  that  is,  less  than  atmospheric  pressure 
to  the  extent  of  from  i  to  10  mm.  of  mercury. 

The  amount  and  relation  of  the  different  pressures  in  the  three  divisions 
of  the  systemic  vascular  apparatus  are  approximately  shown  in  Fig.  157. 


Z  //?e    of 

srsroL/c  pressure 

l/ne  of 
A7EAA/    PRESSURE 

PULSE    Pf^E5SUf(E  » 
Tfye  c//fference  detyreen 
SYSTOLIC 

D/ASTOUC  PRESSURE 

c 


Fig.  157. — A  Diagram  Designed  to  Show  the  Amount  and  the  Relation  of  the  Blood- 
pressure' in  THE  THREE  DIVISIONS  OF  THE  VASCULAR  APPARATUS,  AS  WELL  AS  THE  RELATION  OF 

THE  Diastolic,  the  Mean,  and  the  Systolic  Pressures  in  the  Arterlal  System.  Based  on 
experiments  made  on  dogs.  H.  Heart.  A.  Arteries.  C.  Capillaries.  V.  Large  veins.  O,  O, 
being  the  zero  line  (  =  atmospheric  pressure),  the  pressure  is  indicated  by  the  height  of  the  curve. 
The  numbers  on  the  left  give  the  pressure  (approximately)  in  millimeters  of  mercury,  h.  Pressure 
in  heart,  a.  Arteriole  region  showing  sudden  fall  of  pressure,  c.  The  fall  of  pressure  in  the 
capillaries,     v.  The  negative  pressure  in  the  large  veins. 

RESUME  OF  THE  FACTS  OF  THE  BLOOD-PRESSURE  AND  OF  THE 
FACTORS  WHICH  CAUSE  IT. 


From  a  consideration  of  the  foregoing  facts  and  statements  the  following 
resume  may  be  made:  i.  The  blood  during  its  flow  exerts  a  pressure  against 
the  sides  of  the  blood-vessels.  2,  This  pressure  is  the  resultant  on  the  one 
hand  of  the  intra-ventricular  pressure  developed  at  the  time  of  the  contrac- 
tion, and  on  the  other  hand  of  the  resistance  to  the  forward  movement  of  the 
blood.  3.  The  resistance  is  to  be  sought  for  in  the  cohesion  and  friction  of  the 
molecules  of  the  blood.  4.  The  resistance  is  inversely  proportional  to  the 
diameter  of  the  vessel  and  is  therefore  least  in  the  large  arteries  and  veins  and 
greatest  in  the  arterioles  and  capillaries.  5.  The  pressure  is  highest  in  the 
aorta  where  it  may  amount  in  man  to  150  mm.  of  mercury  above  that  of  the 
atmosphere,  and  lowest  at  the  ends  of  the  vense  cavse  where  it  may  be  no 
greater  than  that  of  the  atmosphere  or  may  be  even  10  mm.  Hg.  below  it. 
6.  The  pressure  falls  from  the  beginning  to  the  end  of  the  vascular  apparatus, 
though  not  progressively,  for  throughout  the  large  vessels  of  the  arterial 
system  it  continues  relatively  high.  7.  The  high  pressure  in  the  aorta  is  due 
to  the  total  resistance  of  the  vascular  apparatus  and  the  pressure  at  any  given 


THE  CIRCULATION  OF  THE  BLOOD.  339 

point  of  the  apparatus  represents  the  resistance  yet  to  be  overcome.  8.  The 
high  pressure  in  the  arterial  system  and  its  marked  fall  at  its  periphery  is 
more  especially  the  result  of  the  very  great  resistance  at  this  point,  known 
as  the  peripheral  resistance,  the  result  of  a  rapid  diminution  in  the  diameter 
of  the  arterioles  and  the  capillary  vessels,  modified  by  the  tonic  contraction 
of  the  arteriole  muscles.  9.  The  pressure  in  the  arterial  system  undergoes 
considerable  variation  both  above  and  below  the  mean  pressure  during  the 
systole  and  diastole  of  the  heart. 

The  Heart. — The  primary  factor  in  the  production  of  the  pressure  is  the 
pumping  action  of  the  heart.  Should  there  be  any  cessation  in  its  activity, 
the  elastic  walls  of  the  arteries  would  recoil  and  force  the  blood  into  the 
veins.  There  would  be  coincidently  a  fall  of  the  pressure  to  that  of  the 
atmosphere.  Even  under  normal  circumstances  this  condition  is  approxi- 
mated during  the  diastole.  The  recoil  of  the  arterial  wall  by  which  the  for- 
ward movement  of  the  blood  is  maintained  is  attended  by  a  fall  in  pressure. 
But  before  this  reaches  any  considerable  extent,  the  heart  again  contracts 
and  forces  its  contained  volume  of  blood  into  the  arteries. 

That  this  may  be  accomplished  it  is  essential  that  the  cardiac  energy 
be  sufficient  not  only  to  drive  a  portion  of  the  blood  through  the  capillaries 
into  the  veins,  but  to  oppose  the  recoiling  arteries,  and  to  distend  them  to 
their  previous  extent,  so  that  the  incoming  volume  of  blood  may  be  ac- 
commodated.    This  at  once  reestablishes  the  pressure  at  its  former  level. 

During  the  contraction  of  the  heart  the  kinetic  energy  is  transformed 
into  potential  energy,  represented  by  the  tense  distended  walls  of  the  arteries. 
With  the  relaxation  of  the  heart  and  the  closure  of  the  semilunar  valves  the 
potential  energy  of  the  arteries  is  again  transformed  into  kinetic  energy, 
represented  by  the  moving  blood.  The  artery  thus  continues  the  work 
of  the  heart  during  its  period  of  inactivity.  The  rapidity  with  which  the 
cardiac  contractions  succeed  each  other  prevents  the  pressure  from  sinking 
below  a  certain  average  level. 

The  Resistance. — The  secondary  factor  is  the  resistance  to  the  flow 
of  blood  through  the  vessels,  the  nature  of  which  has  been  previously  stated. 
So  long  as  the  resistance,  and  especially  that  variable  element  of  it  at  the 
periphery  of  the  arterial  system,  maintains  a  certain  average  value,  so  long  will 
the  pressure  in  each  division  of  the  vascular  apparatus  maintain  an  average  or 
a  physiologic  value.  Should  the  resistance  at  the  periphery  of  the  arterial 
system  vary  in  either  direction,  the  result  of  an  increase  or  a  decrease  in  the 
degree  of  the  contraction  of  the  arteriole  muscle,  there  will  arise  a  change 
in  the  relative  degree  of  pressure  in  each  of  the  three  divisions  of  the  vascular 
apparatus. 

The  Elasticity  of  the  Vessel  Walls. — A  tertiary  factor  is  the  elasticity 
of  the  arterial  wall.  While  it  can  hardly  be  said  that  the  elasticity  is  a  cause 
of  the  pressure,  there  can  be  attributed  to  it  the  capability  of  modifying 
and  assisting  in  the  maintenance  of  the  pressure  at  a  more  or  less  constant 
level;  for  were  it  not  for  this  property  of  the  vessel  wall  the  variations  in 
pressure  during  and  after  the  systole  would  be  far  more  extensive  than  they 
are,  and  would  approximate  the  variations  observed  in  tubes  with  rigid 
walls.  The  elasticity,  moreover,  assists  in  the  equalization  of  the  blood- 
stream, converting  the  intermittent  and  remittent  flow  characteristic  of  the 


340  TEXT-BOOK  OF  PHYSIOLOGY. 

large  arteries  into  the  continuous  equable  stream  characteristic  of  the  capil- 
laries. It  also  permits  of  wide  variations  in  the  amount  of  blood  the  arteries 
can  contain  between  their  minimum  and  maximum  distention. 

VARIATIONS  IN  THE  BLOOD-PRESSURE. 

A.  In  the  Arterial  Pressure. — It  is  evident  from  the  preceding  r,tate- 
ments  that  the  arterial  blood-pressure  as  a  whole  may  be  increased  above  the 
normal,  by: 

1.  An  increase  in  the  rate  or  force  of  the  heart's  contraction. 

2.  An  increase  in  the  peripheral  resistance. 

3.  An  increase  in  both  the  force  of  the  heart  and  the  peripheral  resistance 
and  that  it  may  be  brought  back  to  the  normal  by  a  decrease  in  either  one 
or  both  of  these  factors. 

It  is  also  evident  that  the  arterial  blood-pressure  may  be  decreased  below 
the  normal  by: 

1.  A  decrease  in  the  rate  and  force  of  the  heart's  contraction. 

2.  A  decrease  in  the  peripheral  resistance. 

3.  A  decrease  in  both  the  force  of  the  heart  and  the  peripheral  resistance 
and  that  it  may  be  raised  to  the  normal  by  an  increase  in  either  one  or  both 
of  these  factors. 

If  when  the  arterial  pressure  is  in  a  condition  of  equilibrium  the  heart 
ejects  into  the  arteries  in  a  given  period  of  time  an  increased  c{uantity  of 
blood  as  a  result  of  an  increased  rate  of  contraction,  there  will  be  an  accumu- 
lation of  blood  temporarily  in  the  arteries  and  a  rise  of  pressure  (the  peripheral 
resistance  remaining  the  same),  for  the  reason  that  the  pressure  is  only 
sufficient  to  force  into  the  capillaries  a  given  volume,  in  the  same  period  of 
time.  As  the  pressure  rises  the  velocity  and  the  outflow  will  be  increased 
until  equilibrium  is  restored  though  at  a  somewhat  higher  level.  A  rise 
of  pressure  from  an  increase  in  the  rate  of  the  beat  alone  has  been  questioned, 
for  it  has  apparently  been  demonstrated  that  there  is  a  defmite  relation  be- 
tween the  normal  rate  and  the  volume  discharged  from  the  ventricle,  and 
that  when  the  rate  is  increased,  the  volume  discharged  diminishes  and  hence 
the  pressure  remains  normal  or  even  falls  below  the  normal. 

An  increase  in  the  pressure  is  readily  brought  about  by  an  increase  in  the 
force  or  power  of  the  contraction,  the  frequency  remaining  the  same.  An 
increase  in  the  volume  of  blood  ejected  at  each  contraction  will  necessarily 
lead  to  an  accumulation.  With  the  accumulation  there  goes  an  increased 
distention  of  the  artery  and  a  corresponding  increase  of  pressure.  In  a 
short  time,  therefore,  the  increased  pressure  will  force  out  of  the  arteries 
at  a  higher  rate  of  speed  this  excess  of  blood  until  the  outflow  again  equals  the 
inflow.  This  restores  the  equilibrium  but  establishes  the  mean  pressure  at  a 
higher  level. 

If  the  peripheral  resistance  is  increased  by  a  contraction  of  the  muscle 
walls  of  the  arterioles,  the  frequency  and  force  of  the  heart  remaining  the 
same,  there  will  also  be  an  accumulation  of  blood  in  the  arteries,  an  increased 
distention  and  consequent  rise  of  pressure  (Fig.  158).  The  outflow  of 
blood  will  at  the  same  time  be  diminished.  A  rise  of  pressure  from  this  cause 
much  beyond  the  normal  is  to  a  large  extent  prevented  by  a  simultaneous 


THE   CIRCULATION  OF  THE  BLOOD. 


341 


decrease  in  the  rate  and  force  of  the  heart-beat.  This  is  due  to  a  stimulation 
of  the  peripheral  ends  of  the  depressor  nerve,  and  a  consequent  reflex  stimu- 
lation of  the  cardio-inhibitor  center,  and  not  to  a  direct  action  on  the  heart- 


^mm\\mi(ifmmmmmmmwm!mmmmmsm. 


Fig.  158. — A  Tracing  Showing  an  Increase  in  the  Blood-pressure  in  the  Carotid 
Artery  of  a  Rabbit  Due  to  an  Increase  in  the  Peripheral  Resistance  from  a  Contraction 
OF  THE  Arterioles  Caused  by  Reflex  Stimulation  of  the  Vaso-motor  Center.  The  nerve 
stimulated  was  the  sciatic.  Stimulation  began  at  s.  The  rate  of  the  heart-beat  is  unchanged. 
With  the  cessation  of  the  stimulation  the  blood-pressure  falls  for  the  reverse  reasons. 

muscle,  inasmuch  as  the  effect  is  not  observed  after  division  of  the  vagi. 
When  both  the  force  of  the  heart  and  the  peripheral  resistance  are  simul- 
taneously increased  there  is  a  rapid  increase  in  pressure;  the  former  factor 
tends  to  increase,  the  latter  factor,  to 
decrease,  the  velocity  of  the  outflow. 
According  as  the  one  or  the  other  pre- 
ponderates, will  there  be  an  increase 
or  decrease  in  velocity.  If  they  balance 
each  other,  there  will  be  no  change.  A 
rise  of  pressure  from  a  combination  of 
these  factors  is  rather  a  pathologic  than 
a  physiologic  condition  and  is  observed  in 
certain  diseases  of  the  vascular  apparatus. 
The  converse  of  these  statements  also 
holds  true.  If  when  the  general  arterial 
pressure  is  in  a  condition  of  equilibrium 
the  heart  ejects  into  the  arteries  in  a  given 
period  of  time  a  lessened  quantity  of 
blood,  either  as  a  result  of  a  decrease  in 
the  rate  or  force,  there  will  soon  be  a 
diminution  of  the  arterial  distention  and 
a  consequent  fall  in  pressure  (Fig.  159). 
The  velocity  at  the  same  time  diminishes. 
This  continues  untfl  the  outflow  no  longer 
exceeds  the  inflow.  Equilibrium  will 
again  be  established,  but  the  pressure 
will  be  at  a  lower  level. 

If  the  peripheral   resistance  is  dimin- 
ished by  a  dilatation  of  the  arterioles,  the 
heart's  contractions  remaining  the  same, 
the    existing    pressure    soon    diminishes, 
increases. 


Fig.  159. — A  Tracing  of  the  Blood- 
pressure  IN  THE  Carotid  Artery  of  a 
R.^BBiT,  showing  a  sudden  decrease  in 
the  pressure  due  to  an  arrest  in  the  rate 
and  force  of  the  heart-beat  the  result  of 
stimulating  the  vagus  nerve  f rom  "  on  " 
to  "off."  With  the  cessation  of  the  stimu- 
lation the  pressure  began  to  rise  as  the 
rate  and  the  force  of  the  heart-beat  re- 
turned. (The  abscissa  should  be  20  mm. 
lower.) 

The   outflow  of   blood   at   once 


342  TEXT-BOOK  OF  PHYSIOLOGY. 

As  a  rule  a  diminution  in  peripheral  resistance  is  attended  by  an  in,crease 
in  the  rate  or  force  of  the  heart,  and  this  is  especially  the  case  if  the  pressure 
has  been  above  the  normal. 

When  both  the  force  of  the  heart  and  the  peripheral  resistance  are  simul- 
taneously diminished,  there  will  be  a  rapid  fall  in  pressure.  The  former 
factor  tends  to  decrease,  the  latter  factor  to  increase  the  velocity  of  outflow. 
According  as  the  one  or  the  other  preponderates  will  there  be  a  decrease  or 
an  increase  in  velocity.  If  they  balance  each  other  there  will  be  no  change. 
This  condition  is  also  a  pathologic  rather  than  a  physiologic  condition  and 
observed  in  states  of  profound  depression  due  to  serious  injuries. 

Local  Variations  in  the  Arterial  Blood-supply. — The  variations  in 
pressure  and  velocity  from  variations  either  in  the  activity  of  the  heart  or  in 
the  peripheral  resistance  recorded  in  preceding  paragraphs,  have  reference  to 
the  arterial  system  in  its  entirety;  but  it  is  evident  from  many  facts  that 
similar  variations  take  place  in  special  regions  or  organs  of  the  body.  Thus, 
it  is  a  well-known  fact  that  for  the  exhibition  of  the  functional  activity  of 
every  organ  there  must  be  an  increase  in  the  volume  of  blood  supplied  to  it 
in  each  unit  of  time.  This  is  accomplished  by  an  active  dilatation  of  the 
arterioles  of  the  artery  of  supply,  and  unless  the  area  or  organ  supplied  is 
large,  as  the  splanchnic  area' for  example,  there  will  be  no  necessary  diminu- 
tion in  either  the  general  blood-pressure  or  the  average  velocity.  With  the 
cessation  of  functional  activity,  there  is  no  longer  any  need  for  so  large  a 
blood-supply  and  hence  the  arterioles  contract,  diminish  the  outflow,  and  raise 
the  pressure.  If,  on  the  other  hand,  the  area  to  be  supplied  be  large,  as  the 
splanchnic  area,  the  dilatation  of  the  intestinal  arteries  will  be  attended  by 
such  a  large  inflow  of  blood  that  not  only  will  there  be  a  fall  of  pressure  in 
these  vessels,  but  a  fall  of  pressure  in  other  arteries  as  well,  combined  with  a 
diminution  in  velocity  through  them.  With  the  contraction  of  the  intestinal 
arteries  the  reverse  conditions  at  once  arise.  By  constant  variations  in  the 
;  peripheral  resistance  of  individual  arteries  in  each  and  every  region  of  the 
'body,  and  in  association  with  variations  in  the  rate  or  force  of  the  heart,  the 
'blood  is  shunted  now  into  this,  now  into  that  organ  in  accordance  with  its 
functional  needs.  All  variations  in  peripheral  resistance  are  largely  brought 
about  refiexly  by  the  vas(5-motor  nerves,  the  origin,  distribution,  and  mode  of 
action  of  which  will  be  considered  in  subsequent  paragraphs. 

B.  In  Capillary  Pressure. — The  pressure  in  the  capillaries,  though 
for  the  most  part  possessing  a  permanent  value,  is  subject  to  variations  in 
accordance  with  variations  in  the  pressure  in  either  the  arterial  or  venous 
systems  or  both.  The  marked  difference  in  the  pressure  in  the  large  arteries 
and  the  capillaries  is  partly  due  to  the  resistance  offered  by  the  narrow  arteri- 
oles. If  the  latter  dilate  in  any  given  area,  the  capillary  pressure  increases 
because  of  the  propagation  into  them  of  the  arterial  pressure.  The  reverse 
condition  would  decrease  the  pressure.  On  the  other  hand,  any  interference 
with  the  outflow  from  any  given  area,  due  to  venous  compression,  would 
likewise  increase  the  pressure;  any  factor  which  would,  on  the  contrary, 
favor  the  outflow  would  decrease  the  pressure.  Independent  of  any  change 
in  the  arteriole  resistance,  it  is  evident  that  a  rise  in  arterial  pressure  alone  would 
increase  the  capillary  pressure.  If  both  arterial  and  venous  pressures  rise, 
the  capillary  pressure  increases;  if  both  fall,  it  decreases. 


THE  CIRCULATION  OF  THE  BLOOD.  343 

C.  In  Venous  Pressure. — Independent  of  any  change  in  the  venous 
pressure  in  a  given  area  from  local  or  temporarily  acting  causes — e.g.,  aspira- 
tion of  the  thorax  or  heart,  muscle  contractions,  change  of  position,  etc. — the 
general  venous  pressure  will  be  increased  by  a  decrease  in  the  value  of  those 
factors  which  produce  the  difference  of  pressure  between  the  arteries  and 
veins.  An  increase  in  the  value  of  these  factors  would  necessarily  decrease 
the  pressure. 

Variations  in  the  Relation  of  the  Arterial  and  Venous  Pressures. — 
So  long  as  the  heart  maintains  a  given  rate  and  force  and  the  resistance  at 
the  periphery  of  the  arterial  system  (due  to  the  contraction  of  the  arteriole 
muscle)  a  given  value,  will  the  usual  physiologic  difference  between  the  pres- 
sure in  the  arteries  and  veins  remain  unchanged.  If,  however,  either 
factor  changes  in  one  direction  or  another,  there  will  arise  a  change  in  the 
relative  degree  of  pressure  in  the  different  divisions  of  the  vascular  apparatus. 
Thus  if  the  heart  force  increases  and  a  larger  volume  of  blood  is  discharged 
into  the  arteries  in  a  unit  of  time,  the  amount  of  blood  in  the  venous  system 
diminishes,  and  the  result  is  a  rise  of  the  arterial  and  a  fall  of  the  venous 
pressures.  If,  on  the  contrary,  the  heart  force  decreases  or  the  mitral  valve 
permits  of  a  regurgitation,  a  smaller  volume  of  blood  is  ejected  into  the 
arteries  in  a  unit  of  time,  the  amount  of  blood  in  the  venous  system  increases, 
and  the  result  is  a  fall  of  the  arterial  and  a  rise  of  the  venous  pressure. 

Again  if  the  arteriole  muscle  relaxes  and  a  larger  volume  of  blood  flows 
from  the  arteries  into  the  veins  in  a  unit  of  time,  the  result  will  be  a  fall  of 
arterial  and  a  rise  of  venous  pressure.  If,  on  the  contrary,  the  arterial  muscle 
contracts  and  a  smaller  volume  of  blood  flows  into  the  veins,  the  reverse 
change  of  pressure  obtains. 

The  Determination  of  the  Arterial  Blood-pressure  in  Man. — Inas- 
much as  the  blood-pressure  undergoes  considerable  variation  in  both  physi- 
ologic and  pathologic  conditions  as  well  as  in  response  to  the  action  of  drugs, 
it  seemed  desirable  to  possess  some  means  by  which  an  accurate  knowledge 
of  the  pressure  under  a  variety  of  conditions  could  be  obtained  both  for 
diagnostic  and  therapeutic  purposes.  The  foregoing  method  of  obtaining 
the  blood-pressure  not  being  of  general  application  to  human  beings  for 
obvious  reasons,  special  instruments  have  been  devised  by  which  the  pres- 
sures may  be  determined  at  least  approximately  without  resorting  to  any 
surgical  procedure.  These  instruments  are  termed  sphygmomanometers. 
Some  of  the  many  forms  of  this  instrument  are  adapted  for  obtaining  the 
systolic  pressure  only,  while  others  are  adapted  for  obtaining  either  the  sys- 
tolic or  the  diastolic  pressure,  or  both. 

i:  1  The  principle  involved  in  the  first  group  is  the  application  of  a  hydrostatic 
pressure  to  an  artery,  e.g.,  the  temporal,  radial,  etc.,  until  the  lumen  is  com- 
pletely obliterated  as  indicated  by  the  disappearance  of  the  pulse  beyond  the 
point  of  compression,  and  at  the  same  time  the  registration  of  the  pressure 
applied,  by  means  of  a  mercurial  or  spring  manometer.  The  pressure  just 
sufficient  to  obliterate  the  pulse  or  to  allow  it  to  reappear  after  obliteration, 
is  taken  as  the  systolic  pressure. 

The  principle  involved  in  the  second  group  is  based  on  a  suggestion  of 
Marey,  that  the  maximum  pulsation  of  the  artery  or  the  maximum  distention 
and  recoil  following  a  heart-beat  would  be  most  likely  to  take  place  when 


344 


TEXT-BOOK  OF  PHYSIOLOGY. 


an  elastic  pressure  applied  to  the  outside  of  an  artery  is  just  sufficient  to 
equalize  the  diastolic  pressure  within.  Inasmuch  as  these  pulsations  can 
be  transmitted  to,  taken  up  and  reproduced  by  a  mercurial  column  in  connec- 
tion with  the  pressure  appliances,  it  becomes  possible,  when  the  maximum 
oscillation  of  the  mercurial  column  is  attained,  to  read  off  the  diastolic 
pressure. 

With  either  form  of  apparatus  it  becomes  necessary  to  devise  a  suitable 
elastic  sac  or  tube  enclosed  by  non-elastic  or  rigid  walls  and  capable  of 
being  made  to  encircle  a  linger  or  an  arm,  which  can  in  turn  be  connected 
with  a  pressure  apparatus,  and  with  a  manometer  by  which  any  giv^en 
pressure  can  be  registered. 

One  of  the  best  known  of  the  sphygmomanometers  is  that  of  Mosso 
represented  in  Fig.  i6o.  It  consists  essentially  of  rubber  capsules,  which  are 
contained  within  metallic  tubes  and  into  which  two  fingers  of  each  hand  can 
be  inserted.  This  system  is  connected,  on  the  one  hand,  with  a  pressure 
apparatus,  and,  on  the  other,  with  a  manometer  provided  with  a  scale.     A 


Fig.  i6o. — The  Sphygmom.a.nometer  of  Mosso. 

float  and  writing-pen  record  the  movements  of  the  mercurial  column  on  a 
moving  blackened  surface.  In  using  this  apparatus  the  pressure  is  adjusted 
to  the  point  at  which  the  mercurial  column  exhibits  the  greatest  oscillations. 

Mosso's  interpretation  of  the  results  obtained  with  the  apparatus  was 
that  when  the  greatest  oscillations  of  the  mercurial  column  were  taking  place, 
the  external  pressure  was  just  equal  to  the  mean  arterial  pressure,  the  latter 
being  the  mean  between  the  maximum  pressure  during  the  systole  and  the 
minimum  pressure  during  the  diastole  of  the  heart.  It  was  necessary,  there- 
fore, only  to  take  the  readings  corresponding  to  the  excursions  of  the  mer- 
curial column  and  to  determine  from  them  the  mean  arterial  pressure. 

It  has  been  experimentally  demonstrated,  however,  by  Howell  and  Brush 
that  this  interpretation,  either  for  this  or  any  similar  form  of  apparatus,  is 
not  correct,  but  that  the  maximum  oscillations  take  place  when  the  pressure 
applied  to  the  exterior  of  the  artery  is  just  equal  to  the  pressure  within  the 


THE  CIRCULATION 'OF  THE  BLOOD. 


345 


artery  at  the  end  of  the  cardiac  diastole;  or  in  other  words,  the  pressure  in 
the  manometer  from  which  the  greatest  oscillation  takes  place  indicates 
diastolic  pressure.  These  experimenters  connected  the  right  carotid  artery 
of  a  dog  with  a  mercurial  manometer,  interposing  along  the  course  of  the 
connecting  tube  a  maximum  and  a  minimum  valve.  The  left  carotid 
artery  was  surrounded  by  a  plethysmograph  which  was  connected,  with 
both  a  mercurial  and  a  spring  manometer,  the  former  for  the  purpose 
of  indicating  the  pressure  necessary  to  obtain  the  greatest  oscillation,  the 
latter  for  the  purpose  of  magnifying  and  recording  the  pulsation.  When 
the  observations  were  simultaneously  made  it  was  found  that  the  diastolic 
pressure  in  the  right  carotid  measured  by  the  minimum  manometer  was 


Fig.  i6i. — Stanton's  Sphygmomanometer. 


almost  exactly  equal  to  the  pressure  measured  by  the  manometer  in  connec- 
tion with  the  sphygmomanometer  surrounding  the  left  carotid  artery,  when 
it  was  exhibiting  its  maximum  excursions.  The  difference  in  the  results  of 
the  two  sides  scarcely  exceeded  more  than  one  or  two  millimeters  of  mercury. 
It  was,  therefore,  established  that  the  greatest  oscillations  record  diastolic 
pressure.  It  was  also  shown  by  the  same  investigators  that  Mosso's  appara- 
tus is  not  adapted  for  obtaining  systolic  pressure. 

Among  the  many  forms  of  sphygmomanometers  adapted  for  clinic  pur- 
poses and  with  which  both  systolic  and  diastolic  pressures  may  be  obtained 
is  that  devised  by  Stanton^  (Fig.  i6i).     The  pressure  is  applied  to  the  arm  by 

'  The  following  description  of  this  apparatus  is  abstracted  from  the  Univ.  of  Pa.  Medical 
Bulletin,  Feb.,  1903. 


346  TEXT-BOOK  OF  PHYSIOLOGY. 

the  rubber  armlet  H,  which  is  2,1  inches  wide.  This  is  the  widest  armlet 
that  can  be  adjusted  to  the  average-sized  arm  and  presents  distinct  advan- 
tages over  the  narrow  armlet  hitherto  employed.  This  armlet  is  prevented 
from  expanding  outward  by  a  cuff,  f,  of  double  thick  canvas  with  inserted 
strips  of  tin,  which  is  held  in  place  by  two  straps  which  completely  encircle 
the  cuff.  On  the  rigidity  of  this  depends  to  a  large  extent  the  transmission 
of  pulsation.  The  rubber  armlet  is  connected  by  glass  with  a  stiff-walled 
rubber  tube,  G,  which  in  turn  connects  with  the  manometer.  The  man- 
ometer is  perhaps  the  most  important  part  of  the  apparatus.  It  is  constructed 
entirely  of  metal  except  for  the  glass  tube  containing  the  mercury  column. 
The  chamber  c  communicates  by  means  of  a  metal  tube  with  the  glass 
column  D,  which  is  connected  by  a  screw-thread  at  3,  the  caliber  of  c  being 
approximately  100  times  that  of  d.  The  cap  of  the  chamber,  which  screws 
on,  is  provided  with  a  metal  T  which  is  connected  at  2  with  the  rubber  armlet 
and  at  i  with  the  bulb,  used  as  an  air-pump.  At  a  is  a  stopcock  shutting  the 
rubber  bulb  completely  from  the  rest  of  the  apparatus  while  at  b  is  a  screw- 
valve  which  allows  the  air  to  escape  from  the  closed  system.  When  desired, 
the  manometer  can  be  made  portable  (without  removing  the  mercury)  by 
screwing  the  caps  i  and  2  into  either  end  of  the  T  at  i  and  2.  The  man- 
ometer is  then  tilted  away  from  the  glass  column  d  until  all  the  mercury  has 
run  into  the  chamber,  the  glass  is  then  unscrewed  and  cap  3  screwed  in. 
Before  removing  cap  3  the  manometer  must  always  be  tilted,  else  the  mercury 
will  be  lost.  The  rubber  bulb  is  similar  to  those  found  on  atomizers,  with 
the  addition  of  a  distensible  reservoir  to  obliterate  the  air  pulse. 

In  using  this  apparatus  the  pressure  is  raised  by  the  air-bulb  forcing  air 
into  the  closed  system — distending  the  rubber  armlet  and  with  the  same 
degree  of  force  displacing  the  mercury  in  c,  driving  it  up  the  glass  column  D. 
When  the  pulse  is  no  longer  felt,  the  bulb  still  being  compressed,  the  arm  of 
valve  A  is  turned  until  it  is  at  right  angles  with  the  thumb  and  finger.  The 
valve  B  is  now  slowly  unscrewed  until  the  mercury  column  begins  to  fall.  At 
a  given  level  it  exhibits  a  considerable  oscillation  which  may  be  mistaken 
for  the  actual  systolic  pressure  but  which  is  probably  due  to  the  impact  of  the 
blood  against  the  upper  edge  of  the  rubber  portion  of  the  cuff.  If  the 
column  of  mercury  be  still  further  lowered  so  that  the  pressure  indicated  is 
a  trifle  lower  than  the  systolic  pressure  the  blood  will  be  forced  through  the 
compressed  artery  and  give  rise  to  a  pulse  wave,  which  may  be  felt  at  the 
wrist.  The  highest  excursion  of  the  mercurial  column  noted  by  the  eye  at 
the  moment  the  pulse  reappears  is  regarded  as  the  systolic  pressure. 

The  pressure  is  then  lowered  5  millimeters  at  a  time  and  the  oscillations  of 
the  mercurial  column  noted.  As  the  pressure  is  thus  slowly  lowered  there 
will  come  a  moment  when  the  oscillations  will  attain  a  maximal  value  and 
beyond  which  the  oscillations  again  diminish.  The  lowest  level  of  the 
mercury  column  at  the  time  of  the  greatest  oscillation  is  taken  as  the  diastolic 
pressure. 

Erlanger's  sphygmomanometer  is,  also,  a  most  valuable  instrument  for 
obtaining  both  systolic  and  diastolic  pressure.  It  possesses  an  advantage 
in  that  it  is  provided,  in  addition  to  the  mercurial  manometer,  with  a  tam- 
bour and  lever  by  which  changes  in  pressure  can  also  be  recorded  on  a  re- 
volving cylinder  (Fig.  162).     A  complete    description  of    this    apparatus, 


THE  CIRCULATION  OF  THE  BLOOD. 


347 


the  manner  of  using  it  and  the  results  that  can  be  obtained  with  it  will  be 
found  in  the  Johns  Hopkins  Hospital  Reports,  Vol.  XII. 

With  this  apparatus,  the  lever  often  exhibits  a  considerable  oscillation 
even  when  the  pressure  exerted  on  the  arm   exceeds  the  systolic  pressure. 


Fig.  162. — Erlanger's  Sphygmomanometer. 

It  is  difficult  therefore  to  determine  at  times  the  moment  at  which  the  pres- 
sure indicated  by  the  mercurial  column  just  falls  below  the  systolic  pressure 
and  allows  the  blood  to  pass  through.  A  new  criterion  for  this  determina- 
tion has  been  furnished  by  Erlanger,  with  a  given  speed  of  the  drum,  the  up  and 
down  strokes  of  the  lever  practically  coincide.  But  if  the  speed  of  the  drum 
be  slightly  increased  "so  that  each  wave  subtends  about  i  Jto  2  mm.  of  smoked 


348  TEXT-BOOK  OF  PHYSIOLOGY. 

paper  (this  speed  is  attained  merely  by  removing  the  governor),  the  change 
in  form  of  the  successive  waves  manifests  itself  usually  as  a  more  or  less 
abrupt  separation  of  the  ascending  and  descending  strokes  of  the  pulse  record 
(Fig.  163).  The  phenomenon  may  vary  somewhat  with  the  form  of  the 
pulse  wave  and  may  even  be  obscured  by  fling,  but  there  has  been  no  great 
difiiculty  in  recognizing  it  in  every  case.  It  is  often  very  clear  when  the 
tracing  shows  no  abrupt  increase  in  amplitude  whatsoever.  It  is  just  as 
accurate  an  index  to  the  systolic  pressure  as  the  '  sensory  criterion '  and  that 
of  V.  Recklinghausen.  The  change  in  form  occurs  because,  at  the  moment 
the  pressure  on  the  artery  falls  below  systolic,  blood  succeeds  in  making  its 
way  beneath  the  cuff.  This  must  be  squeezed  out  before  the  lever  can  re- 
turn to  the  base  line,  whereas  at  higher  pressures  the  lever  is  raised  only 
through  the  hydraulic  ram  action  of  the  pulse  wave  upon  the  upper  edge  of 
the  cuff." 

The  conclusions  of  Erlanger  regarding  the  results  of  his  investigations 
with  this  apparatus  may  be  partially  summed  up  in  the  following  statements, 

and  as  they  hold  true  for  other  forms  of 
apparatus  which  determine  both  systolic 
and  diastolic  pressures,  they  are  here  ap- 
pended:   "The  pressure  that    is   deter- 
mined by  occluding  an  artery  is  probably 
the  maximum  end  pressure  of  the  artery 
occluded.     The  pressure  determined  by 
the  method  of  maximal  oscillations  is  the 
Fig.    163.— Tracing    Showing    the    minimum  lateral  pressure  of  the  artery 
:°„"S  ^'^SZ,!l^^7i  il    compressed    and,  therefore,  as  the  mini- 
Noted.— (Erlanger.)  mum  lateral  pressure  IS  the  same  in  all  of 

the  larger  arteries,  the  pulse  pressure, 
determined  when  the  pressures  in  the  brachial  artery  are  observed,  tends 
to  approximate  the  lateral  pulse  pressure  in  the  aorta." 

Any  positive  statement  as  to  the  numerical  values  of  the  different  pres- 
sures is  somewhat  difficult  to  make  inasmuch  as  they  will  vary  within  physi- 
ological limits  in  accordance  with  the  position  of  the  body,  exercise,  charac- 
ter of  psychic  states,  digestion,  temperature,  and  other  conditions.  For 
comparative  investigations  it  is  necessary,  therefore,  to  place  the  subject  of 
the  investigation  in  one  and  the  same  position,  to  apply  the  cuff  to  the  corre- 
sponding arm,  to  use  always  a  uniform  width  of  cuff  and  to  select  the  same 
time  of  day  with  reference  to  meals,  etc. 

It  may  be  stated,  however,  that  in  adult  life  the  systolic  pressure  in  the 
brachial  artery  ranges  from  no  to  135  millimeters  of  Hg.  in  men  and  about 
10  mm.  less  in  women;  the  diastolic  pressure  ranges  from  65  to  no  mm, 
Hg.;  the  pulse  pressure  ranges  from  25  to  40  mm.  Hg. 

The  Auscultatory  Method  of  Determining  the  Blood-Pressure. — 
In  1905  a  new  method  was  introduced  and  described  by  Koj-otkow  for  the 
determination  of  both  the  systolic  and  diastolic  pressures,  which  in  the 
experience  of  clinicians  is  more  accurate  and  satisfactory  in  both  physiologic 
and  pathologic  conditions  than  any  of  the  other  clinical  methods.  (See 
papers  by  Goodman  and  Howell  in  the  Univ.  of  Pa.  Medical  Bulletin,  Nov., 
19 10    and    the  American    Journal   of    the    Medical   Sciences,   September, 


THE  CIRCULATION  OF  THE  BLOOD.  349 

191 1).  It  consists  in  the  interpretation  of  certain  sounds  heard  with  the 
stethoscope,  in  the  artery  under  observation  when  it  is  gradually  released  from  a 
pressure  that  has  obliterated  its  lumen  in  a  given  region. 

In  the  employment  of  this  method  the  brachial  artery  is  selected  and 
compressed  in  the  usual  manner  with  a  wide  cufT  in  connection  with  a 
graduated  mercurial  manometer. 

After  the  pulse  has  been  obliterated  the  stethoscope  is  placed  over  the 
artery  below  the  cuff,  care  being  taken  to  prevent  undue  pressure.  On 
releasing  the  pressure  in  the  cuff  very  gradually  as  in  the  employment  of 
other  methods  a  series  of  sounds  corresponding  with  each  arterial  pulsation 
is  heard  as  the  pressure  falls  from  the  systolic  to  the  diastolic  level.  The  series 
of  events  are  spoken  of  as  phases  of  which  five  are  recognized. 

The  first  phase  is  characterized  by  a  loud  clear-cut  snapping  sound : 
the  second  phase  is  characterized  by  a  series  of  murmurs;  the  third  phase 
by  a  succession  of  loud  clear  snapping  sounds  which  resemble  very  closely 
those  of  the  first  phase  but  are  less  loud;  the  fourth  phase  is  inaugurated  by 
a  sudden  decrease  in  the  intensity  of  the  murmurs  of  the  third  phase  giving 
rise  to  what  is  described  as  a  dull  tone  that  rapidly  becomes  weaker  and  soon 
fades  away;  the  fifth  phase  is  one  of  silence. 

These  phases  which  are  sharply  defined  and  easily  distinguishable  are 
believed  to  be  associated  with  vibrations  of  the  arterial  walls.  The  first 
sound  is  generally  believed  to  be  due  to  the  sudden  distention  of  the  artery, 
by  the  inrush  of  blood  beneath  the  cuff,  and  indicates  the  systolic  pressure 
which  can  be  at  once  observed  by  the  height  of  the  mercury  in  the  manom- 
eter. This  sound  lasts  until  the  pressure  falls  about  14  millimeters.  The 
second  sound,  a  succession  of  murmurs,  is  believed  to  be  caused  by  whirl- 
pool eddies  in  the  blood  stream  as  it  is  propelled  from  the  partially  con- 
stricted artery  into  the  non-constricted  region  below  the  cuff.  These  murmurs 
last  until  the  pressure  falls  about  20  millimeters.  The  third  sound  is  at- 
tributed to  the  vibration  of  the  arterial  wall  but  as  the  lumen  of  the  artery 
is  so  much  greater  than  that  of  the  compressed  portion  the  rapidity  of  the 
current  is  less  and  hence  the  sound  is  neither  so  sharp  nor  pronounced. 
It  lasts  until  the  pressure  falls  about  6  millimeters.  The  transition  from  the 
second  to  the  third  sound  involves  a  fall  of  about  5  millimeters.  The  disap- 
pearance of  the  sounds  is  coincident  with  the  return  of  the  artery  to  its  nor- 
mal size  and  hence  a  cessation  of  the  vibration.  It  therefore  indicates  the 
diastolic  pressure,  which  can  at  once  be  observed  by  the  height  of  the  mer- 
cury in  the  manometer. 

The  systolic  pressure  obtained  by  this  method  corresponds  to  the  first 
sound  that  is  heard  over  the  brachial  artery  and  is  about  130  millimeters; 
the  diastolic  pressure,  corresponds  with  the  cessation  of  all  sounds  and  is 
about  85  millimeters.     The  pulse  pressure  is  therefore  45  millimeters. 

In  pathologic  states  of  the  vascular  apparatus  the  duration  and  intensity 
of  the  sounds  undergo  considerable  modification.  In  some  diseases  they  are 
quite  characteristic  and  hence  have  both  a  diagnostic  and  therapeutic  value. 

THE  VELOCITY  OF  THE  BLOOD. 

From  the  number  of  heart-beats  per  minute,  72,  and  the  amount  of  blood 
discharged  from  the  left  ventricle  at  each  beat,  80  c.c,  it  is  evident  that  the 


350  TEXT-BOOK  OF  PHYSIOLOGY. 

blood  must  be  flowing  through  the  vascular  apparatus  with  a  certain  velocity, 
for  during  the  minute  the  entire  volume  of  blood,  3684  grams,  must  have 
passed  one  and  a  half  times  through  the  heart.  Direct  observation  of  the 
escape  of  blood  from  the  central  end  of  a  divided  artery,  and  from  the  per- 
ipheral end  of  a  divided  vein,  as  well  as  of  the  flow  through  the  capillaries 
as  seen  with  the  microscope,  shows  that  the  velocity  of  the  flow  varies  indif- 
ferent parts  of  the  vascular  apparatus.  In  the  arteries,  moreover,  the  flow 
is  not  quite  uniform,  but  experiences  alternate  acceleration  and  retarda- 
tion with  each  heart-beat.  In  the  capfllaries  and  veins  the  flow  is  contin- 
uous and  uniform,  as  the  conditions  of  the  arterial  walls  are  such  as  to 
completely  overcome  the  intermittency. 

If  the  systemic  vascular  apparatus  be  conceived  of  as  a  system  of  tubes 
which  have  symmetrically  divided  and  subdivided,  and  have  again  united 
and  reunited  in  a  corresponding  manner,  it  is  clear  that  the  total  sectional 
area  will  steadily  increase  from  the  beginning  to  the  middle  of  the  system,  and 
then  as  steadily  decrease  from  the  middle  to  the  end  of  the  system.  In 
such  a  system  the  same  volume  of  blood  must  pass  through  any  given  section 
in  a  unit  of  time  if  the  balance  of  the  circulation  is  to  be  maintained.  As  the 
velocity  of  a  fluid  is  inversely  as  the  sectional  area  of  the  tubes  through  which 
it  flows,  it  follows  that  the  initial  mean  velocity  of  the  blood  in  the  aorta  will 
steadily  decrease  as  it  flows  into  the  steadily  enlarging  stream-bed  until  it  reaches 
a  minimal  value  in  the  middle  of  the  capillary  system;  and  that  it  will  again 
steadily  increase  as  it  flows  into  the  narrowing  stream-bed  until  it  reaches  the 
heart.  The  initial  mean  velocity  of  the  blood  in  the  aorta  will  not  be  attained 
in  the  venae  cavae,  for  the  reason  that  the  total  sectional  area  of  the  latter  is 
somewhat  greater  than  that  of  the  former.  The  same  facts  hold  true  for  the 
pulmonic  vascular  system. 

The  Mean  Velocity  in  the  Aorta. — From  the  well-known  fact  that  the 
velocity  with  which  a  fluid  is  flowing  through  a  tube  may  be  determined 
by  dividing  its  sectional  area  into  the  quantity  discharged  in  a  unit  of  time, 
attempts  have  been  made  to  determine  the  mean  velocity  of  the  blood  at  the 
beginning  of  the  aorta.  If  it  be  assumed  that  the  volume  discharged  at  each 
contraction  is  80  c.c,  and  the  number  of  heart-beats  per  minute  is  72,  the 
total  volume  discharged  per  minute  would  be  5760  c.c,  or  96  c.c.  per 
second.  The  sectional  area  of  the  aorta  at  its  origin  is  6.15  sq.  cm.  On 
the  principle  above  stated,  these  two  factors  would  show  a  velocity  of  156 
mm.  per  second.  This  being  the  case  the  velocity  in  the  aortic  .arch  at 
least  would  be  considerably  less  than  in  the  carotid  artery  as  will  be  stated 
later,  a  fact  which  may  however  be  explained  on  the  assumption  that  owing 
to  the  curvature  of  the  aorta  and  the  extensibility  of  its  walls  the  lateral 
pressure  becomes  very  great;  as  a  result  the  sectional  area  is  increased  and 
the  velocity  diminished.  With  the  cessation  of  the  heart's  activity,  the  elastic 
recoil  gives  an  impetus  to  the  blood  and  increases  its  velocity. 

The  Mean  Velocity  in  the  Arteries. — The  mean  velocity  of  the  blood 
in  the  larger  and  more  superficially  lying  arteries  has  been  determined  by 
Volkmann  with  the  hemodromometer,  by  Ludwig  and  Dogiel  with  the 
Stromuhr,  and  by  other  investigators  with  different  forms  of  apparatus. 

Since  neither  the  blood  nor  any  particle  placed  in  it  can  be  seen  through  the 
walls  of  the  artery,  it  occurred  to  Volkmann  to  intercalate  along  the  course 


THE  CIRCULATION  OF  THE  BLOOD. 


351 


of  a  vessel  a  U-shaped  glass  tube  about  one  meter  in  length  with  a  lumen 

the  diameter  of  that  of  the  selected  vessel,  into  and  through   which   the 

blood  could  be  made  to  flow.     The  mechanic  construction  of  the  apparatus 

is  such  (Fig.  164)  that  the  blood  can  be  made  to  flow  directly  into  the  distal 

portion  of  the  artery  across  the  base  or  indirectly 

by  way  of  the  glass  tube.     Previous  to  the  inter-  )/ 

calation  of  the  tube  it  is  filled  with  serum  or  nor-  i 

mal  saline  solution.     With  the  turning  of  the  cocks 

as  B  the  blood  enters  the  glass  tube  and  drives  the 

serum  ahead  of  it  into  the  arterial  system.     From 

the   difference  in   time   between   the  moment  the 

blood  enters  and  the  moment  it  leaves  the  tube  and 

from    the    capacity    of    the    tube    the   velocity   is 

determined. 

The  Stromuhr  or  rheometer  of  Ludwig  (Fig.  165) 
is  constructed  on  the  same  principle,  but  instead  of 
the    glass    tube    having    the   same    diameter    it  is 


Fig.  164. — A'oLKMAxx's  Hemodromom- 
ETER.     C,  C.  Arterial  cannulas. 


Fig.  165. — Ludwig  and 
Dogiel's  Rheometer.  X,  Y. 
Axis  of  rotation.  A,  B.  Glass 
bulbs.  /?,  ^.  Cannulas  inserted 
in  the  divided  artery,  e,  e^, 
rotates  on  g,  /.     c,  d.  Tubes. 


considerably  enlarged  on  its  two  sides.  The  bulbs  are  fastened  to  a  metallic 
disk  which  rotates  around  an  axis  in  the  metallic  base  which  carries  the  tubes 
to  be  inserted  into  the  arteries.  With  this  device  it  is  possible  to  place  either 
bulb  in  connection  with  the  proximal  end  of  the  artery.     Pre\'ious  to  the 


352 


TEXT-BOOK  OF  PHYSIOLOGY. 


experiment  the  proximal  bulb  is  tilled  with  oil,  the  distal  bulb  with  serum 
or  normal  saline.  On  removing  the  clips  on  the  artery  the  blood  flows  into 
the  proximal  bull?  and  drives  the  oil  into  the  distal  bulb.  As  soon  as  the 
former  is  filled  with  blood  the  bulbs  are  reversed  and  the  same  relative  condi- 
tions are  attained.  This  is  repeated  a  number  of  times.  Knowing  the 
capacity  of  the  bulbs,  and  the  number  of  times  they  are  filled  in  a  given 
period,  the  total  quantity  of  blood  discharged  is  obtained.  This  divided  by 
the  sectional  area  of  the  artery  gives  the  velocity.  The  following  values 
have  thus  been  obtained:  For  the  carotid  of  the  dog,  205  to  357  mm.  per 
second;  for  the  carotid  of  the  horse,  306  mm.;  for  the  metatarsal  artery  of  the 
horse,  56  mm.  (Volkmann).  For  the  carotid  of  rabbits,  94  to  226  mm.;  for 
the  carotid  of  the  dog,  349  to  733  mm.  (Dogiel). 

The  variations  in  the  velocity  of  the  blood  in  the  arteries  during  the  differ- 
ent phases  of  the  cardiac  cycle  have  been  determined  by  Chauveau  and 

Lortet    with    the    hematachometer 


(Fig.  166).  This  consists  of  a  me- 
tallic tube  carrying  a  graduated 
disk.  At  one  point  the  tube  is 
perforated  but  covered  with  a  rub- 
ber band  through  which  passes  an 
index.  When  the  tube  is  inserted 
into  the  divided  ends  of  an  artery, 
the  current  of  blood  strikes  the 
short  arm  of  the  index  and  gives  to 
the  outer  long  arm  a  movement  in 
the  opposite  direction.  The  extent 
of  the  excursion  indicates  the  veloc- 
ity. The  apparatus  is  first  grad- 
uated with  currents  of  water  of 
known  velocity.  With  this  instru- 
ment Chauveau  found  that  in  the 
horse  the  velocity  during  the  systole 
was  520  mm.  per  second,  at  the 
beginning  of  the  diastole  220  mm. 
per  second,  and  during  the  pause 
150  mm.  per  second. 
The  rate  of  flow  in  the  capillary 
It  has  been  estimated  bv 


Fig.  166. — The  Hematachometer  of  Chau- 
veau AND  Lortet.  A,  B.  Tube  inserted  in 
artery.  C.  Lateral  tube  connected  with  a  manom- 
eter.'  b.  Index  moving  in  a  caoutchouc  mem- 
brane, a.     G.  Handle. 


The  Velocity  in  the  Capillaries 

vessels  cannot  be  experimentally  deterrhined 
Vierordt  at  0.5  mm.  per  second  in  his  own  retinal  capillaries;  by  Weber  at 
0.8  mm.  In  frogs  the  velocity  can  be  fairly  well  determined  by  observing 
the  time  required  for  a  corpuscle  to  pass  over  one  or  more  divisions  of  an 
ocular  micrometer.  Weber  calculated  in  this  way  that  the  velocity  is  0.5 
mm.  per  second. 

As  the  velocity  varies  inversely  with  the  sectional  area,  it  becomes  possible 
to  approximately  determine  the  relation  of  the  sectional  area  of  the  capillary 
system  to  that  of  the  aorta  from  the  above-mentioned  velocities.  If  it  be 
assumed  that  the  velocity  in  the  aorta  averages  300  mm.  and  in  the  capillaries 
0.5  mm.  per  second,  then  the  sectional  area  of  the  capillaries  is  to  that  of  the 
aorta  as  600  to  i. 


THE  CIRCULATION  OF  THE  BLOOD. 


353 


The  Velocity  in  the  Veins. — In  the  venous  system  the  velocity  increases 
in  proportion  as  the  sectional  area  decreases.  In  the  jugular  vein  Volkmann 
found  the  velocity  225  mm.  per  second,  which  was  about  one-half  that  in 
the  aorta  of  the  same  animal.  The  reason  for  the  slow  rate  of  movement  in 
the  jugular  vein  is  to  be  found  in  the  fact  that  the  sectional  area  of  the  com- 
bined venae  cavae  is  about  twice  that  of  the  aorta;  hence  the  relation  of  the 
sectional  area  of  the  capillary  system  to  the  sectional  area  of  the  venae  cavae 
is  about  300  to  i. 

The  blood-pressure,  the  velocity  of  the  blood,  the  sectional  area  of  the 
vascular  apparatus,  and  their  relation  one  to  the  other  are  shown  in  Fig.  167. 


Arteries.                                       Capillaries. 
Fig.  167. ,  Blood-pressure.     ,  Wlocity. 


Veins, 
-c — c — o — o,  Sectional  area. 


The  Relations  of  Blood-pressure  and  Velocity. — Though  the  pres- 
sure of  the  blood  bears  a  definite  relation  to  the  velocity  it  must  be  kept  in 
mind  that  it  is  rather  the  difference  in  pressure  between  the  beginning  and 
the  termination  of  the  arterial  system,  rather  than  the  mean  pressure  that 
influences  the  velocity.  Thus,  with  a  given  force  of  the  heart  and  a  given 
peripheral  resistance,  the  velocity  will  have  a  given  value,  and  so  long  as 
these  factors  remain  constant  will  the  velocity  remain  constant,  even  though 
the  mean  pressure  should  fall,  as  from  a  hemorrhage,  or  should  rise,  as  from 
an  injection  of  some  indifferent  fluid. 

If,  however,  the  primary  factors,  viz.,  the  cardiac  force  or  the  peripheral 
resistance,  change  their  values  in  either  the  same  or  opposite  directions, 
there  will  be  a  change  at  once  in  the  velocity.  The  variations  in  pressure 
and  velocity,  both  in  the  same  and  opposite  directions,  which  are  theoretically 
possible  from  a  change  in  the  force  of  the  heart,  or  in  the  peripheral  resistance 
or  both,  are  shown  in  the  following  table  arranged  by  Waller.  The  plus 
sign  indicates  increase,  the  minus  sign,  decrease,  in  effect. 
The  statements  herein  embodied  have  been  established  by  Marey  with  an 
artificial  schema  of  the  circulatory  apparatus,  and  by  Chauveau  and  Lortet 
by  experiments  on  animals  with  the  hemodromograph,  a  specially  devised 
apparatus  for  this  purpose. 
23 


354  TEXT-BOOK  OF  PHYSIOLOGY. 


No.  Heart  Arterioles  Blood-pressure  Blood-flow 


/  Force  constant Resistance  increased ...  -1^  — 

\  Force  constant Resistance  diminished.  —  -|- 


3  /  Force  increased  ....  Resistance  constant. . . 

4  \  Force  diminished  .  .  .  Resistance  constant. . . 

5  /  Force  increased..  '. .  .  .  Resistance  diminished . 

6  \  Force  diminished . . .  .  Resistance  increased. . . 


+ 


{ 


Force  increased Resistance  increased . .  . 

Force  diminished .  .  .     Resistance  diminished . 


-1- 

— 

4- 

+ 

- 

+ 

- 

-1- 

+ 

+ 

_ 

+ 

Though  all  the  relations  between  pressure  and  velocity  in  the  table  are 
possible,  those  which  are  most  physiological  are  probably  5  and  6,  for  in  both 
instances  there  is  a  minimum  alteration  in  pressure,  but  a  maximum  altera- 
tion in  blood  flow  or  velocity.  The  first  instance  is  the  condition  most 
favorable  for  the  functional  activity  of  organs,  for  the  reason  that  the  volume 
of  blood  which  the  organ  receives  in  a  unit  of  time  is  increased  without  any 
change  in  pressure;  and  it  is  an  established  fact  that  within  physiological 
limits  it  is  the  volume  of  blood  which  an  organ  receives  rather  than  the  pres- 
sure under  which  it  is  received,  that  determines  its  activity.  In  the  second 
instance,  on  the  cessation  of  activity  the  velocity  is  decreased  and  the  normal 
condition  restored  without  any  appreciable  change  in  pressure. 

THE  PULSE. 

The  pulse  may  be  defined  as  a  periodic  expansion  and  recoil  of  the  walls 
of  the  arterial  system.  The  expansion  is  caused  by  the  discharge  from  the 
heart  into  the  arteries  of  a  volume  of  blood,  approximately  80  c.c,  during 
the  systole;  the  recoil  is  due  to  the  elastic  reaction  of  the  arterial  walls  on 
the  blood,  driving  it  forward,  into,  and  through  the  capillaries,  during  the 
diastole. 

At  the  close  of  the  cardiac  diastole  the  arterial  system  is  full  of  blood  and 
considerably  distended.  During  the  occurrence  of  the  succeeding  systole, 
a  definite  volume  of  blood  is  again  discharged  into  the  aorta.  The  incoming 
volume  of  blood  is  now  accommodated  by  the  discharge  of  a  portion  of  the 
general  blood  volume  into  the  capillaries  and  by  the  expansion  of  the  arteries 
both  in  a  transverse  and  longitudinal  direction.  The  expansion  naturally 
begins  at  the  root  of  the  aorta  and  at  the  beginning  of  the  systole.  As  the 
blood  continues  to  be  discharged  from  the  heart,  the  expansion  increases  in 
extent;  at  the  same  time  adjoining  segments  of  the  aorta  and  its  branches 
expand  in  quick  succession,  and  by  the  time  the  systole  is  completed  the 
expansion  has  traveled  over  the  entire  arterial  system  as  far  as  the  capillaries. 
With  the  cessation  of  the  systole  and  perhaps  even  before,  the  recoil  of  the 
arterial  walls  at  once  occurs,  beginning  at  the  root  of  the  aorta  and  rapidly 
passing  over  the  arteries  to  the  capillaries. 

The  mode  of  development  as  well  as  the  propagation  of  the  expansion 
and  recoil  movement  of  the  arterial  wall,  which  together  constitute  the  pulse, 
are  illustrated  in  Fig.  168  in  which  A  B  represent  the  artery  subdivided  into 


THE  CIRCULATION  OF  THE  BLOOD. 


355 


six  equal  parts  indicated  by  the  letters  a  to  g.  In  accordance  with  this 
subdivision  of  the  artery  the  systole  of  the  heart  may  be  also  divided  into  six 
parts,  during  the  first  three  of  which  the  heart  increases  in  power,  and  during 
the  last  three  of  which  it  decreases  in  power,  gradually  falling  to  zero.  The 
effect  on  the  arterial  wall  of  the  discharge  of  blood  from  the  ventricle  is 
illustrated  in  the  figure.  During  the  first  one-sixth  of  the  systole  a  certain 
volume  of  blood  is  forced  into  the  artery,  which  at  this  moment  is  already 
full  of  blood.  Of  this  volume  a  portion  rrioves  forward  while  another 
portion  moves  sideways  as  the  arterial  wall  begins  to  expand  under  the 
pressure  of  the  heart.  At  the  end  of  the  first  one-sixth  of  the  systole  the 
condition  of  the  arterial  wall  may  be  represented  by  the  lines  ib.     During 


Fig.  i68. — Diagram  Showing  the  Development  of  a  Pulse  Wave.     (RoUet.) 

the  second  one-sixth  the  artery  expands  still  more  as  the  volume  of  blood 
increases  under  the  increasing  force  of  the  heart,  so  that  at  the  end  of  the 
second  period  the  expansion  of  the  arterial  wall  is  not  only  greater  at  the 
point  a  but  in  addition  has  extended  over  a  greater  length  of  the  artery  so 
that  the  condition  of  the  artery  may  be  represented  by  the  lines  2  c.  Dur- 
ing the  third  sixth  the  same  process  continues;  the  incoming  volume  of  blood 
still  further  expands  the  artery  at  a,  as  well  as  successive  portions  further  on 
as  far  as  d,  so  that  at  the  height  of  the  systolic  power  the  condition  of  the 
artery  may  be  represented  by  the  lines  3  d. 

The  force  of  the  heart  now  begins  to  decline  and  from  this  moment  on  the 
elastic  force  of  the  artery  preponderates  and  in  consequence  the  arterial  wall 
begins  to  recoil  at  the  point  a.  At  the  end  of  the  fourth  sixth  of  the  systole, 
therefore,  the  arterial  wall  at  a,  has  recoiled  to  2,  while  the  expansion  at  a 
has  advanced  to  &  3  where  the  force  of  the  heart  and  the  elastic  force  of  the 
artery  are  equal.  At  this  moment  the  condition  of  the  artery  may  be  repre- 
sented by  the  lines  2,  63,  e.  During  the  two  remaining  sixths  of  the  cardiac 
systole,  the  same  process  continues  until,  through  elastic  recoil,  the  artery 
has  returned  to  its  original  condition  at  a,  and  the  expansion  has  extended 
as  far  as  g,  while  the  height  of  the  expansion  has  advanced  to  d^3  where  the 
force  of  the  systole  and  the  force  of  the  elastic  recoil  balance  each  other.  At 
the  end  of  the  systole  the  condition  of  the  arterial  wall  may  be  represented 
by  the  lines  0,  J3,  0,  which  indicates  that  the  expansion  and  recoil  of  the 
artery,  which  together  constitute  the  pulse,  partake  of  the  form  of  a  wave  the 
length  of  which  is  represented  by  the  line  0, 0,  and  the  height  by  the  distance  d^. 

This  expansion  and  recoil  which  thus  pass  from  the  beginning  to  the 
end  of  the  arterial  system  assumes  the  form  of  a  wave  and  therefore  is  known 
as  the  pulse-wave  or  pulse.  Preceding  and  causing  the  expansion  and  recoil 
of  the  arterial  system  there  is  an  alternate  increase  and  decrease  of  the  gen- 


356  TEXT-BOOK  OF  PHYSIOLOGY. 

eral  blood-pressure,  as  shown  by  the  small  curves  on  a  blood-pressure  tracing, 
and  for  this  reason  the  pressure  which  causes  the  expansion  and  recoil  is 
termed  the  pulse  pressure.  It  is  defined  as  the  rhythmic  change  in  pressure 
at  any  given  point  of  the  arterial  system;  and  in  amount,  is  the  difference 
between  the  diastolic  and  the  systohc  pressures,  at  the  corresponding  points. 
The  volume  of  blood  ejected  from  the  ventricle  is  frequently  termed  the 
pulse  volume. 

The  Velocity  of  Propagation  of  the  Pulse-wave. — The  propagation 
of  the  pulse-wave  from  its  origin  at  the  root  of  the  aorta  to  any  given  point 
of  the  arterial  system  occupies  an  appreciable  period  of  time.  The  difference 
in  time  between  the  systole  and  the  appearance  of  the  pulse-wave  at  the 
dorsal  artery  of  the  foot  can  be  appreciated  by  the  sense  of  touch.  The 
absolute  time  occupied  by  the  wave  in  reaching  this  point  was  determined 
by  Czermak  to  be  0.193  second.  The  rate  at  which  the  wave  is  propagated 
over  the  vessels  of  the  lower  extremity  has  been  estimated  by  the  same  ob- 
server at  1 1. 1 6  meters  per  second,  and  for  the  upper  extremities  at  but  6.7 
meters  per  second.  Other  experimenters  have  obtained  for  the  lower  ex- 
tremities somewhat  different  results,  varying  from  6.5  to  11  meters  per  second. 
Weber's  original  estimate  was  from  7.92  to  9.24  meters  per  second.  The 
slower  rate  of  movement  in  the  vessels  of  the  upper  extremities  has  been 
attributed  to  a  greater  distensibility  of  their  walls,  a  condition  unfavorable 
to  rapid  propagation.  For  this  reason  a  low  arterial  pressure  will  occasion 
a  delay  in  the  appearance  of  the  pulse- wave  in  any  portion  of  the  body;  a 
high  arterial  pressure  will  of  course  have  the  opposite  effect.  The  difference 
in  the  speed  of  the  pulse- wave  and  the  blood-current  shows  that  they  are  not 
identical  and  must  not  be  confounded  with  each  other. 

The  pulse-wave  which  thus  spreads  itself  over  the  entire  arterial  system 
with  each  systole  of  the  heart  can  be  perceived  in  certain  localities  by  the  eye, 
by  the  sense  of  touch,  and  investigated  with  various  forms  of  apparatus  or 
instrumental  means.  The  pulse-wave,  or  at  least  the  elevation  of  the  soft 
tissues  overlying  it,  can  be  seen  in  the  radial  artery,  where  it  passes  across 
the  wrist-joint,  in  the  carotid  artery,  in  the  temporal  artery,  in  the  arteries  of 
the  retina  under  certain  conditions,  with  the  ophthalmoscope.  If  the  ends 
of  the  fingers  are  firmly  placed  over  the  radial  artery,  not  only  the  increase 
and  decrease  of  pressure,  but  also  many  of  the  peculiarities  of  the  pulse- 
wave,  may  be  perceived.  Without  much  difficulty  is  may  be  perceived  that 
the  expansion  takes  place  quickly,  the  recoil  relatively  slowly;  that  the  waves 
succeed  one  another  with  a  certain  frequency,  corresponding  to  the  heart- 
beat; that  the  pulsations  are  rhythmic  in  character,  etc.  InasmiArh  as  the 
individuality  of  the  pulse-wave  varies  at  different  periods  of  life  and  under 
different  physiologic  and  pathologic  conditions,  various  terms  more  or  less 
expressive,  have  been  suggested  for  its  varying  peculiarities.  Thus  the 
pulse  is  said  to  ha  frequent  or  infrequent  according  as  it  exceeds  or  falls  short 
of  a  certain  average  number — 72  per  minute;  quick  or  slow,  according  to  the 
suddenenss  with  which  the  expansion  takes  place  or  strikes  the  fingers; 
hard  or  soft,  tense  or  easily  compressible,  according  to  the  resistance  which  the 
vessel  offers  to  its  compression  by  the  fingers;  large,  full  or  small,  according 
to  the  volume  of  blood  ejected  into  the  aorta,  or,  in  other  words,  the  degree 
of  fullness  of  the  arterial  system. 


THE  CIRCULATION  OF  THE  BLOOD. 


357 


Frequency  of  the  Pulse. — As  the  pulse  or  the  arterial  expansion  and 
recoil  is  the  direct  result  of  the  heart's  action,  its  frequency  must,  under 
physiologic  conditions,  coincide  with  that  of  the  heart.  All  conditions  which 
modify  the  rate  of  the  heart  will  modify  at  the  same  time  the  rate  of  the  pulse. 

The  Sphygmograph. — The  sphygniograph  is  an  apparatus  designed 
to  take  up,  reproduce,  and  record  the  alternate  expansion  and  recoil  of  an 
artery  caused  by  the  temporary  increase  and  decrease  of  pressure  following  each 


Fig.  169.— Von  Frey's  Sphygmogkaph.     G.  S.  Metal  framework.     P.  Button  attached  to  spring. 
F.  Vertical  rod.     U.  Clock-work  which  turns  the  recording  cylinder.     VI.  Time  marker. 

heart-beat.  The  tracing  or  record  obtained  with  it  is  termed  the  pulse- 
curve  or  the  sphygmogram.  Different  forms  of  this  apparatus  have  been 
devised  by  Marey,  Dudgeon,  v.  Frey,  and  many  others.  The  instrument 
of  v.  Frey  is  shown  in  Fig.  169.  This  consists  first  of  a  metal  framework 
GS  by  which  the  apparatus  is  fastened  to  the  arm  and  support  given  to  the 
lever,  recording  surface,  etc.  The  essential  part  is  the  spring  carrying  a  but- 
ton P,  which  is  placed  over  the  artery,  usually  the  radial,  before  it  crosses  the 
wrist-joint.  A  vertical  rod  F  transmits  the 
movement  of  the  spring  to  the  recording  lever; 
the  movements  of  the  latter  are  recorded  on  a 
small  cylinder  inclined  slightly  so  that  the 
upstroke  may  be  vertical.  A  small  electro- 
magnet serves  to  record  the  time  relations  of  the 
changes  in  the  blood-pressure.  An  average 
tracing  taken  from  the  radial  artery  is  shown  in 
Fig.  170.  This,  however,  is  not  a  tracing  of 
the  pulse-wave,  but  rather  a  record  of  the  changes  in  pressure,  their  suc- 
cession and  time  relations,  which  follow  each  beat  of  the  heart.  The 
artery  usually  selected  for  obtaining  a  sphygmogram  is  the  radial.  This 
artery  lies  quite  superficially,  covered  only  by  connective  tissue  and  skin  and 
supported  by  the  fiat  surface  of  the  radial  bone,  conditions  most  favorable 
to  technical  investigation. 

The  sphygmogram  or  pulse-curve  may  be  divided  into  two  portions: 


Fig. 


—The  Pulse-curve  or 
Sphygmogram. 


3S8  TEXT-BOOK  OF  PHYSIOLOGY. 

viz.,  a  line  of  ascent  from  a  to  b,  and  a  line  of  descent  from  b  to  d  (Fig.  170). 
In  normal  tracings  the  former  is  almost  vertical  and  caused  by  the  sudden 
expansion  of  the  artery  immediately  following  the  ventricular  contraction; 
the  latter  is  in  general  oblique,  due  to  the  recoil  of  the  arterial  walls,  occupies 
a  longer  period  of  time,  and  is  marked  by  several  elevations  and  depressions, 
both  of  which  indicate  that  the  restoration  to  equilibrium  is  neither  immediate 
nor  uncomplicated.  One  of  these  elevations  is  quite  constant  and  known  as 
the  dicrotic  wave,  c;  the  depression  or  notch  just  preceding  it  is  known  as  the 
dicrotic  notch.  Pre-  and  post-dicrotic  waves  are  not  infrequently  present. 
The  summit  is  generally  sharp  and  pointed. 

The  vertical  direction  of  the  line  of  ascent  is  taken  as  an  indication  that 
the  arterial  walls  expand  readily,  that  the  blood  is  discharged  quickly,  and 
that  the  ventricular  action  is  not  impeded.  An  oblique  direction  of  the 
line  of  ascent  is  an  indication  that  the  reverse  conditions  obtain.  The  height 
varies  inversely  as  the  arterial  pressure,  other  things  being  equal;  being 
high  with  a  low  pressure,  and  low  with  a  high  pressure. 

The  dicrotic  elevation  shows  that  a  second  expansion  wave  is  developed 
which  interrupts  temporarily  the  recoil  of  the  arterial  walls.  The  origin  of 
this  second  expansion  has  been  the  subject  of  much  investigation,  and  at 
present  it  may  be  said  that  the  question  is  not  fully  decided.  It  is  asserted 
by  some  investigators  that  it  is  central  in  origin,  beginning  at  the  base  of  the 
aorta  and  passing  to  the  periphery;  by  others,  that  it  is  peripheral  in  origin, 
beginning  near  the  capillary  region  and  reflected  to  the  heart.  The  former 
view  is  the  one  more  generally  accepted.  According  to  it,  the  expansion  is 
the  result  of  the  sudden  closure  of  the  aortic  valves,  and  a  backward  surge 
of  the  blood  column  against  them.  The  sudden  arrest  of  the  blood  and 
its  accumulations  again  expands  the  aorta. 

The  dicrotic  notch  is  therefore  taken  as  the  moment  at  which  the  ven- 
tricular systole  ceases  and  the  aortic  valves  close.  From  this  fact  it  is  evi- 
dent that  immediately  after  the  first  expansion  the  pressure  begins  to  fall, 
even  though  the  ventricular  systole  continues,  owing  to  the  discharge  of  blood 
from  the  arterial  into  the  capillary  and  venous  systems.  The  height  of  the 
dicrotic. wave  or  the  depth  of  the  dicrotic  notch  is  increased  by  low  arterial 
tension  and  highly  elastic  arteries.  Both  features  are  diminished  by  the 
reverse  conditions.  The  apex  is  sometimes  rounded  and  even  flat,  indica- 
tive of  a  great  diminution  in  arterial  elasticity.  The  sphygmogram  not 
infrequently  varies  considerably  from  the  normal  type  in  different  pathologic 
conditions  of  the  circulatory  apparatus.  A  consideration  of  these  varia- 
tions does  not  fall  within  the  scope  of  this  work. 

The  Carotid  Pulse. — The  carotid  pulse  can  be  readily  recorded  by 
applying  over  the  carotid  artery,  anterior  to  the  sternocleidomastoid  muscle, 
on  a  level  with  the  thyreoid  cartilage,  a  funnel-shaped  tambour  in  con- 
nection with  a  suitable  recording  tambour  and  lever.  The  sphygmogram 
thus  obtained  resembles  in  all  essential  respects  that  obtained  from  the 
radial  artery.  It  is  often  of  advantage  in  the  investigation  of  certain 
problems  of  the  heart,  both  physiologic  and  pathologic,  to  record  the 
carotid  pulse  and  the  cardiac  impulse  simultaneously. 

The  Venous  Pulse. — By  this  term  is  meant  a  pulsation  of  the  large  veins 
in  the  neighborhood  of  the  heart  but  more  especially  in  the  jugular  veins. 


THE  CIRCULATION  OF  THE  BLOOD.  359 

It  is  caused  by  variations  of  pressure  transmitted  backward  into  the  veins 
during  and  after  the  systole  of  the  auricle.  Though  the  venous  pulsation  is 
not  very  marked  in  physiologic  conditions  it  frequently  becomes  pronounced 
in  certain  pathologic  conditions  of  the  heart. 

The  pressure  variations  in  the  jugular  vein  can  be  recorded  by  applying 
over  the  vein  a  properly  constructed  tambour,  a  glass  funnel  or  a  Mackenzie 
metal  tambour  connected  with  a  suitable  recording  tambour.  A  graphic 
record  of  a  normal  venous  pulse  thus  obtained,  shown  in  Fig.  171,  is  rather 
complicated',  consisting  of  three  positive  and  three  negative  waves  which 
are  related  to  variations  of  pressure  in  the  right  auricle,  the  result  of  the 
successive  contractions  of  the  auricular  and  ventricular  walls  and  the  action 
of  intra-ventricular  structures. 


Fig.  171. — Simultaneous  Tracings  of  the  Jugular  Pulse,  the  Carotid  Pulse,  and  the 
Apex  Beat. — {Bachmann.)  At  the  bottom  of  the  tracing  the  time  is  given  in  the  fiftieths  of  a 
second.  The  vertical  Unes  o,  i,  2,  3,  etc.,  mark  synchronous  points  on  the  curves.  A,  The 
auricular  wave;  5,  the  so-called  c  wave  caused  by  the  systole  of  the  ventricle;  v,  the  stagnation  wave 
caused  by  the  filling  of  the  auricle.  It  will  be  noticed  that  the  c  wave  (marked  5  in  thfe  tracing) 
occurs  at  the  beginning  of  the  ventricular  systole  as  marked  on  the  apex  beat,  and  shortly  before 
the  pulse  in  the  carotid  arter>'.  The  height  of  the  v  wave  is  reached  just  after  the  occurrence  of 
the  dicrotic  notch  on  the  carotid  wave,  and  coincides  with  the  opening  of  the  auriculoventricular 
valves;  Af,  the  negative  wave  caused  by  the  effect  of  the  ventricular  systole;  Vf,  the  negative  wave 
following  the  opening  of  the  auriculoventricular  valves. 

As  the  venous  pulse  is  a  very  evident  symptom  in  some  pathologic  condi- 
tions of  the  heart,  and  as  its  proper  interpretation  assists  in  the  diagnosis  of 
these  conditions,  it  has  become  of  much  significance  in  modern  clinical  medi- 
cine. For  purposes  of  interpretation  it  is  desirable  to  obtain  simultaneously 
graphic  records  not  only  of  the  venous  pulse,  but  of  the  carotid  or  radial 
pulse,  and  of  the  cardiac  impulse  as  well.  In  the  accompanying  figure  171 
these  three  records  are  represented. 

The  generally  accepted  interpretation  of  these  waves  is  as  follows: 
T\it  first  positive  wave,  a,  is  due  to  an  expansion  of  the  vein,  the  result 
of  a  sudden  rise  of  pressure.     As  it  occurs  before  the  ventricular  systole,  it 
is   pre-systolic   in  time  and  caused  by  the  contraction  of  the  auricle,  the 


36o  TEXT-BOOK  OF  PHYSIOLOGY. 

effect  of  which  is  to  cause  a  temporary  retardation  of  the  blood-stream 
flowing  toward  the  auricle  and  hence  a  backward  wave  of  pressure. 

The  first  negative  wave  is  due  to  a  recoil  of  the  veins  following  a  diminu- 
tion of  the  pressure  as  the  blood  again  moves  forward  in  consequence  of  the 
relaxation  of  the  auricular  walls. 

The  second  positive  wave,  c  or  s,  is  also  caused  by  a  wave  of  positive  pres- 
sure in  the  vein,  reflected  from  the  auricle,  though  it  is  not  of  auricular  origin. 
As  it  begins  with  the  ventricular  contraction  and  develops  during  the  closed 
period,  the  protosystolic  period,  i.e.,  between  the  closure  of  the  tricuspid 
valve  and  the  opening  of  the  semilunar  valves  (see  page  282),  it  is  believed 
to  be  due  to  the  bulging  of  the  auriculo-ventricular  valve  into  the  auricular 
cavity,  by  the  still  higher  intra-ventricular  pressure  thus  diminishing  its  size 
and  raising  its  pressure. 

The  second  negative  wave,  Af,  is  due  to  a  marked  fall  of  pressure,  a  col- 
lapse of  the  walls  of  the  vein  and  a  rapid  flow  of  blood  to  the  auricular  cavity. 
These  phenomena  begin  with  the  opening  of  the  semilunar  valves  and  are 
due  in  part  to  the  relaxation  of  the  auricular  walls,  but  more  especially  to  a 
descent  of  the  more  central  portions  of  the  auriculo-ventricular  valve  or 
septum,  into  the  ventricular  cavity  in  consequence  of  the  contraction  of  the 
papillary  muscles.  The  hollow  cone  thus  formed,  enlarges  the  auricular 
cavity,  withdraws  some  of  its  contained  blood,  and  hence  lowers  the  pressure, 
which  leads  to  the  inflow  of  blood  from  the  veins  and  hastens  the  auricular 
fining. 

The  third  positive  wave  v  is  caused  by  a  third  wave  of  pressure  reflected 
from  the  auricle.  It  occurs  toward  the  end  of  the  ventricular  systole  and 
is  probably  due  to  a  slight  retardation  of  the  blood  flow  in  consequence  of 
the  return  of  the  auriculo-ventricular  septum  to  its  normal  position,  the 
result  of  a  relaxation  of  the  papillary  muscles,  when  the  intra-ventricular 
pressure  is  still  higher  than  the  intra-auricular  pressure. 

The  third  negative  wave,  Vf,  is  caused  by  a  third  fall  of  pressure  in  the 
vein  and  appears  very  shortly  after  the  beginning  of  the  ventricular  relaxa- 
tion, and  the  closure  of  the  semilunar  valves.  It  develops  during  the  common 
pause  of  auricles  and  ventricles.  The  fall  of  the  venous  pressure  follows 
the  passage  of  the  blood  from  the  auricle  into  the  ventricle.  It  continues 
during  the  ventricular  filling  but  disappears  on  the  return  of  the  auricular 
contraction. 

The  Volume  Pulse. — If  an  individual  artery  expands  with  each  systole 
and  recoils  with  each  diastole  of  the  heart,  the  same  is  true  of  all  arteries, 
and  as  a  result  the  volume  of  any  organ  or  part  of  the  body  must  undergo 
similar  changes.  To  such  alternate  changes  in  volume  the  term  volume 
pulse  is  given.  The  extent  to  which  an  organ  will  increase  in  volume  will 
depend  to  some  extent  on  its  elasticity.  The  reason  for  the  increase  in 
volume  is  the  resistance  offered  to  the  flow  of  blood  into  and  through  the 
capillaries;  the  decrease  in  volume  to  the  overcoming  of  the  resistance  through 
the  arterial  recoil. 

The  variations  in  volume  may  be  recorded  by  enclosing  the  organ  in  a 
rigid  glass  or  metal  vessel,  which  at  one  point  is  in  communication  with  a 
recording  apparatus,  e.g.,  a  tambour  with  a  lever  or  a  piston  recorder  with 
float  and  writing  point.     The  space  between  the  organ  and  vessel  is  filled 


THE  CIRCULATION  OF  THE  BLOOD. 


361 


with  normal  saline,  air,  or  oil.  Such  an  apparatus  is  known  as  a  plethysmo- 
graph.  Fig.  172.  Many  forms  of  this  apparatus  have  been  devised  in 
accordance  with  the  character  of  the  organ — spleen,  kidney,  etc. — to  be 
investigated,  though  the  principle  underlying  them  is  essentially  the  same. 
In  addition  to  changes  in  volume  due  to  the  heart's  action,  most  organs 
undergo  additional  changes  in  volume  from  vaso-motor  and  respiratory 
causes. 

Indeed  the  plethysmographic  is  the  most  generally  employed  method 
of  showing  the  action  of  vaso-motor  nerves  in  changing  the  contraction  of  the 
arterioles  and  hence  the  outflow  of  blood.     Thus  when  an  organ  is  enclosed 


Fig.  172. — A  Plethysmograph. 

in  a  plethysmograph  and. the  arterial  contraction  increased  by  either  a  direct 
or  reflex  stimulation  of  the  vaso-motor  center  there  will  be  a  rise  in  the 
pressure,  a  diminution  in  the  outflow  of  blood  and  a  decrease  in  the 
volume  of  the  organ  under  observation;  and  on  the  contrary,  if  the  arteriole 
contraction  is  diminished  by  a  direct  or  reflex  inhibition  of  the  vaso-motor 
center  there  will  be  a  fall  of  pressure,  an  increased  outflow  of  blood  and  an 
increase  in  the  volume  of  the  organ.  From  this  it  is  learned  that  the  func- 
tional activity  of  an  organ  which  is  attended  and  conditioned  by  an  increased 
blood-supply  is  always  associated  with  an  increase  in  volume.  On  plethys- 
mographic records  large  undulations  are  frequently  observed  which  are 
regarded  as  of  respiratory  origin. 

THE  CAPILLARY  CIRCULATION 

In  certain  regions  of  the  body  of  many  animals  it  is  possible,  on  account 
of  the  delicacy  and  transparency  of  the  tissues,  to  observe  not  only  the  flow 
of  blood  through  the  smaller  arteries,  capillaries,  and  veins,  but  many  of 
the  phenomena  connected  with  it,  to  which  reference  has  already  been  made. 


362 


TEXT-BOOK  OF  PHYSIOLOGY. 


•The  structures  usually  selected  for  the  observation  of  these  phenomena  are 
the  interdigital  membranes  (Fig.  173),  the  tongue,  the  lung,  the  bladder, 
and  the  mesentery  of  the  frog.  Though  any  one  of  these  structures  will 
afford  an  admirable  view  of  the  blood-flow,  the  mesentery  for  many  reasons 
is  the  most  satisfactory.  For  a  comparison  of  the  phenomena  observed  in 
the  cold-blooded  animals  with  those  in  the  warm-blooded  animals  the  omen- 
tum of  the  guinea-pig  may  be  employed.  If  the  frog  is  the  subject  of  ex- 
periment, it  should  be  slightly  curarized  and  the  brain  destroyed  by  pithing. 
The  animal  is  then  placed  on  a  small  board  capable  of  adjustment  to  the 

stage  of  the  microscope.  The  abdo- 
men is  then  opened  along  the  side 
and  a  loop  of  intestine  withdrawn 
and  placed  around  a  cork  ring  which 
surrounds  an  opening  in  the  side  of 
the  frog  board.  The  loop  of  the  in- 
testine should  be  so  placed  that  it  will 
lie  between  the  observer  and  the  body 
of  the  frog.  The  mesentery  thus  ex- 
posed must  be  kept  moist  with  nor- 
mal saline  solution. 

When  examined  with  low  powers 
of  the  microscope,  arteries,  veins,  and 
capillaries  will  be  found  occupying  the 
field  of  vision.  Their  general  arrange- 
ment, their  size  and  connections,  can 
be  readily  determined.  After  a  few 
preliminary  adjustments  a  region  will 
be  found  in  which  the  blood  is  flowing 
in  opposite  directions.  The  vessel  ap- 
parently carrying  blood  away  from  the 
observer  is  an  artery;  the  vessel  appar- 
ently carrying  blood  toward  the  obser- 
ver is  a  vein;  the  smallest  vessels  are  capillaries.  The  blood  in  the  artery  is 
of  a  brighter  color  than  the  blood  in  the  vein;  the  blood  in  the  capillaries 
is  almost  colorless.  The  arterial  blood-stream  not  infrequently  shows  remit- 
tency,  an  alternate  acceleration  and  retardation,  corresponding  to  each 
heart-beat;  the  capillary  and  venous  streams  are  uniform  and  continuous. 
The  relative  velocities  in  the  three  sets  of  vessels  are  indicated  by  the  move- 
ment of  the  red  corpuscles.  In  the  arteries  they  pass  before  the  eye  so  rapidly 
that  they  cannot  be  distinguished;  in  the  capillaries  they  pass  so  slowly  that 
both  form  and  structure  may  be  determined;  in  the  veins,  though  again 
moving  rapidly,  they  can  often  be  distinguished. 

The  relative  positions  of  the  red  and  white  corpuscles  in  the  blood- 
stream are  also  apparent;  the  former  occupy  the  central,  the  latter  the  per- 
ipheral portion,  at  the  same  time  adhering  to  the  sides  of  the  vessel.  Be- 
tween the  axial  portion  of  the  stream  occupied  by  the  red  corpuscles  and  the 
wall  of  the  vessel  there  is  a  clear  still  layer  of  plasma,  the  result  of  an  adhe- 
sion of  the  plasma  to  the  wall.  It  is  this  feature  which  gives  rise  to  the 
friction  between  successive  layers  of  the  blood-stream,  the  resistance  of  the 


Fig.  173. — The  Vessels  of  the  Frog's 
Web.  a.  Trunk  of  vein,  and  {b,  h)  its 
tributaries  passing  across  the  capillary  net- 
work. The  dark  spots  are  pigment  cells. — 
{Yeo^s  "Physiology.^') 


THE  CIRCULATION  OF  THE  BLOOD. 


363 


blood-flow,  and  the  development  of  the  blood-pressure.  The  relative 
breadth  of  the  still  layer  and  amount  of  friction  are  greater  in  small  than  in 
large  vessels. 

The  volume  of  blood  passing  into  any  given  capillary  area  is  determined 
by  the  degree  of  contraction  of  the  arterioles.  Thus  on  the  application  of 
warm  saline  solution,  which  relaxes  the  arterioles,  there  is  a  large  increase 
in  the  inflow  of  blood;  vessels  previously  invisible  suddenly  come  into  view 
as  the  blood  with  its  corpuscles  passes  into  them.  On  the  application  of 
cold  water,  which  contracts  the  arterioles  and  di- 
minishes the  inflow,  many  of  the  smaller  vessels 
entirely  disappear  from  view.  The  contraction  and 
relaxation  of  the  arterioles  will  therefore  deter- 
mine the  quantity  of  blood  flowing  into  and 
through  the  capillary  system. 

Migration  of  the  White  Corpuscles. — A 
phenomenon  frequently  observed  in  the  capillary 
vessels  of  the  mesentery  or  of  the  bladder  of  the 
frog  is  the  passage  of  the  white  corpuscles  through 
the  walls  into  the  surrounding  lymph-spaces.  To 
this  process  the  term  migration  or  diapedesis  is 
given.  After  the  tissues  have  been  exposed  to  the 
air  for  some  time  or  subjected  to  an  irritant,  the 
vessels  dilate  and  become  distended  with  blood. 
In  a  short  time  the  blood-stream  slows,  and  finally 
comes  to  rest.  The  condition  of  stasis  is  then 
established.  During  the  development  of  this  condi- 
tion the  white  corpuscles  accum.ulate  in  large  num- 
bers along  the  inner  surface  of  the  vessels  and  soon 
begin  to  pass  through  the  vessel-walls.  This  they 
do  by  protruding  a  portion  of  their  substance  and 
inserting  it  into  and  through  the  vessel-wall.  This 
once  accomplished,  the  remainder  of  the  cell  in  due 
time  follows  until  it  has  entirely  passed  out  into  the 
tissue-space.  The  opening  in  the  vessel-wall  now 
closes.  The  successive  steps  in  this  process  are 
shown  in  Fig.  174.  As  this  migration  occurs 
mainly  after  the  circulation  has  ceased  or  when 
the  tissues  present  the  phenomena  of  approaching 
inflammation,  it  is  difficult  to  state  in  how  far  it  is  strictly  a  physiologic 
process. 

The  Venous  Circulation. — The  blood,  having  passed  through  the 
capillary  vessels,  is  gathered  up  by  the  veins  and  conveyed  to  the  right  side 
of  the  heart.  As  the  veins  converge  and  unite  to  form  larger  and  larger 
trunks  the  sectional  area  gradually  diminishes,  and  hence  the  velocity  of  the 
blood-flow  increases,  though  it  never  attains  the  velocity,  even  in  the  venae 
cavae,  that  it  had  in  the  aorta,  for  the  reason  that  the  sectional  area  of  the 
venae  cavas  is  considerably  larger  than  that  of  the  aorta.  The  pressure  also 
is  very  low  in  the  larger  veins  because  the  friction  still  to  be  overcome  is 
relatively  very  slight. 


Fig.  174. — Diagram  to 
SHOW  Various  Stages  in 

THE  diapedesis  OR  MI- 
GRATION OF  White  Cor- 
puscles. 


364  TEXT-BOOK  OF  PHYSIOLOGY. 

The  capacity  of  the  venous  system  is  considerably  greater  than  that  of 
the  arterial  system,  as  there  are  usually  two  and  even  three  veins  accom- 
panying each  artery.  This,  taken  in  connection  with  its  greater  disten- 
sibility,  makes  of  the  venous  system  a  reservoir  in  which  blood  can  be  stored. 
On  this  reservoir  the  arterial  system  can  call  for  that  amount  of  blood 
necessary  for  the  maintenance  of  its  normal  volume  and  pressure,  and  into 
it  any  excess  can  be  discharged.  The  relative  amounts  of  blood  contained 
in  the  two  systems  are  regulated  by  the  degree  of  contraction  of  the  arteriole 
muscles  and  this  in  turn  by  the  vaso-motor  nerves.  The  movement  of  the 
blood  through  the  veins  is  accomplished  by  the  cooperation  of  several 
forces,  reference  to  which  will  be  made  in  a  following  paragraph. 

THE  PULMONIC  VASCULAR  APPARATUS. 

The  pulmonic  vascular  apparatus  consists  of  a  closed  system  of 
vessels  extending  from  the  right  ventricle  to  the  left  auricle,  and  includes 
the  pulmonary  artery,  capillaries,  and  pulmonary  veins.  In  its  anatomic 
structure  and  physiologic  properties  it  closely  resembles,  with,  the  systemic 
apparatus. 

The  stream-bed  widens  from  the  beginning  of  the  pulmonary  artery  to 
the  middle  of  the  capillary  system;  it  again  narrows  from  this  point  to  the 
terminations  of  the  pulmonary  veins. 

The  movement  of  the  blood  from  the  beginning  to  the  end  of  the  system 
is  due  to  a  difference  of  pressure  between  these  two  points,  the  result  of  the 
friction  between  the  blood  and  the  vascular  walls.  The  pressure  in  the  pul- 
monary artery  of  the  dog  has  been  shown  by  Beutner  to  be  about  one-third 
that  in  the  aorta;  by  Bradford  and  Dean  to  be  one -fifth.  Wiggers  has 
recently  shown  that  in  normally  breathing  dogs  with  arterial  pressures  rang- 
ing from  no  to  112  mm.  of  mercury,  the  maximal  or  systolic  pressure  in 
the  pulmonary  artery  averaged  36  mm.,  and  the  minimal  or  diastolic 
averaged  5  mm.  The  reason  for  the  low  pressure  may  be  found  in  the 
large  size  and  rich  development  of  the  pulmonary  capillaries  and  the 
imperfect  development  of  an  arteriole  muscle  at  the  periphery  of  the 
pulmonary  artery,  the  result  of  which  is  a  diminution  in  the  friction. 
Inasmuch  as  the  friction  is  relatively  low,  the  work  of  the  right  heart  is 
less  than  that  of  the  left  heart  and  hence  its  walls  are  not  so  well  developed. 
The  pulmonary  pressure  being  low  the  intraventricular  pressure  of  the 
right  heart  is  relatively  low  as  compared  with  that  of  the  left  heart.  The 
velocity  of  the  blood-stream  in  each  of  the  three  divisions  of  the  system 
can  not  well  be  determined.  The  time  occupied  by  a  particle  of  blood  in 
passing  from  the  right  to  the  left  ventricle  has  been  estimated  at  one- 
fourth  the  time  required  to  pass  from  the  left  to  the  right  ventricle.  As- 
suming the  latter  to  be  thirty  seconds,  the  former  would  be  seven  and 
one-half  seconds. 

The  capillary  vessels  are  spread  out  in  a  very  elaborate  manner  just 
beneath  the  inner  surface  of  the  pulmonary  air-cells,  and  form,  by  their 
close  relation  to  it,  a  mechanism  for  the  excretion  of  carbon  dioxid  and  the 
absorption  of  oxygen.  The  extent  of  the  capillary  surface  is  very  great. 
It  has  been  estimated  at  200  square  meters.     The  amount  of  blood  flowing 


THE  CIRCULATION  OF  THE  BLOOD.  365 

through  this  system  hourly  and  exposed  to  the  respiratory  surface  is  about 
800  Hters.  The  reason  for  the  existence  of  the  pulmonary  circulation  is 
the  renewal  of  the  oxygen  in  the  blood  and  the  elimination  of  the  carbon 
dioxid;  for  the  accomplishment  of  both  objects  ample  provision  is  here 
made.  The  flow  of  blood  through  the  cardio-pulmonary  vessels  is  subject 
to  variation  during  both  inspiration  and  expiration  in  consequence  of  their 
relation  to  the  respiratory  apparatus.  The  mechanism  by  which  these 
variations  are  produced  will  be  considered  in  the  chapter  devoted  to 
Respiration. 

FORCES  CONCERNED  IN  THE  CIRCULATION  OF  "THE  BLCOD. 

1.  The    Contraction   of  the   Heart. — The   primary    forces   which   keep 

the  blood  flowing  from  the  beginning  of  the  aorta  to  the  right  side  of  the 
heart  and  from  the  beginning  of  the  pulmonary  artery  to  the  left  side  are 
the  contractions  of  the  left  and  right  ventricles  respectively.  This 
is  evident  from  the  fact  that  each  ventricle  at  each  contraction  not 
only  overcomes  the  pressure  in  the  aorta  and  pulmonary  artery,  the 
sum  of  all  resistances,  but  imparts  a  given  velocity  to  the  blood.  Since 
the  pressure  continuously  falls  from  the  beginning  to  the  end  of  each 
system,  it  follows  that  the  blood  must  flow  from  the  point  of  high  to 
the  point  of  low  pressure.  During  the  interval  of  the  heart's  activity, 
the  walls  of  the  arteries,  to  which  the  heart's  energy  was  largely  trans- 
ferred, now  take  up  and  continue  the  work  of  the  heart,  and  by  recoil- 
ing drive  the  blood  forward  and  into  the  venous  system.  Though 
the  heart's  energy  is  probably  sufficient  to  drive  the  blood  into  the 
opposite  side  of  the  heart,  it  is  supplemented  by  other  forces — e.g.: 

2.  Muscle  Contraction. — As  a  result  of  the  relation  which  the  veins  bear 

to  the  muscles  in  all  parts  of  the  body  it  is  clear  that  with  the  contrac- 
tion and  relaxation  of  the  muscles  there  will  be  exerted  an  intermittent 
pressure  on  the  veins.  With  each  contraction,  the  blood  on  the  prox- 
imal side  will  at  once  be  driven  forward  with  increased  velocity,  while 
that  on  the  distal  side  will  be  retarded,  will  accumulate  and  distend 
the  veins,  owing  to  the  closure  of  the  valves;  with  the  relaxation  of  the 
muscle  the  elastic  and  contractile  tissues  in  the  walls  of  the  veins  will 
come  into  play  and  force  the  blood  forward. 

3.  Thoracic  Aspiration. — The   inspiratory   movement   aids   the   flow   of 

blood  through  the  venae  cavae  and  their  tributaries.  With  each  inspira- 
tion the  pressure  within  the  thorax  but  outside  the  lungs  undergoes  a 
diminution  more  or  less  pronounced  in  accordance  with  the  extent  of 
the  movement.  As  a  result,  the  blood  in  the  large  veins  outside  of  the 
thorax,  being  subjected  to  a  pressure  greater  than  that  in  the  thorax, 
flows  mxore  rapidly  toward  the  heart.  With  each  expiration  the  re- 
verse obtains. 

4.  Action  of  the  Valves. — It  is  quite  probable  that  gravity  opposes  to 

some  extent  the  flow  of  blood  through  the  veins  below  the  level  of  the 
heart.  This  opposition  to  the  upward  flow  is  largely  prevented  by 
the  valves,  for  each  retardation  is  imrrtediately  checked  by  their  closure 
and  support  given  to  the  column  of  blood.     The  influence  of  gravity 


366  TEXT-BOOK  OF  PHYSIOLOGY. 

is  shown  when  the  relation  of  the  arm  to  the  heart  is  changed.     Thus, 
if  the  arm  be  allowed  to  hang  passively  by  the  side  of  the  body,  the 
veins,  as  seen  on  the  back  of  the  hand,  will  become  distended  with 
blood.     If  now  the  arm  be  raised,  the  blood  will  flow  rapidly  toward 
the  heart,  as  shown  by  the  rapid  emptying  of  the  veins. 
Work  Done  by  the  heart.^ — ^The  work  which  the  left   ventricle  per- 
forms at  each  contraction  when  it  discharges  its  contained  volume  of  blood 
into  the  aorta  is: 

1.  To  overcome  the  total  resistance  of  the  systemic  vascular  apparatus. 

2.  To  impart  velocity  to  the  blood. 

The  resistance  may  be  expressed  in  terms  of  aortic  pressure.  The 
pressure  in  the  aorta  has  not  been  absolutely  determined,  though  for  many 
reasons  it  may  be  assumed  to  be  about  150  mm.  Hg.,  or  its  equivalent,  a 
column  of  blood  1.93  meters  in  height.  If  the  volume  of  blood  which  the 
heart  discharges  is  assumed  to  be  8t^  grams,  the  work  done  may  be  calculated 
by  multiplying  the  weight  by  the  height  to  which  it  is  raised:  viz., 
0.083  X  1.93  =  0.16019  kilogrammeter. 

The  velocity  of  the  blood  in  the  aorta  has  been  approximately  estimated 
at  0.5  meter  per  second.  The  work  done  in  imparting  this  velocity  to  83 
grams  is  estimated  by  squaring  the  velocity  and  dividing  by  the  accelerating 
force  of  gravity  (^^^i)  ^^^  multiplying  the  quotient  by  0.083.  The 
value  of  the  fraction  given  above  represents  the  distance  a  body  would  have 
to  fall  to  acquire  this  velocity:  viz.,  0.0127  meter.  The  work  done  is  there- 
fore 0.083X0.0127,  or  0.01054  kilogrammeter. 

The  entire  work  of  the  left  ventricle  is  the  sum  of  these  two  amounts, 
or  0.17073  kilogrammeter.  Assuming  that  the  heart  beats  72  times  per 
minute,  the  work  done  daily  would  be  0.17073X72X60X24,  or  1 7701.3 
kilogrammeters.  The  right  ventricle  approximately  performs  one-third  of 
this  amount  of  work  in  overcoming  the  resistance  offered  by  the  pulmonary 
system  and  in  imparting  velocity  to  its  contained  volume  of  blood.  The 
work  of  the  entire  heart  would  therefore  be  for  the  twenty-four  hours  about 
23,600  kilogrammeters. 

THE  NERVE  MECHANISM  OF  THE  VASCULAR  APPARATUS. 

By  this  expression  is  meant  a  combination  of  nerves  and  nerve-centers 
by  which  the  rate  and  force  of  the  heart  contractions  and  the  contraction 
of  the  arteriole  muscles  are  maintained.  It  includes  the  cardiac  nerves 
(the  cardio-accelerator  and  the  cardio-inhibitor)  and  the  vascular  or  vaso- 
motor nerv^es  (the  vaso-augmentor  or  constrictor  and  the  vaso-inhibitor  or 
dilatator  nerves).  The  function  of  this  mechanism  is  to  maintain  the 
high  blood-pressure  characteristic  of  the  arterial  system,  and  to  regulate 
from  moment  to  moment,  the  quantity  of  blood  flowing  into  and  out  of  organs 
in  accordance  with  their  functional  activities.  The  cardiac  nerves  have 
been  considered  on  pages  310-31 1. 

Arterial  Tonus. — The  arteries,  especially  those  in  the  peripheral  region 
of  the  arterial  system,  possess  a  well-defined  layer  of  non-striated  muscle- 
fibers  arranged  in  a  circular  direction  or  at  right  angles  to  the  long  axis  of 
the  vessel.     In   the  physiologic   condition  these  arterioles  are  distended 


THE  CIRCULATION  OF  THE  BLOOD.  367 

beyond  the  natural  condition  by  the  side  pressure  of  the  blood  flowing 
through  them,  at  the  same  time  the  muscle  fibers  are  in  a  state  of  continuous 
contraction,  more  or  less  pronounced,  and  give  to  the  arteries  a  certain 
average  caliber  which  permits  a  definite  volume  of  blood  to  flow  through 
them  in  a  given  unit  of  time.  To  this  condition  of  the  arterial  wall  the 
term  tonus  is  applied. 

The  cause  of  this  tonic  contraction  is  not  definitely  known.  It  has 
been  attributed  to  the  action  of  local  nerve-ganglia,  to  the  pressure  of  blood 
from  within,  to  the  influence  of  organic  substances  in  the  blood,  the  prod- 
ucts of  gland  activity:  e.g.,  adrenalin  or  epinephrin. 

This  tonic  contraction  of  the  vascular  muscle  is  subject  to  increase  or 
decrease,  augmentation  or  inhibition,  in  accordance  with  the  action  of  various 
agents.  An  augmentation  of  the  contraction  will  result  in  a  decrease  of 
the  caliber  and  a  reduction  in  the  outflow  of  blood.  An  inhibition  of  the 
contraction  or  relaxation  will  result  in  an  increase  both  of  the  caliber  and 
outflow  of  blood.  The  small  arteries  thus  determine  the  volume  of  blood 
passing  to  any  given  area  or  organ  in  accordance  with  its  functional 
activities. 

The  Vaso-motor  Nerves. — The  activities  of  the  vascular  muscle  are 
regulated  by  the  central  nerve  system  through  the  intermediation  of  nerve- 
fibers,  termed  vaso-motor  nerves.  Of  these  there  are  two  kinds,  one  which 
increases  or  augments  the  contraction,  the  vaso-constrictors  or  vaso-aug- 
mentors;  and  another  which  decreases  or  inhibits  the  contraction,  the  vaso- 
dilatators or  v.aso-inJiibitors. 

The  vaso-motor  nerves  of  both  classes,  unlike  the  ordinary  motor  nerv^es, 
do  not  pass  directly  to  the  muscle-fiber,  but  indirectly  by  way  of  the  ganglia 
of  the  sympathetic  nerve  system.  In  these  ganglia  the  vaso-motor  nerves, 
which  come  from  the  central  nerve  system,  terminate,  breaking  up  into 
tufts,  which  arborize  around  the  nerve-cells.  From  the  cells  of  these  gang- 
lia new  nerve-fibers  arise  which  then  pass  without  interruption  to  their 
final  destination. 

The  nerve-fibers  W'hich  emerge  from  the  central  nerve  system  are  ex- 
tremely fine  in  caliber  and  medullated;  those  w^hich  emerge  from  the  sym- 
pathetic ganglia  are  equally  fine,  but  non-medullated.  The  former  are  termed 
pre-ganglionic  or  autonomic,  the  latter  post- ganglionic  or  sympathetic  fibers. 
The  ganglion  in  which  the  pre-gangUonic  fibers  end  is  not  necessarily  found  in 
the  pre- vertebral  or  lateral  chain;  it  may  be  found  in  the  collateral  or  even  in 
the  peripheral  group  of  ganglia.     (See  sympathetic  or  autonomic  nerve  system.) 

The  Vaso-constrictor  Nerves. — The  vaso-cmistrictor  nerves  take  their 
origin  from  nerve-cells  located  in  the  anterior  horns  and  lateral  gray  matter 
of  the  spinal  cord.  They  emerge  from  the  cord  in  company  with  the  fibers 
that  compose  the  ventral  roots  of  the  spinal  nerv^es  from  the  second  or  third 
lumbar  nerves  inclusive.  A  short  distance  from  the  cord  they  leave  the  ventral 
root  as  the  white  rami  communicantes  and  enter  the  pre-vertebral  or  lateral 
sympathetic  ganglia.  From  the  results  of  many  observations  and  experi- 
ments it  is  probable  that  the  great  majority  of  the  vaso-constrictor  nerves 
terminate  in  these  ganglia;  that  is  to  say,  it  is  here  that  the  pre-ganglionic 
fibers  arborize  around  the  contained  nerve-cells.  From  the  nerve-cells 
new  fibers  arise,  the  post-ganglionic,  which  pass  to  the  blood-vessels  of  the 


368  TEXT-BOOK  OF  PHYSIOLOGY. 

skin  of  the  head  and  face,  to  the  blood-vessels  of  the  skin  of  the  upper  and 
lower  extremities  and  trunk  and  to  the  blood-vessels  of  the  thoracic  and 
abdominal  viscera. 

The  vasoconstrictors  for  the  head  and  face  emerge  from  the  spinal 
cord  in  the  first  four  thoracic  nerves,  thence  pass  successively  by  way  of  the 
white  rami  communicantes  into  and  through  the  ganglion  stellatum  (the 
first  thoracic),  the  annulus  of  Vieussens,  the  inferior  cervical  ganglion,  the 
sympathetic  cord  to  the  superior  cervical  ganglion,  around  the  cells  of  which 
they  arborize.  From  this  ganglion  the  new  fibers  follow  the  carotid  artery 
and  its  branches  to  their  terminations. 

The  vaso-constrictors  for  the  fore-limbs  emerge  from  the  cord  in  the  roots 
of  the  fourth  to  the  tenth  thoracic  nerves  inclusive.  Through  the  white 
rami  they  pass  into  the  sympathetic  chain,  after  which  they  take  an  upward 
direction  and  terminate  around  the  cells  of  the  ganglion  stellatum.  From 
this  ganglion  the  new  fibers  enter,  by  way  of  the  gray  rami  communicantes, 
the  trunks  of  the  cervical  nerves  which  unite  to  form  the  brachial  plexus 
and  by  this  route  pass  to  the  blood-vessels  of  the  skin  and  possibly  of  the 
muscles  of  the  fore-limb. 

The  vaso-constrictors  for  the  hind-limbs  emerge  from  the  cord  in  the 
roots  of  the  eleventh  dorsal  to  the  second  or  third  lumbar  nerves  inclusive. 
They  then  pass  through  the  white  rami  to  the  lower  lumbar  and  upper 
sacral  ganglia.  Thence  by  way  of  the  gray  rami  they  pass  into  the  nerve- 
trunks  which  unite  to  form  the  sacral  nerves  and  by  this  route  pass  to  the 
blood-vessels  of  the  skin  and  possibly  the  muscles  of  the  hind  limb. 

The  vaso-constrictors  for  the  walls  of  the  trunk  of  the  body  emerge 
from  the  spinal  cord  between  the  second  thoracic  and  third  lumbar  nerves. 
They  then  pass  by  way  of  the  white  rami  into  the  thoracic  and  lumbar 
ganglia  around  the  cells  of  which  they  arborize.  From  these  ganglia  new 
fibers  arise  which  pass  by  way  of  the  gray  rami  into  the  thoracic  and  lum- 
bar nerves  and  directly  to  the  blood-vessels  of  the  skin. 

The  vaso-constrictors  for  the  viscera  of  the  abdominal  cavity  after 
emerging  from  the  spinal  cord  pass  by  vv^ay  of  the  branches  that  unite  to 
form  the  splanchnic  nerves  directly  into  the  collateral  ganglia,  the  semilunar, 
the  superior  mesenteric,  the  inferior  mesenteric,  and  the  sacral.  From 
these  ganglia  an  elaborate  network  of  non-medullated  fibers  passes  to  the 
blood-vessels  of  the  stomach,  intestines,  and  other  viscera.  The  great 
splanchnic  nerve  is  one  of  the  most  important  vaso-constrictor  trunks  of  the 
body,  on  account  of  the  large  vascular  area  it  controls. 

The  existence,  course,  distribution,  and  functions  of  the  vaso-constrictor 
nerves  have  been  determined  by  a  variety  of  methods,  physiologic  and  anat- 
omic. Stimulation  of  the  nen'e-trunks  under  appropriate  conditions 
gives  rise  to  a  contraction,  division  to  a  dilatation  of  the  blood-vessels. 
The  physiologic  continuity  of  the  pre-ganglionic  fibers  with  the  nerve-cells 
of  the  sympathetic  ganglia  has  been  shown  by  the  intra-vascular  injection 
or  the  local  application  of  nicotin.  This  agent,  as  shown  by  Langley,  has 
a  selective  action  on  the  arborizations  of  the  pre-ganglionic  fibers,  and  when 
given  in  sufficient  doses  suspends  their  conductivity;  hence  stimulation  of 
pre-ganglionic  fibers  is  without  effect,  though  stimulation  of  the  post- 
ganglionic fibers  is  followed  by  the  usual  contraction. 


THE  CIRCULATION  OF  THE  BLOOD.  369 

The  following  will  serve  as  illustrations  of  the  functions  of  vaso- 
constrictor nerA-es.  Division  of  the  great  splanchnic  is  followed  by  a  marked 
dilatation  of  the  blood-vessels  of  the  intestinal  tract  and  a  decided  fall  in 
blood-pressure;  stimulation  of  the  peripheral  end  by  their  contraction  and  a 
marked  rise  in  blood-pressure.  Division  of  the  cervical  cord  of  the  sym- 
pathetic is  followed  by  dilatation  of  the  blood-vessels  of  the  side  of  the  head; 
stimulation  of  the  peripheral  end  by  their  contraction. 

The  Vaso-dilatator  Nerves. — The  vaso-Jilatalor  nerves  have  their 
origin  for  the  most  part  as  generally  believed  in  nerve-cells  situated  in 
the  region  of  the  spinal  cord  included  between  the  origins  of  the  second 
dorsal  to  the  second  lumbar  nerves  inclusive,  though  they  are  not^confined 
to  this  region.  Some  vaso-dilatator  libers  have  their  origin  in  the  medulla 
oblongata,  others  in  the  sacral  region  of  the  spinal  cord. 

\'aso-dilatator  fibers  have  been  found  in  association  with  vaso-constrictor 
fibers  in  most  of  the  nerve  trunks  of  the  body  though  they  are  perhaps  not 
so  abundant.  Thus  the  results  of  experimentation  indicate  that  they  are 
present  in  the  cervical  portion  of  the  sympathetic  in  the  nerve  trunks  of 
the  upper  and  lower  limbs,  in  the  nerve  trunks  of  the  body  walls,  and  in  the 
splanchnics.  The  exact  path,  however,  through  which  they  pass  from  the 
spinal  cord  to  the  peripheral  ners'e  trunks  has  not  in  all  cases  been  positively 
determined.  The  cell  stations  also  in  which  the  pre-ganglionic  fibers 
terminate  have  not  in  all  cases  been  located.  There  are  reasons  for  the 
opinion,  however,  that  they  follow  somewhat  the  same  paths  as  the  vaso- 
constrictor libers. 

The  vaso-dilatator  libers  that  arise  in  the  medulla  oblongata  pass  out  in 
the  trunk  of  the  pars  intermedia  or  nerve  of  Wrisberg  and  in  the  trunk  of 
the  glosso-pharyngeal  ners-e.  Those  libers  which  are  contained  in  the  nerve 
of  Wrisberg,  enter,  after  a  short  course,  the  trunk  of  the  facial  nerve  and 
through  its  branches  the  great  petrosal  and  the  chorda  tympani,  are  ulti- 
mately distributed  as  pre-ganglionic  fibers  to  the  spheno-palatine  and  sub- 
maxillary and  sublingual  ganglia  respectively.  From  the  spheno-palatine 
ganglion  cells  post-ganglionic  fibers  are  distributed  to  the  blood-vessels  of  the 
mucous  membrane  of  the  nasal  chambers  posteriorly,  and  to  adjacent  regions. 
From  the  submaxillary  and  sublingual  ganglia  post-ganglionic  fibers  pass  to 
the  blood-vessels  of  the  submaxillary  and  sublingual  glands. 

The  vaso-dilatator  fibers  that  are  contained  in  the  glosso-pharyngeal 
nerve  pass  through  the  tympanic  plexus  by  way  of  Jacobson's  nerve  to  the 
otic  ganglion,  around  the  cells  of  which  their  end  branches  arborize;  from 
the  cells  of  this  ganglion  post-ganglionic  fibers  pass  to  the  walls  of  the 
blood-vessels  of  the  parotid  gland  and  of  the  cheek  and  gums. 

The  vaso-dilatators  that  emerge  from  the  sacral  region  of  the  spinal 
cord,  pass  by  way  of  the  second  and  third  sacral  nerves  as  preganglionic 
fibers  to  ganglia  situated  near  the  blood-vessels  supplying  in  both  sexes, 
the  organs  of  generation  and  adjacent  structures.  From  the  ganglia  post- 
ganglionic fibers  are  distributed  to  the  walls  of  these  blood-vessels. 

The  existence,  course,  distribution,  and  functions  of  the  vaso-dilatator 
fibers  have  been  determined  by  the  same  methods  employed  as  in  the  investi- 
gation of  the  vaso-constrictors.     Thus  division  and  appropriate  stimulation 
of  the  peripheral  end  of  the  cervical  sympathetic  will  be  followed  by  a  con- 
24 


370  TEXT-BOOK  OF  PHYSIOLOGY. 

gestive  dilatation  of  the  blood-vessels  of  the  upper  and  lower  lips,  gums, 
cheeks,  nasal  mucous  membrane,  and  corresponding  cutaneous  regions. 
Stimulation  of  peripheral  nerve  trunks,  providing  the  stimulation  is  not  too 
rapid  will  be  followed  by  a  dilatation  of  the  blood-vessels  and  an  increase  in  the 
volume  of  the  limb.  Stimulation  of  the  peripheral  end  of  the  divided 
chorda  tympani  nerve  is  at  once  followed  by  an  active  dilatation  of  the 
blood-vessels  of  the  submaxillary  gland.  The  inflow  of  blood  is  so  great  that 
the  gland  becomes  bright  red  in  color.  Its  tissues  being  unable  to  appropriate 
all  the  oxygen,  the  blood  emerges  in  the  veins  almost  arterial  in  character. 
Stimulation  of  the  peripheral  ends  of  the  divided  nervi  erigentes  is  followed 
by  similar  effects  in  the  blood-vessels  of  the  corpora  cavernosa.  Slow  stimu- 
lation, once  per  second,  of  the  peripheral  end  of  a  divided  sciatic  nerve  is 
followed  by  dilatation  of  the  blood-vessels  of  the  leg. 

From  these  and  many  other  facts  of  a  similar  character  it  is  probable 
that  the  blood-vessels  of  each  organ  are  under  the  control  of  two  antagonistic 
classes  of  nerve-fibers,  one  augmenting  the  degree  of  their  contraction, 
the  vaso-constrictors,  the  other  diminishing  it  through  inhibition,  the 
vaso-inhibitors.  Through  the  cooperative  antagonism  of  these  two  classes 
of  nervTs  the  caliber  of  the  blood-vessels  and  thereby  the  volume  of  the 
blood  is  accurately  adapted  to  the  needs  of  each  organ  both  during  rest 
and  during  activity.  It  is  also  to  the  alternate  activity  of  these  nerves  that 
the  variations  occurring  from  time  to  time  in  the  volume  of  organs  are  to  be 
attributed. 

A  general  vaso-dUatator  center  has  never  been  located  and  there  are 
many  reasons  for  thinking  that  such  a  center  has  no  anatomic  existence. 
There  are,  however,  special  or  local  vaso-dilatator  centers  in  the  medulla 
oblongata  and  in  various  regions  of  the  spinal  cord  especially  in  the  sacral 
region. 

Antidromic  Vaso-dilatator  Nerve-fibers.  Though  it  has  been  generally  be- 
lieved that  the  vaso-dilatator  nerves  for  the  blood  vessels  of  the  limbs  and  trunk 
arise  from  nerve  cells  in  the  ventral  horns  of  the  grey  matter;  that  they  pass  out- 
ward through  the  ventral  roots  of  the  thoracic  and  lumbar  nerves,  that  they 
belong  to  the  efferent  system  of  nerves,  yet  these  facts  have  never  been  positively 
determined.  While  this  may  be  the  correct  interpretation  doubt  has  been  thrown 
upon  it  by  the  investigations  of  Bayliss.  From  the  results  of  a  long  series  of  ex- 
periments this  investigator  concludes  that  special  vaso-dialtator  nerves  for  the 
regions  of  the  body  just  mentioned,  do  not  leave  the  spinal  cord  in  the  ventral 
roots;  that  the  vaso-dilatation  observed  on  stimulation  of  the  mixed  spinal  nerve 
is  due  to  the  presence  of  nerve  fibers  that  do  not  differ  from  the  ordinary 
afferent  or  sensor,  posterior  or  dorsal  root  fibers;  that  these  nerve  fibers  moreover 
have  their  origin  in  the  nerve  cells  of  the  ganglia  of  the  dorsal  roots.  From  the 
fact  that  they  transmit  nerve  impulses  to  blood  vessels  in  a  direction  contrary  to 
that  of  other  afferent  nerve  fibers,  the  term  antidromic  has  been  given  to  them. 
The  centers  from  which  they  arise  are  capable  apparently  of  being  aroused  to 
activity  by  impulses  transmitted  to  them  from  other  regions  of  the  body.  These 
statements  are  based  on  the  following  facts:  Stimulation  of  the  peripheral  ends 
of  the  divided  dorsal  roots  of  the  upper  thoracic  and  lumbo-sacral  nerves  gives 
rise  to  vascular  dilatation  in  the  upper  and  lower  limbs;  separation  from  the  cord 
is  not  followed  by  their  degeneration,  hence  they  are  not  efferent  nerves;  extirpa- 
tion of  the  ganglia  of  the  dorsal  roots  is,  however,  followed  by  their  degeneration, 
hence  their  trophic  centers  are  in  these  ganglia.     Whether  the  blood-vessels  of 


THE  CIRCULATION  OF  THE  BLOOD. 


371 


the  abdorminal  viscera  which  apparently  receive  vaso-dilatator  nerve  impulses 
are  supplied  by  nerves  having  the  foregoing  origin  and  action  is  a  subject  for 
further  investigation. 

Physiologic  Properties.— The  vaso-constrictors  and  the  vaso-dilatators 
differ  somewhat  in  their  physiologic  properties,  as  shown  by  the  results 
of  experiment.  Thus,  when  a  mixed  nerve,  i.e.,  one  containing  both  classes 
of  fibers — e.g.,  the  sciatic — is  stimulated  with  frequently  repeated  induced 
currents,  the  constrictor  effect  is  the  more  pronounced,  the  dilatator  effect 
being  wanting  or  prevented;  w'hen  stimulated  with  slowly  repeated  induced 
currents,  the  dilatator  effect  is  the  more  pronounced.  These  different  effects 
are  strikingly  shown  in  Fig.  175,  ^  and  B. 


A  B 

Fig.  175. — ^Plethvsmogr-ams  of  the  Hixd-leg  of  the  Cat  following  .Stimulation  of 
THE  Sciatic  Nerve.  In  .4  the  rate  of  stimulation  was  sixteen  per  second,  in  B  one  per  second 
for  fifteen  seconds. 

In  the  experiment  of  which  these  tracings  arc  the  result  the  leg  of  a  cat  was 
enclosed  in  a  plethysmograph  and  the  variations  in  volume  due  to  dilatation 
or  contraction  of  the  vessels,  following  stimulation  of  the  sciatic  nerve,  were 
recorded  by  means  of  tambour  and  lever  on  a  slowly  revolving  cylinder.  In 
A  the  fall  of  the  curve  indicates  a  diminution  of  volume,  from  contraction  of 
blood-vessels  following  a  rate  of  stimulation  of  the  sciatic  nerve  of  16  per 
second  for  fifteen  seconds.  In  B  the  rise  of  the  curve  indicates  an  increase 
in  volume  from  dilatation  of  the  vessels  following  a  rate  of  stimulation  of  i 
per  second  for  fifteen  seconds  (Bowditch  and  Warren).  With  different  rates 
of  stimulation  somewhat  different  results  are  obtained. 

After  di\'ision  of  a  mixed  nerve  the  vaso-constrictors  degenerate  and  lose 
their  influence  over  the  blood-vessel  in  from  four  to  five  days,  the  vaso-- 
dilatators  in  from  seven  to  ten  days,  as  sho^vn  by  the  response  to  electrical 
stimulation. 

When  a  nerve  is  cooled,  the  vaso-constrictors  lose  their  irritability  before 
the  vaso-dilatators. 

Vaso-motor  Constrictor  Centers. — The  nerve-cells  thoughout  the 
spinal  cord  from  which  the  vaso-constrictor  nerves  take  their  origin  may  be 
regarded  as  nerv'e-centers  which  through  their  related  nerv'e-fibers  cause  a 
varying  degree  of  contraction  of  the  arteriole  muscle.  In  how  far  these 
centers  are  independent  in  their  activity  it  is  difficult  to  state.  From  the 
results  of  experiments  that  have  been  made  with  a  view  of  isolating  these 
centers,  such  as  di\dsion  of  the  cord  at  different  levels,  it  is  fairly  well  proven 


372  TEXT-BOOK  OF  PHYSIOLOGY. 

that  they  respond  to  nerve  impulses  transmitted  to  them  from  dilTerent  regions 
of  the  body,  as  shown  by  the  contraction  of  blood-vessels.  This  is  especially 
true  of  lower  animals  such  as  the  frog  and  it  may  possibly  be  true  of 
mammals.  Though  it  is  probable  that  the  spinal  vaso-constrictor  cells 
possess  a  certain  degree  of  tonicity,  nevertheless  they  are  subordinate  in 
their  activity  to  and  dominated  by  a  group  of  nerve  cells  in  the  upper  part 
of  the  floor  of  the  fourth  ventricle  and  termed  for  this  reason  the  medullary 
bulbar  vasoconstrictor  center. 

Though  the  blood-pressure  falls  to  a  very  low  level  after  the  separa- 
tion of  the  medulla  from  the  spinal  cord,  the  animal,  if  properly  cared  for, 
may  survive  the  operation  and  live  for  a  considerable  time.  Under  these 
circumstances  the  arteries  gradually,  recover  their  former  degree  of  con- 
traction. This  is  accepted  as  evidence  that  the  nerve  cells  in  the  spinal 
cord  have  acquired  an  independent  activity,  or  developed  an  activity  that 
had  hitherto  been  dormant.  After  this,  these  nerve  centers  can  be  excited 
to  actix'ity  by  ner\e  impulses  transmitted  from  the  periphery. 

The  Medullary  or  Bulbar  Vaso-Constrictor  Centers. — The  existence  of 
such  a  dominating  center  has  been  determined  experimentally:  thus  if  a 
definite  region  of  the  medulla  oblongata  is  punctured  or  in  anyway  destroyed 
there  is  an  immediate  dilatation  of  the  blood-vessels  throughout  the  body 
and  a  fall  of  blood-pressure  below  one-half  or  one-third  of.  the  normal  value. 
This  region  has  a  width  of  one  and  a  half  millimeters  and  extends  longitudin- 
ally for  a  distance  of  four  or  nve  millimeters,  terminating  at  a  point  four 
millimeters  above  the  tip  of  the  calamus  scriptorius.  Because  of  the  effects 
that  follow  the  destruction  of  this  area  the  anatomic  existence  of  a  general 
vaso-constrictor  center  has  been  assumed. 

A  transection  of  the  medulla  above  the  upper  limit  of  this  area  is  without 
effect  on  the  blood-pressure.  A  similar  section  below  it,  however,  is  at  once 
followed  by  vascular  dilatation,  a  loss  of  vascular  tone,  and  a  general  fall  of 
blood-pressure.  Subsequent  stimulation  of  the  peripheral  end  of  the  divided 
medulla,  the  animal  being  curarized  and  artificial  respiration  maintained,  will 
give  rise  to  a  marked  contraction  of  the  blood-vessels  and  a  rise  of  blood- 
pressure  up  to  and  far  beyond  the  normal  value. 

If  the  experimental  lesion  is  limited  to  the  area  mentioned  in  the  foregoing 
paragraph,  the  vascular  dilatation  also  passes  away  after  a  time,  the  blood- 
vessels regain  their  normal  tone,  and  the  pressure  again  rises.  These  and 
the  foregoing  facts  indicate  that  there  is  in  the  gray  matter  beneath  the  floor 
of  the  fourth  ventricle  a  restricted  area  composed  of  nerve-cells,  which  main- 
tains through  efferent  nerve-fibers  the  tonus  of  the  blood-vessels  by  virtue 
of  its  dominating  influence  over  the  vaso-motor  centers  in  the  cord,  and 
which  is  therefore  to  be  regarded  as  the  general  vaso-motor  (constrictor) 
center.  The  vaso-motor  centers  throughout  the  cord  are  to  be  regarded  as 
subsidiary  centers.  The  nerve-fibers  which  transmit  the  regulative  nerve 
impulses  from  the  general  to  the  subsidiary  centers  are  to  be  found  in  the 
lateral  columns  of  the  spinal  cord. 

The  Tonic  Activity  of  the  General  Vaso-constrictor  Center. — 
Since  the  blood-vessels  maintain  a  more  or  less  constant  tone,  it  is  assumed 
that  the  vaso-motor  center  is  in  a  state  of  continuous  tonic  activity  or 
tonus,  and  as  a  result  continuously  discharging  nerve  impulses  through  vaso- 


THE  CIRCULATION  OF  THE  BLOOD.  373 

constrictor  nerves  to  the  blood-vessels.  The  causes  of  this  activity  or  tonicity 
have  been  difficult  to  formulate.  In  how  far  the  activity  of  the  center  is 
maintained  by  the  chemic  character  of  the  blood  and  lymph  by  which  it  is 
surrounded  and  in  how  far  by  a  continuous  inflow  of  nerve  impulses 
transmitted  from  all  regions  of  the  body  is  not  readily  determinable.  The 
following  facts  will  show  that  both  factors  are  probably  involved. 

Direct  Stimulation  of  the  Vaso-motor  Centers. — The  general  vaso- 
motor (constrictor)  center  at  least  is  markedly  influenced  by  the  quantity  and 
quality  of  blood  and  lymph  circulating  around  and  through  it.  If  the  blood- 
supply  to  the  medulla  and  associated  structures  be  diminished  by  compression 
of  the  carotid  arteries,  the  activity  of  the  center  is  at  once  increased,  as  shown 
by  increased  vascular  contraction  and  a  rise  of  pressure.  Restoration  of 
the  blood-supply  is  followed  by  a  return  of  the  center  to  its  normal  degree  of 
activity.  Increased  blood-supply,  as  in  cerebral  hyperemia,  is  attended  by  a 
fall  in  blood-pressure  indicating  a  decrease  in  the  activity  of  the  center.  A 
diminution  in  the  percentage  of  oxygen  or  an  increase  in  the  percentage 
of  COj  in  the  blood  will  increase  the  activity  of  the  center.  In  asphyxia 
especially,  the  center  is  extremely  excitable,  as  shown  by  a  rise  of  the  arterial 
pressure.  The  subsidiary  centers  in  the  spinal  cord  are  influenced  by 
corresponding  conditions. 

Reflex  Stimulation  of  the  Vaso-motor  Centers. — The  results  of 
experiment  make  it  certain  that  the  degree  of  vascular  contraction  maintained 
by  the  vasoconstrictor  centers  can  be  increased  or  decreased  by  nerve  im- 
pulses transmitted  to  the  cord  and  medulla  from  the  periphery  or  from 
the  brain.  The  effect  may  be  general,  or  local  and  confined  to  the  area 
from  which  the  impulses  arise.  The  following  experiments  may  be  cited 
as  illustrations: 

Stimulation  of  the  central  end  of  a  divided  posterior  root  of  a  spinal  nerve 
gives  rise  to  increased  vascular  contraction,  as  shown  by  the  rise  of  blood- 
pressure.  Stimulation  of  the  central  end  of  the_ divided  sciatic  will  give  rise 
to  opposite  results,  according  to  the  strength  of  the  stimulus,  weak  stimuli 
producing  dilatation,  strong  stimuli  producing  contraction  of  the  vessels. 
Stimulation  of  the  central  end  of  the  divided  vagus  gives  rise  to  dilatation  of 
the  vessels  of  the  lips,  cheeks,  and  nasal  and  palatal  mucous  membranes. 
Stimulation  of  the  tongue  is  followed  by  dilatation  of  the  vessels  of  the  submax- 
illary gland.  Stimulation  of  certain  branches  of  the  vagus  nerve  is  followed 
by  a  passive  dilatation  of  blood-vessels  and  a  marked  fall  of  pressure. 

A  satisfactory  explanation  of  these  different  results  is,  however,  wanting. 
By  some  investigators  it  is  believed  that  the  usual  variations  in  the  arteriole 
contraction  are  the  outcome  of  corresponding  variations  in  the  activity  of 
the  general  vaso-constrictor  center  the  result  of  nerve  impulses  coming 
through  afferent  nerves. 

The  preceding  statements  as  to  the  effects  on  the  degree  of  vascular  con- 
traction, and  hence  on  the  blood-pressure  which  follow  stimulation  of  differ- 
ent afferent  nerv^es,  has  lead  to  the  assumption  that  there  are  in  most  afferent 
nerves  two  classes  of  nerve-fibers,  though  perhaps  in  varying  proportions,  one 
of  which  when  in  activity  augments,  the  other  of  which  when  in  activity  inhibits 
the  activity  of  the  vaso-constrictor  center.  The  former  class  is  generally 
termed  pressor  or  excitator,  the  latter  depressor  or  inhibitor  fibers. 


374 


TEXT-BOOK  OF  PHYSIOLOGY. 


,  It  is  possible,  therefore,  that  under  physiologic  conditions,  physiologic 
stimuli  act  on  the  peripheral  terminations  of  either  the  one  or  the  other; 
according  as  they  do  will  the  center  be  augmented  or  inhibited  in  its  activity, 
and  followed  by  either  an  increase  or  a  decrease  in  the  degree  of  the  previous 
vascular  contraction. 

Again  it  may  be  assumed,  from  the  results  of  experimentation  on  afferent 
nerves,  that  the  physiologic  stimuli  may  act  simultaneously  on  the  periph- 
eral terminations  of  both  classes 
of  fibers  and  that  the  vaso-con- 
strictor  center  is  acted  on  by  the 
two  antagonistic  influences.  In 
this  assumption  the  resultant 
effect  on  the  blood-vessels,  viz., 
increased  or  decreased  contrac- 
tion, will  be  the  resultant  of  their 
action  on  the  vaso-constrictor 
center.  If  the  stimuli  act  pre- 
ponderantly on  the  depressor 
fibers  the  center  will  be  depres- 
sed and  the  vessels  will  dilate; 
if  they  act  preponderantly  on 
the  pressor  fibers  the  center  will 
be  stimulated  and  the  vessels 
will  contract. 

Inasmuch  as.  the  vascular 
dilatation  is  often  greater  than 
the  dilatation  which  follows  divi- 
sion of  the  vaso-constrictor  fibers 
themselves,  it  has  been  assumed 
by  some  that  the  general  vascular 
tonus,  as  well  as  its  variations 
from  time  to  time,  is  the  result- 
ant of  the  simultaneous  activity 
and  variations  in  activity  of  both 
vaso-constrictor  and  vaso-dilata- 
tor  centers;  that  in  the  afferent 
nerves  there  are  two  sets  of  fibers, 
one  of  which  when  stimulated 
augments  the  activity  of  the  vaso- 
constrictor center  and  inhibits 
the  activity  of  the  vaso-dilatator 
center;  the  other  of  which  aug- 


car.a. 
fnf.c.ff. 


Fig.  176. — Diagram  showing  the  Origin 
AND  Relation  of  the  Depressor  Nerve  in  the 
'Rabbit.  Depr.  n.,  depressor  nerve;  vag.  n.,  vagus 
nerve;  sup.  1.  n.,  superior  laryngeal  nerve;  inf.  e.g., 
inferior  cervical  ganglion;  sym.  n.,  sympathetic  nerve; 
car.  a.,  carotid  artery;  dig.  m.,  digastric  muscle; 
hyp.  n.,  hypoglossal  nerve;  sup.  c.  g.,  superior  cer- 
vical ganglion;  inf.  1.  n.,  inferior  laryngeal  nerve. 


ments  the  activity  of  the  vaso-dilatator  center  and  inhibits  the  activity  of 
the  vaso-constrictor  center.  The  result,  either  contraction  or  dilatation, 
which  follows  stimulation  of  their  peripheral  terminations  will  depend  on 
the  character  of  the  physiologic  stimulus. 

In  those  particular  instances  in  which  stimulation  of  the  peripheral 
terminations  of  afferent  nerves,  associated  with  the  nervi  erigentes  and 
chorda  tympani,  is  followed  by  active  dilatation  of  the  blood-vessels,  it  has 


THE  CIRCULATION  OF  THE  BLOOD.  375 

been  assumed  that  there  are  afferent  nerA'e-fibers  which  directly  stimulate 
or  augment  the  activity  of  a  special  vaso-dilatator  center  and  for  this  reason 
should  be  termed  "reflex  vaso-dilatator  nerA-es"  (Hunt). 

Tiie  Influence  of  Emotional  States. — The  vaso-constrictor  centers 
are  capable  of  being  influenced  in  their  activities  by  emotional  states, 
doubtless  as  a  result  of  the  arrival  of  ner\'e  impulses  from  the  cortex  of  the 
cerebrum.  Thus  it  is  well  known  that  fear  causes  a  contraction  of  the  blood- 
vessels of  the  head  and  face  and  that  shame  causes  a  dilatation  of  the  same 
vessels.  With  the  cessation  or  the  disappearance  of  the  emotional  state, 
the  blood-vessels  return  to  their  former  degree  of  contraction.  The  vaso- 
dilatator centers  in  the  medulla  and  in  the  sacral  region  of  the  spinal  cord  are 
influenced  in  a  similar  manner  by  emotional  states. 

The  Depressor  Nerve. — A  striking  illustration  of  the  depressor  or  in- 
hibitor action  of  afferent  nerves  upon  the  vaso-constrictor  center  is  furnished 
by  the  result  of  stimulation  of  a  branch  of  the  vagus,  the  so-called  "depressor 
nerve."  In  the  rabbit,  Fig.  176,  there  is  a  small  nerve  formed  by  the  union 
of  a  branch  from  the  trunk  of  the  vagus  with  a  branch  from  the  superior 


Fig.  177. — Fall  OF 'Blood-pressure  from  Excitatiox  of  the  Depressor  Nerve.  The 
cylnder^as  stopped  in  the  middle  of  the  curve  and  the  excitation  maintained  for  seventeen  min- 
utes.    The  line  of  zero  pressure  (0,0)  should  be  30  mm.  lower  than  here  shown. — (Bayliss.) 

laryngeal.'  The  peripheral  distribution  of  this  ner^'e  is  over  the  wall  of  the 
ventricle  and  perhaps  to  some  extent  to  the  structures  of  the  aorta  near  its 
origin.  A^ similar  anatomic  arrangement  is  met  with  in  the  horse,  pig,  and 
hedge-hog.  In  some  other  animals,  as  the  dog,  it  is  bound  up  in  the  vago- 
sympathetic. In  man  it  is  also  present,  though  shortly  after  its  origin  it 
enters  the  trunk  of  the  vagus.  Division  of  this  nen'e  is  without  effect 
on  either  the  heart  or  the  vessels.  Stimulation  of  the  peripheral  end  has 
neither  an  accelerator  nor  an  inhibitor  action  on  the  heart.  Stimulation  of 
the  central  end  is  followed  by  a  fall  in  blood-pressure,  frequently  to  a  level 
below  one-half  the  normal  value;  at  the  same  time  there  is  a  diminution, 
brought  about  reflexly,  in  the  rate  of  the  heart-beat  (Fig.  177).  The  fall  in 
pressure,  however,  is  not  due  to  this  cause,  for  it  occurs  equally  well  after 
division  of  all  the  cardiac  nerves.  For  this  reason  the  nerve  was  termed  the 
depressor  nerve  of  the  vaso  motor  center. 

On  exposure  of  the  abdominal  cavity,  it  is  observed  during  stimulation  of 
the  depressor  that  there  is  a  notable  dilatation  of  the  intestinal  vessels. 


376  TEXT-BOOK  OF  PHYSIOLOGY. 

From  this  fact  it  was  assumed  that  the  action  of  the  depressor  nerve  was  to 
lower  the  general  pressure  through  reflex  dilatation  of  these  vessels.  It  has 
been  shown  by  Porter  and  Beyer  that  if  the  splanchnics  are  divided  and  the 
peripheral  end  stimulated  so  as  to  maintain  the  tonus  of  the  intestinal  vessels, 
and  hence  the  general  pressure,  stimulation  of  the  depressor  ner\'e  will 
nevertheless  be  followed  by  a  fall  of  the  blood-pressure  almost  as  great  as 
when  the  splanchnics  are  intact.  From  this  it  is  evident  that  the  depressor 
nerve  is  related  to  centers  which  influence  the  vascular  apparatus  in  its 
entirety.  It  has  been  supposed  that  through  it  the  heart  can  protect  itself 
from  injurious  results  of  an  excessive  rise  of  arterial  pressure. 

Thus,  when  the  intra-cardiac  pressure  or  the  intra-aortic  pressure 
rises  beyond  a  normal  amount  from  increased  resistance,  the  peripheral 
terminations  of  this  nerve  are  stimulated  with  the  result  that  the  vaso-motor 
center  is  inhibited  and  the  arterioles  relaxed.  Through  this  means  the 
pressure  falls  and  the  work  of  the  heart  is  lessened. 


CHAPTER  XV. 

RESPIRATION. 

Respiration  is  a  process  by  which  oxygen  is  introduced  into,  and  carbon 
dioxid  removed  from,  the  body.  The  assimilation  of  the  former  and  the 
evolution  of  the  latter  take  place  in  the  tissues  as  a  part  of  the  general 
process  of  nutrition.  Without  a  constant  supply  of  oxygen  and  an  equally 
constant  removal  of  the  carbon  dioxid,  those  chemic  changes  which  under- 
lie and  condition  all  life  phenomena  could  not  be  maintained. 

The  general  process  of  respiration  may  be  considered  under  the  following 
headings,  viz.: 
I;  The  anatomy  and  general  arrangement  of  the  respiratory  apparatus. 

2.  The  mechanic  movements  of  the  thorax  by  which  an  interchange  of 

atmospheric  and  intra-pulmonary  air  is  accomplished. 

3.  The  chemistry  of  respiration,  the  changes  in  composition  undergone  by 

the  air,  blood,  and  tissues. 

4.  The  nerve  mechanism  by  which  the  respiratory  movements  are  main- 

tained. 

THE  RESPIRATORY  APPARATUS. 

The  respiratory  apparatus  consists  essentially  of: 

1.  The  lungs  and  the  air-passages  leading  into  them :  viz. ,  the  nasal  chambers, 

mouth,  pharynx,  larynx,  and  trachea. 

2.  The  thorax  and  its  associated  structures. 

The  nasal  chambers  are  the  natural  entrances  for  the  inspired  air. 
Their  complicated  structure  slightly  retards  the  movement  of  the  air,  in 
consequence  of  which  its  temperature  and  moisture  are  adjusted  to  the 
physiologic  conditions  for  the  lower  respiratory  passages.  The  mouth, 
though  frequently  serving  as  an  entrance  for  air,  is  not  primarily  a  respira- 
tory passage.  Both  the  nasal  chambers  and  the  mouth  communicate 
posteriorly  with  the  pharynx,  in  which  the  respiratory  and  the  deglutitory 
passages  cross  each  other,  the  former  leading  directly  into  the  larynx. 

The  larynx  is  a  complicated  mechanism  serving  the  widely  different 
though  related  functions  of  respiration  and  phonation.  It  consists  of  a 
framework  of  cartilages,  articulating  one  with  another,  united  by  ligaments 
and  moved  by  muscles;  it  is  covered  externally  wdth  fibrous  tissue  and  lined 
with  mucous  membrane.  The  superior  opening  of  the  larynx,  the  glottis, 
is  triangular  in  shape,  the  base  being  directed  upward  and  forward,  the 
apex  downward  and  backward.  The  inclination  of  the  glottic  opening  is 
almost  vertical. 

The  cavity  of  the  larynx  is  partially  subdivided  by  the  interposition  of 
the  vocal  bands  into  a  superior  and  an  inferior  portion.  The  opening, 
bounded  by  the  vocal  bands,  is  also  triangular  in  shape,  though  in  this  case 

377 


378 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  base  is  directed  backward,  and  the  apex  forward.   (See  chapter  on  Voice 
and  Speech.) 

The  introduction  of  the  vocal  bands  narrows  at  this  level  the  air-passage 
and  to  some  extent  interferes  with  the  free  entrance  of  air.  According  to 
the  investigations  of  Semon,  the  area  of  the  air-passage  above  and  below 
the  phonatory  apparatus  is  about  200  sq.  mm.;  while  the  area  bounded 
by  the  vocal  apparatus  is  but  155  sq.  mm.  during  quiet  respiration. 


Fig.  178. — Trachea  and  Bronchial  Tubes,  i,  2,  Larynx.  3,  3.  trachea.  4.  Bifurcation 
of  trachea.  5.  Right  bronchus.  6.  Left  bronchus.  7.  Bronchial  division  to  upper  lobe  of  right 
lung.  8.  Division  to  middle  lobe.  9.  Division  to  lower  lobe.  10.  Division  to  upper  lobe  of  left 
lung.  II.  Division  to  lower  lobe.  12,  12,  12,  12.  Ultimate  ramifications  of  bronchi.  13,13,13, 
13.  Lungs,  represented  in  contour.     14,  14.  Summit  of  lungs.     15,  15.  Base  of  lungs. — (Sappey.) 

The  trachea  is  a  tube,  some  12  centimeters  in  length,  from  one-half  to 
three-fourths  of  a  centimeter  in  breadth,  extending  from  the  lower  border 
of  the  larynx  to  a  point  opposite  the  fifth  dorsal  vertebra.  It  consists  of 
an  external  fibrous  and  an  internal  mucous  membrane,  between  which  is 
a  series  of  superposed  C-shaped  arches  or  rings  of  elastic  cartilage,  some 
18  or  20  in  number.  Between  the  fibrous  and  mucous  coats  posteriorly, 
and  occupying  the  space  between  and  attached  to  the  free  ends  of  the  car- 
tilages, there  is  a  layer  of  transversely  arranged  non-striated  muscle-fibers, 
known  as  the  tracheal  muscle.  The  alternate  contraction  and  relaxation  of 
this  muscle  would  by  varying  the  distance  between  the  ends  of  the  cartil- 
ages, either  diminish  or  increase  the  caliber  of  the  trachea.     The  surface  of 


RESPIRATION.  379 

the  mucous  membrane  is  covered  by  a  layer  of  stratified  columnar  ciliated 
epithelium  (Fig.  179).  In  the  submucous  tissue  there  are  a  number  of  glands 
the  ducts  of  which  open  on  the  free  surface. 

Opposite  the  fifth  dorsal  vertebra  the  trachea  divides  into  a  right  and  a 
left  bronchus.  Each  bronchus  again  subdivides  into  two  or  three  branches, 
which  penetrate  the  corresponding  lung. 

The  lungs,  in  the  physiologic  condition,  occupy  the  greater  part  of  the 
cavity  of  the  thorax.  They  are  separated  from  each  other  by  the  contents 
of  the  mediastinal  space:  viz.,  the  heart,  the  large  blood-vessels,  the  esoph- 
agus, etc.  Each  lung  is  somewhat  pyramidal  in  shape  with  the  apex 
directed  upward.  The  outer  surface  is  convex  and  corresponds  to  the 
general  conformation  of  the  thorax.  The  inner  surface  is  concave  and  accom- 
modates the  contents  of  the  mediastinal  space.  The  under  surface,  of  the 
lung  is  concave  and  rests  on  the  diaphragm.  The  posterior  border  is  con- 
vex; the  anterior  border  is  thin.  At 
about  the  middle  of  the  inner  surface  of 
the  lung  the  blood-vessels  which  connect 
the  heart  with  the  interior  of  the  lung 
enter  and  leave  in  company  with  the 
branches  of  the  bronchi,  bronchial  arter- 
ies, veins,  nerves,  and  lymphatics. 

A  histologic  analysis  of  the  lung  shows 
it  to  consist  of  the  branches  of  the 
bronchi,  their  subdivisions  and  ultimate 
terminations,  blood-vessels,  lymphatics 
and  nerves,   imbedded  in  a  stroma  of 

fibrous  and  elastic  tissue.     The  anatomic 

,    ^.  1  •  -L  i.1  4.        J.  r.  Fig.   I -I,       I      '.-'     1  SE  Section  of 

relations  which  these  structures  bear  one  ^he  Trachea  of  a  KijTEN.-{Stiriing.) 
to  another  is  as  follows: — • 

Within  the  substance  of  the  lung  the  bronchi  divide  and  subdivide, 
giving  origin  to  a  large  number  of  smaller  branches,  the  bronchial  tubes, 
which  penetrate  the  lung  in  all  directions.  With  this  repeated  subdivision 
the  tubes  become  narrower,  their  walls  thinner,  their  structure  simpler. 
In  passing  from  the  larger  to  the  smaller  tubes  the  cartilaginous  arches 
become  shorter  and  thinner,  and  finally  are  represented  by  small  angular 
and  irregularly  disposed  plates.  In  the  smallest  tubes  the  cartilage  entirely 
disappears.  With  the  diminution  of  the  caliber  of  the  tube  and  a  decrease 
in  the  thickness  of  its  walls,  there  appears  a  layer  of  non-striated  muscle- 
fibers,  the  so-called  bronchial  muscle,  between  the  mucous  and  submucous 
tissues,  which  completely  surrounds  the  tube  and  becomes  especially 
well  developed  in  those  tubes  devoid  of  cartilage.  The  fibrous  and  mucous 
coats  at  the  same  time  diminish  in  thickness. 

Bronchial  Innervation. — -The  bronchial  muscles  are  presumably  in  a 
state  of  tonic  contraction  and  impart  to  the  bronchial  tubes  a  certain  average 
caliber  best  adapted  for  respiratory  purposes.  Experimental  investigations 
indicate  that  they  are  innerv-ated  by  efferent  fibers  of  the  vagus  nerve  (broncho- 
constrictors  and  possibly  broncho-dilatators)  inasmuch  as  stimulation  of  this 
nerve  is  usually  followed  by  a  contraction  of  the  muscles  and  a  narrowing  of 
the  lumen  of  the  bronchial  system.     These  muscles  may  also  be  thrown  into 


38o 


TEXT-BOOK  OF  PHYSIOLOGY. 


increased  activity  by  the  inhalation  of  irritating  gases  and  into  a  tetanus  by 
pathologic  causes  as  seen  in  the  various  forms  of  asthma. 

When  the  bronchial  tube  has  been  reduced  to  the  diameter  of  about  one 


Bronchiole 


Infundibulum 


Air-cell. 

Fig.  i8o. — Scheme  of  a  Bronchiole  Ter- 
minating IN  Alveolar  Passages,  those  Leading 
into  Infundibula  beset  with  Air-cells. — 
{Landois  and  Stirling.) 


Fig.  i8i. — Single  Lob- 
ule OF  Human  Lung.  a. 
Alveolar  passage,  b.  Cav- 
ity of  lobule  or  infundib- 
ulum. c.  Pulmonarj'  sacs. 
— (Dalton.) 


millimeter,  it  is  known  as  a  bronchiole  or  a  terminal  bronchus.  From  the 
sides  of  the  terminal  bronchus  and  from  its  final  termination  there  is  given 
ofi  a  series  of  short  branches  which  soon  expand  to  form  lobules  or  alveoli 

(Fig.  1 80).  The  cavity  of  the 
alveolus  is  termed  the  infundib- 
ulum. From  the  inner  sur- 
face of  the  alveolus  and  of  the 
passageway  leading  into  it,  there 
project  thin  partitions  which 
subdivide  the  outer  portion  of 
the  general  cavity  or  infundib- 
ulum into  small  spaces,  the  so- 
called  air-sacs  or  air-ctlls  (Fig. 
181).  The  wall  of  the  alveolus 
is  extremely  thin  and  consists  of 
fibro-elastic  tissue,  supporting  a 
very  elaborate  capillary  network 
Fig.  182.— Section  of  Silvered  Lung  of  Kitten,  of  blood-vessels.  The  bron- 
INCLUDING  Portions  of  Infundibulum  and  Ai^^^^  chial  SVStem  as  far  as  the  alveo- 
SAC     a.  Small    polvhedral    epithelial    cells    covering  ^     -  •     i-       j  i  -i-        j 

the  wall  of  the  infundibulum.  b.  Fibro-elastic  lar  passages  IS  Imed  by  Ciliated 
framework,     c.  Large  flattened  epithelial  plates  lining    epithelium.      The    air-SaCS    are 

tl"-(p'i!^soT)  ''^''^  ^''  '"""^^  ^''°"^'  °^  '""'^^  ''"'   lined  by  fiat  epithelial  plates  of 

irregular  shape,  termed  the  res- 
piratory epithelium  (Fig.  182).  The  alveoli  are  united  one  to  another  by 
fibro-elastic  tissue. 

The  bronchial  arteries  which  supply  nutritive  material  to  the  pulmonary 


RESPIRATION.  381 

structures  arise  from  the  aorta  as  a  rule,  though  sometimes  from  an  inter- 
costal artery.  Each  lung  receives  two  arteries  which  accompany  the  bronchi 
as  far  as  the  distal  ends  of  the  alveolar  passages.  From  the  capillary  net- 
work formed  out  of  the  terminals  of  these  arteries,  two  systems  of  veins  arise, 
one  of  which  returns  the  blood  from  the  larger  tubes  and  empties  it  into  the 
azygos  vein;  the  other  of  which  returns  the  blood  from  the  smaller  tubes  and 
the  alveolar  passages,  and  empties  it  into  the  pulmonary  veins.  The  blood 
in  the  pulmonary  veins,  though  largely  arterialized,  nevertheless  contains 
some  venous  blood  derived  from  the  veins  arising  from  the  capillary  network 
of  the  bronchial  arterioles. 

The  nerv^es  distributed  to  the  muscle-fibers  of  the  bronchial  arteries,  and 
of  the  bronchial  tubes  and  to  the  mucous  membrane,  are  derived  from  the 
vagus  and  the  sympathetic  and  enter  the  substance  of  the  lung  at  and  around 
its  root. 

In  consequence  of  the  presence  of  the  elastic  tissue,  the  lungs  are  disten- 
sible and  elastic.  After  removal  from  the  body  the  elastic  tissue  at  once 
recoils,  forcing  out  a  portion  of  the  con-  a 

tained  air.     The  condition   of  the  lung  f^§/^ 

is  now  one  of  collapse.     Under  pressure,  p  v. %Bw-----pa 

however,   the   lung   can   be  readily  dis-  //^, 

tended    or    inflated.     These    properties  .-s^^ 7/4  xnN^^^^ 

endure   for  a   long  period  after  death,  ^^^T^/'^^^^  ^^^^ 

if  not  indefinitely,  if  the  lungs  are  prop-       ^^^^        ^^^^^T  W^ 

erly  preserx-ed.     The    capacity    of    the       ^^^        ^^   ^^  e!^® 

lungs  can  be  made  to  vary  within  rather  ^  a  «Jm  ^  ^  ^^fe 
wide  limits  in  virtue  of  the  presence  of      ^^         ^^ft       M--,^  ^ 

the  elastic  tissue.  ^^  ^P       ^^^         ^^ 

The    Pulmonary  Blood-vessels. —       ^^fe)  ^^  ^^  ^^^ 

The  pulmonary  artery  which  conducts  the  ^^^^^  ^^^^^ 

venous  blood  from  the  heart  to  the  lungs  ^^^  183.-THE  Relation  of  the 
divides  beneath  the  arch  of  the  aorta  into  pulmonary  Artery,  PA,  and  the 
a  right  and  a  left  branch.     Each  branch     Pulmonary  Vein,  PV,  to  the  Lobules, 

•  ■u    •*-  uj-    ■  •  4.         4.U     1  4.      A.\.     B.  The  Bronchiole. 

with  Its  subdivisions  enters  the  lung  at 

the  hilum  in  company  with  the  larger  di^'isions  of  the  bronchi.  Yvlthin 
the  lung  the  arteries  divide  and  subdivide  in  a  manner  corresponding 
to  that  of  the  bronchial  tubes,  which  they  follow  to  their  ultimate  ter- 
minations. As  the  pulmonary  lobules  are  approached,  a  small  arterial 
branch  plunges  into  the  wall  of  the  lobule  (Fig.  183),  in  which  it  forms 
an  elaborate  capillary  network  which  surrounds  and  embraces  the  air- 
sacs  on  all  sides.  As  this  network  is  to  subserve  the  respiratory  exchange 
of  gases  it  lies  nearer  the  inner  than  the  outer  surface  of  the  lobule  and  in 
close  relation  to  the  respiratory  epithelium.  The  air  and  blood  are  thus 
brought  into  intimate  relationship,  being  separated  only  by  the  respiratory 
epithelium  and  the  wall  of  the  capillary  vessel.  The  blood  emerging  from 
the  capillary  vessels  is  conducted  by  a  corresponding  converging  system  of 
vessels,  the  pulmonary  veins,  out  of  the  lungs  and  into  the  left  auricle  of  the 
heart.  The  main  function  of  the  pulmonary  apparatus  and  the  pulmonary 
division  of  the  circulatory  apparatus  is  to  afford  a  ready  means  for  the 
exhalation  of  the  carbon  dioxid  and  the  absorption  of  oxygen.     In  conse- 


382 


TEXT-BOOK  OF  PHYSIOLOGY. 


quence  of  this  exchange  of  gases  the  blood  changes  in  color  from  dark  bluish- 
red  to  scarlet  red.  The  relations  of  the  heart  and  its  vessels  to  the  lungs  and 
bronchial  tubes  are  shown  in  Fig.  184. 

The  Thorax. — The  thorax,  in  which  the  respiratory  organs  and  their 
associated  structures  are  lodged,  is  conic  in  shape,  though  somewhat  com- 
pressed from  before  backward.  Its  apex  is  directed  upward,  its  base  down- 
ward. The  walls  of  the  thorax  are  composed,  first,  of  a  bony  framework 
or  skeleton  and,  second,  of  muscles  and  fascia.     The  bony  framework  is 


Fig.  184. — Bronchi  and  Lungs,  Posterior  View,  i,  i.  Summit  of  lungs-  2,  2.  Base  of 
lungs.  3.  Trachea.  4.  Right  bronchus.  5.  Division  to  upper  lobe  of  lung.  9.  Division  to  lower  lobe. 
10.  Left  branch  of  pulmonary  artery,  ii.  Right  branch.  12.  Left  auricle  of  heart.  13.  Left 
superior  pulmonary  vein.  14.  Left  inferior  pulmonary  vein.  15.  Right  superior  pulmonary  vein. 
16.  Right  inferior  pulmonary  vein.  17.  Inferior  vena  cava.  18.  Left  ventricle  of  heart.  19. 
Right  ventricle. — (Sappey.) 


formed  posteriorly  by  the  thoracic  vertebrae  and  the  posterior  extremities  of 
the  ribs,  laterally  by  the  ribs,  and  anteriorly  by  the  costal  cartilages  and  the 
sternum.  The  superior  opening,  through  which  pass  the  trachea,  esophagus, 
and  blood-vessels,  is  oval  in  outline  and  measures  from  side  to  side  about 
12.5  cm.,  and  from  before  backward  about  6.25  cm.  The  inferior  opening 
is  of  large  size,  but  irregular  in  its  boundaries  from  the  upward  inclination  of 
the  ribs  and  the  dowmward  projection  of  the  sternum. 

The  ribs,  which  form  a  large  part  of  the  thoracic  walls,  constitute  a  series 
of  bony  arches  attached  posteriorly  to  the  vertebrae  and  anteriorly  to  the 
sternum  through  the  intermediation  of  their  cartilages.  The  last  two  form 
an  exception.  The  ribs  are  somewhat  twisted  upon  themselves  and  pursue 
an  oblique  direction  from  above  downward  and  forward.  As  a  result  the 
anterior  extremity  lies  at  a  lower  level  than  the  posterior.  The  costal 
cartilages  are  directed  upward  and  forward,  with  the  exception  of  the  upper 


RESPIRATION. 


383 


three,  which  are  almost  horizontal. 
The  general  arrangement  and  appearance 
of  the  thorax  are  shown  in  Fig.  185. 

The  costo-vertebral  and  costo-chon- 
dral  and  the  chondro-sternal  articulations 
are  diarthrodial  in  character  and  endow 
the  thoracic  walls  with  a  considerable 
degree  of  mobility.  The  costo-vertebral 
joints  are  two  in  number,  the  first  being 
formed  by  the  beveled  head  of  the  rib 
and  the  bodies  of  the  two  adjoining  ver- 
tebrae; the  second,  by  the  tubercle  of  the 
rib  and  the  transverse  process.  The 
costo-chondral  and  the  chondro-sternal 
articulations,  as  their  names  imply,  are 
formed  by  the  ribs,  cartilages,  and 
sternum. 

The  muscles  which  complete  the 
formation  of  the  thoracic  walls  are  as 
follows:  the  diaphragm,  the  intercostales 
externi  and  interni,  the  levatores  costa- 
rum,  the  triangularis  sterni,  and  the 
infra-costales. 

The  diaphragm  is  the  musculo-mem.- 
branous  sheet  which  closes  the  inferior 


Fig.  186. — Diaphragm,  Inferior  Aspect,  i.  Anterior 
and  middle  leaflet  of  central  tendon.  2.  Right  leaflet.  3. 
Left  leaflet.  4.  Right  crus.  5.  Left  cms.  6,  6.  Intervals 
for  phrenic  nerves.  7.  Muscular  fibers,  from  which  the 
ligamenta  arcuata  originate.  8.  Muscular  fibers  that  arise 
from  the  inner  surface  of  the  six  lower  ribs.  9.  Fibers 
that  arise  from  ensiform  cartilage.  10.  Opening  for  inferior 
vena  cava.  11.  Opening  for  esophagus.  12.  Aortic  open- 
ing. 13,  13.  Upper  portion  of  transversalis  abdominis, 
turned  upward  and  outward.  14.  Anterior  leaflet  of  trans- 
versalis aponeurosis.  15,  15.  Quadratus  lumborum.  16, 
16.  Psoas  magnus.     17.  Third  lumbar  vertebra. 


Fig.  1S5. — Thorax,  Anterior  View 
I.  ^Manubrium  sterni.  2.  Gladiolus.  3 
Ensiform  cartilage  of  xiphoid  appendix 
4.  Circumference  of  apex  of  thorax.  5 
Circumference  of  base.  6.  First  rib 
7.  Second  rib.  8,  8.  Third,  fourth 
fifth,  sixth,  and  seventh  ribs.  9.  Eighth 
ninth,  and  tenth  ribs.  10.  Eleventh  and 
twelfth  ribs.     11,   11.  Costal  cartilages. 


opening  of  the  thorax 
and  completely  sepa- 
rates its  cavity  from  that 
of  the  abdomen.  It  con- 
sists of  two  muscles 
which  arise  from  the 
bodies  of  the  first  three 
or  four  lumbar  verte- 
brae and  neighboring 
fascia,  from  the  border 
of  the  six  lower  ribs,  and 
from  the  ensiform  car- 
tilage (Fig.  186).  From 
this  extensive  origin  the 
muscle-fibers  pass  cen- 
trally to  be  inserted  into 
a  common  tendon.  As 
the  direction  of  the  fibers 
is  from  below  upward 
and  inward,  the  dia- 
phragm is  somewhat 
dome-shaped.  Its  infe- 
rior border  is  for  a  short 
distance  in  contact  with 
the  sides  of  the  thorax. 


584 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  intercostales  externi,  eleven  in  number  on  each  side,  occupy  the 
spaces  between  the  ribs  to  which  they  are  attached  from  the  tubercle  to  the 
.anterior  extremity  (Figs.  187  and  188).  Their  fibers,  which  are  arranged 
in  parallel  bundles,  are  directed  from  above  downward  and  from  behind 

forward.  The  point  of  attach- 
ment, therefore,  of  any  given 
bundle  of  fibers  to  the  rib 
above,  lies  nearer  the  vertebral 
column,  nearer  the  fulcrum, 
than  the  point  of  attachment 
below. 

The  intercostales  interni, 
eleven  in  number,  occupy  the 
spaces  between,  and  are  at- 
tached to  the  ribs  from  the 
tubercle  to  the  anterior  extrem- 
ity of  the  cartilages.  Their 
fibers,  which  are  also  arranged 
in  parallel  bundles,  are  direc- 
ted from  above  downward  and 
backward  (Figs.  187  and  188). 
The  portions  of  the  internal 
intercostals  between  the  carti- 
lages are  frequently  termed  in- 
tercartilaginei. 

The  levatores  costarum  are 
twelve  in  number  on  either  side. 
They  arise  from  the  tips  of  the 
transverse  processes  of  the  last 
cer^dcal  and  the  thoracic  verte- 
brae with  the  exception  of  the 
last.  From  the  point  of  origin 
the  fibers  pass  downward  and 
outward  in  a  diverging  manner 
to  be  inserted  into  the  ribs 
between  the  tubercle  and  the 
angle.  Their  action,  as  their 
name  implies,  is  to  elevate  the 
posterior  portion  of  the  ribs. 
The  triangularis  sterni  arises  from  the  side  of  the  posterior  surface  of  the 
lower  third  of  the  sternum  and  is  inserted  by  fleshy  slips  into  the  cartilages 
of  the  ribs  from  the  second  to  the  sixth. 

From  the  fact  that  the  inferior  opening  of  the  thorax  as  well  as  the  inter- 
costal spaces  are  completely  closed  by  the  foregoing  muscles,  and  from  the 
further  fact  that  the  superior  is  closed  by  fascia  except  at  those  points  through 
which  pass  the  trachea,  blood-vessels  and  esophagus,  the  cavity  of  the  thorax 
is  absolutely  air-tight. 

The  Pleurae. — Each  lung  is  surrounded  by  a  closed  invaginated  serous 
sac,  the  pleura,  of  which  the  inner  portion  is  reflected  over  and  is  closely 


Fig.  187. — Showing  the  Situation,  the  Points 
OF  Attachment,  and  Direction  or  the  Intercostal 
Muscles,  i.  The  intercostales  externi.  2.  The  inter- 
costales interni.     3.  The  intercartilaginei. — {Deaver.) 


RESPIRATION, 


385 


adherent  to  the  surface  of  the  entire  lung  as  far  as  its  root;  the  outer  portion 
is  retiected  over  the  inner  wall  of  the  thorax,  the  superior  surface  of  the  dia- 
phragm, and  the  viscera  of  the  mediastinum.  Under  normal  conditions 
these  two  layers  of  the  pleura,  the  visceral  and  parietal,  are  in  contact,  or 
at  most  separated  only  by  a  thin  capillary  layer  of  lymph.  The  presence  of 
this  fluid  prevents  appreciable  friction  as 
the  two  surfaces  play  against  each  other 
in  consequence  of  the  movements  of  the 
lungs. 


THE  MECHANIC  MOVEMENTS  OF  THE 
THORAX. 

The  blood  receives  oxygen  from,  and 
yields  carbon  dioxid  to,  the  alveoli  of  the 
lungs,  as  it  flows  through  the  pulmonic 
capillaries.  That  this  exchange  of  gases 
may  continue,  it  is  of  primary  importance 
that  the  air  within  the  alveoli  be  renewed 
as  rapidly  as  it  is  vitiated.  This  is  accom- 
plished by  an  alternate  increase  and  de- 
crease in  the  capacity  of  the  thorax,  ac- 
companied by  corresponding  changes  in 
the  capacity  of  the  lungs.  During  the  for- 
mer there  is  an  inflow  of  atmospheric  air 
(inspiration),  during  the  latter  an  out-flow  of  intra-pulmonic  air  (expira- 
tion). The  continuous  recurrence  of  these  two  movements  brings  about 
that  degree  of  pulmonic  ventilation  necessary  to  the  normal  exchange  of 
gases  betw^een  the  blood  and  the  air.  The  two  movements  together  con- 
stitute a  respiratory  act  or  cycle. 

In  the  course  of  the  respiratory  cycles  the  thorax  presents  alternately  a 
short  period  of  rest — viz.,  between  the  end  of  an  expiration  and  the  beginning 
of  an  inspiration — and  a  relatively  long  period  of  activity,  including  both 
inspiration  and  expiration.  The  former  may  be  regarded  as  the  static,  the 
latter  as  the  dynamic  condition  of  the  thorax.  In  the  static  condition,  the 
thorax  and  its  contained  and  associated  organs  sustain  a  definite  relation 
one  to  another;  in  the  dynamic  conditions  these  relations  undergo  a  change 
the  extent  of  which  is  proportional  to  the  extent  of  the  movements.^ 


Fig.  188. — View  from  bf.hixd  of 
Four  Dorsal  V  e  r  t  e  b  r  .e  and 
Three  Attached  Ribs,  showing 
THE  Attachment  of  the  Elevator 
Muscles  of  the  Ribs  and  the 
Intercostals.  I.  Long  and  short 
elevators.  2.  External  intercostal.  3. 
Internal  intercostal. — {Allen  Thomson.) 


THE  STATIC  CONDITION. 

Relation  of  the  Thoracic  Organs. — Intra-pulmonic  Pressure :  Intra- 
thoracic Pressure. — In  the  static  condition  of  the  thorax  the  lungs,  by 
virtue    of    their  distensibility,  completely  fill  all    parts  of  the  thorax  not 

'  It  is  a  matter  of  discussion  as  to  whether  or  not  there  is  an  absolute  cessation  of  movement 
of  the  thoracic  walls  at  the  end  of  e.xpiration.  A  graphic  record  of  the  movement  shows  that 
if  there  is  no  absolute  cessation,  the  moVement  is  so  slight  that,  for  the  purposes  here  intended, 
a  pause  may  be  admitted.  With  this  admission  it  is  however,  recognized  that  the  forces,  both 
elastic  and  muscular,  which  are  always  acting  on  the  thoracic  walls,  though  in  opposite  directions, 
have  not  ceased  to  act,  but  have  become  so  nearly  equal  that  for  a  brief  period  they  are  practically 
in  a  condition  of  equilibrium,  during  which  the  thoracic  walls  are  stationar}-. 


386 


TEXT-BOOK  OF  PHYSIOLOGY. 


occupied  by  the  heart  and  great  blood-vessels  (Fig.  189).  This  condition  is 
maintained  by  the  pressure  of  the  air  within  the  lungs,  the  intra-piilmonic 
pressure,  which  with  the  respiratory  passages  open,  is  that  of  the  atmosphere, 
760  mm.  Hg.  This  relation  persists  so  long  as  the  thorax  remains  air- 
tight. If  the  skin  and  muscles  covering  an  intercostal  space  be  removed  the 
lung  can  be  seen  in  close  contact  with  the  parietal  layer  of  the  pleura  gliding 
by  with  each  inspiration  and  expiration.  If,  however,  an  opening  be  now 
made  in  the  pleura  sufficient  to  admit  air,  the  lung  immediately  collapses  and 
a  pleural  cavity  is  established.  The  pressure  of  air  within  and  without  the 
lung  counterbalancing,  at  the  moment  the  air  is  admitted,  the  elastic  tissue 
at  once  recoils  and  forces  a  large  part  of  the  air  out  of  the  lung.  This  is  a  proof 
that  in  the  normal  condition,  the  lungs,  distended  by  atmospheric  pressure 
from  within,  are  in  a  state  of  elastic  tension  and  ever  endeavoring  to  pull  the 


Fig.  189. — Section  of  Thorax  with  the  Lungs,*  Heart,  and  Principal  Vessels.  5. 
Catheter  introduced  into  the  pleural  space  and  connected  with  a  manometer. — {After  Morat  and 
Doyen.) 

visceral  layer  of  the  pleura  away  from  the  parietal  layer.  That  they  do  not 
succeed  in  doing  so  is  due  to  the  fact  that  the  atmospheric  pressure  from 
without  is  prevented  from  acting  on  the  lung  by  the  firm  unyielding  walls  of 
the  thorax. 

Intra-thoracic  Pressure. — As  a  result  of  the  elastic  tension  of  the  lungs  a 
fractional  part  of  the  intra-pulmonary  pressure,  760  mm.  Hg.,  is  counter- 
balanced or  opposed,  so  that  the  heart  and  great  vessels  and  other  intra-thoracic 
viscera  are  subjected  to  a  pressure  somewhat  less  than  that  of  the  atmosphere; 
the  amount  of  this  pressure  will  be  that  of  the  atmosphere  less  that  exerted  by 
the  elastic  tissue  of  the  lung  in  the  opposite  direction,  expressed  in  terms  of 
millimeters  of  mercury.  In  the  thorax  but  outside  the  lungs,  there  then 
prevails  a  pressure,  negative  to  the  pressure  inside  the  lungs  and  which  is 
known  as  the  intra-thoracic  pressure. 


RESPIRATION.  387 

The  amount  of  this  intra-thoracic  pressure  can  be  approximate!}'  deter- 
mined in  several  ways.  Thus,  if  shortly  after  death  a  mercurial  manometer 
be  inserted  air-tight  into  the  trachea  of  a  human  being  and  the  thorax  opened, 
the  lungs  will  recoil  and  compress  their  contained  air.  The  mercurial 
manometer  will  at  once  show  an  excess  of  pressure  in  the  trachea  of  about 
6  mm.  This  was  taken  by  Bonders  as  a  measure  of  the  force  with  which 
the  lungs  endeavor  to  recoil.  The  intra-thoracic  pressure  would  be,  there- 
fore, atmospheric  pressure,  760  mm.,  less  6  mm.,  or  754  mm.  Hg.  Another 
method  is  to  insert  a  rubber  catheter  through  a  small  opening  in  an  intercostal 
space  into  the  thoracic  cavity.  The  air  which  enters  through  the  open  ex- 
tremities of  the  catheter  and  leads  to  a  collapse  of  the  lungs  may  be  subse- 
quently aspirated,  when  the  lung  returns  to  its  normal  position.  The 
catheter  is  then  placed  in  connection  wdth  a  water  manometer.  On 
establishing  a  communication  between  them,  by  the  turning  of  a  stopcock, 
the  water  will  rise  in  the  proximal  and  fall  in  the  distal  limb  of  the  manometer, 
indicating  a  pressure  in  the  thorax  negative  to  that  in  the  lung.  The  differ- 
ence in  the  level  of  the  water  in  the  two  limbs  of  the  manometer,  expressed  in 
millimeters, of  mercury,  would  also  represent  the  force  w4th  which  the  elastic 
tissue  strives  to  recoil,  and  the  extent  to  which  it  opposes  the  atmospheric 
pressure.  This  subtracted  from  the  atmospheric  pressure  would  give  the 
intra-thoracic  pressure.  In  the  living  dog  this  latter  is  less  than  the  former, 
to  the  extent  of  from  3.5  to  5.5  mm.  For  the  same  reason  the  superior  surface 
of  the  diaphragm  also  experiences  a  pressure  less  than  that  of  the  atmosphere. 
Owing  to  the  soft  and  yielding  character  of  the  abdominal  walls  the  atmospheric 
pressure  is  transmitted  through  the  abdominal  organs  to  the  inferior  surface 
of  the  diaphragm.  The  pressure  being  greater  from  below  than  above,  the 
diaphragm  is  forced  upward  until  it  assumes  the  dome-like  appearance  it 
usually  presents.     (These  relations  are  shown  in  Fig.  189.) 

The  cause  of  the  negativity  of  the  intra-thoracic  pressure  is  connected 
with  the  change  in  the  relation  of  the  lungs  to  the  thorax  attending  the  first 
inspiration.  Previous  to  birth  the  walls  of  the  alveoli  and  bronchioles  are 
collapsed  and  in  apposition.  The  larger  bronchial  tubes  in  all  probability 
contain  fluid.  The  lungs  therefore  are  devoid  of  air  (atelectatic),  and, 
having  a  specific  gravity  greater  than  water,  readily  sink  when  placed  in  this 
fluid.  The  capacity  of  the  thorax  does  not  exceed  the  volume  of  the  lungs. 
With  the  first  inspiration,  however,  the  thoracic  walls  take  a  new  position. 
The  air  at  once  rushes  into  the  lungs  and  distends  them.  But  as  the 
capacity  of  the  thorax  even  at  the  end  of  the  expiration  is  now  greater  than 
the  volume  which  the  lungs  would  assume  unless  distended,  there  at  once 
arises  the  elastic  recoil  in  the  opposite  direction,  the  condition  which  gives  rise 
to  the  negativity  of  the  pressure  in  the  thoracic  cavity.  It  is  also  probable 
that  as  the  child  develops,  the  thorax  grows  more  rapidly  than  the  lungs, 
giving  rise  to  a  condition  which  would  increase  and  accentuate  the  elastic 
tension  and  thus  increase  the  negativity  of  the  intra-thoracic  pressure. 

THE  DYNAMIC  CONDITION. 

In  the  dynamic  condition  the  thorax  and  its  contained  organs  undergo  a 
series  of  movements  in  consequence  of  which  the  relations  of  the  static  con- 


388  TEXT-BOOK  OF  PHYSIOLOGY. 

dition  are  changed.     To  these  movements  the  term  respiratory  has  been  given, 
as  a  result  of  which  the  ventilation  of  the  lungs  is  accomplished. 

The  Respiratory  Movements. — The  respiratory  movements  consist  of 
an  alternate  increase  and  decrease  in  the  capacity  of  the  thorax,  accompanied 
by  corresponding  changes  in  the  lungs,  the  two  movements  being  known  as 
inspiration  and  expiration  respectively.  During  the  increase  in  the  thoracic 
capacity,  the  air  passively  flows  into  the  lungs;  during  the  decrease  in  the 
thoracic  capacity,  the  air  passively  flows  out  of  the  lungs.  In  both  move- 
ments the  lungs  play  an  entirely  passive  part,  their  movements  being  deter- 
mined by  the  pressure  of  air  within  them  and  by  the  outward  movement  of 
the  thoracic  walls,  with  which  they  are  in  close  contact. 

1.  Inspiration  is  an  active  process,  the  result  of  muscle  activity. 

2.  Expiration  is  a  passive  process,  the  result  mainly  of  the  recoil  of  the 

elastic  tissue  of  the  walls  of  the  thorax  and  abdomen  and  of  the  elastic 
tissue  of  the  lungs. 

In  inspiration  the  thorax  is  enlarged  in  all  its  diameters:  viz.,  vertical, 
transverse,  and  antero-posterior.  In  expiration  the  thorax  is  diminished  in 
all  its  diameters  as  it  returns  to  its  former  condition. 

Inspiratory  Muscles. — The  muscles  which  from  their  origin,  direction, 
and  insertion  contribute  to  the  enlargement  or  expansion  of  the  thorax  are 
quite  numerous,  and  include  those  muscles  which  enter  into  the  formation 
of  the  thoracic  walls  (intrinsic  muscles),  as  well  as  certain  muscles  which, 
having  their  origin  elsewhere,  are  attached  to  the  thoracic  walls  at  different 
points  (extrinsic  muscles),  though  the  extent  to  which  they  are  called  into 
activity  depends  on  the  necessity  for  either  tranquil  or  energetic  inspirations. 
The  gradations  between  a  minimum  and  a  maximum  inspiration  are  very 
sHght,  and  it  is  dificult  to  state  at  what  particular  instant  any  given  muscle 
begins  to  act.  It  is  customary,  however,  to  divide  the  muscles  into  two  groups : 
(i)  Those  active  in  the  average  or  ordinary  inspirations,  and  (2)  those  active 
in  maximum  or  extraordinary  inspirations.  Among  the  muscles  active 
in  ordinary  inspirations  may  be  mentioned  the  diaphragm,  the  intercostales 
externi,  the  inter cartilaginei,  the  levatores  costarum,  the  scaleni,  and  the  ser- 
ratus  posticus  superior.  Among  the  muscles  active  in  extraordinary  inspira- 
tions may  be  mentioned,  in  addition  to  the  foregoing,  the  sterno-cleido- 
mastoideus,  the  trapezius,  and  the  pectorales  minor  and  major. 

The  vertical  diameter  is  increased  by  the  contraction  and  descent  of  the 
diaphragm,  and  more  especially  of  its  lateral  muscular  portions.  At  the 
end  of  an  expiration  the  diaphragm  is  relaxed,  and  the  lower  portion  closely 
applied  to  the  walls  of  the  thorax.  At  the  beginning  of  an  inspiration  the 
muscle-fibers  contract,  shorten,  and  approximate  a  straight  line,  whereby 
not  only  is  the  convexity  of  the  diaphragm  diminished,  but  that  portion  in 
contact  with  the  thorax  is  drawn  away,  thus  making  a  large  free  space 
triangular  in  shape,  termed  the  complementary  pleural  space,  into  which 
the  lateral  and  posterior  portions  of  the  lungs  at  once  descend.  The  attach- 
ment of  the  central  tendon  of  the  diaphragm  to  the  pericardium  prevents 
any  marked  descent  of  this  portion  except  in  forcible  inspiratory  efforts 
(Fig.  190).  The  vertical  diameters  are  thus  enlarged,  though  unequally  in 
different  regions  of  the  thorax. 

As  the  diaphragm  descends  it  displaces  the  abdominal  viscera,  forcing 


RESPIRATION. 


389 


them  downward  against  the  abdominal  walls,  which  advance  and  become 
more  convex.  In  forcible  inspiration  the  diaphragm,  acting  from  the  central 
tendon  as  the  more  fixed  point,  would  draw  the  lower  portion  of  the  thorax 
inward  were  this  not  prevented  by  the  outward  pressure  of  the  displaced 
viscera. 

Coincidently  with  the  descent  of  the  diaphragm  and  the  partial  removal 
of  the  pressure  on  the  under  surface  of  the  lung,  the  intra-pulmonic  air  ex- 
pands. As  it  expands  it  distends  the  lungs  in  the  vertical  direction,  causing 
them  to  follow  the  diaphragm  and  to  occupy  the  so-called  complementary 
pleural  space.  With  the  expansion  of  the  intra-pulmonic  air  there  is  a  fall 
in  its  pressure  below  the  atmospheric  pressure  to  be  followed  immediately 
by  an  inflow  of  air  until  atmospheric  pressure  is  again  established.  This 
occurs  at  the  end  of  the  inspiration. 

The  antero-posterior  and  transverse  diameters  are  increased  by  the  eleva- 
tion and  outward  rotation  of  the  ribs  and  an  advance  of  the  sternum,  both 
movements  made  possible  by  the  construction  and  arrangement  of  the  costo- 
vertebral and  costo-chondral  and  chondro-sternal  articulations.  The 
construction  of  these  articulations  is  such  as  to  permit  at  the  first  a  slight 
elevation  and  depression  of  the  head 
of  the  rib,  and  at  the  second  a  glid- 
ing of  the  tubercle  on  the  transverse 
process.  The  axis  around  which  the 
rib  rotates  practically  coincides  with 
the  axis  of  the  rib  neck,  which  in  the 
upper  part  of  the  thorax  is  almost 
horizontal,  in  the  lower  part  some- 
what sagittal  in  direction.  Hence 
when  the  ribs  are  elevated  the  upper 
part  of  the  thorax  increases  in  its  an- 
tero-posterior, the  lower  part  in  its 
transverse  diameters.  At  the  same 
time,  the  lower  portion  of  the  sternum 
is  pushed  forward  and  upward  by 
the  elevation  of  the  anterior  extremity 
of  the  ribs  and  the  widening  of  the  angle  of  the  costo-chondral  articulation. 
With  the  elevation  of  the  ribs  there  goes  an  eversion  or  outward  rotation 
which  still  further  increases  the  transverse  diameters.  Coincidently  with 
the  increase  in  the  transverse  and  antero-posterior  diameters  of  the  thorax, 
and  the  partial  removal  of  the  pressure  on  the  lateral  surfaces  of  the  lungs 
there  is  also  an  additional  expansion  of  the  intra-pulmonic  air.  As  it  expands 
it  distends  the  lungs,  causing  them  to  occupy  the  available  space  thus  estab- 
lished. With  the  expansion  of  the  intra-pulmonic  air  there  is  a  still  further 
fall  of  pressure  and  an  additional  inrush  of  air.  Between  the  descent  of  the 
diaphragm  and  the  elevation  of  the  ribs  and  the  advance  of  the  sternum  the 
volume  of  air  necessary  for  the  ordinary  respiratory  needs  is  introduced  into 
the  lungs. 

This  elevation  and  outward  rotation  of  the  ribs  is  the  resultant  of  the  coop- 
eration of  the  following  muscles,  viz.:  the  intercostales  externi,  the  intercar- 
tilaginei,  the  levalores  costarum,  the  scaleni  and  the  serratus  posticus  superior. 


Fig.  190. — DiAGR.'\M  Showing  the  Position 

AND  Sa^PE  OF  THE  DIAPHRAGM   AT   REST   a  AND 

During  Inspir.^tion  a'  and  h. — (Boruttau.) 


39° 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  action  of  the  external  intercostal  muscles,  as  well  as  the  action  of 
the  intercartilaginei  muscles,  has  been  a  subject  of  much  discussion.  Some 
investigators  have  maintained  that  they  are  elevators  of  the  ribs,  and  there- 
fore inspiratory;  others  that  they  are  depressors  of  the  ribs,  and  therefore 
expiratory  in  function.  At  the  present  time  the  general  consensus  of  opinion 
is  that  the  former  view  is  the  one  most  in  accordance  with  the  facts.  In 
the  following  explanation  as  to  their  action,  their  relation  to  the  ribs  and  to 
the  cartilages,  must  be  recalled  to  mind.  The  relation  of  the  external  inter- 
costals  is  such  that  the  point  of  attachment  of  any  given  bundle  of  fibers 
to  the  rib  above  lies  nearer  the  vertebral  column,  nearer  the  fulcrum, 
than  the  point  of  attachment  to  the  rib  below.  The  relation  of  the  inter- 
cartilaginei to  the  cartilages  is  such  that  the  point  of  attachment  of  any 
given  bundle  of  fibers  to  the  cartilage  above  lies  nearer  the  sternum,  nearer 
the  fulcrum,  than  the  point  of  attachment  to  the  cartilage  below.  The 
situation  of  the  muscles  and  the  shortness  of  their  fibers  render  it  extremely 
difhcult  to  obtain  myographic  tracings  which  would  elucidate  their  action 
in  elevating  the  ribs  and  cartilages. 

A  clear  conception  of  their  action,  however,  may  be  arrived  at  by  the 
study  of  the  schematic  model  first  presented  by  Hamberger.     Fig.  191.     In 


Fig.  191. — Diagrams  Illustrating  the  action  of  the  External  Intercostal  and  In- 
tercartilaginei Muscles. 

this  model  v-v'  is  a  vertical  support  carrying. two  freely  movable  parallel 
bars  rr',  united  .  at  their  opposite  ends  with  two  other  freely  movable 
and  parallel  bars  cc,  carried  by  a  second  vertical  supports,  representing 
respectively  the  vertebral  column,  two  adjoining  ribs,  two  adjacent  cartilages, 
and  the  sternum.  Diagram  A  shows  the  position  of  the  different  parts  at 
the  end  of  expiration  and  B  their  position  at  the  end  of  inspiration.  The 
parallel  bars  are  joined  to  each  other  by  elastic  bands  ei  and  ic  having  the 
direction  of  and  representing  the  external  intercostal  and  intercartilaginei 
muscles,  respectively.  The  bars  are  depressed  to  sufficiently  elongate  and 
tense  the  elastic  bands  thus  and  imitate  the  condition  of  the  muscles  in  so 
far  as  tension  is  concerned  prior  to  their  contraction.  On  releasing  the 
bars  the  elastic  bands  at  once  recoil  and  the  bars  representing  ribs  and 
cartilages  are  raised.  Although  the  elastic  forces,  acting  in  opposite  direc- 
tions as  indicated  by  the  arrows  are  equal,  the  bars  are  yet  raised  for  the 
reason  that  in  accordance  with  the  parallelogram  of  forces,  the  component 


RESPIRATION.  391 

acting  upward  on  the  long  arm  of  the  lever  preponderates  over  the  com- 
ponent acting  downward  on  the  short  arm  of  the  lever.  This  taken  in  con- 
nection with  the  fact  that  the  distance  between  the  adjoining  bars  is  fixed, 
leads  not  only  to  an  elevation  of  the  bars,  but  to  a  widening  of  the  angle 
between  them  and  an  advance  of  the  second  vertical  support.  The  action 
of  these  bands  thus  disclose  and  illustrate  the  action  of  both  the  external 
intercostal  and  intercartilagenei  muscles.  It  must  therefore  be  concluded 
that  these  muscles  are  the  elevators  of  the  ribs  and  cartilages  and  hence, 
inspiratory  in  function.  Of  late  the  correctness  of  Hamberger's  view  has 
been  confirmed  by  experinients  on  living  animals. 

The  levatores  costarum,  as  is  evident  from  their  points  of  origin  and  in- 
sertion, elevate  the  ribs  posteriorly. 

The  scalenus  muscles,  anticus,  medius,  and  posticus,  arise  from  the  trans- 
verse processes  of  the  cervical  vertebrae,  and  after  pursuing  a  downward  and 
forward  direction  are  inserted  into  the  sternal  end  of  the  first  and  second  ribs. 
The  action  of  the  first  two,  at  least,  is  to  elevate  the  first  rib  and  thus  establish 
a  fixed  point  from  which  the  intercostal  muscles  can  act.  The  posticus  has 
doubtless  a  similar  action  on  the  second  rib. 

The  serratus  posticus  superior,  a  quadrilateral  sheet  of  muscle-fibers, 
arises  mainly  from  the  spines  of  the  last  cervdcal  and  first  and  second  thoracic 
vertebrae.  The  anterior  extremity  is  serrated  and  attached  to  the  outer 
surfaces  of  the  second,  third,  fourth,  and  fifth  ribs  beyond  the  angle.  The 
action  of  the  muscle  is  the  elevation  of  the  ribs  to  which  it  is  attached. 

In  forcible  or  extraordinary  inspirations,  whereby  the  capacity  of  the 
thorax  is  still  further  increased,  the  foregoing  muscles  are  reinforced  by  the 
sterno-cleido-niastoideus,  the  trapezius,  and  the  pectorales  minor  and  major. 
Their  functions  will  become  apparent  from  a  consideration  of  their  origins 
and  insertions. 

Expiratory  Forces  and  Muscles. — Expiration,  as  pre\dously  stated,  is 
a  passive  process  brought  about  by  the  recoil  of  the  elastic  tissues  of  the 
thoracic  and  abdominal  walls,  and  of  the  lungs,  all  of  which  have  been 
stretched  and  made  tense  during  inspiration.  With  the  cessation  of  the  in- 
spiratory effort  the  elastic  forces,  assisted  by  the  weight  of  the  ribs,  sternum, 
and  soft  tissues,  return  the  thorax  to  its  former  condition.  The  result  is  a 
diminution  of  all  the  diameters  of  the  thorax.  The  vertical  diameter  is 
diminished  by  the  recoil  of  the  tense  abdominal  walls,  the  replacement  of  the 
abdominal  organs  and  the  consequent  ascent  of  the  diaphragm  to  its  former 
position.  The  transverse  and  antero-posterior  diameters  are  diminished  by 
the  descent  of  the  ribs,  sternum,  and  lungs.  Coincident  with  the  return  of  the 
thoracic  walls  to  their  former  condition  there  is  a  recoil  of  the  elastic  tissue 
of  the  lungs,  in  consequence  of  which  there  is  a  compression  of  the  intra- 
pulmonic  air.  With  its  compression  there  is  a  rise  of  pressure  above  atmos- 
pheric and  at  once  there  is  an  outflow  of  intra-pulmonic  air  until  atmospheric 
pressure  is  again  established  at  the  end  of  expiration. 

It  is  somewhat  uncertain  if  a  normal  expiratory  movement  necessitates 
active  muscle  contraction.  If,  however,  there  is  any  impairment  of  the 
elasticity  of  the  lungs  or  ribs,  or  any  interference  with  the  free  exit  of  the 
intra-pulmonary  air,  it  is  highly  probable  that  the  elastic  forces  are  assisted 
bv  the  internal  intercostal  and  triangularis  sterni  muscles.     It  has  been  in- 


392  TEXT-B(30K  OF  PHYSIOLOGY. 

sisted  upon  also  that  while  the  recoil  of  the  elastic  tissues  is  effective  in  the 
early  stages  of  an  expiration,  it  is  ineffective  in  the  later  stages.  Hence  there 
arises  a  necessity  for  muscle  assistance. 

The  action  of  the  internal  intercostals  is  less  clearly  understood  than  that 
of  the  external  intercostals.  If,  however,  we  consider  the  direction  of  these 
muscles  as  indicated  in  Fig.  191,  diagram,  A  by  the  dotted  line  n,  ii,  it  would 
seem  that  their  action  would  be  the  opposite  of  that  of  the  external  inter- 
costals— that  is,  it  would  be  to  depress  the  ribs.  By  the  shortening  of  the 
muscles,  the  two  forces,  indicated  by  the  direction  of  the  arrows,  are  equal 
and  opposite,  but  as  the  component  acting  on  the  long  arm  of  the  lever  pre- 
ponderates over  that  acting  on  the  short  arm  of  the  lever,  the  ribs  are 
depressed.  If  this  is  the  case  these  muscles  must  therefore  be  expiratory  in 
function.  The  action  of  the  band  is  supposed  to  disclose  and  illustrate  the 
action  of  the  muscle. 

The  triangularis  sterni  muscle,  judging  from  its  anatomic  relations,  in  all 
probability  assists  in  expiration  by  depressing  the  cartilages  to  which  it  is 
attached  and  as  a  further  result  depressing  the  anterior  extremities  of 
the  ribs. 

Forced  Expiration. — After  the  elastic  forces  have  ceased  to  act  and  the 
normal  expiratory  movement  has  been  brought  to  a  close,  the  thorax  can  be, 
to  a  considerable  extent,  still  further  diminished  in  all  its  diameters  by  the 
contraction,  through  volitional  effort,  of  abdominal  and  thoracic  muscles. 
To  this  decrease  in  the  capacity  of  the  thorax,  as  a  result  of  which  a  much 
larger  volume  of  air  is  expelled  from  the  lungs  than  during  passive  expiration, 
the  term  forced  expiration  has  been  given.  With  the  cessation  of  muscle 
activity  the  elastic  forces  of  the  now-compressed  thoracic  walls,  aided  by  the 
return  of  the  upwardly-displaced  abdominal  organs,  at  once  restore  the 
thoracic  walls  to  the  position  they  had  attained  at  the  end  of  passive  expira- 
tion. Of  the  muscles  active  in  forced  expiration  in  addition  to  the  inter- 
costales  interni  and  the  triangularis  sterni,  the  following  may  be  mentioned, 
viz.:  the  abdominales,  the  serratiis  posticus  inferior,  and  the  quadratus 
liimhoriim. 

The  conjoint  action  of  these  muscles  is  to  diminish  the  convexity  of  the 
abdominal  walls  and  to  exert  a  pressure  on  the  abdominal  organs.  These, 
taking  the  line  of  least  resistance,  are  forced  upward  against  the  inferior 
surface  of  the  diaphragm,  which  in  consequence  becomes  more  strongly 
curved  and  ascends  higher  into  the  thorax.  The  vertical  diameter  of  the 
thorax  is  thus  diminished.  'Acting  from  the  pelvis  as  a  fixed  point,  these 
muscles  will  also  draw  downward  and  inward  the  lower  end  of  the  ster- 
num and  the  lower  ribs  and  diminish  the  antero-posterior  and  transverse 
diameters. 

Movements  of  the  Lungs. — As  the  thorax  is  enlarging  in  all  its  diam- 
eters during  inspiration,  through  muscle  activity,  the  lungs  are  correspondingly 
enlarging  in  all  their  diameters,  by  virtue  of  their  distensibility,  through  the 
pressure  of  the  air  within  them.  The  lungs  must  therefore  move  downward, 
outward  and  forward.  That  this  is  the  case  is  made  evident  both  by  an 
examination  of  the  lungs  through  an  intercostal  space  after  removal  of  the 
skin  and  intercostal  muscles,  and  by  the  methods  of  percussion.  The  inferior 
border  of  each  lune;  descends  from  the  lower  border  of  the  sixth  to  the 


RESPIRATION.  393 

eleventh  rib,  inserting  itself  into  the  space  developed  between  the  thorax 
and  diaphragm  as  the  latter  contracts  and  is  drawn  away  from  the  former. 
In  consequence  of  the  lateral  expansion  the  anterior  border  of  each  lung 
advances  toward  the  middle  line  until  the  heart  is  almost  covered.  With 
the  beginning  and  continuance  of  expiration  the  lungs  exhibit  a  reverse 
movement  which  continues  until  they  reach  their  original  position.  At  all 
times,  however,  the  movements  of  the  lungs  are  entirely  passive  and  deter- 
mined by  the  movements  of  the  thorax. 

The  Changes  in  the  Relation  of  the  Thoracic  Organs,  and  in  the  In- 
tra-pulmonic  and  Intra-thoracic  Pressures. — In  the  dynamic  condition,  as 
previously  stated,  the  relations  of  the  thoracic  organs  undergo  a  change  as 
well  as  the  intra-pulmonic  and  intra-thoracic  pressures.  Thus  during 
inspiration  the  diaphragm  descends,  the  ribs  ascend  and  outwardly  rotate 
and  the  sternum  advances,  the  result  of  which  is  an  enlargement  in  the 
diameters  of  the  thorax.  Coincidently  with  the  enlargement  of  the  thorax 
through  muscle  activity  there  goes  a  corresponding  increase  in  the  size  and 
capacity  of  the  lungs,  in  consequence  of  the  expansion  and  pressure  of  the 
air  in  the  pulmonary  alveoli. 

During  expiration  the  diaphragm  ascends  in  consequence  of  the  return 
of  the  displaced  abdominal  viscera,  the  ribs  descend  and  inwardly  rotate 
and  the  sternum  recedes  from  the  recoil  of  the  elastic  tissues,  the  result  of 
which  is  a  diminution  in  the  diameters  of  the  thorax.  Coincidently  wuth  the 
diminution  of  the  thorax  there  goes  a  decrease  in  the  size  and  capacity  of  the 
lungs  in  consequence  of  the  recoil  of  their  elastic  tissue  whereby  the  air  in 
the  lungs  is  compressed. 

The  intra-pulmonic  pressure  in  consequence  of  the  alternate  expansion 
and  compression  of  the  intra-pulmonic  air  also  undergoes  a  considerable 
variation. 

During  inspiration  the  intra-pulmonic  air  expands.  With  the  expansion 
its  pressure  falls;  but  though  it  is  now  less  than  atmospheric  pressure  it  is 
yet  much  greater  than  the  opposing  force  of  the  lung  tissue.  As  a  result  of 
the  fall  of  intra-pulmonic  pressure,  there  is  a  rapid  iniiow  of  air  which  con- 
tinues until  atmospheric  pressure  is  restored;  that  is,  at  the  end  of  the 
inspiration. 

During  expiration  the  intra-pulmonic  air  becomes  compressed.  With 
the  compression  its  pressure  rises  above  that  of  the  atmosphere  and  in  conse- 
quence there  is  a  rapid  outflow  of  air,  which  continues  until  atmospheric 
pressure  is  again  restored;  that  is,  at  the  end  of  the  expiration.     (Fig.  ig2,  A.) 

The  cause  for  the  fall  of  intra-pulmonic  pressure  during  inspiration  and 
the  rise  during  expiration  is  to  be  found  in  the  resistance  offered  by  the  air- 
passages  to  the  movement  of  the  air,  throughout  their  entire  extent,  and 
especially  at  the  level  of  the  vocal  bands.  The  greater  the  resistance,  from 
whatever  cause,  physiologic  or  pathologic,  the  greater  the  variations  of 
the  pressure.  If  the  inspiratory  and  expiratory  movements  take  place  slowly 
the  intra-pulmonic  pressure  may  scarcely  vary  in  either  direction. 

In  quiet  inspiration  the  fall  of  pressure,  as  indicated  by  a  manometer 
inserted  into  one  nostril,  seldom  amounts  to  more  than  1.5  mm.  of  Hg.,  the 
rise  in  expiration,  2.5  to  3  mm.  of  Hg.  In  forcible  inspiratory  and  expiratory 
efforts  these  limits  mav  be  largelv  exceeded.     Thus  it  was  found  by  Donders 


394  TEXT-BOOK  OF  PHYSIOLOGY. 

that  with  one  nostril  closed  and  a  mercurial  manometer  inserted  into  the 
other  the  pressure  by  voluntary  efforts  could  be  made  to  fall  57  mm.  during 
inspiration  and  to  rise  87  mm.  during  expiration.  The  changes  in  intra- 
pulmonic  pressure  are  graphically  represented  in  the  upper  half  of  Fig.  192. 
The  intra-thoracic  pressure  also  varies  during  both  inspiration  and  expira- 
tion. As  the  thorax  enlarges  and  the  intra-pulmonic  pressure  falls,  the 
recoil  of  the  elastic  tissue  increases,  with  the  result  of  still  further  dimin- 
ishing the  intra-thoracic  pressure,  until  its  maximum  is  reached  near 
the  end  of  the  inspiration.  The  fall  of  intra-thoracic  pressure  at 
the  end  of  a  quiet  inspiration  reaches  to  about  9  mm.  Hg.  In 
forcible  inspiratory  efforts  this  fall  in  intra-thoracic  pressure  may 
amount  to  30  or  40  mm.  of  Hg.  As  the  thorax  again  diminishes  and  the 
intra-pulmonic  pressure  rises  above  the  atmospheric  pressure,  the  recoil 
of  the  elastic  tissue  is  again  opposed,  with  the  result  of  increasing  the 


-£l213--''^  -^-  Expiration 

7S8 

A.  Intra-pulmonic  Pressures. 

760  mm  1~  ^^^^P^^'^^^^^'^  -^^Ezpimiio/i 


7JI 

B.  Intra-thoracic  Pressure. 

Fig.  192. — Representing  the  Changes,  i,  in  the  Intra-pulmonic,  and  2,  in  the  Intra-tho- 
racic Pressures  during  Inspiration  and  Expiration. 

intra-thoracic  pressure,  until  the  former  condition  of  pressure  has 
been  regained '  at  the  end  of  the  expiration.  Neither  the  fall  nor 
the  subsequent  rise  of  the  intra-thoracic  pressure  takes  place,  however, 
in  a  steadily  progressive  manner  for  the  following  reasons:  If  a  tracing 
were  made  of  the  variations  in  the  circumference  of  the  thorax  during  a 
respiratory  movement  it  would  resemble  in  its  main  features  the  tracing  in 
Fig.  194,  and  variations  in  any  linear  dimension  of  the  lung  would  be  of 
course  in  the  same  proportion.  This  amount  of  elongation  of  elastic  tissue 
in  any  direction  would  likewise  be  proportional  to  the  force  of  elastic  recoil. 
Therefore  the  intra-thoracic  pressure  would  vary  from  a  uniform  decrease 
and  increase  just  as  the  curve  of  Fig.  194  varies  from  uniform  straight  lines. 
The  changes  in  intra-thoracic  pressure  are  graphically  represented  in 
Fig.  192,  B. 

Tke  intra-thoracic  pressure  and  its  variations  iniiuence  favorably  the 
flow  of  lymph  through  the  thoracic  duct  (see  page  222),  as  well  as  the  flow 
of  blood  from  the  extra-thoracic  veins  into  the  intra-thoracic  veins,  the  right 
side  of  the  heart,  and  the  cardio-pulmonary  vessels.  (See  paragraphs  at  the 
end  of  this  chapter.) 

The  succession  of  events  in  the  thorax  at  the  time  of  a  respiratory  act 
may  be  summarized  as  follows: 


RESPIRATION.  395 

During  Inspiration. 

1.  Enlargement  of  the  thoracic  diameters  by  muscle  action, 

2.  Increase  in  the  negativity  of  the  intra-thoracic  pressure. 

3.  Expansion  of  intra-pulmonic  (alveolar)  air. 

4.  Expansion  of  the  lungs. 

5.  Lowering  of  the  intra-pulmonic  air  pressure  below  the  atmospheric 

air  pressure. 

6.  Inflow  of  atmospheric  air,  in  consequence  of  its  higher  pressure, 

until   the    intra-pulmonic    air    pressure    rises    to     that   of    the 
atmosphere. 
During  Expiration. 

1.  Diminution  of  the  thoracic  diameters  by  the  action  of  elastic  forces. 

2.  Decrease  in  the  negativity  of  the  intra-thoracic  pressure. 

3.  Recoil  of  the  lungs. 

4.  Compression  of  the  intra-pulmonic  (alveolar)  air. 

5.  Rise  of   intra-pulmonic  air    pressure    above    the    atmospheric  air 

pressure. 

6.  Outflow  of  intra-pulmonic  air,  in  consequence  of  its  higher  pres- 

sure, until  the  intra-pulmonic  air  pressure  falls  to  that  of  the 
atmosphere. 

Respiratory  Movements  of  the  Upper  Air-passages. — The  resistance 
to  the  entrance  of  air  into  and  through  the  respiratory  tract  is  much  dimin- 
ished by  respiratory  movements  of  the  nares  and  larynx  which  are  associated 
and  occur  synchronously  with  the  movement  of  the  thorax. 

The  nares  at  each  inspiration  are  dilated  by  the  outw^ard  movement  of 
their  alee  or  wings,  the  result  of  muscle  activity.  At  each  expiration  they 
are  diminished  by  the  return  of  their  cartilages  through  the  play  of  elastic 
forces.  The  larynx,  as  shown  by  observation  with  the  laryngoscope,  exhibits 
corresponding  movements  of  the  vocal  membranes.  Their  introduction  at 
this  level  naturally  narrows  the  tract,  and  would  interfere  with  both  the 
entrance  and  the  exit  of  air  were  they  not  kept  widely  asunder  during  the 
time  they  are  not  required  for  purposes  of  phonation.  This  is  accomplished 
by  the  tonic  contraction  of  the  posterior  crico-arytenoid  muscles,  which  are 
entirely  respirator}^  in  function. 

It  is  not  infrequently  stated  that  these  membranes  exhibit  considerable 
oscillations,  outward  and  inward,  corresponding  to  the  periods  of  inspira- 
tion and  expiration.  The  statements  of  the  majority  of  laryngologists  do 
not  favor  this  view.  During  tranquil  breathing  the  membranes  are  widely 
separated  and  almost  stationary,  seldom  moving  in  either  direction  more  than 
a  few  millimeters.  In  labored  respirations  these  movements  are  naturally 
increased  in  extent.  The  irregular  movements  of  the  membranes  occasioned 
by  the  unskilful  use  of  the  laryngoscope,  especially  with  nervous  patients, 
are  not  to  be  regarded  as  strictly  physiologic.  The  respiratory  space  in 
quiet  breathing  is  an  isosceles  triangle,  with  a  length  of  20  mm.  and  a  width 
at  the  base  of  15.5  mm.  with  an  area  of  155  mm. 

Respiratory  Types. — Observation  of  the  respiratory  movements  in  the 
two  sexes  shows  that  while  the  enlargement  of  the  thoracic  cavity  is  accom- 
plished both  by  the  descent  of  the  diaphragm  (as  shown  by  the  protrusion  of 
the  abdomen)  and  the  elevation  of  the  thoracic  walls,  the  former  movement 


396 


TEXT-BOOK  OF  PHYSIOLOGY. 


preponderates  in  the  male,  the  latter  in  the  female,  giving  rise  to  what  has  been 
termed  in  the  one  case  the  diaphragmatic  or  abdominal  and  in  the  other  the 
thoracic  or  costal  type  of  respiration.  The  cause  of  this  greater  mobility 
and  activity  of  the  thorax  in  the  female  has  been  a  subject  of  much  discus- 
sion. It  has  been  attributed,  on  the  one  hand,  to  the  necessity  for  a  physi- 
ologic adjustment  between  respiration  and  child-bearing,  and  therefore  a 
specific  sex  peculiarity;  on  the  other  hand,  it  has  been  attributed  to  persistent 
constriction  of  the  waist,  in  consequence  of  which  the  full  play  of  the  dia- 
phragm is  prevented  and  the  burden  of  inspiration  is  thrown  on  the  thoracic 
muscles.  It  has  been  assumed  that  if  inspiration  were  confined  in  women 
to  the  diaphragm,  there  would  arise  in  the  latter  stages  of  gestation  such  an 
increase  in  intra-abdominal  pressure  that  not  only  would  respiratory  ex- 
changes be  interfered  with,  but  fetal  life  might  be  unfavorably  influenced, 
if  not  endangered.  Modern  investigations  have  not  confirmed  this  assump- 
tion, but,  on  the  contrary,  have  corroborated  the  view^  that  the  preponderance 
of  thoracic  movement  is  due  to  the  influences  of  dress  restrictions,  for  with 
their  removal  the  so-called  costal  type  of  breathing  entirely  disappears. 
While  gestation  may  lead  to  a  greater  activity  of  the  thorax,  this  is  but  tem- 
porary, for  with  its  termination  there  is  a  return  to  the  diaphragmatic  type 
of  breathing. 

Number  of  Respirations  per  Minute. — The  number  of  respirations 
which  occur  in  a  unit  of  time  varies  with  a  variety  of  conditions,  the  most 
important  of  which  is  age.  The  results  of  the  observations  of  Quetelet  on 
this  point,  which  are  generally  accepted,  are  as  follows: 

Age.  Respirations  per  Minute.  Age.  Respirations  per  Minute. 

o-  I  year, 44  20-25  years, 18.7 

5  years, 26  25-30  years, 150 

15-20  years, 20  30~5o  years, 17.0 

From  these  observations  it  may  be  assumed  that  the  average  number  of 
respirations  in  the  adult  is  eighteen  per  minute,  though  varying  from  moment 
to  moment  from  sixteen  to  twenty.  During  sleep,  however,  the  respiratory 
movements  often  diminish  in  number  as  much  as  30  per  cent.,  at  the  same 
time  diminishing  in  depth. 

Rhythm. — Each  respiratory  act  takes  place  normally  in  a  regular 
methodic  manner,  each  event  occurring  in  a  definite  sequence  and  occupy- 


"^"^^^^^^^S 


Fig.  193. — Pneumograph. — (Fitz.) 

ing  the  same  relative  period  of  time.  This  rhythm,  however,  is  not  infre- 
quently temporarily  disturbed  by  emotions,  volitional  acts,  muscle  activity, 
phonation,  changes  in  the  composition  of  the  blood,  etc.;  with  the  removal 
of  these  disturbing  factors,  the  respiratory  mechanism  soon  returns  to  its 
normal  condition. 


RESPIRATION. 


397 


A  graphic  representation  of  the  excursions  of  the  thoracic  walls,  rhythmic 
or  otherwise,  is  obtained  by  fastening  to  the  thorax  an  apparatus,  a  stethom- 
eter  or  a  pnen?nograph,  which  by  means  of  a  tambour  takes  up  and  trans- 
mits the  movement  to  a  second  tambour  provided  with  a  recording  lever. 
A  simple  form  of  pneumograph,  suggested  by  Fitz  (Fig.  193),  consists  of  a 
coil  of  wire  two  and  a  half  centimeters  in  diameter  and  about  40  centimeters  in 
length,  enclosed  by  thin  rubber  tubing,  one  end  of  which  is  closed,  the  other 
placed  in  communication  either  with  a  tambour  and  lever  or  with  a  piston 
recorder.  By  means  of  an  inelastic  cord  or  chain  the  apparatus  is  securely 
fastened  to  the  chest.  With  each  inspiration  the  spring  is  elongated,  the  air 
within  the  system  is  rarefied,  and  as  a  result  the  lever  falls;  with  each  expira- 
tion the  reverse  conditions  obtain  and  the  lever  rises.  If  the  lever  be  ap- 
plied to  the  recording  surface  of  a  moving  cylinder,  a  curve  of  the  thoracic 
movement,  a  pneumatogram,  is  obtained  (Fig. 
194),  from  which  it  is  apparent  that  inspira- 
tion takes  place  more  abruptly  and  occupies 
a  shorter  period  of  time  than  expiration;  that 
expiration  immediately  follows  inspiration, 
but  that  there  is  a  slight  pause  between  the 
end  of  the  expiration  and  the  beginning  of  the 
inspiration.  The  time  relations  of  the  two 
movements  can  be  obtained  by  a  magnet-signal 
actuated  by  an  electric  current  interrupted 
once  a  second.  The  ratio  of  inspiration  to 
expiration  has  been  represented  as  5  to  6.  or 
6  to  8. 


INSR 


Fig.  194. 


A  Pneumatogram.  —  {After 
Marey.) 


Fig. 


195. — A  Spirometer.- 
{Boruttau.) 


Volumes  of  Air  Breathed. — The  volumes  of  air  which  enter  and  leave 
the  lungs  with  each  inspiration  and  expiration  naturally  vary  with  the  ex- 
tent of  the  movement,  though  four  at  least  may  be  determined:  (i)  that  of  an 
ordinary  inspiration;  (2)  that  of  an  ordinary  expiration;  (3)  that  of  a  forced 
inspiration;  (4)  that  of  a  forced  expiration. 

The  apparatus  employed  for  the  determination  of  these  different  volumes 
is  the  spirometer,  a  modification  of  the  gasometer.  The  form  introduced  by 
Jonathan  Hutchinson,  of  which  Fig.  195  is  a  modification,  consists  of  two 
metallic  cylinders,  one  containing  water,  the  other  containing  air,  the  latter 
being  inserted  into  the  former.  The  air  cylinder  is  balanced  by  a  weight  so 
accurately  that  it  remains  stationary  in  any  position.  A  tube,  penetrating 
the  base  of  the  water  cylinder,  is  continued  upward  through  and  above  the 
level  of  the  water.  The  air-space  above  is  thus  placed  in  free  communica- 
tion with  the  external  air.     A  stopcock  at  the  outer  end  of  this  tube  prevents 


398  TEXT-BOOK  OF  PHYSIOLOGY. 

the  escape  of  the  air  when  this  is  not  desirable.  To  the  free  end  of  the  tube 
a  rubber  tube  provided  with  a  suitable  mouthpiece  is  attached,  through  which 
air  can  be  breathed  into  or  out  of  the  air  cylinder.  With  each  inspiration 
the  air  cylinder  descends;  with  each  expiration  it  ascends.  A  scale,  on  the 
side  support,  graduated  in  cubic  inches  or  centimeters,  indicates  the  volume 
of  air  inspired  or  expired. 

With  an  apparatus  of  this  character  Hutchinson,  from  a  long  series  of 
observations,  defined  and  determined  the  above-mentioned  four  volumes  as 
follows : 

1.  The  tidal  volume,  that  which  flows  into  and  out  of  the  lungs  with  each 

inspiration  and  expiration,  which  varies  from  20  to  30  cubic  inches  (330 
to  500  c.c). 

2.  The  complemenial  volume,  that  which  flows  into  the  lungs,  in  addition  to 

the  tidal  volume,  as  a  result  of  a.  forcible  inspiration,  and  which  amounts 
to  about  no  cubic  inches  (1800  c.c). 

3.  The  reserve  volume,  that  which  flows  out  of  the  lungs,  in  addition  to  the 

tidal  volume,  as  a  result  of  3,  forcible  expiration,  and  which  amounts  to 
about  100  cubic  inches  (1650  c.c). 

After  the  expulsion  of  the  reserve  volume  there  yet  remains  in  the 
lungs  an  unknown  volume  of  air  which  serves  the  mechanic  function  of 
distending  the  air-cells  and  alveolar  passages,  thus  maintaining  the  . 
conditions  essential  to  the  free  movement  of  blood  through  the  capil- 
laries and  to  the  exchanges  of  gases  between  the  blood  and  alveolar  air. 
As  this  volume  of  air  cannot  be  displaced  by  volitional  effort,  but 
resides  permanently  in  the  alveoli  and  bronchial  tubes  though  constantly 
undergoing  renewal,  it  was  termed — 

4.  The  residual  volume,  the  amount  of  which  is  difficult  of  determination, 

but  has  been  estimated  by  different  observers  at  914  c.c,  1562  c.c, 
198c  c.c 

The  Vital  Capacity  of  the  Lungs. — From  foregoing  statements  it  is 
clear  that  the  thorax  and  lungs  are  capable  of  a  maximum  degree  of  expan- 
sion, at  which  moment  the  lungs  contain  their  maximum  volume  of  air. 
This  volume,  whatever  it  may  be,  represents  the  entire  capacity  of  the  lungs 
in  the  physiologic  condition,  and  includes  the  tidal,  the  complemental,  the 
reserve,  and  the  residual  volumes.  Mr.  Hutchinson,  however,  defined  the 
vital  or  respiratory  capacity  of  the  lungs  as  the  amount  of  air  which  can  be 
expelled  by  the  most  forcible  expiration  after  the  most  forcible  inspira- 
tion, this  therefore  excludes  the  residual  volume.  The  vital  capacity 
was  supposed  to  be  an  indication  of  an  individual's  respiratory  power,  not 
only  in  physiologic  but  also  in  pathologic  conditions.  Though  averaging 
about  230  cubic  inches  (3770  c.c.)  for  an  individual  5  feet  7  inches  in  height, 
the  vital  capacity  varies  with  a  number  of  conditions,  the  most  important  of 
which  is  stature.  It  is  found  that  between  5  and  6  feet  the  capacity  in- 
creases 8  inches  (130  c.c.)  for  each  inch  increase  in  height. 

The  Total  Volume  of  Air  Breathed  Daily. — For  the  solution  of  certain 
problems  connected  with  ventilation  it  is  necessary  to  determine  the  total 
volume  of  air  taken  into  the  lungs  in  the  course  of  24  hours.  This  can 
be  determined  approximately  if  the  two  factors,  the  average  volume  of  air 
taken  into  the  lungs  at  each  inspiration,  and  the  average  number  of  res- 


RESPIRATION. 


399 


pirations  per  minute  be  known.  If  it  be  accepted  that  the  inspired  volume 
varies  from  328  to  492  c.c.  and  that  the  respiratory  frequency  averages  18  per 
minute,  then  the  total  volume  breathed  would  amount  to  from  8500  to 
12,752  liters. 

The  volume  changes  of  the  thorax  indicated  by  the  volumes  of  air  en- 
tering and  leaving  the  lungs  can  be  not  only  determined  but  graphically 
represented  by  means  of  an  apparatus  similar  in  principle  to  the  spirometer, 


Fig.  196. — Gad's,  PNEUiLvxoGRj^iPH. 

devised  by  Gad  and  known  as  the  pneumatograph  or  aeroplethysmo graph 
(Fig.  196).  This  consists  of  a  quadrangular  box  with  double  walls,  the 
space  between  which  is  filled  with  water.  The  center  of  the  box  is  an  air 
chamber.  A  thin  walled  mica  box  sinks  into  the  water.  Posteriorly  it  is 
attached  to  and  rotates  around  an  axis,  which  permits  of  an  elevation  or 
depression  of  the  anterior  portion.  It  is  also  carefully  counterpoised.  A 
light  lever  attached  to  the  mica  box  records  its  movements.     The  interior 

^JOO 

■fOOO 

3SOO 

3000 

2J00 

2000 

iSOO 

1000 

SOO 

O 

Fig.  197. — Diagram  Representing  the  Volume  Changes  of  the  Thorax  and  Lungs. — 

{After  Boruttau.) 

of  the  box  communicates  by  a  tube  with  a  large  reservoir  into  which  the 
individual  breathes,  the  object  being  to  prevent  a  too  rapid  vitiation  of  the 
air.  Inspiration  causes  the  lever  to  descend,  expiration  to  ascend.  Previous 
graduation  of  the  apparatus  is  necessary  to  determine  the  volumes  breathed. 
A  graphic  record  of  the  volume  changes  is  shown  in  Fig.  197. 

Respiratory  Sounds. — On  applying  the  ear  over  the  trachea  and 
bronchi  there  is  heard  during  both  inspiration  and  expiration  a  well-defined 
sound,  loud,  harsh,  and  blowing  in  character,  which  from  its  situation  is 


/A 

/    \ 

1   Vol.    \ 

ma/ 

M/-^\/\___/V 

> 

Xf 

\tompie/ 
ym/ital/ 

€ci/u.r.c/.f(/. 

n 

\i 

V          ) 

400  TEXT-BOOK  OF  PHYSIOLOGY. 

known  as  the  bronchial  sound.  It  is  especially  well  heard  between  the 
scapulae  above  the  fourth  dorsal  vertebra.  This  sound  is  produced  in  the 
larynx,  for  with  its  separation  from  the  trachea  the  sound  disappears.  The 
cause  of  the  sound  is  to  be  found  in  the  narrowing  of  the  air-passage  at  the 
level  of  the  vocal  membranes,  though  the  mechanism  of  its  production  is  uncer- 
tain. On  applying  the  ear  to  almost  any  portion  of  the  chest- wall,  but  especially 
to  the  infrascapular  area,  there  is  heard  during  both  inspiration  and  expiration 
a  delicate,  sighing,  rustling  sound,  which  from  its  supposed  seat  of  origin, 
the  air-vesicles  or  -cells,  is  known  as  the  vesicular  sound.  This  sound  is 
supposed  to  be  due  to  the  sudden  expansion  of  the  air-cells  during  inspiration 
and  to  the  friction  of  the  air  in  the  alveolar  passages. 

THE  CHEMISTRY  OF  RESPIRATION. 

The  general  metabolic  process  as  it  takes  place  in  the  tissues  involves 
the  assimilation  of  oxygen  and  the  evolution  of  carbon  dioxid.  The  former 
is  the  first,  the  latter  the  last,  of  a  series  of  chemic  changes  the  continuance  of 
which  is  essential  to  the  maintenance  of  all  life  phenomena.  A  constant 
supply  of  oxygen  and  an  equally  constant  removal  of  carbon  dioxid  are 
necessary  conditions  for  tissue  activity.  The  blood  is  the  medium  by  which 
the  oxygen  is  transported  from  the  lungs  to  the  tissues  and  the  carbon  dioxid 
from  the  tissues  to  the  lungs.  The  respiratory  movements  constitute  the 
means  by  which  the  oxygen  of  the  air  is  brought  into,  and  the  carbon  dioxid 
expelled  from,  the  lungs  into  the  surrounding  air. 

The  exchanges  between  blood  and  tissues  constitute  internal  respiration, 
in  contradistinction  to  the  thoracic  movements  by  which  the  air  is  brought 
into  relation  with  the  blood,  and  which  constitute  external  respiration.  The 
transfer  of  the  oxygen  by  the  blood  from  the  interior  of  the  lungs  to  the  tissues, 
and  of  the  carbon  dioxid  from  the  tissues  to  the  interior  of  the  lungs,  is  the 
outcome  of  a  series  of  chemic  changes  which  are  related  to  the  exchange  of 
gases  between  the  air  in  the  lungs  and  the  blood,  on  the  one  hand,  and 
between  the  blood  and  tissues,  on  the  other. 

In  consequence  of  the  many  and  complex  chemic  changes  which  attend 
these  gaseous  exchanges,  there  arise  changes  in  composition  of: 

1.  The  air  breathed. 

2.  The  blood,  both  arterial  and  venous. 

3.  The  tissue  elements  and  the  lymph  by  which  they  are  surrounded. 

The  investigation  of  the  nature  of  these  changes,  the  mechanism  of  their 
production,  and  their  quantitative  relations  constitute  the  subject-matter  of 
the  chemistry  of  respiration. 

CHANGES  IN  THE  COMPOSITION  OF  THE  AIR. 

Experience  teaches  that  the  air  during  its  sojourn  in  the  lungs  undergoes 
such  a  change  in  composition  that  it  is  rendered  unfit  for  further  breathing. 
Chemic  analysis  has  shown  that  this  change  involves  a  loss  of  oxygen,  a  gain 
in  carbon  dioxid,  watery  vapor,  and  organic  matter.  For  the  correct  under- 
standing of  the  phenomena  of  respiration  it  is  essential  that  not  only  the 
character  but  the  extent  of  these  changes  be  known.  This  necessitates  an 
analysis  of  both  the  inspired  and  expired  airs,  from  a  comparison  of  which 
certain  deductions  can  be  made. 


RESPIRATION.  401 

The  results  which  have  been  obtained  are  represented  in  the  following 
table: 

Inspired  Air.  Expired  Air. 

[Oxygen, 20.80.  f  Oxygen, 16.02. 

100    J  Carbon  dioxid, traces.  I  Carbon  dioxid, .  .     4.38. 

vols.  ]  Nitrogen, 79 .20.  ,     \  Nitrogen, 79  .60. 

[  Watery  vapor, variable.  '  j  Watery  vapor, .  .  saturated. 

[  Organic  matter. .   a  trace. 

These  analyses  indicate  that  under  ordinary  conditions  the  air  loses 
oxygen  to  the  extent  of  4.78  per  cent,  and  gains  carbon  dioxid  to  the  extent 
of  4.38  per  cent. ;  that  it  gains  in  nitrogen  to  the  extent  of  0.4  per  cent,  and  in 
watery  vapor  from  its  initial  amount  to  the  point  of  saturation,  as  well  as  in 
organic  matter.  It  is  to  these  changes  in  their  totality  that  those  disturbances 
of  physiologic  activity  are  to  be  attributed  which  arise  when  expired  air  is 
re-breathed  for  any  length  of  time  without  having  undergone  renovation. 

Special  forms  of  apparatus  have  been  devised  for  the  collection  and  analy- 
sis of  gases.  Their  construction  as  well  as  the  methods  of  analysis  involved 
are  complicated  and  need  not  be  described  in  this  connection.  The  presence 
of  the  carbon  dioxid,  however,  may  be  readily  shown  by  breathing  through 
a  glass  tube  into  a  vessel  containing  barium  or  calcium  hydrate  solution.  The 
turbidity  which  immediately  follows  is  due  to  the  formation  of  barium  or 
calcium  carbonate,  which  can  be  due  only  to  the  presence  of  carbon  dioxid. 
That  this  turbidity  is  not  due  to  the  carbon  dioxid  normally  present  in  the 
air  is  shown  by  the  fact  that  the  solution  remains  clear  until  the  passage  of 
the  atmospheric  air  has  been  maintained  for  some  time.  From  the  percent- 
age loss  of  oxygen  and  gain  in  carbon  dioxid,  the  total  oxygen  absorbed  and 
carbon  dioxid  exhaled  may  be  approximately  calculated.  Thus,  if  the 
volume  of  air  breathed  daily  be  accepted  at  either  8500  or  12,752  liters,  and 
the  percentage  loss  of  oxygen  be  4.78,  the  total  oxygen  absorbed  may  be 
obtained  by  the  rule  of  simple  proportion,  e.g.: 

100  :  4.78  ::    8,500  :  ,r  =  4o6  liters  or  580  grams^ 
Or 

100  :  4. 78  ::  12,752  :  .^  =  609  liters  or  870  grams. 

By  the  same  method  the  total  carbon  dioxid  exhaled  is  found  to  be  either 
372  liters  or  735  grams,  or  558  liters  or  1103  grams;  volumes  in  both  instances 
which  agree  very  well  with  volumes  obtained  by  other  methods. 

For  the  fact  that  only  558  liters  of  carbon  dioxid  are  exhaled  as  compared 
with  609  liters  of  oxygen  absorbed,  it  is  evident  that  not  all  of  the  oxygen 
unites  with  carbon  to  form  carbon  dioxid  and  that  the  remainder  of  the 
oxygen  must  unite  with  some  other  element.  As  there  is  usually  an  excess 
of  water  eliminated  over  that  introduced  into  the  body,  it  is  highly  probable 
that  the  oxygen  combines  with  free  hydrogen  to  form  water.  The  relative 
amounts  of  the  oxygen  so  utilized  are  not  fixed  but  variable,  and  depend  on 
the  equality  and  c^uantity  of  the  foods,  excercise,  etc.  The  ratio  of  the 
volume  of  the  carbon  dioxid  exhaled  to  the  volume  of  oxygen  absorbed  is 
known  as  the  respirator v  quotient,  and  is  usually  represented  by  the  symbol 

CO 

^  .    Thus  in  the  foregoing  analysis  the  respiratory  quotient  is  0.916. 

The  gain  in  nitrogen  is  a  variable  factor,  ranging  from  zero  to  0.9  per 

'  I  liter  of  oxygen  weighs  1.4298  grams;   i  liter  of  carbon  dioxid  weighs  1.977  grams. 
26 


402  TEXT-BOOK  OF  PHYSIOLOGY. 

cent.     This  gain  is  probably  of  accidental  occurrence,  due  to  absorption 
from  the  large  intestine,  in  which  decomposition  of  nitrogen-holding  com- 
pounds is  taking  place.     It  is  generally  believed  that  free  nitrogen  plays  no 
part  in  any  phenomenon  of  combination  or  decomposition  within  the  body. 
The  gain  in  watery  vapor  will  depend  on  the  amount  previously  present 
in  the  air.     This  is  conditioned  by  the  temperature.     With  a  rise  in  tempera- 
ture the  percentage  of  water  increases;  with  a  fall,  it  decreases.     By  breath- 
ing into  a  vessel  containing  pumice  stone  saturated  with  sulphuric  acid,  the 
vapor  may  be  collected.     The  difference  observed  between  the  weight  before 
and  after  breathing  is  an  indication  of  the  amount  by  weight  of  water  exhaled 
during  the  time  of  breathing.     It  has  been  calculated  that  the  amount  of 
water  exhaled  daily  varies  between  300  and  500  grams.     Though  invisible 
at  ordinary  temperatures,  it  becomes  visible  at  low  temperature  as  soon  as 
it  emerges  from  the  respiratory  tract.     The  loss  of  heat  is  followed  by  a  con- 
densation of  the  vapor,  which  appears  at  once  as  a  cloudy   precipitate. 
The  gain  in  organic  matter  is  also  variable.     The  amount  present  is  not 
sufficient  to  permit  of  a  thorough  chemic  analysis,  but  there  are  reasons  for 
believing  that  it  belongs  to  the  proteid  group  of  bodies.     If  it  accumulates 
in  the  air,  especially  at  high  temperatures,  it  readily  undergoes  decomposition, 
with  the  production  of  offensive  odors.     Traces  of  free  ammonia  have  also 
been  found  in  the  expired  air.     In  addition  to  these  chemic  changes,  the 
air  experiences  physical  changes;  e.g.,  a  rise  in  temperature  and  an  increase 
in  volume.     The  risfe  in  temperature  can  be  shown  by  breathing  through  a 
suitable  mouthpiece  into  a  glass  tube  containing  a  thermometer.     By  this 
means  it  has  been  shown  that  inspired  air  at  20°  C.  rises  in  temperature  to 
37°  C. ;  at  6.3°  to  29.8°  C.     The  increase  in  the  temperature  will  depend  upon 
that  of  the  air  inspired  and  the  time  it  remains  in  the  lungs.     If  retained  a 
sufficient  length  of  time  it  will  always  become  that  of  the  body.     As  a  result 
of  .the  heat  absorption  the  expired  air  increases  in  volume  about  one-ninth 
of  that  of  the  inspired  air.     When  corrected  for  temperature  and  pressure 
and  freed  from  aqueous  vapor,  the  volume  of  the  expired  air  is  less  than 
that  of  the  inspired  air  by  about  one  two-hundred  and  fiftieth. 

The  Composition  of  the  Alveolar  Air. — The  foregoing  statement  of  the 
composition  of  the  expired  air,  derived  in  part  from  the  upper  air-passages, 
trachea,  and  bronchi,  does  not  necessarily  represent  the  composition  of  the 
alveolar  air.  It  is  very  probable  that  the  percentage  of  carbon  dioxid  is 
greater,  the  percentage  of  oxygen  less,  in  the  latter  than  in  the  former.  This 
is  made  evident  by  collecting  in  several  portions  the  expired  air  as  it  escapes 
from  the  respiratory  tract  and  subjecting  it  to  analysis.  The  last  portion 
always  contains  a  larger  amount  of  carbon  dioxid  and  a  smaller  amount  of 
oxygen  than  the  first  portion.  The  determination  of  the  composition  of  the 
alveolar  air  is  extremely  difficult.  It  has  been  estimated  to  contain  from 
5  to  6  per  cent,  of  carbon  dioxid  and  from  14  to  18  per  cent,  of  oxygen. 

Pulmonary  Ventilation. — It  is  owing  largely  to  this  inequality  of 
volumes  and  consequently  of  the  "partial  pressures"  of  these  two  gases  in 
the  trachea  and  alveoli  that  the  degree  of  ventilation  necessary  for  the  ex- 
change of  gases  between  lungs  and  air  is  maintained.  Though  the  respira- 
tory movements  doubtless  create  currents  in  the  air-passages  which  carry, 
on  the  one  hand,  a  portion  of  the  inspired  air  directly  into  the  alveoli,  and, 


RESPIRATION.  403 

on  the  other  hand,  carry  a  portion  of  the  alveolar  air  directly  out  of  the  body, 
other  portions  find  their  way  into  and  out  of  the  alveoli  in  accordance  with  the 
laws  of  diffusion.  If  the  pressure  of  the  oxygen  in  the  trachea  is  158  mm. 
Hg.  and  in  the  alveoli  approximately  122  mm.  Hg.,  diffusion  downward 
will  take  place.  Equilibrium,  however,  is  never  established,  as  the  oxygen 
is  continually  disappearing  by  passing  into  the  blood.  On  the  contrary,  if 
the  carbon  dioxid  pressure  in  the  alveoli  is  approximately  28  to  40  mm.  Hg., 
and  in  the  trachea  0.3  mm.  Hg.,  diffusion  will  take  place  upward.  Equilib- 
rium will  never  be  established,  however,  as  the  carbon  dioxid  is  constantly 
coming  out  of  the  blood.  Pulmonary  ventilation  may  also  be  aided  by 
those  alternate  changes  in  volume  of  the  heart,  great  vessels,  and  lungs 
occurring  as  the  result  of  the  heart-beat  and  producing  the  so-called  cardio- 
pneumatic  movements. 

CHANGES  IN  THE  COMPOSITION  OF  THE  BLOOD. 

The  blood  which  flows  into  the  lungs  through  the  pulmonary  artery  is 
dark  bluish-red,  that  which  flows  from  the  lungs  into  the  pulmonary  veins 
is  scarlet  red,  in  color.  The  blood  is  changed,  while  flowing  through  the 
lung  capillaries,  from  the  venous  to  the  arterial  condition.  As  the  air  in  the 
lungs  gains  carbon  dioxid  and  loses  oxygen,  it  is  fair  to  assume  that  what 
the  air  gains  the  blood  loses,  and  what  the  air  loses  the  blood  gains.  In 
other  words,  the  blood,  while  passing  through  the  lungs,  is  changed  from 
venous  to  arterial  by  the  loss  of  carbon  dioxid  and  the  gain  of  oxygen.  The 
change  in  color  of  venous  blood  from  dark  bluish  to  scarlet  red  is  strikingly 
shown  by  shaking  it  in  a  test-tube  with  oxygen  or  atmospheric  air. 

The  blood  which  flows  into  the  tissues  through  the  arteries  is  scarlet  red, 
that  which  flows  from  the  tissues  into  the  veins  is  bluish-red  in  color.  The 
blood  while  flowing  through  the  tissue  capillaries  is  changed  from  the  arterial 
to  the  venous  condition.  Since  arterial  blood  when  deprived  of  oxygen 
becomes  bluish-red,  the  indication  is  that  the  change  in  color  is  associated 
with,  if  not  entirely  due  to,  the  escape  of  oxygen  into  the  tissues.  The 
constant  elimination  of  carbon  dioxid  from  the  blood  into  the  lungs  indicates 
that  the  carbon  dioxid  is  as  constantly  passing  from  the  tissues  through  the 
capillary  walls  into  the  blood. 

These  considerations  are  confirmed  by  the  results  of  analyses  which  have 
been  made  of  both  venous  and  arterial  blood.  The  presence  of  gas  in  the 
blood  is  demonstrated  by  subjecting  it  under  appropriate  conditions  to  the 
vacuum  of  the  mercurial  air-pump,  into  which  it  at  once  escapes.  From 
100  volumes,  an  average  of  60  volumes  of  gas  at  standard  pressure,  760  mm. 
Hg.  and  temperature  0°  C,  can  thus  be  obtained. 

Gases  of  the  Blood. — An  analysis  of  the  volumes  of  gas  removed  from 
both  venous  and  arterial  blood  shows  that  each  consists  of  oxygen,  carbon 
dioxid,  and  nitrogen,  though  in  different  amounts.  An  average  composi- 
tion of  the  gases  extracted  from  dog's  blood  obtained  from  the  right  ventricle 
and  carotid  artery  is  given  in  the  following  table: 

Venous  blood  |  g^^g^'dioxid,  1 !  1 ! ! !     45  vot        ^'^^"^^^  ^^?°^  |  c2bon  dioxid;. '.     !o  vot 
100  vols.       I  Nitrogen, 1-2  vols.  ^°°  ^°^'-      [  Nitrogen, 1-2  vols. 


404  TEXT-BOOK  OF  PHYSIOLOGY. 

The  changes  produced  in  the  blood  by  respiration,  both  external  and  in- 
ternal, become  apparent  from  a  comparison  of  these  analyses.  The  venous 
blood  while  passing  through  the  lungs  gains  from  eight  to  eleven  volumes 
per  cent,  of  oxygen  and  loses  five  volumes  per  cent,  of  carbon  dioxid.  The 
arterial  blood  while  passing  through  the  tissues  loses  oxygen  and  gains 
carbon  dioxid  in  corresponding  amounts.  The  volume  of  nitrogen  is  not 
appreciably  changed. 

The  Relation  of  the  Gases  in  the  Blood. — The  mechanism  by  which 
the  gases  become  associated  with  the  blood  at  the  moment  of  their  entrance 
into  it,  and  again  become  dissociated  just  prior  to  their  exit  from  it,  as  well 
as  their  relation  to  the  blood  while  in  transit,  will  be  more  readily  understood 
after  reference  to  a  few  elementary  facts  relative  to  the  absorption  of  gases 
by  liquids  in  general  and  the  conditions  of  temperature  and  pressure  by 
which  it  is  inl^uenced. 

It  is  well  known  that  liquids  will  absorb  or  dissolve  at  any  constant 
pressure  unequal  volumes  of  different  gases  in  accordance  with  their  solubili- 
ties and  with  variations  in  temperature.  Water,  for  example,  will  absorb, 
in  accordance  with  the  foregoing  conditions,  oxygen,  carbon  dioxid,  and 
nitrogen,  as  well  as  many  other  gases.  The  volume  of  any  gas  thus  absorbed 
is  known  as  the  coejjicient  of  absorption,  and  may  be  defined  as  the  number 
of  cubic  centimeters  of  the  gas  which  one  cubic  centimeter  of  water  will 
absorb  when  the  gas,  in  contact  with  the  water,  stands  under  a  pressure  of 
one  atmosphere  or  760  mm.  of  mercury  and  at  a  temperature  of  0°  C.  The 
volume  absorbed,  however,  varies  inversely  as  the  temperature.  Thus  at 
0°  C.  the  volume  of  oxygen  absorbed  by  one  volume  of  water  is  0.0489  c.c; 
of  carbon  dioxid  1.7 13  c.c;  of  nitrogen  0.0234  c.c.  With  a  rise  of  tempera- 
ture, the  pressure  remaining  constant,  the  absorptive  power  of  water  for  each 
of  these  gases  diminishes.  Thus  at  15°  C,  the  volumes  of  oxygen,  carbon 
dioxid  and  nitrogen  absorbed  are  0.0310  c.c,  1.0025  c.c.  and  0.0168  c.c. 
respectively.  Though  the  volume  of  the  gas  absorbed  diminishes  as  the 
temperature  rises,  it  is  independent  of  pressure,  for  no  matter  to  what  extent 
the  pressure  may  vary  the  volume  absorbed  is  always  the  same.  (Law  of 
Henry.) 

If  the  weight  of  the  gas  absorbed  be  considered  rather  than  the  volume 
(that  is  the  product  of  the  volume  and  the  density  or  the  number  of  molecules 
in  the  volume),  then  the  temperature  remaining  constant,  the  weight  of  the 
volume  absorbed  increases  and  decreases  proportionately  as  the  pressure 
rises  and  falls.  Thus  at  a  pressure  of  760  mm.  of  mercury  and  at  a  tempera- 
ture of  0°  C,  the  volume  of  oxygen  absorbed  by  one  volume  of  water  is 
0.0489  c.c;  at  1520  mm.  of  mercury,  the  same  volume  is  absorbed  but  its' 
weight  is  doubled.  If  the  pressure  falls  below  760  mm.  of  mercury  the 
same  volume  is  absorbed  but  its  weight  is  diminished.  (Law  of  Dalton.) 
Because  of  the  foregoing  facts,  it  is  necessary  in  all  gaseous  determina- 
tions to  reduce  for  purposes  of  comparison  the  obtained  volumes  to  standard 
temperature  (0°  C.)  and  pressure  (760  mm.  of  mercury). 

When  the  liquid  is  once  saturated  with  a  gas  at  a  constant  pressure 
and  temperature,  there  is  coincidently  with  the  entrance  of  the  gas  into  the 
liquid,  an  equivalent  exit  of  the  gas  from  it,  though  the  volume  retained  in  the 
liquid  remains  constant.     The  reason  for  this  fact  is,  that  under  the  condi- 


RESPIRATION.  •        405 

tions,  the  volume  of  the  gas  dissolved  by  the  liquid  though  small  in  amount 
exerts  a  pressure  in  the  opposite  direction  equivalent  to  the  pressure  acting 
upon  the  liquid.  If  one  cubic  centimeter  of  water  absorbs  0.0489  c.c.  of 
oxygen  at  760  mm.  and  0°  C,  this  volume  will  exert  a  pressure  opposite 
in  direction  of  760  mm.  of  mercury.  For  this  reason  the  entrance  and  exit  of 
the  gas  are  equal  and  opposite. 

If  water  be  exposed  to  atmospheric  air  consisting  of  oxygen,  carbon 
dioxid,  and  nitrogen  in  the  ordinary  proportions,  at  any  given  temperature 
and  pressure,  the  water  will  absorb  unequal  volumes  of  each  of  the  three 
gases.  The  pressure  under  which  each  gas  is  absorbed  is  a  part  only, 
however,  of  the  total  atmospheric  pressure  at  the  time.  The  pressure 
exerted  by  any  one  of  these  gases  is  known  as  its  ''partial  pressure," 
and  depends  on  the  percentage  volume  of  the  gas  present.  If  atmospheric 
air  contains  at  standard  pressure  and  temperature  79.15  volumes  percent, 
of  nitrogen,  its  partial  pressure  will  be  "^^^^^  of  760,  or  601.54  mm.  Hg.; 
if  the  air  contains  0.04  volume  per  cent,  of  carbon  dioxid  and  20.85  volumes 
■per  cent,  of  oxygen,  the  partial  pressure  of  each  will  be  0.30  mm.  Hg.  and 
158.46  mm.  Hg.  respectively.  The  absorption  of  each  gas  is  independent  of 
all  the  rest,  and  is  the  Same  for  nitrogen,  for  example,  as  if  it  alone  were 
present  at  a  pressure  of  601.54  mm.  Hg. 

Again,  if  water  holding  in  solution  a  certain  volume  of  a  gas — carbon 
dioxid,  for  example — be  exposed  to  an  atmosphere  containing  but  0.04 
volume  per  cent,  of  carbon  dioxid,  and  having  therefore  a  pressure  of  but 
0.3  mm.  Hg.,  the  gas  will  at  once  begin  to  leave  the  water,  and  continue  to 
do  so  until  the  pressure  of  the  carbon  dioxid  in  the  atmosphere  balances  the 
pressure  of  the  gas  in  the  w^ater,  at  which  moment  the  escape  of  the  gas 
ceases.  The  pressure  of  a  gas  in  a  liquid  is  equal  to  that  pressure  in  milli- 
meters of  mercury  of  the  same  gas  in  the  atmosphere  which  is  required  to  keep 
it  in  solution.  What  is  true  for  the  carbon  dioxid  is  true  for  any  other  gas 
that  may  be  in  solution.  .  If  a  liquid  has  a  greater  density  than  water,  as 
from  the  presence  of  inorganic  salts,  the  absorptive  power  under  standard 
conditions  of  temperature  and  pressure  becomes  less.  It  is  for  this  reason  that 
blood-plasma  contains  less  oxygen,  nitrogen,  and  carbon  dioxid  than  water. 

It  will  be  recalled  that  the  blood  yields  up  its  gases  when  subjected  to  the 
vacuum  of  the  mercurial  pump;  that  is,  to  a  diminution  or  complete  removal 
of  the  atmospheric  pressure.  From  this  it  might  be  inferred  that  the  gases 
are  merely  held  in  solution  by  pressure,  and  at  once  escape  the  moment 
they  are  exposed  to  a  space  in  which  there  is  a  very  slight  or  a  total  absence 
of  pressure.  In  other  words,  that  the  absorption  of  gases  by  the  blood  and 
their  escape  from  it  follow  the  law  of  pressure  as  stated  in  foregoing  para- 
graphs. It  is  therefore  necessary  to  test  this  supposed  condition  of  the  gases 
in  the  blood  by  subjecting  the  latter  to  gradually  diminishing  pressures, 
with  a  view  of  determining  in  how  far  the  discharge  of  the  gases  follows  the 
law  of  falling  pressures.  For  convenience  the  conditions  of  each  gas  will 
be  considered  separately. 

Oxygen. — If  blood  is  subjected  to  a  succession  of  pressures  progressively 
less  than  the  standard,  it  is  found  that  though  oxygen  is  evolved,  its  evolution 
is  not  in  accordance  with  the  law  of  partial  pressures;  that  is,  in  proportion 
to  the  diminution  of  pressure.     Within  wide  limits — e.g.,  from  760  to  332 


4o6  TEXT-BOOK  OF  PHYSIOLOGY. 

mm.  atmospheric  pressure,  to  which  correspond  oxygen  pressures  of  159 
and  70  mm.  respectively — there  is  but  a  sUght  increase  in  the  amount  of 
oxygen  evolved;  and  it  is  not  until  the  pressure  of  the  oxygen  falls  below 
the  latter  that  it  begins  to  be  liberated  in  large  amounts.  From  this  on,  the 
oxygen  continues  to  be  liberated  with  decreasing  pressures,  until  the  zero  point 
is  reached,  when  all  gaseous  discharge  ceases.  Coincidently  the  blood 
changes  in  color  from  a  bright  red  to  a  deep  bluish-red.  It  is  evident  from 
the  results  of  this  procedure  that  the  condition  of  the  oxygen  in  the  blood 
is  but  to  a  slight  extent  one  of  physical  absorption.  The  indications  are 
that  the  union  is  of  the  nature  of  a  chemic  combination. 

If  the  red  corpuscles  are  removed  from  the  blood  and  the  plasma  alone 
treated  in  the  manner  above  described,  it  will  be  found  that  the  oxygen 
liberated  now  follows  the  law  of  partial  pressure.  The  amount  so  liberated, 
however,  is  small — about  one  per  cent,  of  the  total  oxygen  of  the  blood. 
The  agent  therefore  which  holds  the  oxygen  in  combination  is  the  red  corpus- 
cle, or  more  exactly,  the  hemoglobin,  which  constitutes  about  32  per 
cent,  of  its  volume.  This  is  proved  by  the  fact  that  a  solution  of  gas-free 
hemoglobin  of  a  strength  equivalent  to  that  of  the  blood  (14  per  cent.), 
exposed  to  oxygen  under  a  gradually  increasing  pressure  from  zero  up  to  50  to 
70  mm.  pressure,  will  absorb  large  quantities  of  oxygen;  beyond  this  point  the 
amount  absorbed  is  again  small  in  comparison.  At  70  mm.  pressure  the 
hemoglobin  is  almost  saturated.  Coincidently  with  this  absorption  the  hemo- 
globin changes  in  color  from  bluish-red  to  scarlet-red  and  changes  from  hemo- 
globin to  oxyhemoglobin.  The  reverse  method,  that  of  subjecting  oxy- 
hemoglobin to  gradually  diminishing  pressures,  yields  opposite  results,  that 
is,  the  oxygen  becomes  dissociated  and  the  force  by  which  this  is  accomplished 
is  known  as  the  force  of  dissociation.  As  one  gram  of  hemoglobin  com- 
bines with  1.34  c.c.  of  oxygen,  and  as  the  percentage  of  hemoglobin  is  13.50 
to  14,  it  is  evident  that  there  is  sufficient  hemoglobin  to  combine  with 
practically  all  the  oxygen  usually  present  in  the  blood.  Thus  the  hemoglobin 
in  100  c.c.  of  blood  would  hold  in  combination  18.76  c.c.  of  oxygen.  This, 
together  with  the  one  c.c.  held  in  solution  in  plasma,  would  equal  the  volume 
obtained  in  the  vacuum  of  the  air-pump. 

The  union  of  the  oxygen  with  the  hemoglobin  is  therefore  largely  chemic 
in  character,  dependent  however  on  pressure.  About  one  per  cent,  is 
physically  absorbed  by  or  dissolved  in  the  plasma;  the  remainder  is  chemi- 
cally combined  with  the  henioglobin. 

The  association  or  combination  of  oxygen  is  favored  by  a  pressure  of  at 
least  30  to  50  mm.  Hg.  and  upward;  the  dissociation,  by  diminution  of 
pressure.  In  the  conversion  of  hemoglobin  into  oxyhemoglobin  two  an- 
tagonistic forces  are  at  work,  heat  and  chemic  affinity.  The  former 
tends  to  prevent,  the  latter  to  favor,  the  union.  Chemic  affinity  increases 
with  the  influence  of  mass,  that  is,  in  proportion  to  the  number  of  atoms  in  a 
unit  of  volume,  with  the  density,  and  with  the  partial  pressure  of  the  oxygen. 
Diminution  of  pressure  reduces  the  mass  influence  and  permits  the  heat  to 
bring  about  dissociation  (Bunge).  The  following  table  by  Hiifner  shows 
the  relative  proportion  of  hemoglobin  and  oxyhemoglobin  in  blood  contain- 
ing 14  per  cent,  hemoglobin  and  exposed  to  air  at  gradually  diminishing 
pressures : 


RESPIRATION.  407 


Atmospheric  Pressure 

Partial  P 

ressure  of  Oxygen 

Hemoglobin 

Oxyhemoglobin 

in  mm.  Hg. 

in 

mm.  Hg. 

Percentage. 

Percentage. 

760 

159-3 

1.49 

98.51 

524.8 

no 

2.14 

97.86 

357 

8 

75 

3-II 

96.89 

238 

5 

50 

4.60 

95-40 

119 

3 

25 

8.79 

91  .21- 

47 

7 

10 

19.36 

80.64 

23 

8 

5 

32.51 

67.49 

0 

0 

0.0 

100.00 

0.00 

Carbon  Dioxid. — The  blood  yields  up  its  contained  carbon  dioxid  to  the 
vacuum  of  the  gas-pump  as  completely  as  it  does  its  oxygen.  The  same  is 
not  the  case,  however,  if  the  red  corpuscles  are  first  removed  and  the  ex- 
periment made  with  either  plasma  or  serum.  Even  at  zero  pressure  the  fluid 
contains  carbon  dioxid,  as  shown  by  its  liberation  on  the  addition  of  some 
weak  acid,  as  tartaric  or  phosphoric,  an  indication  that  it  exists  in  a  state  of 
firm  combination.  The  same  result  follows  the  addition  of  the  red  blood- 
corpuscles,  which  act  in  a  manner  similar  to  the  acids  just  mentioned. 
This  property  of  the  corpuscles  has  been  attributed  to  hemoglobin,  and 
especially  when  in  the  state  of  oxyhemoglobin.  It  is  for  this  reason  that 
blood  yields  all  its  carbon  dioxid  to  the  vacuum  of  the  gas-pump. 

The  limit  of  pressure  at  which  the  plasma  ceases  to  absorb  carbon 
dioxid  physically  and  begins  to  combine  it  chemically  is  not  very  clearly 
defined.  It  has  been  estimated  that  of  the  entire  amount,  38  to  45  volumes, 
only  about  2.5  volumes  are  so  absorbed,  the  remainder  being  in  a  condition 
of  both  loose  and  stable  combination. 

An  analysis  of  the  serum,  and  presumably  of  the  plasma,  shows  the 
presence  of  sodium  salts,  with  which  the  carbon  dioxid  could  enter  into  com- 
bination, viz.:  sodium  carbonate  and  dibasic  sodium  phosphate.  The 
sodium  is  thus  partly  divided  between  carbonic  acid  and  phosphoric  acid. 
The  amount  of  the  sodium  which  falls  to  carbon  dioxid  will  depend  on  the 
mass  influence  of  the  latter;  that  is,  its  partial  pressure. 

At  its  origin  in  the  tissues  the  carbon  dioxid  acquires  a  considerable 
tension,  and  its  mass  influence  is  correspondingly  large.  On  entering  the 
blood  it  combines  with  sodium  carbonate,  with  the  formation  of  sodium 
bicarbonate,  as  shown  in  the  following  equation: 

Na,C03  +  C02  +  H20'=2NaHC03. 

At  the  same  time,  having  a  greater  mass  influence  than  the  phosphoric  acid, 
it  will  withdraw  from  the  dibasic  sodium  phosphate  one-half  of  its  sodium, 
with  the  formation  of  sodium  bicarbonate  and  monobasic  sodium  phosphate, 
as  shown  in  the  following  equation: 

NajHPO,  +  C02-l-H20=NaHC03-FNaH2PO,. 

With  the  difl'usion  of  the  carbon  dioxid  from  the  blood  into  the  alveoli  its 
tension  in  the  venous  blood  falls,  its  mass  influence  diminishes,  while  that  of 
the  phosphoric  acid  relatively  increases.  As  a  result,  the  sodium  is  withdrawn 
from  the  sodium  bicarbonate,  an  additional  liberation  of  carbon  dioxid  takes 
place  and  dibasic  sodium  phosphate  is  re-formed.  The  association  or  com- 
bination of  the  carbon  dioxid  with  the  basic  salts  depends  on  its  partial 
pressure;  dissociation  in  the  lungs,  on  a  diminution  of  pressure. 


4o8  TEXT-BOOK  OF  PHYSIOLOGY. 

Nitrogen. — This  gas  exists  in  both  arterial  and  venous  blood  in  a  state 
of  solution.  There  is  no  evidence  that  it  enters  into  combination  with  any 
other  element. 

Tension  of  the  Gases  in  the  Blood. — It  will  be  recalled  that  a  liquid 
holding  in  solution  one  or  more  gases  will  on  exposure  to  an  atmosphere 
composed  of  the  same  gases  either  give  up  or  absorb  volumes  varying  in 
amount  and  in  accordance  with  their  partial  pressures  until  equilibrium  is 
established.  If  the  pressure  of  any  one  gas  in  the  atmosphere  is  greater  than 
the  pressure  of  the  same  gas  in  the  lic[uid,  it  is  absorbed;  if  the  pressure  is 
less  the  gas  is  discharged.  Knowing  the  pressure  of  the  gases  in  percent- 
ages of  an  atmosphere,  at  the  beginning  and  the  end  of  an  experiment,  the 
original  tension  or  pressure  of  the  gases  in  the  lic^uid  can  be  easily  calculated. 
On  this  principle  various  forms  of  apparatus  known  as  aerotonometers  have 
been  devised  by  which  the  tension  of  the  gases  in  the  blood  can  be  determined. 

These  appliances  consist  essentially  of  a  glass  tube  containing  oxygen, 
carbon  dioxid,  and  nitrogen  in  known  amounts  and  tensions.  The  blood 
from  an  animal  is  then  allowed  to  flow  directly  from  an  artery  or  vein  into  the 
tube.  As  it  flows  down  its  sides  in  a  thin  layer  it  presents  a  large  surface 
to  the  action  of  the  contained  gases.  In  the  aerotonometer  of  Fredericq 
the  blood,  made  non-coagulable  by  the  injection  of  peptone,  is  returned 
from  the  opposite  extremity  of  the  tube  to  the  animal.  This  enables  the 
experiment  to  be  continued  for  an  hour  or  more.  A  knowledge  of  the 
tensions  of  the  blood  gases  is  of  interest,  as  it  affords  a  clue  to  the  mechanism 
by  which  the  interchange  takes  place  between  the  lungs  and  the  blood,  on  the 
one  hand,  and  the  blood  and  tissues,  on  the  other.  The  results,  however,  of 
different  observers  are  not  sufficiently  in  accord  to  permit  of  positive 
deductions. 

In  the  well-known  experiments  of  Strassburger,  the  tension  of  the 
oxygen  in  the  arterial  blood  of  the  dog  was  found  to  be  29.64  mm.  Hg.,  or 
3.9  per  cent,  of  an  atmosphere,  and  in  the  venous  blood  22.04  rn^n-  Hg.,  or 
2.9  per  cent.  The  tension  of  the  carbon  dioxid  in  the  venous  blood  was 
found  to  be  41.14  mm.  Hg.,  or  5.4  per  cent,  of  an  atmosphere,  and  in  the 
arterial  blood  21.8  mm.  Hg.,  or  2.8  per  cent.  Very  different  results  have 
been  obtained  by  Fredericq  with  the  aerotonometer  devised  by  him  and  by 
the  employment  of  a  method  different  from  that  of  Strassburger.  Thus  he 
states  that  the  oxygen  tension  in  the  pulmonary  alveoli  is  136  mm.  Hg.,  or 
18  per  cent,  of  an  atmosphere  while  in  the  arterial  blood  it  is  106  mm.  Hg., 
or  14  per  cent.;  while  the  carbon-dioxid  tension  in  the  tissues  varies  from 
45  to  68  mm.  Hg.,  or  from  6  to  9  per  cent,  of  an  atmosphere;  while  in  the 
venous  blood  it  varies  from  30  to  41  mm.  Hg.,  or  from  3.8  to  5.4  per  cent, 
and  in  the  pulmonary  alveoli  it  is  about  21  mm.  or  2.8  per  cent. 

CHANGES  IN  THE  COMPOSITION  OF   THE  TISSUES  AND  LYMPH. 

From  previous  statements  the  inferences  can  be  drawn  that  the  oxygen 
leaves  the  blood  as  the  latter  flows  through  the  capillaries;  that  it  passes 
through  the  capillary  wall  into  the  surrounding  lymph  and  so  to  the  tissue- 
cells;  that  it  oxidizes  food  materials  in  the  tissue-cells  whereby  the  potential 
energy  of  the  former  is  liberated  as  kinetic  energy;  that  the  carbon  dioxid 


RESPIRATION.  409 

so  evolved  passes  into  the  lymph  and  through  the  wall  of  the  capillary  into 
the  blood. 

While  this  is  doubtless  the  case,  the  presence  of  free  oxygen  in  the  tissues 
can  not  be  demonstrated  by  the  usual  methods  of  gas  analysis.  Only  in  the 
saliva  and  in  the  blood  of  the  placental  umbilical  vein  can  it  be  shown 
that  oxygen  has  direcdy  passed  through  the  capillary  wall.  For  this  reason 
it  has  been  claimed  by  a  few  investigators  that  oxygen  does  not  leave  the 
blood,  but  that  the  field  of  its  activity  as  an  oxidizing  agent  is  limited  to  the 
blood-current,  where  it  meets  with  and  oxidizes  easily  reducible  substances 
entering  from  the  tissues.  On  this  view  the  potential  energy  of  the  food 
would  be  liberated  by  mere  decomposition  or  cleavage  in  consequence  of 
cell  activity. 

Nevertheless  many  facts  from  the  fields  of  comparative  physiology  and 
physiologic  chemistry  combine  to  support  the  view  that  oxygen  is  absolutely 
necessary  to  the  maintenance  of  the  Hfe  of  all  tissue-cells.  Though  they 
will  continue  to  manifest  their  characteristic  activities — e.g.,  contraction  on 
the  part  of  a  muscle,  secretion  by  a  gland,  the  conduction  of  a  nerve  impulse 
by'the  nerve,  etc. — for  a  variable  length  of  time  after  oxygen  is  prevented 
from  gaining  access  to  them,  nevertheless  they  will  in  due  time  die. 

The  necessity  for  oxygen  on  the  part  of  the  tissues  and  the  avidity  with 
which  they  absorb  it,  is  shown  by  their  power  of  reducing  pigments  such  as 
alizarine  blue.  If  this  pigment  be  injected  into  the  blood-vessels  of  an 
animal  and  the  animal  killed  in  about  ten  minutes,  it  will  be  found  that  while 
the  blood  exhibits  a  deep  blue  color  the  tissues  present  their  usual  colors. 
But  after  exposure  to  the  air  or  to  free  oxygen  the  latter  also  acquire  the 
characteristic  blue  color.  The  explanation  offered  for  this  fact  is  that  the 
tissues  in  their  need  for  oxygen  absolutely  extract  it  from  the  pigment, 
reducing  it  to  a  colorless  compound,  which,  however,  on  exposure  recom- 
bines  with  oxygen  and  regains  the  original  color. 

Though  free  oxygen  cannot  be  shown  to  be  present  in  the  tissues,  there 
are  many  reasons  for  believing  that  it  is  continually  passing  into  them  by  way 
of  the  lymph-stream.  Its  rapid  disappearance  would  indicate  that  it  is 
immediately  utilized  for  the  production  of  carbon  dioxid  (which  is  improb- 
able on  other  grounds) ,  or  that  the  tissues  possess  a  capacity  for  oxygen  storage, 
of  placing  it  in  reserve  under  some  combination  or  other,  by  which  it  can  be 
securely  retained  until  required  for  oxidation  purposes.  This  is  rendered 
probable  from  the  fact  that  the  carbon  dioxid  evolved  at  any  given  moment 
is  not  necessarily  dependent  on  the  oxygen  just  absorbed,  for  if  oxygen  be 
withheld  from  a  nutritive  fluid  which  is  being  artificially  circulated  through 
a  recently  isolated  organ,  carbon  dioxid  will  continue  to  be  discharged  for 
some  time.  A  muscle,  or  even  a  living  animal — e.g.,  a  frog — placed  in  an 
atmosphere  of  pure  nitrogen  will  remain  active  and  evolve  CO2  even  for 
several  hours. 

Naturally  the  absorption  of  oxygen  and  the  discharge  of  carbon  dioxid 
and  the  changes  of  composition  which  are  incident  to  nutrition  will  be  most 
marked  in  those  tissues  characterized  by  the  greatest  degree  of  physiologic 
activity.  Muscle-tissue  exhibits  these  changes  to  a  greater  degree  than 
bone.  Tissues  with  intermediate  degrees  of  activity  should  exhibit  corre- 
sponding degrees  of  respiratory  change.     Experiment  confirms  this  view. 


4IO 


TEXT-BOOK  OF  PHYSIOLOGY. 


Thus,  loo  grams  each  of  muscle,  spleen,  and  broken  bone  from  a  recently 
living  animal  exposed  to  the  air  for  twenty-four  hours  absorbed  respectively 
50.8  C.C.,  27.3  c.c,  and  17.2  c.c,  of  oxygen,  while  each  discharged  during  the 
same  period  56.8  c.c,  15.4  c.c,  and  8.1  c.c  of  carbon  dioxid  respectively. 
In  another  series  of  experiments  by  a  different  observer  100  grams  of  muscle 
absorbed  in  three  hours  23  c.c.  of  oxygen,  and  100  grams  of  bone  5  c.c  of 
oxygen.  Both  tissues  discharged  carbon  dioxid  in  amounts  proportional  to 
the  oxygen  absorbed.  The  same  respiratory  changes  may  be  more  satis- 
factorily demonstrated  by  passing  blood  through  the  tissues  of  isolated 

Atmospheric  Air. 

0x-jJ8 mm.Iic/,  or  ^0.8 S  pc/'  ceuJ , 

COy0.3m?n.//6/,  or  0.0^  per  cent, 

of  a?i  atmosphere . 


Ox'Tens/.off 
J  per  cent. 


CO;t  -Tension 
J. 5  per  cent. 


4jz-77nsifm 

J30mmNff^ 

r/ per  cent.  \ 
\C/Jy  Tension 

.38  ?n  t/ilji// 


Alveolas 


Denems 


Arter/'al 


Blood. 


Btoad. 


Oz-P^nsi^/n  O.OO^'/mPn 

(Parens io/i  iSt)68m.mPff 

6  to  3  per  ce/it. 


Ox-Tension. 
J4 per  cent 


CO2  -Tension 

38  Jti,  ?n.    Tip, 

Sper  ee/zt 


Tissues. 

Fig.  198. — Diagram  showing  the  Relative  Tension  of  Oxygen  and  Carbon  Dioxid  in 

THE  LXJNGS,  in  THE  BlOOD,  AND  IN  THE  TISSUES. 

organs  and  the  tissues  of  recently  living  animals.  The  analysis  of  the  blood 
before  and  after  perfusion  shows  a  loss  of  oxygen  and  a  gain  m  carbon 
dioxid. 

Tension  of  the  Gases  in  the  Tissues!— As  the  presence  of  free  oxygen 
cannot  be  demonstrated,  its  tension  there  must  be  regarded  as  nil.  The 
tension  of  the  carbon  dioxid  is  quite  high,  though  difficult  of  exact  deter- 
mination. It  has  been  estimated  at  from  45  to  68  mm.  Hg.,  or  from  6  to  9 
per  cent,  of  an  atmosphere. 

The  variations  of  tension  or  pressure  of  these  two  gases  in  the  lungs,  in 
different  parts  of  the  vascular  apparatus,  and  in  the  tissues,  and  their  rela- 


RESPIRATION.  411 

tions  to  each  other,  are  shown  in  Figure  198,  expressed  in  mm.  Hg.  and 
percentages  of  an  atmosphere. 

The  Mechanism  of  the  Gaseous  Exchange.— In  these  pressure  differ- 
ences sufficient,  cause  is  found  for  the  exchange  of  the  gases.  The  oxygen 
pressure  in  the  alveoh  being  in  excess  of  that  in  the  blood,  the  gas  passes 
through  the  thin  alveolo-capillary  wall  into  the  plasma.  As  the  oxygen 
pressure  in  the  plasma  rises  and  approximates  that  in  the  alveoli,  a  portion  of 
the  oxygen  combines  with  the  hemoglobin  until  the  latter  is  almost  saturated. 
The  corpuscle  is  then  carried  through  the  arterial  system  surrounded  by 
oxygen  under  a  definite  pressure  which  is  sufficient  to  keep  the  absorbed 
oxygen  in  union  with  the  hemoglobin.  On  passing  into  the  systemic  capil- 
laries, the  blood  enters  a  region  in  which  the  oxygen  tension  in  the  surround- 
ing tissues  is  nil.  At  once  the  oxygen  dissolved  in  the  plasma  passes  through 
the  capillary  wall  into  the  surrounding  tissue-spaces.  The  pressure  removed 
from  the  corpuscle,  a  dissociation  of  the  oxygen  and  of  the  hemoglobin  takes 
place,  after  which  the  dissociated  oxygen  also  passes  through  the  capillary 
wall  into  the  surrounding  lymph  and  so  to  the  tissue-cells  where  it  is  stored 
and  utilized.  On  passing  into  the  venous  system  the  dissociation  of  the 
oxygen  and  the  hemoglobin  is  checked  by  the  rise  of  oxygen  pressure  in  the 
plasma.  On  reaching  the  lungs  the  oxygen  again  passes  into  the  blood 
until  the  former  condition  is  regained. 

The  sojourn  of  the  blood  in  the  capillaries  being  short,  the  oxyhemoglo- 
bin can  part  with  but  a  portion  of  its  oxygen,  sufl&cient,  however,  to  satisfy 
the  needs  of  the  tissues. 

The  carbon  dioxid  pressure  in  the  tissues  being  in  excess  of  that  in  the 
blood,  it  passes  through  the  capillary  wall  into  the  blood,  where  it  exists  in  the 
free  and  combined  states.  On  passing  into  the  pulmonic  capillaries  the 
blood  enters  a  region  in  which  the  carbon  dioxid  in  the  alveoli  is  less  than 
in  the  blood.  At  once  a  diffusion  and  dissociation  of  the  carbon  dioxid 
takes  place  through  the  alveolo-capillar}-  wall  until  equilibrium  is  established. 
This,  however,  is  of  very  short  duration,  for  the  carbon  dioxid  so  eliminated 
is  rapidly  removed  from  the  lungs  by  the  respiratory  movements. 

While  diffusion,  in  response  to  physical  and  chemic  conditions,  thus 
plays  a  large  part  in,  and  is  sufficient  to  account  for,  the  exchanges  of  gases, 
it  is  possible  that  the  alveolar  or  respiratory  epithelium  may  also  play  an 
essential  role.  It  is  believed  by  some  investigators  that  it  is  active  in  both 
the  absorption  of  oxygen  and  the  excretion  of  carbon  dioxid.  This  view  has 
been  suggested  as  a  means  of  interpreting  the  results  of  the  experiments  of 
more  recent  investigators,  made  with  a  view  of  determining  the  tension  of  the 
blood  gases.  It  was  found  by  Bohr  that  the  tension  of  the  oxygen  in  arterial 
blood  was  often  as  high  as  loi  to  144  mm.  Hg.,  and  in  many  instances  higher 
than  the  tension  of  the  oxygen  in  the  trachea,  while  the  carbon  dioxid  tension 
in  the  trachea  was  higher  than  in  the  blood.  Haldane  and  Smith  by  a  dif- 
ferent method  found  an  oxygen  tension  in  the  arterial  blood  of  200  mm.  Hg. 
If  these  results  should  prove  to  be  correct,  though  they  are  at  present  subject 
to  considerable  criticism  and  not  generally  accepted,  some  other  force  than 
diffusion  would  have  to  be  found  to  explain  the  facts.  It  would  then  remain 
to  be  determined  in  how  far  the  alveolar  epithelium  could  be  regarded  as 
an  active  agent  in  both  absorption  and  excretion  in  opposition  to  pressure. 


412  TEXT-BOOK  OF  PHYSIOLOGY. 

THE  TOTAL  RESPIRATORY  EXCHANGE. 

The  total  quantities  of  oxygen  absorbed  and  carbon  dioxid  discharged 
by  a  human  being  in  twenty-four  hours  are  measures  of  the  intensity  of  the 
respiratory  process,  and  an  indication  of  the  extent  and  character  of  the 
chemic  changes  attending  all  life  phenomena.  Their  determination  and 
their  relation  to  each  other  are  matters  of  interest  and  importance.  The 
quantities  which  have  been  obtained  by  different  observers  are  the  outcome 
of  calculations  based  on  certain  groups  of  data  and  of  experiments  made  with 
special  forms  of  apparatus. 

Thus  from  the  total  air  breathed  daily,  estimated  from  the  amounts 
obtained  during  a  longer  or  shorter  period  by  experiments  with  spirometric 
apparatus,  and  from  the  percentage  loss  of  oxygen  and  gain  of  carbon  dioxid 
shown  by  an  analysis  of  the  respired  air,  it  can  be  calculated  at  least  ap- 
proximately what  the  total  amounts  of  oxygen  absorbed  and  carbon  dioxid 
exhaled  must  be.  If  it  be  assumed  that  the  minimum  daily  volume  of  air 
breathed  is  8500  hters  and  the  maximum  volume  12,752  Hters,  and  the 
percentage  loss  of  oxygen  is  4.78,  then  the  total  volume  of  oxygen  absorbed 
is  406  liters  (580  grams)  or  609  liters  (870  grams).  By  the  same  method  the 
total  carbon  dioxid  exhaled  daily  is  found  to  be  either  372  liters  (735  grams) 
or  558  liters  (1103  grams).  The  direct  experiments  which  have  been  made 
with  specially  devised  forms  of  apparatus,  both  on  human  beings  and  animals, 
have  yielded  similar  results.  With  those  forms  which  are  adapted  for  both 
human  beings  and  animals — Scharling's,  Pettenkofer  and  Voit's — it  is  only 
possible,  however,  to  determine  theamountof  carbon  dioxid  and  water  exhaled 
and  from  them  to  calculate  the  amount  of  oxygen  absorbed.  This  is  done  by 
deducting  the  loss  in  weight  by  the  man  or  animal  during  the  experiment 
from  the  combined  weights  of  the  carbon  dioxid  and  water  discharged. 
The  difference  represents  the  oxygen  absorbed. 

The  Pettenkofer- Voit  apparatus  (Fig.  199)  consists  essentially  of  a 
chamber  large  enough  to  admit  a  man  and  capable  of  being  made  air-tight 
with  the  exception  of  an  inlet  for  air  for  breathing  purposes.  The  res- 
pired air  is  drawn  through  a  tube  and  measured  by  a  large  meter  turned  by  a 
water  or  gas  motor.  By  means  of  a  side  tube  a  fractional  quantity  of  the 
main  column  of  air  is  diverted  to  an  absorption  apparatus  by  a  small  pump. 
This  air  first  passes  into  a  vessel  containing  H^SO^,  by  which  the  water  is 
collected;  then  into  long  tubes  containing  barium  hydroxid,  by  which  the 
carbon  dioxid  is  absorbed;  thence  into  a  small  meter,  by  which  its  amount  is 
registered.  From  the  amount  of  water  and  carbon  dioxid  thus  obtained 
the  amounts  of  both  in  the  total  air  breathed  are  calculated.  The  water  and 
carbon  dioxid  previously  present  in  the  air  are  simultaneously  determined  by 
a  corresponding  absorption  apparatus  and  deducted  from  the  amounts  ob- 
tained from  the  respired  air.  As  the  apparatus  is  traversed  constantly  by  a 
column  of  air  of  normal  composition  and  the  waste  products  removed  as  rapidly 
as  discharged,  the  experiment  can  be  continued  for  periods  varying  from 
six  to  twenty-four  hours  without  detriment  to  the  subject  of  the  experiment. 

With  those  forms  adapted  only  for  animals — Regnault's  and  Reiset's,  or 
Jolyet  and  Regnard's — it  is  possible  to  determine  simultaneously  the  ab- 
sorption of  oxygen  and  the  discharge  of  carbon  dioxid.     As  the  apparatus 


RESPIRATION.  413 

employed  is  completely  closed,  the  carbon  dioxid  must  be  removed  as  soon 
as  discharged  and  the  oxygen  renewed  as  soon  as  absorbed.  The  former  is 
accomplished  by  the  aspiratory  action  of  moving  bulbs  containing  an  alkali, 
the  latter  by  a  steadily  acting  pressure  on  a  reservoir  of  oxygen.  This 
apparatus  consists  essentially  of  a  glass  bell-jar  in  which  the  animal  is 
placed.  This  is  brought  into  connection  by  tubes,  on  the  one  hand,  with 
the  oxygen  reservoir,  and,  on  the  other  hand,  wdth  the  aspiratory  bulbs, 
kept  in  motion  by  some  form  of  motor.  The  construction  of  each  of  these 
forms  of  apparatus  is  so  complex,  the  conduct  of  an  experiment  and  the 
final  determination  of  the  results  so  complicated,  that  a  detailed  description 
would  be  out  of  place  in  a  work  of  this  character.^ 

Of  the  results  obtaind  by  these  and  other  methods  a  few  are  given  in 
the  following  table: 

Oxygen    Absorbed.  Observer.  Carbon    Dioxid    Discharged 

746  grains.  \'ierordt.  876  grams. 

700  grams.  Pettenkofer  and  Voit.  800  granas. 

663  grams.  Speck.  770  grams. 

The  amounts  of  oxygen  absorbed  in  Pettenkofer  and  \'oit's  experiments 
varied  from  594  to  1072  grams;  of  carbon  dioxid  exhaled,  from  686  to  1285 
grams. 

In  all  these.results  it  is  evident  on  examination  that  the  volume  of  oxygen 
absorbed  is  always  greater  than  the  volume  of  carbon  dioxid  exhaled,  or, 
what  amounts  to  the  same  thing,  the  weight  of  the  oxygen  absorbed  is  always 
greater  than  the  weight  of  the  oxygen  entering  into  the  formation  of  the 
carbon  dioxid  exhaled.  The  reason  for  this  difference  between  the  amounts 
of  oxygen  in  the  inspired  air  and  in  the  CO,  cxhaledis  found  in  the  fact  that 
on  a  mixed  diet — one  containing  fat — a  portion  of  the  oxygen  is  utilized  in 
the  oxidation  of  the  surplus  hydrogen  of  the  fat  with  the  formation  of 
water.  Under  such  a  diet  the  respiratory  quotient  is  always  less  than  unity, 
usually  0.907.  On  a  purely  carbohydrate  diet — one  in  which  there  is  no 
surplus  hydrogen — all  the  oxygen  will  combine  with  carbon  and  be  returned 
as  carbon  dioxid,  and  hence  the  respiratory  quotient  will  be  unity.  The 
respiratory  C[Uotient  therefore  indicates  the  extent  to  which  the  oxygen 
absorbed  is  utilized  in  oxidizing  carbon,  on  the  one  hand,  and  hydrogen, 
on  the  other. 

Since  the  total  oxygen  absorbed  and  carbon  dioxid  discharged  will  vary 
considerably  wdth  the  size  of  the  animal,  it  is  customary,  for  purposes  of 
comparison,  to  reduce  all  total  results  to  the  unit  of  body-weight  (one  kilo- 
gram) and  to  the  unit  of  time  (one  hour). 

Respiratory  Activity. — The  activity  or  the  intensity  of  the  respiratory 
process  may  be  measured  either  by  the  oxygen  absorbed  or  by  the  carbon 
dioxid  discharged.  But  as  the  carbon  dioxid  is  more  easily  estimated  than 
the  oxygen,  it  is  usually  taken  as  the  index  of  the  activity,  though  there  are 
reasons  for  believing  that  it  would  be  more  accurately  indicated  or  repre- 
sented by  the  oxygen. 

Whatever  factor  may  be  accepted  as  the  measure,  it  is  certain  that  the 
respiratory  activity  varies  in  different  tissues  in  accordance  with  their  func- 

'  Both  forms  of  apparatus  are  in  use  in  the  Physiological  Laboratory  of  the  Jefferson  Medical 
College  and  are  fully  described  by  Prof.  H.  C.  Chapman  in  his  text-book  on  Physiolog}-,  to  which 
the  reader  is  directed  for  further  information. 


414  TEXT-BOOK  OF  PHYSIOLOGY. 

tional  activities,  being  least  in  bones  and  greatest  in  muscles.  This  is  shown 
by  the  relative  amounts  of  oxygen  absorbed  and  carbon  dioxid  discharged 
by  equal  amounts  of  each  of  these  and  other  living  tissues  in  twenty-four 
hours,  as  given  in  the  following  table: 

QUANTITY  OF  O2  AND  COj  ABSORBED  AND  EXHALED  DURING  TWENTY-FOUR 
HOURS,  IN  CUBIC  CENTIMETERS. 

Oxygen  Carbon  Dioxid 

By  100  Grams  of:  Absorbed.  Exhaled. 

Muscle, 50.8  c.c.  56.8  c.c. 

Brain, 45 .8  c.c.  42 .8  c.c. 

Kidneys, 37  -o  c.c.  15 .6  c.c. 

Spleen, 27.3  c.c.  15.4  c.c. 

Testicles, 18 .3  c.c.  27  . 5  c.c. 

Pounded  bones 17  . 2  c.c.  8.1  c.c. 

The  total  respiratory  change  therefore  of  the  body  as  a  whole  is  the  resultant 
of  the  respiratory  changes  of  its  individual  organs  and  tissues,  and  is  condi- 
tioned by  all  influences  which  retard  or  hasten  their  activity.  Among 
these  influences  the  more  important  are  the  following: 

Muscle  Activity. — As  the  muscles  constitute  a  large  part  of  the  body, 
about  40  per  cent.,  and  as  muscle-tissue  absorbs  and  discharges  relatively 
large  quantities  of  oxygen  and  carbon  dioxid,  it  is  readily  apparent  that  an 
increase  in  their  activity  would  be  followed  or  attended  by  an  increase  in  the 
respiratory  exchange.  In  passing  from  a  condition  of  body  repose  to  one  of 
marked  activity  there  ought  to  be  an  increase  in  the  amount  of  oxygen 
absorbed  and  CO2  discharged.  Pettenkofer  and  Voit  found  that  a  man  in 
repose  who  absorbed  daily  807.8  grams  of  oxygen  and  discharged  930 
grams  COj  absorbed  during  work  1006  grams  of  oxygen  and  discharged  1137 
grams  of  CO2.  Edward  Smith,  who  estimated  only  the  CO2,  found  that  a 
man  in  repose  who  discharged  carbon  dioxid  at  the  rate  of  161. 6  c.c.  per 
minute  increased  the  amount  while  walking  at  the  rate  of  two  and  three 
miles  an  hour  to  569  c.c.  and  851  c.c.  respectively.  Similar  results  have  been 
obtained  by  other  investigators. 

Digestive  Activity. — The  activity  of  the  aUmentary  canal,  involving 
contraction  of  its  muscle  coat  through  its  entire  length  as  well  as  secretion 
of  its  related  glands  called  forth  by  the  ingestion  of  food,  materially  influences 
the  absorption  of  oxygen  and  discharge  of  carbon  dioxid,  independent  of  the 
increase  due  to  the  oxidation  of  food  materials  after  absorption.  It  was 
found  that  in  a  fasting  man  a  dose  of  sodium  sulphate  increased  the  absorp- 
tion of  oxygen  as  much  as  17  per  cent,  and  the  discharge  of  CO 2  24  per  cent. 
(Lowy).  It  is  difhcult  to  determine  how  much  of  the  increase  after  a  meal 
is  therefore  due  to  food  oxidation  and  how  much  to  functional  activity 
of  the  canal  itself.  The  consumption  of  nitrogenized  meals,  however,  has 
a  greater  effect  than  non-nitrogenized  meals. 

Temperature. — A  rise  in  temperature  of  the  surrounding  air  has  as 
an  effect  diminution  in  the  amounts  of  oxygen  consumed  and  carbon 
dioxid  discharged.  A  fall  in  temperature  has  the  opposite  effect.  Thus 
a  cat  at  a  temperature  of  3.2°  C.  consumed  during  a  period  of  six  hours 
21.39  grams  of  oxygen  and  discharged  22  grams  of  carbon  dioxid,  while  at 
a  temperature  of  29.6°  C.  the  corresponding  amounts  for  the  same  period  of 


RESPIRATION.  415 

time  were  for  oxygen  13.9  grams  and  for  carbon  dioxid  13.12  grams.  Lavoi- 
sier and  Sequin,  having  reference  only  to  the  oxygen,  found  that  a  man 
at  a  temperature  of  15°  C.  consumed  38.31  grams  of  oxygen,  while  at  a  tem- 
perature of  32.8°  C.  the  corresponding  amount  was  but  35  grams.  Similar 
results  have  been  obtained  by  other  observers  with  different  animals.  The 
explanation  of  these  facts  is  to  be  found  in  the  increased  activity  of  all 
physiologic  mechanisms  coincident  with  a  fall,  and  in  the  decreased  activity, 
coincident  with  a  rise  in  temperature.  The  lower  temperatures  act  as  a 
stimulus  to  the  peripheral  terminations  of  the  ner\'e  system,  bringing  about 
reflexly  increased  activity  of  the  body  at  large.  The  muscles  especially  are 
not  only  reflexly  but  volitionally  excited  to  greater  activity.  This  leads 
naturally  to  an  increase  in  the  consumption  of  oxygen  and  in  the  production 
of  carbon  dioxid  and  in  the  evolution  of  heat. 

In  cold-blooded  animals  the  respiratory  exchange  is  influenced  in  a 
manner  the  reverse  of  that  observed  in  warm-blooded  animals.  With  a 
rise  of  external  temperature  and  a  corresponding  rise  of  body-temperature 
the  discharge  of  carbon  dioxid  steadily  increases.  Thus  a  frog  in  an  atmo- 
sphere at  0°  C.  with  a  body-temperature  of  1°  C.  discharged  per  kilogram 
per  hour  4.31  c.c.  of  carbon  dioxid;  in  an  atmosphere  of  35°  C.  with  a  body> 
temperature  of  34°  C.  there  was  a  discharged  325  c.c.  per  kilo  per  hour. 
Intermediate  temperatures  were  attended  by  corresponding  increases  in 
the  amounts  of  CO 2  discharged.  The  reason  for  this  difference  in  the  two 
classes  of  animals  is  probably  to  be  found  in  the  cold-blooded  animals,  in 
the  want,  of  a  self-adjusting  heat-regulating  mechanism. 

Age.  —In  early  youth,  as  a  result  partly  of  the  more  pronounced  activity 
of  the  nutritive  energies  and  partly  of  a  cutaneous  surface  relatively  greater, 
as  compared  with  the  mass  of  the  body,  than  in  adult  life,  the  absorption  of 
oxygen  and  the  discharge  of  carbon  dioxid  are  greater  both  absolutely  and 
relatively.  Thus,  in  a  boy  of  nine  and  a  half  years  with  a  weight  of  22 
kilograms  it  was  found  that  in  twenty-four  hours  there  was  a  discharge  of 
carbon  dioxid  amounting  to  488  grams,  or  0.92  gram  per  kilo  per  hour,  and 
in  man  with  a  weight  of  65.5  kilograms  there  was  a  discharge  of  804.72 
grams,  or  0.51  gram  per  kilo  per  hour. 

THE  NERVE  MECHANISM  OF  RESPIRATION. 

The  nerve  mechanism  by  which  the  respiratory  muscles  are  excited  to 
action  is  extremely  complex  and  involves  the  action  of  both  afferent  and  effer- 
ent nerves  and  their  related  nerve-centers  in  the  central  nerve  system.  For 
the  free  introduction  of  air  into  the  lungs  it  is  essential  that  the  nasal  and 
laryngeal  passages  and  the  cavity  of  the  thorax  be  simultaneously  enlarged. 
The  muscles  by  which  these  results  are  accomplished  have  already  been 
mentioned  and  described.  Their  simultaneous  and  coordinate  contraction 
implies  the  coordinate  activity  of  nerve-centers  and  their  related  motor 
nerves;  thus  the  action  of  the  nasal  and  laryngeal  muscles  (the  dilatator 
naris  and  the  posterior  crico-arytenoid)  involves  the  acti\dty  of  the  facial  and 
inferior  laryngeal  nerves  respectively,  the  centers  of  origin  of  which  lie  in 
the  gray  matter  beneath  the  floor  of  the  fourth  ventricle;  the  diaphragm  and 
intercostal  muscles  involve  respectively  the  activity  of  the  phrenic  and 
intercostal  nerves,  the  centers  of  origin  of  which  lie  in  the  anterior  horn  of 


4i6  TEXT-BOOK  OF  PHYSIOLOGY. 

the  gray  matter  of  the  spinal  cord  at  a  level,  for  the  phrenic,  of  the  fourth, 
fifth,  and  sixth  cervical  nerves,  and  for  the  intercostals.at  the  level  of  the 
upper  thoracic  nerves.  Division  of  any  one  of  these  nerves  is  followed  by 
paralysis  of  its  related  muscle. 

Inspiratory  Center. — The  coordinate  contraction  of  the  inspiratory 
muscles  implies  a  practically  simultaneous  discharge  of  nerve  impulses  from 
each  of  the  foregoing  nerve-centers,  accurately  graduated  in  intensity  in 
accordance  with  inspiratory  needs.  This  has  been  supposed  to  necessitate 
the  existence  in  the  central  nerve  system  of  a  single  group  of  nerve-cells  from 
which  nerve  impulses  are  rhythmically  discharged  and  conducted  to  the 
previously  mentioned  nerve-centers  in  the  medulla  oblongata  and  spinal 
cord,  by  which  they  are  in  turn  excited  to  activity.  To  this  group  of  cells  the 
term  "inspiratory  center"  has  been  given. 

For  the  free  exit  of  air  from  the  lungs  it  is  essential  not  only  that  the  air- 
passages  be  open,  but  that  the  air  in  the  lungs  be  compressed  until  its  pressure 
rises  above  that  of  the  atmosphere.  This  is  accomplished  by  the  recoil  of  the 
elastic  tissue  of  the  lungs  and  thorax,  the  return  of  the  displaced  abdominal 
organs  aided  by  atmospheric  pressure,  and  the  contraction  of  the  expiratory 
muscles.  In  how  far  muscle  action  is  necessary  for  expiratory  purposes 
will  depend  on  the  resistance  offered  to  the  outflow  of  air  and  on  the  degree 
of  efficiency  of  the  elastic  forces. 

Expiratory  Center. — The  simultaneous  and  coordinate  activity  of  the 
expiratory  muscles  in  impeded  expirations  also  involves  the  action  of  motor 
nerves  and  nerve-centers.  The  simultaneous  and  coordinate  discharge  of 
nerve  impulses,  also  graduated  in  intensity  for  expiratory  needs,  apparently 
implies  the  existence  in  the  central  nerve  system  of  a  single  center  from  which 
nerve  impulses  are  rhythmically  discharged  which  excite  and  coordinate 
the  lower  nerve-centers.  To  this  group  of  cells  the  term  "expiratory  center" 
has  been  given.  The  two  centers  taken  together  constitute  the  so-called 
"respiratory  center." 

The  anatomic  existence,  however,  of  a  definite  group  of  cells  which 
initiates  the  respiratory  movements  has  not  as  yet  been  demonstrated. 
Nevertheless  there  is  in  the  dorsal  portion  of  the  medulla  oblongata,  at  the 
level  of  the  sensory  end-nucleus  of  the  vagus  nerve,  a  region  the  sudden  de- 
struction of  which  on  one  side  is  followed  by  a  cessation  of  respiratory  move- 
ments on  the  corresponding  side,  though  they  continue  on  the  opposite  side, 
a  fact  which  indicates  that  the  area,  though  acting  as  a  unit,  is  bilateral. 
The  bilateral  character  of  the  area  is  also  shown  by  the  continuance  of  the 
respiratory  movements  on  both  sides  after  longitudinal  division  of  the 
medulla.  Destruction  of  the  entire  region  is  followed  by  a  complete  cessation 
of  respiratory  activity  and  death  of  the  animal.  For  this  reason  the  term 
'•noeud  vital"  was  applied  to  it.  In  this  area  the  respiratory  center  was 
located.  It  has,  however,  been  shown  by  Gad  that  if  this  area  be  gradually 
destroyed  by  cauterization  the  respiratory  movements  do  not  cease,  but 
continue  until  the  cauterization  has  reached  a  point  far  forward  in  the 
formatio  reticularis,    in  which  the  respiratory  center  was   assumed  to  lie. 

Though  its  existence  has  not  been  anatomically  determined  beyond 
c[uestion,  it  is  permissible  to  speak  of  the  central  mechanism  as  a  "center" 
located  in  the  medulla  oblongata. 


RESPIRATION.  417 

The  Cause  of  the  Rhythmic  Activity  of  the  Inspiratory  Center. — 

It  has  long  been  a  subject  of  discussion  as  to  whether  the  periodic  activity 
of  the  inspiratory  center  is  automatic  or  autochthonic  (Gad)  in  character, 
expressive  of  the  idea  that  the  rhythmic  discharge  of  nerve  impulses  is  due 
to  some  stimulating  agent  generated  in  the  nerve-cells  of  the  center  itself,  the 
activity  of  which  is  conditioned  by  the  gaseous  condition  of  the  blood;  or 
whether  it  is  reflex  in  character,  that  is,  due  to  the  action  of  nervT  impulses 
received  from  different  regions  of  the  body  through  afferent  nerves.  The 
solution  of  this  problem  has  apparently  been  settled  by  experiments  the 
object  of  which  was  the  division  of  all  afferent  nerve-paths  that  might  have 
central  connections  with  the  center.  The  results  of  experiments  of  this 
character  are  somewhat  as  follows :  When  the  vagus  nervTs  are  divided  the 
respiratory  movements  at  once  diminish  in  numbers  per  minute  but  at  the 
same  time  increase  in  depth  and  amplitude.  The  number  of  respiratory 
movements  under  these  circumstances  varies  in  different  animals  from  four 
to  eight  per  minute,  a  rate  which  continues  practically  constant  so  long  as  the 
animal  lives,  which  may  be  a  period  varying  from  a  few  days  to  several 
weeks.  The  relative  duration  of  the  respiratory  phases  also  undergoes  a 
change,  inspiration  becoming  longer  than  expiration  and  at  the  same  time 
becoming  more  or  less  spasmodic  in  character. 

Inasmuch  as  it  is  a  familiar  observ^ation  that  the  normal  rate  of  the  respira- 
tory movement  is  frequently  disturbed  by  nerve  impulses  transmitted  to  the 
center,  through  afferent  nerves  other  than  the  vagi,  as  well  as  from  higher 
centers  in  the  brain,  section  of  the  vagi  has  been  supplemented  by  a  trans- 
verse section  of  the  spinal  cord  at  the  level  of  the  first  dorsal  nerve,  by  section 
of  the  dorsal  roots  of  the  cervical  nerves  and  by  a  transverse  section  of  the 
region  of  the  brain  just  posterior  to  the  corpora  quadrigemina,  a  series  of  pro- 
cedures which  practically  isolates  the  center  from  all  transmitted  impulses. 
Nevertheless,  the  inspiratory  center  still  continues  to  discharge  nerve  im- 
pulses to  the  respiratory  muscles  at  a  rate  not  differing  much  from  that 
witnessed  after  section  of  the  vagi.  At  most  the  diminution  in  the  rate 
will  not  be  more  than  two  or  three  more  per  minute.  The  results  of  these 
experimental  procedures  would  seem  to  indicate  that  the  fundamental  rate 
of  discharge  of  nerve  impulses  is  approximately  from  four  to  six  per  minute. 
This  conclusion  has  been  strengthened  by  the  results  of  experiments  designed 
to  suspend  the  activity  for  some  minutes  by  the  withdrawal  of  the  blood 
by  temporarily  occluding  the  blood-vessels  passing  to  the  head.  With  the 
resuscitation  of  the  center,  after  the  release  of  the  blood-stream  and  at  a 
time  when  there  are  reasons  for  believing  that  the  afferent  paths  are  still 
incapable  of  conduction,  the  initial  rate  of  discharge  was  practically  constant, 
about  four  per  minute  in  the  cat.  The  same  result  was  observed  in  some 
instances  in  cats  when  in  addition  to  producing  anemia  the  vagi  as  well 
as  the  region  posterior  to  the  corpora  quadrigemina  were  divided  (Stewart). 

It  may  therefore  be  assumed  that  the  respiratory  center  possesses  an  in- 
dependent automatic  rhythm  which  is,  however,  much  slower  than  that 
characteristic  of  it  when  all  afferent  paths  leading  to  it  are  intact. 

Accepting  the  statement  that  the  fundamental  rhythm  of  the  inspiratory 
center  is  automatic — that  is,  due  to  a  stimulus  generated  within  itself — 
the  question  at  once  arises  as  to  the  nature  of  the  stimulating  agent.  By 
27 


4i8  TEXT-BOOK  OF  PHYSIOLOGY. 

some  investigators  it  has  been  assumed  that  the  stimulus  is  connected  with 
the  content  or  pressure  of  oxygen,  by  others  with  the  content  or  pressure  of 
carbon  dioxid,  and  that  the  variations  in  the  respiratory  rhythm  arc  depend- 
ent on  variations  in  the  pressure  of  one  or  the  other  of  these  two  gases. 
As  a  result  of  a  long  series  of  experiments  made  on  animals  and  human 
beings,  with  the  respiratory  nerve  mechanism  intact,  it  is  now  the  generally 
accepted  opinion  that  the  more  efhcient  cause  for  the  respiratory  rhythm 
is  an  increase  in  the  pressure  of  carbon  dioxid  in  the  blood  and  hence  in  the 
center  itself  rather  than  a  decrease  in  the  pressure  of  the  oxygen.  Whether 
the  pressure  of  the  carbon  dioxid  be  the  efhcient  cause  or  not  of  the  funda- 
mental respiratory  rhythm  there  is  abundant  evidence  that  the  activity  or  the 
irritability  of  the  center  is  modified  to  an  extraordinary  extent  by  variations 
in  the  pressure  of  the  carbon  dioxid  when  the  nerve  system  is  intact.  Proofs 
in  support  of  this  statement  will  be  given  in  a  subsequent  paragraph. 

The  first  inspiration  after  birth  is  supposed  to  be  due  to  the  direct 
stimulation  of  the  respiratory  center  by  the  increase  in  the  carbon  dioxid 
present  in  the  blood,  though  it  may  be  aided  by  the  cooling  of  the  skin  due 
to  vaporization  of  the  amniotic  fluid. 

Reflex  Stimulation  of  the  Inspiratory  Center. — Whether  the  inspira- 
tory center  is  automatic  in  character  or  not,  it  may  be  influenced  directly 
by  nerve  impulses  descending  from  the  brain  in  consequence  of  volitional 
acts  or  emotional  states,  and  indirectly  by  nerve  impluses  brought  to  it 
from  the  general  periphery  through  various  afferent  nerves,  in  consequence  of 
agencies  acting  on  their  peripheral  terminations:  e.g.,  cold  applied  to  the  skin, 
irritating  gases  to  the  nasal  and  bronchial  mucous  membrane,  distention  and 
collapse  of  the  pulmonary  alveoli. 

Of  all  afferent  nerves,  the  vagus  appears  to  be  the  most  influential  in 
maintaining  the  normal  rhythmic  discharge  of  nerve  impulses  from  the 
inspiratory  center,  as  shown  by  the  effects  that  follow  their  separation  from 
the  center.  (Fig.  igg.)  Thus,  if  whfle  the  animal  is  breathing  regularly 
and  quietly  both  vagi  are  cut,  the  respiratory  movements  become  much  slower, 
falling  perhaps  to  one-third  their  original  number  per  minute.  At  the  same 
time  the  inspirations  become  deeper  and  somewhat  spasmodic  in  character. 
The  duration  of  the  inspiratory  movement  is  also  increased  beyond  that 
of  the  expiratory  movement.  If  now  the  central  end  of  the  divided  vagus 
be  stimulated  with  weak  faradic  currents,  the  respiratory  movements  are 
again  increased  in  frequency  and  their  depth  diminished  until  the  normal 
rate  is  restored.  With  the  cessation  of  the  stimulation  the  former  condition 
at  once  returns.  This  would  indicate  that  in  the  physiologic  state  afferent 
impulses  are  ascending  the  vagus  fibers  which  influence  the  rate  of  discharge 
from  the  inspiratory  center,  or,  in  other  words,  inhibit  the  inspiratory 
discharge  and  lead  to  an  expiratory  movement  sooner  than  would  other- 
wise be  the  case.  If,  however,  the  stimulation  is  increased  in  strength, 
the  inspiratory  movement  gradually  so  exceeds  the  expiratory  that  the  mus- 
cles pass  into  the  tetanic  state  and  the  chest-walls  come  to  rest  in  the  con- 
dition of  forced  inspiration.  The  vagus  apparently  contains  fibers  which 
are  capable  of  so  exciting  or  augmenting  the  activity  of  the  inspiratory  center, 
and  therefore  the  extent  of  the  inspiratory  movement,  as  to  lead  to  the  con- 
dition of  tetanus  of  the  inspiratory  muscle.     If,  on  the  other  hand,   the 


RESPIRATION. 


419 


central  end  of  the  divided  superior  laryngeal  nerve  be  stimulated  with 
induced  electric  currents,  the  opposite  effect  is  produced:  viz.,  an  excess  of 
the  expiratory  over  the  inspiratory  movement  until  the  chest-walls  come  to 
rest  in  the  condition  of  passive  expiration.  The  superior  laryngeal  nerve 
apparently  contains  fibers  which  gradually  inhibit  activity  of  the  inspiratory 
center  and  hence  the  inspiratory  movement. 


meet.  ob. 


Fig.  199. — Dl^gkam  Showing  the  Relation  of  the  Pulmonary  Fibers  of  the  Vagus  to 

THE    iNSPIR.'iTORY    CENTER    AND    THE    CONNECTIONS    OF   THE    LaTTER    WITH   THE    PHRENIC   AND 

Intercostal  Nerve  Centers  and  Their  Related  Muscles. — (G.  Bachman).  med.  ob.  Medulla 
oblongata,  sp.  c.  Spinal  cord.  p.  v.  r.  Pulmonary  vagus  nerve,  excitator  and  inhibitor,  in  sp.  c. 
Inspiratory  center,  phr.  c.  Phrenic  nerve  centers,  phr.  n.  Phrenic  nerve,  int.  n.  c.  Intercostal 
nerve  centers,    int.  c.  n.  Intercostal  nerves,   ext.  int.  c.  m.  External  intercostal  muscles. 


The  same  result,  an  expiratory  standstill,  not  infrequently  follows  strong 
stimulation  of  the  divided  vagi,  and  always  after  the  administration  of 
large  doses  of  chloral. 

The  results  of  these  experiments  would  seem  to  indicate  that  the  vagus 
nerve  contains  two  classes  of  nerve-fibers,  one  of  which,  when  stimulated, 
inhibits  and  regulates  the  discharge  of  nerve  energy  from  the  inspiratory 


420 


TEXT-BOOK  OF  PHYSIOLOGY. 


Positive 
ventilation. 


center,  and  thereby  the  extent  and  frequency  of  the  inspiratory  movement; 
the  other  of  which  when  stimulated,  excites  or  augments  the  discharge  of 
nerve  energy  from  the  inspiratory  center  and  thereby  leads  to  an  increase 
in  the  depth  or  amplitude  of  the  inspiratory  movement.  According  as 
the  one  or  the  other  of  these  two  classes  of  fibers  are  excessively  stimu- 
lated, will  the  inspiratory  center  be  inhibited  or  augmented  in  its  activity 
to  such  an  extent  that  the  chest-walls  will  come  to  rest  in  the  first  in- 
stance in  the  state  of  expira- 
tory standstill,  in  the  second 
instance  in  the  state  of  inspira- 
tory standstill. 

The  stimulus  adequate  to 
the  excitation  of  the  pulmon- 
ary terminations  of  the  vagus 
nerve-fibers  in  the  physiologic 
condition  was  formerly  be- 
lieved to  be  the  chemic  action 
of  carbon  dioxid;  it  is  now 
believed  to  be  a  mechanic 
action,  the  result  of  the  alternate  distention  and  collapse  of  the  walls  of  the 
pulmonary  alveoli.  Thus,  it  has  been  shown  by  Head  that  if  the  lungs 
are  actively  inflated  (positive  ventilation)  there  will  be  produced  an  in- 
hibition of  the  inspiratory  and  an  augmentation  of  the  expiratory  movement 
until  the  inspiratory  muscles  are  completely  relaxed  as  indicated  by  the 
relaxation  of  the  diaphragm,  the  movements  of  which  are  simultaneously 


Fig.  200. — Positive  Ventilation  {Head).  Under 
the  influence  of  positive  ventilation,  the  inspiratory 
contractions  of  the  diaphragm  become  less  and  less 
till  they  disapoear  completely. 


Seconds. 


Fig.  201.^ — Negative  Ventilation.  (Head).  At  a  negative  ventilation  was  commenced. 
The  expiratory  relaxation  of  the  diaphragm  is  seen  to  become  more  and  more  incomplete,  until  it 
finally  enters  into  continued  contraction. 


recorded  (Fig.  200),  a  result  similar  in  all  respects  to  that  produced  by 
stimulation  of  the  superior  laryngeal  nerve.  On  the  other  hand,  if  the 
lungs  are  collapsed  by  the  artificial  withdrawal  of  air  (negative  ventilation) 
there  will  be  produced  an  augmentation  of  the  inspiratory  and  an  inhibition 
of  the  expiratory  movements  until  the  inspiratory  muscles  are  in  a  condi- 
tion of  tetanic  contraction  as  indicated  by  the  contraction  of  the  dia- 
phragm (Fig.  201)  and  by  the  state  of  the  thorax  which  is  that  characteristic 
of  extreme  inspiration,  a  result  similar  in  all  respects  to  that  produced  by 
moderate  stimulation  of  the  central  end  of  the  divided  vagus. 

A  satisfactory  explanation  of  the  action  of  the  respiratory  mechanism 


RESPIRATION.  421 

is  very  difficult  to  present.  Theories  vary  in  accordance  with  the  estimate 
of  an  investigator  as  to  the  degree  of  automaticity  of  the  inspiratory  center, 
of  the  effects  of  vagus  stimulation  and  as  to  the  extent  to  which  the  expira- 
tory center  is  involved  with  the  activity  of  the  inspiratory  center  either 
simultaneously  or  successively. 

If  it  is  assumed  that  the  inspiratory  center  is  automatic  and  in  a  state  of 
continuous  excitation  the  result  of  the  action  of  carbon  dioxid  in  the  blood 
circulating  around  it,  then  it  is  only  necessary  to  assume  the  existence,  in 
the  trunk  of  the  vagus  of  one  set  of  nerve-fibers,  viz.,  inhibitor  fibers, 
the  central  terminations  of  which  arborize  around  the  inspiratory  center 
and  the  function  of  which  is  to  check  or  inhibit  the  action  of  the  inspiratory 
center  and  thus  permit  of  an  expiratory  movement.  The  inhibitor  fibers 
are  supposed  to  be  stimulated  peripherally  by  the  expansion  of  the  lungs. 
With  the  recoil  of  the  lungs  the  inhibitor  effect  gradually  dies  away,  while  the 
inherent  excitation  of  the  inspiratory  center  again  returns,  to  be  followed 
by  another  discharge  of  nerve  impulses  and  a  new  inspiratory  movement, 
which  will  in  turn  be  again  inhibited  as  the  inhibitor  fibers  are  stimulated 
by  the  expanding  lung.  This  explanation  is  in  accordance  with  the  results 
which  follow  stimulation  of  the  superior  laryngeal  nerve  or  the  trunk  of 
the  vagus  with  induced  electric  currents  of  moderate  intensity. 

If  it  is  assumed,  on  the  contrary,  that  the  inspiratory  center  is  not  in  a 
state  of  constant  excitation  leading  to  a  frequent  periodic  discharge  of  nerve 
impulses,  but  requires  the  arrival  of  a  stimulus  to  call  forth  its  normal 
activity,  then  this  theory  does  not  suffice,  inasmuch  as  it  leaves  out  of  con- 
sideration the  presence  of  nerve-fibers  in  the  vagus  which  increase  or  aug- 
ment the  acti\dty  of  the  inspiratory  center;  and  that  such  fibers  are  present  is 
apparently  indicated  by  the  effects  of  stimulation  of  the  central  end  of  the 
vagus  nerve  with  moderately  strong  induced  electric  currents  and  from  the 
experiments  of  Hering  and  Breuer,  and  later  of  Head.  These  observers 
assume,  therefore,  that  in  addition  to  the  inhibitor  fibers  there  are  also  present  in 
the  vagus  excitator  fibers,  the  central  terminations  of  which  are  in  relation 
with  the  inspiratory  center  also  (Fig.  199);  and  just  as  the  inhibitor  fibers 
are  stimulated  by  the  expansion  of  the  lungs  so  the  excitator  fibers  are  stimu- 
lated in  turn  by  the  recoil  of  the  lungs.  The  nerve  impulses  thus  developed 
ascend  to  the  inspiratory  center,  excite  it,  and  call  forth  a  new  inspiration 
sooner  than  it  would  otherwise  take  place.  According  to  this  view  the 
respiratory  mechanism  is  self-regulative  and  maintained  by  the  alternate 
expansion  and  recoil  of  the  lungs. 

Many  experimenters,  however,  find  difficulty  in  accepting  the  view  that 
the  recoil  of  the  lungs  should  stimulate  nerve  endings  and  hence  this  theory 
has  not  received  general  acceptance. 

Another  explanation  which  is  satisfactory  in  many  respects  has  been 
presented  by  Meltzer.  This  investigator  asserts  also  the  existence  in  the 
trunk  of  the  vagus  the  two  classes  of  nerve-fibers,  the  inhibitor  and  the 
excitator;  but  that  for  some  reason  they  do  not  respond  to  stimulation  at 
the  same  time  as  shown  by  the  effects  which  follow;  the  inhibitor  fibers 
respond  first  and  the  excitator  fibers  somewhat  later.  Therefore  when 
they  are  stimulated  simultaneously  the  primary  effect  is  an  inhibition  of 
the  inspiratory  center  followed  by  an  expiratory  movement.     The  secondary 


422  TEXT-BOOK  OF  PHYSIOLOGY. 

effect  is  a  stimulation  of  the  inspiratory  center  followed  by  a  new  inspiratory 
movement.  In  this  view  expansion  of  the  lungs  stimulates  both  the  inhibitor 
and  the  excitator  fibers,  but  during  the  expansion  and  for  a  short  time  after, 
the  eflect  of  the  inhibitor  stimulation,  viz.,  cessation  of  inspiration  and  the 
advent  of  expiration,  alone  manifests  itself.  With  the  cessation  of  expira- 
tion, the  inhibitor  stimulation  dies  away  and  the  late  effect  or  the  long  after- 
effect of  the  excitator  stimulation,  viz.,  a  new  inspiration,  manifests  itself. 
This  author  assumes  the  surface  of  the  lung  to  be  the  peripheral  organ  of  the 
respiratory  reflexes. 

When  it  is  assumed  that  both  inspiratory  and  expiratory  centers  cooper- 
ate in  a  respiratory  movement,  as  they  do  in  labored  respiration  either 
simultaneously  or  successively,  the  difficulties  of  the  problem  are  manifestly 
much  greater.  In  this  case  it  may  be  supposed  that  aft'erent  impulses,  de- 
veloped during  the  expansion  of  the  lung,  inhibit  the  inspiratory  while  aug- 
menting the  expiratory  center,  and  that  impulses  developed  during  the  recoil 
of  the  lungs  inhibit  the  expiratory  while  stimulating  the  inspiratory  center. 

The  Effect  of  a  Change  in  the  Pressure  of  the  Blood  Gases  on  the 
Activity  of  the  Inspiratory  Center. — It  has  long  been  known  that  the  in- 
spiratory center  is  very  sensitive  to  a  change  in  the  composition  of  the  blood  in 
so  far  as  its  gaseous  constituents  are  concerned.  So  long  as  the  composition 
remains  normal  the  center  retains  its  normal  irritability  and  rhythm.  As 
stated  in  a  previous  paragraph  it  has  been  a  subject  of  discussion  as  to  whether 
the  center  is  more  responsive  to  an  increase  in  the  pressure  of  the  carbon 
dioxid  or  to  a  decrease  in  the  pressure  of  the  oxygen.  As  the  outcome  of 
a  long  series  of  experiments  it  is  now  the  generally  accepted  opinion  that  an 
increase  in  the  percentage  and  pressure  of  the  carbon  dioxid  in  the  blood 
and  hence  in  the  center  itself  is  more  efficient  in  raising  the  irritability  of  the 
center  than  a  decrease  in  the  percentage  and  pressure  of  the  oxygen.  Thus 
if  an  animal  is  caused  to  inhale  air  containing  but  2  per  cent,  of  COj  more 
than  normal  the  respiratory  movements  will  be  increased  in  frequency  and 
depth,  while  a  corresponding  diminution  in  the  percentage  of  oxygen  will 
be  without  effect. 

It  has  been  shown  by  Haldane  and  Priestley  that  when  an  individual  was 
breathing  normal  air  and  the  rate  of  the  respiratory  movement,  14  per  minute, 
the  average  depth  was  637  c.c.  and  the  total  ventilation  was  8.918  liters  per 
minute.  On  raising  the  percentage  of  the  CO,  in  the  inspired  air  from 
0.04  per  cent,  to  0.79  per  cent,  the  average  depth  increased  to  739  c.c.  and 
the  total  ventilation  to  10.346  liters  per  minute,  the  rate  remaining  the  same. 
When  the  percentage  of  the  CO 2  was  raised  to  2  per  cent,  the  average  depth 
increased  to  864  c.c,  the  rate  to  15,  and  the  total  ventilation  to  12.960  liters 
per  minute;  and  when  the  CO 2  in  the  inspired  air  was  raised  to  6  per  cent, 
the  average  depth  was  increased  to  2104  c.c,  the  rate  to  27  per  minute,  and 
the  total  ventilation  to  56.808  liters.  The  results  of  these  experiments  indicaet 
that  an  increase  in  the  percentage  of  the  COj  in  the  inspired  air  leads  to  an 
increase  in  the  percentage  and  pressure  of  the  CO2  in  the  arterial  blood 
and  hence  in  the  inspiratory  center,  as  a  result  of  which  the  center  becomes 
more  irritable  and  discharges  its  energy  more  frequently  and  to  a  greater 
degree  as  shown  by  the  increase  in  the  rate  and  the  depth  of  the  inspiratory 
movement. 


RESPIRATION.  423 

The  same  observers  have  also  shown  that  when  an  individual  is  caused 
to  inhale  air  the  percentage  of  the  oxygen  of  which  had  been  reduced  from 
20  to  13  and  therefore  to  about  8  per  cent,  in  the  alveolar  air  instead  of  about 
15  per  cent,  no  particular  change  in  either  the  frequency  or  the  depth  of 
the  inspiratory  movements  was  noticed,  but  when  the  percentage  of  the 
oxygen  was  lowered  below  this  amount  the  inspiratory  center  became  more 
irritable  as  shown  by  an  increase  in  the  rate  and  depth  of  the  inspiratory 
movement.  As  a  rule  the  oxygen  percentage  in  the  alveolar  air  must  be 
reduced  fully  one-half  and  thereby  the  percentage  and  pressure  of  the  oxygen 
in  the  arterial  blood  fully  one-third  before  the  respiratory  center  is  stimulated 
to  increased  activity.  A  reason  assigned  for  this  result  is  the  presence  in 
the  blood  of  some  non-oxidized  metabolic  product,  probably  lactic  acid,  that 
is  acting  as  the  stimulus.  All  recent  experimental  work  confirms  the  view 
that  the  specific  stimulus  to  the  inspiratory  center  is  the  normal  pressure  of 
the  CO 2  in  the  blood  and  so  responsive  is  it  to  this  agent  that  an  increase 
in  even  0.2  per  cent,  in  the  alveolar  air  is  sufficient  to  almost  double  the 
respiratory  ventilation. 

MODIFICATIONS  OF  THE  RESPIRATORY  RHYTHM. 

The  character  of  the  respiratory  movements  is  materially  modified  by  a 
change  in  the  quantitative  and  qualitative  composition  of  the  air  and 
blood  as  well  as  by  changes  of  a  pathologic  nature  of  the  respiratory  appa- 
ratus itself. 

Eupnea. — So  long  as  the  air  retains  its  normal  composition  and  the 
respiratory  mechanism  its  structural  integrity,  so  long  do  the  respiratory 
movements  exhibit  a  normal  rhythm  and  frequency.  To  the  condition  of 
easy .  tranquil  breathing  the  term  eupnea  is  given.  In  this  condition  the 
percentages  of  oxygen  and  carbon  dioxid  in  the  blood  are  such  as  to  favor  at 
least  the  rhythmic  discharge  of  nerve  impulses  to  the  respiratory  mus- 
cles, of  sufficient  energy  and  frequency  for  the  maintenance  of  normal 
respiration. 

Hyperpnea. — The  normal  rate  of  the  respiratory  movements  is  increased 
by  a  rise  in  body-temperature,  by  active  exercise,  and  by  emotional  states. 
Whatever  the  cause,  the  increase  in  rate  and  probably  in  depth  is  termed 
hyperpnea. 

Febrile  states  characterized  by  a  rise  in  the  temperature  of  the  blood 
increase  considerably  the  respiratory  activity.  This  is  due  in  all  probability 
to  a  warming  of  the  respiratory  center,  in  consequence  of  which  its  excitabil- 
ity is  heightened;  for  surrounding  the  carotid  arteries  with  warm  tubes  and 
heating  the  blood  on  its  way  to  the  medulla  has  the  same  effect.  It  is  also 
possible,  however,  that  the  high  temperature  of  febrile  conditions  may  inter- 
fere with  the  absorbing  power  of  hemoglobin,  and  thus  by  diminishing  the 
quantity  of  oxygen  absorbed  lead  to  more  frequent  respirations.  To  the 
hyperpnea  induced  by  heat  the  term  thermo-polypnea  is  frequently  given. 

Muscle  activity,  especially  if  it  is  ^^olent  and  indulged  in  by  those  unac- 
customed to  exercise,  is  generally  followed  by  increased  rate  and  depth  of 
breathing,  and  not  infrequently  it  is  attended  with  such  extreme  difficulty 
that  the  condition  approximates  that  of  dyspnea.     This  condition  is  attrib- 


424  TEXT-BOOK  OF  PHYSIOLOGY. 

uted  to  the  production  and  discharge  into  the  blood  of  metabolic  products 
which  act  as  stimuli  to  the  respiratory  center  and  thus  increase  its  activity, 
Of  these  metabolic  products  CO,  is  undoubtedly  one  of  the  most  efficient, 
as  stated  in  foregoing  paragraphs.  Emotional  states  temporarily  increase 
respiratory  activity.     With  their  disappearance  the  normal  condition  returns. 

Apnea. — Apnea  may  be  defined  as  a  temporary  cessation  of  the  respira- 
tory movements.  It  may  be  developed  by  rapid  and  deep  inspirations  due 
to  volitional  efforts,  by  rapid  mechanic  inflation  of  the  lungs,  and  by  stimu- 
lation of  various  afferent  nerves.  If  one  volitionally  breathes  rapidly  and 
deeply  for  a  period  varying  from  two  to  ten  minutes,  it  will  be  found  on 
cessation  of  the  effort  that  a  condition  of  apnea  is  established  which  may 
last  for  from  thirty  seconds  to  several  minutes.  One  experimenter  succeeded 
after  forcible  inspiration  for  two  and  a  half  minutes,  in  establishing  in 
himself  an  apnea  that  lasted  for  several  minutes  before  there  was  the  slightest 
desire  to  breathe.  Before  the  cessation  of  the  apnea,  the  face  became  pale 
and  corpse-like,  indicative  of  a  marked  condition  of  anoxhemia.  If  the 
lungs  of  an  animal  be  rapidly  inflated  through  a  cannula  inserted  in  the 
trachea,  a  similar  condition  is  developed.  Whether  the  apnea  be  estab- 
lished by  volitional  efforts  or  by  mechanic  inflation,  the  respiratory  move- 
ments gradually  return.  At  first  they  are  feeble  but  soon  increase  in 
amplitude  and  frequency  until  the  normal  is  reached.  At  one  time  the 
apnea  that  results  from  rapid  ventilation  of  the  lungs,  whether  volitional  or 
mechanical,  was  attributed,  on  the  assumption  that  a  deficiency  of  oxygen 
in  the  arterial  blood  is  the  physiologic  stimulus  to  the  activity  of  the  inspira- 
tory center,  to  an  excess  of  oxygen  in  the  blood,  the  result  of  the  forced 
ventilation,  complete  saturation  of  the  plasma  and  the  hemoglobin,  in 
consequence  of  which  the  inspiratory  center  remained  inactive.  The 
apneic  state  is  at  present  attributed,  on  the  assumption  that  carbon  dioxid 
in  the  arterial  blood  is  the  physiologic  stimulus  to  the  inspiratory  center,  to 
a  diminution  in  the  percentage  of  the  carbon  dioxid  in  the  alveolar  air  (4  per 
cent,  or  less),  in  the  blood, and  therefore  in  the  center,  the  result  of  the  forced 
ventilation.  The  increased  ventilation  eliminates  the  carbon  dioxid  to  such 
an  extent  that  the  percentage  and  pressure  in  the  blood  is  insufficient  to 
arouse  the  center  to  activity.  To  the  condition  of  the  blood  that  results 
from  this  rapid  ventilation,  viz.,  a  diminished  percentage  of  CO,,  the  term 
acapnia  has  been  given.  An  apnea  which  is  thus  developed  is  termed  apnea 
chemica  or  apnea  vera.  As  previously  stated,  stimulation  of  certain  afferent 
nerves,  especially  the  vagus,  will  induce  a  similar  cessation  of  the  respiratory 
movements.  Thus  if  the  central  end  of  the  divided  vagus  be  stimulated, 
the  thorax  will  come  to  rest  in  the  state  characteristic  of  deep  expiration 
from  inhibition  of  the  inspiratory  center.  Inasmuch  as  stimulation  of  the 
vagus  causes  an  apnea  resembling  that  caused  by  rapid  inflation  of  the  lungs, 
it  has  been  suggested  that  in  the  development  of  apnea  the  inspiratory  center 
is  inhibited  in  its  activity  simultaneously  with  the  elimination  of  the  CO 2, 
from  the  mechanic  stimulation  of  the  pulmonary  terminations  of  the  vagus. 
An  apnea  caused  by  stimulation  of  the  vagus  is  termed  apnea  vagi  or  apnea 
inhihitoria. 

In  the  apnea  that  results  from  voluntary  or  mechanic  inflation  of  the 
lungs  it  is  difficult  to  state  in  how  far  the  condition  is  due  to  a  diminution 


RESPIRATION.  425 

in  the  pressure  of  the  CO,  and  in  how  far  to  a  stimulation  of  the  vagus. 
But  inasmuch  as  apnea  can  be  estabUshed,  though  not  of  such  long  duration, 
after  division  of  the  vagus  nerves,  the  probabilities  are  that  the  diminished 
percentage  of  the  CO 2  is  the  main  cause. 

Dyspnea. — Excessive  and  laborious  respiratory  movements  constitute 
a  condition  known  as  dyspnea.  Movements  of  this  character  indicate  that 
the  blood  contains  a  greater  percentage  of  CO,  than  normal  or  a  diminished 
percentage  of  oxygen.  In  either  case  the  excitability  of  the  respiratory  center 
is  abnormally  heightened.  Of  the  two  conditions,  the  former  is  by  far  the 
more  common.  While  it  is  true  that  a  deficiency  of  oxygen  in  the  arterial 
blood  gives  rise  to  an  increase  in  the  rate  and  depth  of  the  respiratory  move- 
ments, this  does  not  arise  until  the  deficiency  of  the  oxygen  falls  to  about 
one-third  of  the  normal.  On  the  other  hand,  an  increase  of  even  0.2  per 
cent,  of  CO,  in  the  alveolar  air  will  almost  double  the  respiratory  activity. 

A  deficiency  in  the  amount  or  the  quality  of  the  hemoglobin  is  usually 
attended  with  more  or  less  dyspnea.  These  conditions  of  the  blood  may  be 
caused:  (i)  By  all  those  pathologic  conditions  of  the  respiratory  apparatus 
which  limit  the  free  entrance  of  oxygen  into  and  the  free  exit  of  carbon 
dioxid  from  the  blood;  (2)  by  those  alterations  in  the  composition  of  the  air 
and  subsequently  in  the  blood  which  arise  when  the  individual  is  confined  in  a 
space  of  moderate  size  with  imperfect  ventilation. 

Asphyxia. — If  the  state  of  the  blood  observed  in  dyspnea  be  exaggerated 
■ — that  is,  if  the  increase  in  the  percentage  of  carbon  dioxid  become  more 
marked — the  respiratory  movements  become  more  laborious.  A  con- 
tinuance of  this  changed  composition  of  the  blood  eventuates  in  death. 
Before  this  occurs  the  individual  exhibits  a  succession  of  phenomena,  to  the 
totality  of  which  the  term  asphyxia  is  given. 

Asphyxia  may  be  caused :  (i)  By  a  sudden  interference  with  the  entrance 
of  oxygen  into  and  the  exit  of  carbon  dioxid  from  the  blood,  as  in  drowning, 
occlusion  of  the  trachea  from  any  cause,  double  pneumothorax,  etc.  (2) 
By  confinement  in  a  small  space  the  air  of  which  speedily  undergoes  a  loss 
of  oxygen  and  an  accumulation  of  carbon  dioxid.  In  the  first  instance 
death  may  occur  in  a  few  minutes;  in  the  second  instance  it  may  be  postponed 
several  hours  or  more,  the  time  varying  with  the  size  of  the  space. 

The  succession  of  phenomena  presented  by  an  individual  in  the  asphyxi- 
ated condition  is  as  follows:  Increased  rate  and  depth  of  the  respiratory 
movements,  passing  rapidly  from  hyperpnea  to  dyspnea,  with  an  active  con- 
traction of  all  the  muscles  concerned  in  respiration,  ordinary  and  extraor- 
dinary; a  blue,  cyanosed  condition  of  the  face  from  the  rapid  accumulation 
of  carbon  dioxid  and  disappearance  of  the  oxygen  of  the  blood;  a  diminution 
in  the  depth  of  inspiration  and  an  increase  in  the  force  and  extent  of  ex- 
piration, followed  by  general  convulsions;  collapse,  characterized  by  un- 
consciousness, loss  of  the  reflexes,  relaxation  of  the  muscles,  a  weak  action  of 
the  heart,  a  disappearance  of  the  pulse,  and  death.  As  shown  by  observation 
of  the  circulatory  apparatus  in  artificially  induced  asphyxia,  there  is  primarily 
an  increase  in  the  activity  of  the  heart,  soon  followed  by  retardation;  a  rise 
of  blood-pressure  in  the  early  stages  and  a  fall  to  zero  after  collapse  has  set 
in.  The  retardation  and  final  cessation  of  the  heart,  as  well  as  the  rise  of 
the  blood-pressure,  are  to  be  attributed  to  stimulation  of  the  cardio-inhibi- 


426  TEXT-BOOK  OF  PHYSIOLOGY. 

tory  and  vaso-motor  centers  from  the  accumulation  of  the  carbon  dioxid. 
With  the  exhaustion  of  the  nerve-centers,  there  is  a  general  relaxation  of  the 
skeletal  muscles,  the  cardiac  muscle,  a  fall  of  the  blood-pressure,  and  dilata- 
tion of  the  pupils. 

The  Cheyne-Stokes  Respiration. — -A  modification  of  the  respiratory 
movements  characterized  by  periods  of  rest  alternating  with  periods  of 
activity  was  described  in  1818  and  in  1854  by  the  two  writers  whose  names  it 
bears.  The  periods  of  rest  vary  in  duration  from  twenty  to  thirty  seconds; 
the  periods  of  activity  from  thirty  to  sixty  seconds  and  may  include  from 
twenty  to  thirty  respiratory  movements. 

Each  period  of  rest  of  the  respiratory  mechanism  is  closed  by  the  appear- 
ance of  a  slight  shallow  respiratory  movement,  which  is  immediately  fol- 
lowed by  a  second,  slightly  deeper,  and  this  in  turn  by  a  third,  a  fourth,  a 
fifth,  and  so  on,  each  becoming  deeper  than  the  preceding  until  a  certain 
maximum  is  reached,  after  which,  each  succeeding  movement  gradually 


Fig.  202. — Tr.\cing  Showing  the  Cheyne-Stokes  Form  of  Respiration. — {Hill.) 

diminishes  in  depth  until  finally  the  movement  becomes  imperceptible  and  a 
new  period  of  rest  supervenes.  A  graphic  representation  of  the  Cheyne- 
Stokes  type  of  respiration  is  shown  in  Fig.  202.  This  type  of  respiration  is 
frequently  an  accompaniment  of  certain  pathologic  conditions,  e.g.,  uremic 
states,  cerebral  hemorrhage,  heart  diseases,  arteriosclerosis,  etc.,  though  no 
satisfactory  explanation  of  it  has  yet  been  presented.  A  similar  though  far 
less  marked  periodicity  in  the  respiratory  movements  is  frequently  observed 
during  sleep,  especially  in  children.  A  periodicity  can  also  be  developed 
by  dividing  transversely  the  medulla  oblongata  just  above  the  calamus 
scriptorius,  which  either  injures  the  respiratory  center  or  removes  from  it 
some  cerebral  influence. 


THE  EFFECT  OF  THE  RESPIRATORY  MOVEMENTS  ON  THE  FLOW  OF 
BLOOD  THROUGH  THE  INTRA-THORACIC  VESSELS,  AND  ON 
THE  ARTERIAL  PRESSURE. 

I.  On  the  Intra-thoracic  Vessels.— The  forces  which  cause  the  air 
to  flow  into  and  out  of  the  lungs  will  at  the  same  time  and  in  a  similar  way 
cause  the  blood  of  the  extra-thoracic  veins  to  flow  into,  through,  and 
out  of  the  intra-thoracic  vessels.  From  the  tendency  of  the  pulmonary 
elastic  tissue  to  recoil,  the  blood-vessels  in  the  thorax  at  the  end  of  an  expira- 


RESPIRATION.  427 

tion  sustain  a  positive  pressure,  the  intra-thoracic  pressure  (see  page  386), 
about  six  millimeters  of  mercury  less  than  that  in  the  lungs,  or,  in  other  words, 
a  pressure  negative  to  that  of  the  atmosphere  by  six  millimeters.  As  a  result 
the  blood  in  the  systemic  vessels  under  atmospheric  pressure  will  flow 
steadily  toward  the  intra-thoracic  veins,  the  vense  cavae,  and  the  right 
side  of  the  heart,  i.e.,  from  a  point  of  high  to  a  point  of  low  pressure.  During 
inspiration  there  is  a  decrease  in  the  intra-thoracic  pressure,  the  decrease 
being  proportional  to  the  extent  of  the  inspiration.  With  this  decrease 
of  pressure,  the  intra-thoracic  veins  expand  and  their  internal  pressure  falls. 
As  the  systemic  or  extra-thoracic  veins  are  subjected  to  atmospheric  pressure, 
the  blood  in  these  vessels  is  forced,  by  reason  of  the  difference  of  pressure 
between  these  two  regions,  to  flow  more  rapidly  and  freely  into  the  intra- 
thoracic veins  and  right  side  of  the  heart.  The  right  heart  being  more 
generously  filled  with  blood  will  discharge  a  larger  volume  with  each  con- 
traction into  the  pulmonary  artery. 

Coincident  with  these  efl^ects  a  similar  eft'ect  is  produced  in  the  arterioles 
and  capillaries  of  the  pulmonary  alveoli.  Situated  between  the  two  elastic 
layers  of  the  alveolar  wall,  embedded  in  a  meshwork  of  connective  tissue, 
the  pressure  to  which  they  are  subjected  at  the  end  of  an  expiration  will 
also  be  a  few  millimeters  less  than  the  intra-pulmonic  pressure;  and  at  the 
end  of  an  inspiration  it  will  be  considerably  less.  With  the  inspiration  there- 
fore there  will  occur  a  dilatation  of  these  vessels,  and  hence  a  larger  flow  of 
blood  through  them  and  into  the  pulmonary  veins.  The  left  heart,  being 
in  consequence  more  generously  filled  with  blood,  will  discharge  a  larger 
volume  into  the  aorta  at  each  contraction.  During  expiration  the  flow  of 
blood  through  the  intra-thoracic  vessels  will  be  diminished  for  the  reverse 
reasons. 

2.  On  the  Arterial  Pressure. — An  examination  of  a  tracing  of  the  arterial 
pressure  will  show  that  it  is  characterized  by  small  undulations  due  to  the 
cardiac  beat  and  large  undulations  due  to  the  respiratory  movements, 
inspiration  being  accompanied  by  a  rise,  expiration  by  a  fall  of  pressure. 
These  results  are  readily  accounted  for  by  the  difference  in  the  volume  of 
blood  discharged  by  the  left  heart  into  the  aorta  during  the  time  of  the  two 
movements.  If  a  tracing  of  the  respiratory  movements  and  of  the  blood- 
pressure  be  taken  simultaneously,  it  will  be  found  that  the  rise  of  pressure 
does  not  exactly  correspond  with  inspiration,  nor  the  fall  of  pressure  with 
expiration;  for  a  certain  period  after  the  beginning  of  an  inspiration  the 
pressure  continues  to  fall,  and  for  a  certain  period  after  the  beginning  of  an 
expiration  the  pressure  continues  to  rise.  During  the  remainder  of  the 
period,  however,  inspiration  is  attended  by  a  rise,  expiration  by  a  fall  of 
pressure.  The  explanation  of  these  results  lies  in  the  fact  that  at  the  begin- 
ning of  the  inspiration,  when  the  vessels  dilate,  the  blood-flow  momentarily 
slows;  the  left  heart  continuing  to  discharge  small  volumes  into  the  aorta, 
the  pressure  continues  to  fall.  So  soon  as  the  left  heart  begins  to  be  better 
filled,  the  pressure  at  once  begins  to  rise.  At  the  end  of  an  expiration  the 
flow  of  blood  into  the  left  heart  continues  and  the  pressure  rises,  but  with  the 
return  of  the  intra-thoracic  pressure  the  vessels  diminish  in  caliber,  the 
volume  of  blood  transmitted  by  them  becomes  less,  the  output  of  the  left  heart 
declines,  and  the  pressure  falls. 


428  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Traube-Hering  Waves. — Under  certain  experimental  conditions 
the  arterial  blood-pressure  tracing  exhibits,  in  addition  to  the  usual  respira- 
tory variations,  certain  longer  rhythmic  variations  more  or  less  wave-like  in 
character,  which  are  known  as  Traube-Hering  waves.  They  can  be  devel- 
oped on  a  blood-pressure  tracing  by  injecting  magnesium  sulphate  or  mor- 
phine into  the  circulation,  by  tying  the  cerebral  arteries,  etc.  These  waves 
indicate  a  periodic  contraction  and  dilatation  of  the  blood-vessels,  the  result 
of  a  stimulation  of  the  vaso-motor  centers. 


CHAPTER  XVI. 
ANIMAL  HEAT. 

The  chemic  changes  which  take  place  in  the  tissues  and  organs  of  the 
living  body  and  which  underHe  all  manifestations  of  life  are  attended  by  the 
evolution  of  heat.  In  consequence  of  this  each  animal  acquires  a  certain 
body-temperature. 

In  man,  as  well  as  in  other  mammals  and  in  birds,  the  chemic  changes 
are  extremely  active  and  the  evolution  of  heat  very  great.  Through  some 
special  heat-regulating  mechanism,  by  which  heat-production  and  heat- 
dissipation  are  kept  in  equilibrium,  these  animals  have  acquired  and  main- 
tain within  limits  a  constant  temperature  which  is  independent  of  and  gen- 
erally above  that  of  the  surrounding  atmosphere.  As  the  temperature  of 
these  animals  is  high  and  perceptible  to  the  sense  of  touch,  they  were  origi- 
nally designated  "warm-blooded"  animals.  As  this  temperature  is  con- 
stant notwithstanding  the  great  variations  in  external  temperature  during 
the  summer  and  winter  seasons,  they  are  more  appropriately  termed  con- 
stant-temperatured  or  homoio-thermoiis  animals.  The  intensity  of  the  body- 
temperature  determined  by  the  insertion  of  a  thermometer  in  the  rectum 
varies  in  different  classes  of  mammals  from  37.2°  C.  to  40°  C.  The 
causes  of  this  variation  are  doubtless  connected  with  peculiarities  of  organ- 
ization. In  birds  the  rectal  temperature  is  usually  higher,  varying  from 
40.9°  C.  in  the  pigeon  to  44°  C.  in  the  titmouse  and  the  swift. 

In  reptiles,  amphibians,  and  fish  chemic  changes  as  a  rule  are  not  very 
active  and  heat-production  relatively  slight.  As  they  are  devoid  of  a  suffi- 
ciently active  heat-regulating  mechanism,  the  temperature  of  the  body  is 
largely  dependent  on  that  of  the  medium  in  which  they  live,  though  it  is 
always  one  or  more  degrees  above  it.  In  winter  the  body-temperature  of 
frogs,  for  example,  may  decline  as  low  as  8.9°  C,  the  temperature  of  the 
surrounding  medium  being  6.7°  C.  When  subjected  to  temperatures  below 
zero,  the  temperature  of  the  body  may  fall  below  the  freezing-point  also, 
when  the  lymph  and  fluids  of  the  body  become  ice.  Though  apparently 
dead,  the  gradual  elevation  of  the  temperature  restores  their  vitality.  In 
summer-time,  on  the  contrary,  the  body-temperature  may  attain  to  38°  C. 
Similar  variations  have  been  observed  in  other  animals.  As  the  temperature 
of  these  animals  is  low  and  perceptibly  below  that  of  our  own  bodies,  they 
were  originally  termed  "cold-blooded"  animals;  as  their  temperature  is 
inconstant,  varying  with  the  temperature  of  the  surrounding  medium,  they 
are  more  appropriately  termed  "variable-temperatured"  or  poikilo-ther- 
mous  animals. 

THE  TEMPERATURE  OF  THE  HUMAN  BODY. 

The  determination  of  the  temperature  of  the  human  body  under  the 
changing  conditions  of  life  is  a  matter  of  the  greatest  physiologic  and 
clinical  interest.     The  temperature  of  the  superficial  portions  of  the  body 

429 


430  TEXT-BOOK  OF  PHYSIOLOGY. 

may  be  obtained  by  the  introduction  of  a  thermometer  into  the  mouth, 
the  rectum,  the  vagina,  or  the  axilla.  As  a  result  of  many  observa- 
tions it  has  been  found  that  the  temperature  of  the  rectum  is,  on 
the  average,  37.2°  C;  that  the  mouth,  36.8°  C;  that  of  the  axilla,  36.9°  C. 
Owing  to  radiation  and  conduction,  the  surface  temperature  is 
lower  than  that  of  either  the  mouth  or  rectum,  and  varies  to  a  slight  extent 
in  different  regions  of  the  body:  e.g.,  at  a  room-temperature  of  20°  C. 
the  skin  of  the  pectoral  region  has  a  temperature  of  34.7°;  that  of  the 
cheek,  34.4°;  that  of  the  calf,  33.6°;  that  of  the  tip  of  the  ear,  only 
28.8°,  etc. 

In  the  interior  of  the  body,  especially  in  organs  in  which  oxidation  takes 
place  rapidly,  and  which  at  the  same  time  are  protected  by  their  anatomic 
surroundings  from  rapid  radiation,  the  temperature  is  higher  than  that 
observ^ed  in  the  rectum.  From  an  investigation  of  the  temperature  of  the 
blood  as  it  emerges  from  the  liver,  the  muscles,  the  brain,  alimentary  canal, 
etc.,  it  is  evident  that  these  organs  have  a  higher  temperature  than  the 
rectum. 

As  the  chemic  changes  underlying  physiologic  activity  vary  in  intensity 
and  extent  in  different  regions  of  the  body,  there  would  be  marked  varia- 
tions in  their  temperature  were  it  not  that  the  blood,  having  a  large  ca- 
pacity for  heat-absorption,  distributes  the  heat  almost  uniformly  to  all  por- 
tions of  the  body,  so  that  at  a  short  distance  beneath  the  surface  the  tem- 
perature varies  but  a  few  degrees. 

In  the  dog  the  temperature  of  the  blood  in  the  aorta  and  in  its  principal 
branches  is  approximately  38.3°  C.  In  passing  through  the  systemic  cap- 
illaries the  temperature  falls  from  radiation  and  conduction  to  surface 
temperature,  to  again  rise  as  the  venous  blood  approaches  the  deeper  regions 
of  the  body.  In  the  neighborhood  of  the  renal  veins  and  in  the  superior 
vena  cava,  the  temperature  is  again  that  of  the  aorta.  In  the  portal  vein 
the  temperature  rises  to  40.2°  C;  in  the  hepatic  vein,  to  40.6°  C.  In  the 
right  ventricle,  owing  to  the  admixture  of  blood  from  different  localities 
having  different  temperatures,  the  temperature  falls  to  38.2°  or  40.4°.  In 
passing  through  the  pulmonary  capillaries  the  temperature  of  the  blood 
again  falls,  so  that  in  the  left  ventricle  it  will  register  from  38°  C.  to  40.2°  C. 
There  is  thus  usuallv  a  difference  between  the  two  sides  of  the  heart  of  about 
0.2°  C. 

Variations  in  the  Mean  Temperature. — The  mean  temperature  of 
the  human  body  for  twenty-four  hours,  which  for  the  mouth  and  the  rectum 
may  be  accepted  at  36.7°  C.  and  37.2°  C.  respectively,  is  subject  to  variations 
from  a  variety  of  circumstances,  such  as  age,  periods  of  the  day,  food,  exer- 
cise, etc. 

Age. — At  birth  the  temperature  of  the  infant  is  slightly  higher  than 
that  of  the  mother,  registering  in  the  rectum  about  37.5°  C.  In  a  few  hours 
it  rapidly  declines  to  about  36.5°,  to  be  followed  in  the  course  of  twenty-four 
hours  by  a  rise  to  the  normal  or  slightly  beyond.  During  childhood  the 
temperature  gradually  approximates  that  of  the  adult.  In  old  age  the  tem- 
perature rises,  as  a  rule,  and  attains  a  maximum  at  eighty  years  of  37.4°  C. 

Periods  oj  the  Day. — The  observations  of  Jtirgensen  show  that  there  is 
a  diurnal  variation  in  the  mean  temperature  of  from  0.5°  C.  to  1.5°  C,  the 


ANIMAL  HEAT.  431 

maximum  occurring  late  in  the  afternoon,  from  5  to  7  o'clock,  the  minimum 
early  in  the  morning,  from  4  to  7  o'clock.  This  diurnal  variation  in 
the  mean  temperature  is  related  to  corresponding  variations  in  many  other 
physiologic  processess,  and  its  causes  are  to  be  found  in  the  ordinary  habits 
of  life  as  regards  the  time  of  meals,  periods  of  exercise,  sleep,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the  tempera- 
ture but  slightly — not  more  than  0.5°  C.  InsufiEiciency  of  food  lowers 
the  temperature;  total  withdrawal  of  food,  as  in  starvation,  is  followed  by 
a  steady  though  slight  decline,  until  just  preceding  the  death  of  the  animal, 
when  it  falls  abruptly  to  from  6°  to  8°  C.  Cold  drinks  lowey,  hot  drinks 
raise  the  temperature.  Food  and  drinks,  however,  only  temporarily  change 
the  mean  temperature,  and  after  a  short  period  equilibrium  is  restored 
through  the  activity  of  the  heat-regulating  mechanism.  Alcoholic  drinks 
lower  the  temperature  about  0.5°  C.  In  large  toxic  doses  in  persons  un- 
accustomed to  their  use  the  temperature  may  be  lowered  several  degrees. 
This  is  attributed  not  to  a  diminution  in  heat-production,  but  rather  to  an 
increase  in  heat-dissipation  (Reichert)  from  increased  action  of  the  heart, 
dilatation  of  the  blood-vessels  of  the  skin,  and  increased  activity  of  the 
sweat-glands. 

Exercise. — The  temperature  may  be  raised  by  active  muscular  exercise 
from  1°  to  1.5°  C.  as  a  result  of  increased  activity  in  chemic  changes  in  the 
muscles  themselves.  A  rise  beyond  this  point  is  prevented  by  the  increased 
activity  of  the  circulatory  apparatus,  the  removal  of  the  heat  to  the  surface, 
and  its  rapid  radiation. 

External  Temperature. — The  external  temperature  influences  but  slightly 
the  mean  temperature  of  the  human  body.  In  the  tropic,  as  well  as  in  the 
arctic  regions,  notwithstanding  the  change  in  the  temperature  of  the  air, 
the  temperature  of  the  body  remains  almost  constant.  The  same  is  true  for 
the  seasonal  variations  in  the  temperature  of  the  temperate  regions. 

THE  SOURCE  AND  TOTAL  QUANTITY  OF  HEAT  PRODUCED. 

The  Source  of  Heat. — The  immediate  source  of  the  body-heat  is  to  be 
found  in  the  chemic  changes  which  take  place  in  all  the  tissues  and  organs 
of  the  body.  Each  contraction  of  a  muscle,  each  act  of  secretion,  each 
exhibition  of  nerve-force,  is  accompanied  by  the  evolution  of  heat.  The 
chemic  changes  are  for  the  most  part  of  the  nature  of  oxidations,  the  union 
of  oxygen  with  the  elements,  carbon  and  hydrogen,  of  the  food  principles 
either  before  or  after  they  have  become  constituents  of  the  tissues.  The 
ultimate  source  of  the  body-heat  is  the  latent  or  potential  energy  in  the  food 
principles,  which  was  absorbed  from  the  sun's  energy  and  stored  up  during 
the  growth  of  the  vegetable  world.  In  the  metabolism  of  the  animal  body 
the  food  principles  are  again  reduced  through  oxidation,  directly  or 
indirectly,  to  relatively  simple  bodies,  such  as  urea,  carbon  dioxid,  and 
water,  with  a  liberation  of  a  large  portion  of  their  contained  energy  which 
manifests  itself  as  heat  and  rnechanic  motion. 

The  Total  Quantity. — -The  total  quantity  of  heat  liberated  in  the  body 
daily  may  be  approximately  determined  in  at  least  two  ways:  (i)  By  deter- 
mining experimentally  the  heat  values  of  different  food  principles  by  direct 
oxidation;  (2)  by  collecting  and  measuring  with  a  suitable  apparatus,  a  cal- 


432  TEXT-BOOK  OF  PHYSIOLOGY. 

orimeter,  the  heat  evolved  by  the  oxidation  of  the  food  within,  and  dissipated 
from,  the  body  daily. 

I.  Direct  Oxidation. — The  amount  of  heat  which  any  given  food  prin- 
ciple will  yield  can  be  determined  by  burning  a  definite  amount — e.g.,  i 
gram — to  carbon  dioxid  and  water  and  ascertaining  the  extent  to  which 
the  heat  thus  liberated  will  raise  the  temperature  of  a  given  amount  of  water, 
e.g.,  I  kilogram.  The  amount  of  heat  may  be  expressed  in  gram  or  kilo- 
gram degrees  or  calories;  a  gram  calorie  or  kilogram  Calorie  being  the  amount 
of  heat  required  to  raise  the  temperature  of  a  gram  or  a  kilogram  (looo 
grams)  of  water  i°  C.  The  apparatus  employed  for  this  purpose  is  termed 
a  calorimeter,  which  consists  essentially  of  a  closed  chamber,  in  which  the 
oxidation  takes  place,  surrounded  by  a  water-jacket.  The  rise  in  tempera- 
ture of  the  water  indicates  the  amount  of  heat  produced. 

The  results  obtained  by  investigators  employing  different  calorimeters 
and  different  food  principles  of  the  same  class  vary,  though  within  narrow 
limits:  e.g.,  i  gram  casein  yields  5.867  kilogram  Calories;  i  gram  of  lean 
beef,  5,656;  I  gram  of  fat,  9.353,  9423,  9.686  Calories;  i  gram  of  starch  or 
sugar,  4. 1 16,  4.182,  4.479,  etc.,  Calories.  These  results  are,  however, 
physical  values,  and  indicate  the  quantity  of  heat  such  quantities  of  foods 
give  rise  to  when  completely  oxidized  to  carbonic  acid  and  water.  In  the 
human  body  the  carbohydrates  and  the  fats,  with  the  exception  of  the  small 
portion  which  escapes  digestion,  are  reduced  to  carbon  dioxid  and  water, 
and  hence  practically  liberate  as  much  heat  as  they  do  when  oxidized  outside 
the  body.  The  proteins,  however,  are  only  reduced  to  the  stage  of  urea.  As 
this  compound  is  capable  of  further  reduction  in  the  calorimeter  to  carbon 
dioxid  and  water,  with  the  liberation  of  heat,  the  quantity  of  heat  it  contains 
must  therefore  be  deducted  from  the  physical  heat  value  of  the  protein. 
According  to  Rubner,  i  gram  of  urea  will  yield  2.523  kilogram  Calories. 
As  about  one-third  of  a  gram  of  urea  results  from  the  oxidation  of  i  gram 
of  protein,  the  amount  of  heat  to  be  deducted  from  the  heat  value  of  the 
protein  is  \  of  2.523,  or  0.841  Calories.  It  has  also  been  shown  by  the  same 
investigator  that  some  of  the  ingested  protein  is  found  in  the  feces,  the  heat 
value  of  which  must  also  be  determined  and  deducted.  This  having  been 
done,  the  physiologic  heat  value  becomes  4.124  Calories. 

The  following  estimates  give  approximately  the  number  of  kilogram 
Calories  which  should  be  liberated  within  the  body  when  the  proteid  is  burned 
to  the  stage  of  urea,  and  the  fat  and  carbohydrate  to  the  stage  of  carbon  dioxid 
and  water: 

I  gram  of  protein 4-124  Calories 

I  gram  of  fat 9-353  Calories 

I  gram  of  carbohydrate   4.1 16  Calories 

The  total  number  of  kilogram  calories  yielded  by  the  various  diet  scales 
can  be  readily  determined  by  multiplying  the  quantities  of  the  food  prin- 
ciples consumed  by  the  foregoing  factors.  The  diet  scale  of  Vierordt,  for 
example,  yields  the  following: 

120  grams  of  proteid 494.88  Calories 

90  grams  of  fat ■ 841.77  Calories 

330  grams  of  starch 1358.28  Calories 

Total 2694.93  Calories 


ANIMAL  HEAT. 


433 


The  total  Calories  obtained  from  other  diet  scales  would  be  as  follows: 
Ranke's,  2335;  Voit's,  3387;  Moleschott's,  2984;  Atwater's,  3331;  Hultgren's, 
3436.  These  numbers  indicate  theoretically  the  total  heat-production  in 
the  body  daily. 

2.  Calorimetric  Measurements. — By  this  method  the  heat  dissipated 
from  the  body  of  an  animal  is  directly  collected  and  measured,  and  the 
amount  so  obtained  is  taken  as  a  measure  of  the  heat  evolved  by  the  oxidation 
of  the  food.  A  calorimeter  is  therefore  an  apparatus  for  the  direct  estimation 
of  the  quantity  of  heat  dissipated  from  the  body  in  given  time.  The  sub- 
stance employed  for  collecting  and  measuring  the  heat  is  either  water  or 
air.  The  calorimeters  in  general  use  consist  essentially  of  two  metallic 
boxes  placed  one  within  the  other,  though  separated  by  a  space  sufficiently 
large  to  hold  a  definite  amount  of  water  (Fig.  203).  The  animal  is  placed 
in  the  inner  box,  which  is  also  provided  with  tubes  for  the  entrance  of  fresh 
and  the  exit  of  expired  air.  The  heat  radiated  is  absorbed  by  the  water  and 
its  temperature  raised.  To 
prevent  loss  by  radiation 
and  to  render  it  independ- 
ent of  changes  in  the 
surrounding  temperature 
the  calorimeter  is  sur- 
rounded by  a  poorly  con- 
ducting material,  such  as 
wool.  The  temperature  of 
the  animal  is  taken  at  the 
beginning  and  the  end  of 
the  experiment.  If  the  tem- 
perature of  the  animal  re- 
mains the  same  at  the  end 
of  the  experiment,  then  the 
heat  absorbed  by  the  water 
represents  the  amount  pro- 
duced by  the  animal.  If, 
on  the  contrary,  the  tem- 
perature of  the  animal  rises 
or  falls,  the  number  of  calories  so  retained  or  lost  must  be  added  to  or  sub- 
tracted from  the  amount  absorbed  by  the  calorimeter.  In  the  determination 
of  the  absolute  amount  of  heat  retained  or  lost  by  the  animal  above  or 
below  the  initial  temperature,  as  well  as  that  absorbed  by  the  materials  of 
the  apparatus  in  these  various  instances,  the  water  equivalent  of  the  tissues 
of  the  animal  and  the  materials  of  the  calorimeter  must  be  obtained,  and 
then  added  to  or  subtracted  from,  as  the  case  may  be,  the  amount  of  water 
in  the  calorimeter,  and  the  amount  thus  obtained  multiplied  by  its  rise  in 
temperature.  In  properly  conducted  experiments  in  which  the  sources 
of  error  are  reduced  to  a  minimum  there  is  a  very  close  correspondence 
between  the  total  physiologic  heat  value  of  the  food  and  the  amount  col- 
lected by  the  calorimeter.  Thus,  in  an  experiment  detailed  by  Rubner,  a 
dog  was  given  during  twelve  days  228.06  grams  of  protein  and  340.4  grams 
of  fat,  the  physical  heat  value  of  which  was  estimated  at  4419  Calories.  The 
28 


Fig.  203.  —  Water  Calorimeter  of  Dulong.  D 
and  D'.  Tubes  for  the  entrance  and  exit  of  air.  T  and  T'. 
Thermometers  for  ascertaining  the  temperature  of  the 
water.  S.  A  mechanic  contrivance  for  stirring  the  water 
for  the  purpose  of  distributing  the  absorbed  heat  uni- 
formly. To  prevent  the  esca])e  of  heat  with  the  expired 
air,  the  tube  D'  is  wound  many  times  in  the  water-space 
beneath  the  animal  cage. 


434  TEXT-BOOK  OF  PHYSIOLOGY. 

urine  and  feces  during  this  period  were  collected  and  their  heat  value 
determined,  which  amounted  to  305  Calories.  The  heat  which  theoretic- 
ally should  have  been  produced  was  41 19  Calories.  During  the  experiment 
the  calorimeter  actually  absorbed  3958  Calories,  a  difference  between  the 
theoretic  and  experimental  results  of  156  Calories;  thus  of  the  total  energy 
liberated  96  per  cent,  appeared  as  heat. 

Calorimetric  experiments  on  man  corresponding  to  those  made  by 
Rubner  on  dogs  have  not  been  successful,  owing  purely  to  technical  diffi- 
culties. Various  attempts  have  been  made,  however,  to  determine  the  daily 
heat-dissipation.  Liebermeister  immersed  a  man  in  a  bath  with  a  tem- 
perature lower  than  that  of  the  man's  body.  From  the  rise  in  temperature 
of  the  water  it  was  calculated  that  the  man  produced  daily  3525  Calories. 
Leyden  placed  the  leg  alone  of  a  man  in  a  calorimeter.  In  one  hour  6 
Calories  were  absorbed.  Assuming  that  the  total  superficial  area  of  the  body 
was  fifteen  times  that  of  the  leg,  he  calculated,  taking  into  consideration 
various  sources  of  error,  that  the  entire  body  would  produce  daily  2376 
Calories.  Ott,  employing  a  water  calorimeter,  found  that  the  body  of  a 
man  produced  103  Calories  during  an  afternoon,  or  at  the  rate  of  2472  Cal- 
ories daily.  These  and  similar  experiments,  while  not  free  from  many 
objections,  furnish  results  which  indicate  that  the  heat  dissipated  from  the 
body  approximates  the  physiologic  heat  values  of  the  foods. 

HEAT-DISSIPATION  AND  REGULATION  OF  THE  TEMPERATURE. 

Heat-dissipation. — From  the  preceding  statements  it  is  evident  that 
the  body  is  continually  liberating  heat  in  amounts  daily  far  in  excess  of 
that  necessary  for  the  maintenance  of  the  body-temperature.  Should 
this  heat  be  retained,  the  temperature  of  the  body  would  be  raised  at 
the  end  of  twenty-four  hours  an  additional  18°  or  20°  C. — a  temperature 
far  in  excess  of  that  compatible  with  the  maintenance  of  physiologic  pro- 
cesses. That  the  body  may  be  kept  at  the  mean  temperature  of  37°  C.  it 
is  essential  that  the  heat  liberated  be  dissipated  as  fast  as  it  is  produced, 
or  to  state  the  problem  in  another  way,  the  heart  dissipated  by  the  body 
must  be  replaced  by  an  equal  amount  liberated,  if  equilibrium  of  temperature 
is  to  be  maintained.  The  dissipation  of  the  heat  is  accomplished  in  several 
ways:  (i)  In  warming  the  food  and  drink  to  the  temperature  of  the  body. 
(2)  In  warming  the  inspired  air  to  the  same  temperature.  (3)  In  the  eva- 
poration of  water  from  the  lungs.  (4)  In  evaporating  water  from  the  skin. 
(5)  In  radiation  and  conduction  from  the  skin.  The  quantities  of  heat  lost 
to  the  body  by  these  different  processes  it  is  difficult  for  obvious  reasons  to 
accurately  determine,  and  the  estimates  usually  given  must  be  regarded  only 
as  approximative. 

Assuming  2500  Calories  to  be  an  average  of  heat  liberated  during  a  day 
of  repose,  the  losses,  in  the  ways  stated  above,  may  be  given  as  follows: 
I.  In  Warming  Food  and  Drink. — The  average  temperature  of  food  and 
drink  is  about  12°  C;  the  amount  of  both  together  is  about  3  kilograms; 
the  specific  heat  of  food  about  0.8  that  of  water.  The  absorption 
of  body-heat  therefore  by  the  food  amounts  approximately  to  3X0.8 
X25°  C.  =  6o  Calories=2.8  per  cent.     With  the  removal  of  the  end- 


ANIMAL  HEAT.  435 

products  of  the  foods  and  drink  from  the  body  an  equal  amount  of 
heat  is  carried  out. 

2.  In  Warming  the  Inspired  Air. — The    average   temperature  of    the   air 

is  12°  C;  the  amount  of  inspired  air,  about  15  kilograms;  the  specific 
heat  of  air,  0.26.  The  absorption  of  body-heat  by  the  air  until  it  at- 
tains the  temperature  of  the  body  will  therefore  amount  to  15  X0.26X  25° 
=  97.5  Calories  =  3.8  per  cent.  The  expired  air  removes  from  the 
body  a  corresponding  amount. 

3.  In  the  Evaporation  of  Water  from  the  Lungs. — The  quantity  of  water 

evaporated  from  the  lungs  may  be  estimated  at  400  grams;  as  each 
gram  requires  for  its  evaporation  0.582  Calorie,  the  quantity  of  heat 
lost  by  this  channel  would  be  400X0.582  =  232.8  Calories==9.4  per 
cent. 

4.  In  the  Evaporation  of  Water  from  the  Skin. — The  quantity  of  water  evapor- 

ated from  the  skin  may  be  estimated  at  660  grams,  causing  a  loss  of 
heat  by  this  channel  of  660X0.582  =  384.1  Calories=i5.3  per  cent. 

5.  In  Radiation  and  Conduction  from  the  Skin. — The  amount  of  heat  lost 

by  this  process  can  be  indirectly  determined  only  by  subtracting  the 
total   amount   lost   by   the   above-mentioned   channels  from  the  total 
amount  produced.     Thus,  2500—7774.4=1725.6  Calories  =  69  per  cent, 
would  represent  the  average  amount  lost  by  radiation  and  conduction. 
Regulation    of    the   Mean    Temperature. — In    order  that  the    mean 
temperature  of  the  body  may  remain  practically  constant,  the  heat  dissipated 
must  be  exactly  balanced  by  the  heat  liberated.     Should  there  be  any  want 
of  correspondence  between  the  two  processes,  there  would  arise  either  an 
increase  or  a  decrease  in  the  mean  temperature.     As  both  heat-production 
and  heat-dissipation  are  variable  factors,  dependent  on  a  variety  of  internal 
and  external  conditions,  their  adjustment  is  accomplished  by  a  complex  self- 
regulating  mechanism  involving  muscle,   vascular,  and  secretor  elements, 
coordinated  by  the  nerve  system. 

Heat-Production. — Heat-production  varies  in  intensity  and  amount, 
in  accordance  with  a  number  of  conditions,  but  principally  with  variations 
in  physiologic  activity,  the  quantity  and  quality  of  the  food,  and  changes  in 
the  external  temperature.  It  will  be  recalled  that  all  muscles  possess  tonicity 
by  which  is  meant  a  slight  degree  of  contraction,  the  result  of  the  continuous 
arrival  of  nerve  impulses  through  efferent  nerves  discharged  from  motor 
nerve-cells  in  the  spinal  cord  this  discharge  being  maintained  largely  by 
nerve  impulses  coming  through  afferent  nerves  from  the  muscles  themselves, 
the  joints,  tendons,  and  skin.  As  a  result  of  this  slight  but  constant  stimula- 
tion of  the  spinal  cord,  the  metabolic  changes  in  muscle  material  are  main- 
tained at  a  certain  level,  with  a  corresponding  liberation  of  heat.  The  chief 
result  of  the  tonicity  would  thus  be  the  production  of  heat.  Any  physiologic 
condition  that  leads  to  a  greater  discharge  of  nerve  impulses  from  the  spinal 
cord  and  hence  increased  muscle  activity,  must  be  attended  by  increased 
heat  production.  Therefore  work  and  exercise  of  all  kinds  which  involve 
a  more  rapid  contraction  of  the  skeletal  muscles  is  attended  with  increased 
heat  production.  The  consumption  of  foods  that  have  a  higher  potential 
heat  value  also  contribute  to  the  amount  of  heat  produced.  Foods  have 
different  physiologic  heat  values.     If  the  food  consumed  contains  much 


436  TEXT-BOOK  OF  PHYSIOLOGY. 

potential  energy  and  quantity  consumed  be  larger  than  the  average  daily 
requirements,  there,  will  be  an  increase  in  heat-production.  A  lowering 
of  the  external  temperature,  as  in  the  winter  season,  leads  to  increased  heat- 
production  through  stimulation  of  the  nerve-centers.  When  all  these  condi- 
tions, increased  muscle  activity,  increased  amount  of  food  with  high  potential 
energy,  and  a  low  external  temperature  coexist,  heat-production  attains  its 
maximum,  amounting  to  as  much  as  4726  Calories  daily  (Hultgren).  In 
winter  time,  the  lowering  of  the  external  temperature  leads  through  reflex 
stimulation  of  the  spinal  motor  centers  to  a  larger  discharge  of  nerve 
impulses  to  muscles  leading  to  increased  activity  and  increased  heat- 
production. 

Heat-Dissipation. — Heat-dissipation  varies  in  rapidity  in  accordance 
with  variations  of  a  number  of  factors,  but  principally  with  variations  in  the 
external  temperature  and  the  activity  of  the  perspiratory  apparatus.  The 
heat  is  dissipated  mainly  by  the  skin,  about  85  per  cent.,  in  consequence  of 
radiation  and  conduction  and  by  the  evaporation  of  the  sweat.  The  loss 
by  this  channel  as  well  as  from  the  lungs  is  dependent  for  the  most  part  on 
a  difference  of  temperature  of  the  surrounding  air  and  of  the  body.  If  the 
surrounding  temperature  is  high,  there  is  an  increase  in  the  activity  of  both 
the  circulatory  and  respiratory  mechanisms,  brought  about  by  the  central 
nervous  system.  In  addition  to  an  increased  action  of  the  heart,  the 
blood-vessels  of  the  skin  dilate,  and  deliver  to  the  surface  a  larger  volume 
of  blood  in  a  given  time,  thus  increasing  the  conditions  favorable  to 
radiation.  The  sweat  glands  at  the  same  time  are  stimulated  to 
increased  activity  by  the  central  nerve  system  (the  sweat  centers  in  the 
spinal  cord)  by  the  action  of  nerve  impulses  transmitted  from  the  skin 
developed  by  the  action  of  the  higher  temperature  on  the  nerve 
endings;  hence,  in  consequence  of  the  additional  volumes  of  blood  brought  to 
the  skin  a  larger  amount  of  sweat  is  secreted,  which  speedily  undergoes 
evaporation.  As  each  gram  of  water  for  its  evaporation  requires  0.582 
of  a  calorie,  it  is  evident  that  increased  secretion  of  sweat  favors  heat-dissipa- 
tion. The  nerve-centers  influencing  the  activity  of  the  sweat-glands  may  be 
stimulated  not  only  reflexly,  but  directly  by  an  excess  of  heat  in  the  blood. 
If,  however,  the  atmosphere  itself  possesses  a  high  percentage  of  moisture, 
evaporation  from  the  body  is  much  diminished  and  the  value  of  sweating  as 
a  means  of  lowering  the  body-temperature  is  much  impaired.  Evaporation 
is  hastened  by  air  in  motion.  Hastened  respiratory  movements  and  the 
dilatation  of  blood-vessels  of  the  respiratory  surface  also  increase  the  evapora- 
tion of  water  from  the  lungs  and  thus  occasion  a  greater  loss  of  heat. 

If  on  the  contrary,  the  external  temperature  falls  there  is  a  decrease  in 
the  physiologic  activity  of  the  skin  from  a  contraction  of  the  blood-vessels, 
a  diminution  of  the  blood-supply,  and  a  cessation  in  the  secretion  of  sweat. 
The  blood,  being  prevented  from  coming  to  the  surface,  is  retained  in  the 
deeper  portion  of  the  body,  and  in  consequence  the  conditions  for  radiation 
are  diminished.  These  variations  in  the  cutaneous  circulation  in  reponse 
to  variations  in  the  external  temperature  are  brought  about  by  the  vaso- 
motor nerve  mechanism;  and  as  they  take  place  with  extreme  promptness 
heat-dissipation  and  heat-production  are  quickly  adjusted  and  the  mean 
temperature  maintained. 


ANIMAL  HEAT.  437 

Radiation  from  the  skin  is  modified  to  some  extent  by  clothing.  An 
excess  of  clothing  diminishes,  a  diminution  of  clothing  increases  radiation. 
The  quality  of  clothing  is  also  an  important  factor.  Wool  is  a  poor 
conductor  of  heat  but  a  good  absorber  and  retainer  of  moisture,  and  hence 
is  adapted  for  cold  weather.  Linen  and  cotton  possess  the  opposite  quali- 
ties, and  hence  are  adapted  for  warm  weather.  Radiation  from  the  skin  is 
somewhat  interfered  with  by  subcutaneous  fat,  the  extent  of  the  interference 
being  dependent  on  its  amount. 

The  foregoing  estimates  as -to  the  amounts  of  heat  produced  have  refer- 
ence only  to  the  body  in  repose.  When  the  body  passes  into  a  state  of 
muscle  activity,  there  is  at  once  a  notable  increase  in  heat-production  in  con- 
sequence of  the  increase  in  the  activity  of  the  chemic  changes  which  underlie 
body  activity,  as  shown  by  the  increase  in  the  consumption  of  oxygen  and 
the  production  of  carbon  dioxid.  Not  all  of  the  potential  energy  set  free, 
however,  appears  as  heat;  for,  if  the  muscles  are  engaged  in  doing  work  a 
part  of  the  energy,  which  would  otherwise  manifest  itself  as  heat  is  converted 
into  mechanic  motion.  From  the  work  done  during  a  period  of  eight  hours 
it  has  been  estimated  1  hat  about  500  Calories  are  so  transformed  or  utilized. 
Hirn  calculated  from  an  average  of  five  experiments  that  a  man  weighing 
67  kilos  in  repose  produced  154.4  Calories  per  hour  and  absorbed  30.7  grams 
of  oxygen  per  hour;  but  when  engaged  in  active  muscle  movements  pro- 
duced 271.2  Calories  and  absorbed  119.84  grams  of  oxygen  per  hour.  The 
increase  in  heat-production  per  hour  during  activity  was  thus  almost  doubled, 
though  the  sum  total  produced  daily  in  which  there  was  a  working  period 
of  eight  or  ten  hours  was  only  about  one-third  more  than  during  a  day  of 
repose.  During  sleep  there  is  a  greatly  diminished  heat-production,  not  more 
than  40  calories  per  hour  being  produced.  The  preceding  data  may  be 
tabulated  as  follows  (Martin) : 

Day  of  Rest.  Day  of  Work. 


Heat  units  (Calories)  pro-    \  Rest  i6  hrs.     Sleep  8  hrs.     Rest  8  hrs.     Work  8  hrs.     Sleep  8  hrs. 
duced /  2470.4  320  1235.2  2169.6  320 

2790.4  3724-8 


CHAPTER  XVn. 
SECRETION. 

Secretion. — Secretion  is  a  term  applied  to  a  process  by  which  complex 
fluids  are  formed  from  the  constituents  of  the  lymph  which  is  separated 
from  the  blood-stream  by  the  activities  of  the  endothelial  cells  of  the  capil- 
lary wall,  as  the  blood  flows  through  the  capillary  blood-vessels.  In  this 
process  the  endothelial  cell  is  aided  by  the  physical  forces,  diffusion,  osmosis, 
and  filtration.  This  separated  or  secreted  material  may  be  utilized  in  several 
ways: 

1.  For  the  repair  of  the  tissues,  for  growth,  for  the  liberation  of  energy. 

2.  For  the  elaboration  or  production  by  specialized  organs  of  a  variety  of 

complex  fluids  of  widely  different  application.  The  fluids  thus  formed 
are  utilized  for  the  most  part  to  meet  some  special  need  of  the  body. 
All  such  fluids  are  termed  secretions. 

All  secretions  are  products  of  the  activities  of  epithelial  cells  covering 
a  flat,  or  lining  a  more  or  less  complexly  involuted,  membrane  which  in 
each  instance  may  be  termed  a  secretor  organ.  As  the  fluids  for  the  most 
part  are  poured  out  on  the  surface  of  the  body,  they  have  been  termed 
external  secretions:  e.g.,  mucus,  saliva,  gastric  juice,  milk,  sebaceous  matter, 
etc.  Within  recent  years  it  has  been  demonstrated  that  the  epithelium  of 
certain  organs,  for  example,  of  those  which  do  not  possess  a  duct,  such  as 
the  thyroid,  adrenals,  hypophysis,  etc.,  also  produces  certain  specific  con- 
stituents which  are  however  returned  to  the  blood,  and  which  in  some  un- 
known but  yet  favorable  way  influence  the  general  nutrition.  To  such 
products  of  these  organs  the  term  internal  secretions  has  been  given. 

The  blood,  in  addition  to  its  nutritive  constituents,  contains  a  number  of 
principles,  derived  from  the  tissues,  which  are  to  be  regarded  as  waste  pro- 
ducts, the  outcome  of  the  katabolic  activity  of  the  tissues  and  apparently 
of  no  further  use  to  the  body.  If  retained,  they  would  seriously  if  not 
fatally  interfere  with  the  normal  physiologic  activities  of  the  different  tissues. 
They  are  therefore  removed  by  specialized  organs  after  their  separation  from 
the  blood-stream.  The  waste  products  in  solution  thus  removed  are  not 
capable  of  being  utilized  for  any  specific  purpose,  and  are  'therefore  termed 
excretions:  e.g.,  urine,  perspiration,  etc.  Excretion,  also,  is  performed 
by  the  activities  of  epithelial  cells  aided  by  the  physical  forces  of  diffusion, 
osmosis  and  filtration;  and  though  a  distinction  is  made  between  the  two 
classes  of  fluids,  no  sharp  line  can  be  drawn  between  the  cell  processes  which 
take  place  in  secretor  and  excretor  organs. 

All  secretor  organs  may  be  divided  into — • 

1.  Epithelial. 

2.  Reticular  and  vascular,  the  latter  term  indicating  merely  their  relation 

to  blood-vessels. 

438 


SECRETION.  439 

The  Epithelial  Secretor  Organs. — The  epithelial  secretor  organ 
consists  primarily  of  a  thin  delicate  homogeneous  membrane,  one  side  of 
which  is  covered  with  a  layer  of  epithelial  cells  and  the  other  side  of  which 
is  closely  invested  by  a  network  of  capillary  blood-vessels,  lymph-vessels, 
and  nerves.  Though  the  epithelial  cells  have  a  general  histologic  resem- 
blance one  to  another,  their  physiologic  function  varies  in  different  situations, 
in  accordance  probably  with  their  ultimate  chemic  structure,  a  fact  which 
determines  the  difference  in  the  character  of  the  secretions. 

The  epithelial  secretor  organs  may  consist  of  a  single  layer  of  cells  or  a 
group  of  cells,  and  may  be  subdivided  into — 

1.  Secreting  membranes. 

2.  Secreting  glands. 

The  secreting  membranes  are  the  mucous  membranes  lining  the 
gastro-intestinal,  the  pulmonary,  and  the  genito-urinary  tracts,  and  the 
serous  membranes  lining  closed  cavities,  such  as  the  pleural,  pericardial, 
peritoneal,  and  synovial  membranes. 

The  mucous  membranes  are  soft  and  velvety  in  character  and  are  com- 
posed of  a  condensed  connective  tissue  forming  a  basement  membrane 
beneath  which  is  a  layer  of  blood-vessels  and  muscle-fibers,  and  on  which 
is  a  layer  of  epithelium,  the  histologic  as  well  as  physiologic  characters  of 
which  vary  in  different  situations.  The  mucus  secreted  by  the  various 
epithelial  forms  will  very  naturally  possess  a  somewhat  diff'erent  composition, 
according  to  the  locality  in  which  it  is  formed.  In  a  general  way  it  may 
be  said  that  mucus  is  a  pale,  semitransparent,  alkaline  fluid,  containing 
leukocytes  and  epithelial  cells.  It  is  composed  chemically  of  water,  mineral 
salts  and  an  albuminoid  body,  mucin,  to  the  presence  of  which  it  owes  its 
viscidity.  Much  of  the  mucus  is  secreted  by  the  goblet  cells  on  the 
surface  of  the  mucous  membranes.  The  principal  varieties  of  mucus  are 
the  nasal,  bronchial,  vaginal,  urinary,  and  gastro-intestinal. 

The  serous  membranes  are  composed  of  thin  membrane  formed  by  a 
condensation  of  connective  tissue  and  covered  by  a  single  layer  of  large, 
flat,  nucleated  cells  with  irregular  margins.  These  membranes  enclose 
what  are  practically  large  lymph  sacs  or  spaces,  and  the  fluid  they  contain 
resembles  lymph  in  all  respects  and  is  practically  identical  with  it  It  serves 
to  diminish  friction  when  the  viscera  they  enclose  move  over  one  another. 
The  most  important  of  the  serous  membranes  are  the  pleural,  pericardial, 
and  peritoneal. 

The  synovial  membranes  in  and  around  joints  resemble  serous  membranes. 
The  cells  covering  them,  however,  secrete  a  clear,  colorless  fluid  resembling 
lymph,  but  differing  it  in  containing  a  mucin-like  substance,  a  nucleo-albu- 
min,  which  imparts  to  it  considerable  viscidity.  This  synovial  fluid  serves  to 
diminish  friction  between  the  opposing  surfaces  of  the  bones  as  they  glide 
over  one  another  during  movement. 

Other  secretions,  such  as  the  aqueous  and  vitreous  humors  of  the  eye, 
the  fluid  of  the  internal  ear,  the  cerebrospinal  fluid,  etc.,  will  be  considered 
in  connection  with  the  organs  with  which  they  are  associated,  as  have  been 
the  digestive  secretions. 

The  secreting  glands  are  formed  of  the  same  histologic  elements  as 
the  secreting  membranes.     They  are  formed  by  an  involution  of  the  mucous 


440  TEXT-BOOK  OF  PHYSIOLOGY. 

membrane  or  skin  the  epithelium  of  which  is  variously  modified  structurally 
and  functionally  in  the  various  situations  in  which  they  are  formed.  Like 
the  membranes  themselves,  the  glands  are  invested  by  capillary  blood- 
vessels and  supplied  with  lymph-vessels  and  nerves,  of  which  the  latter 
are  in  direct  connection  with  the  blood-vessels  and  epithelial  cells.  The 
interior  of  each  gland  is  in  communication  with  the  free  surface  by  one  or 
more  passageways  known  as  ducts. 

These  glands  may  be  classified  according  as  the  involution  is  cylindrical 
or  dilated  as — 

1.  Tubular.  The  hilndar  glands  may  be  simple — e.g.,  sweat-glands, 
intestinal  glands,  fundus  glands  of  the  stomach;  or  compound — e.g.,  kidney, 
testicle,  salivary,  and  lachrymal  glands. 

2.  Alveolar.  The  alveolar  glands  may  also  be  simple — e.g.,  the  seba- 
ceous glands,  the  ovarian  follicles,  meibomian  glands;  or  compound,  as 
the  mammary  glands  and  salivary  glands. 

For  the  production  of  a  secretion  it  is  necessary  that  the  plasma  of  the 
blood,  the  common  material,  be  delivered  to  the  lymph-spaces  with  which 
the  epithelial  cells  are  in  close  relation.  The  processes  involved  in  the  pass- 
age of  the  plasma  across  the  capillary  wall  have  already  been  considered 
in  connection  with  the  production  of  lymph.  They  include  the  physical 
processes,  diffusion,  osmosis,  and  filtration  combined  with  a  secretor  activity 
of  the  cells  of  the  capillary  wall.  The  question  as  to  which  of  these  pro- 
cesses is  the  more  active  is  yet  a  subject  of  investigation. 

As  the  chemic  composition  and  the  chemic  features  of  the  organic  con- 
stituents of  all  secretions  have  been  demonstrated  to  be  the  outcome  of 
metabolic  processes  going  on  within  the  epithelial  cells,  it  must  be  assumed 
at  least  that  these  differences  are  correlated  with  differences  in  the  histo- 
logic features  and  molecular  structure  of  the  epithelium.  The  discharge  of 
the  secretion  is,  as  a  rule,  intermittent;  that  is,  there  are  periods  of  inactivity 
or  rest.  In  rest  more  especially  the  epithelial  cells,  after  the  assimilation 
of  lymph,  accumulate  within  themselves  such  characteristic  products  as 
globules  of  mucin,  granules  which  apparently  are  the  antecedents  of  the  diges- 
tive enzymes,  granules  of  glycogen,  globules  of  fat,  sugar,  and  proteins, 
as  in  the  case  of  the  mammary  gland.  In  how  far  all  these  compounds  are 
the  result  of  secretor  activity  or  of  a  cell  degeneration  and  disintegration  it  is 
impossible  to  state  in  the  light  of  present  knowledge.  During  the  period 
of  gland  rest  the  blood-supply  to  the  gland  is  merely  sufficient  for  nutritive 
purposes.  When  the  occasion  arises  for  gland  activity,  the  blood-vessels, 
under  the  influence  of  the  vaso-motor  nerves,  dilate  and  deliver  to  the  gland 
an  amount  of  blood  far  beyond  that  required  for  nutritive  purposes.  As  a 
result  the  gland  becomes  red  and  vascular  and  the  blood  emerging  by  the 
veins  frequently  retains  its  customary  arterial  color.  The  increased  blood- 
supply  favors  a  rapid  transudation  of  water  and  salts  into  the  lymph-spaces 
from  which  they  are  speedily  absorbed  and  transmitted  by  the  epithelial 
cells  into  the  interior  of  the  gland  lumen.  Coincident  with  the  passage 
of  water  through  the  cell,  the  organic  constituents  are  extruded  from  the  end 
of  the  cell  bordering  the  lumen  to  become  dissolved,  or  in  the  case  of  fat  to 
be  suspended,  in  the  water.  The  secretion  thus  formed  accumulates  and 
with  the  rise  of  pressure  which  inevitably  follows  it  at  once  passes  into  the 


SECRETION. 


441 


ducts  to  be  discharged  on  the  surface  of  the  mucous  membrane  or  skin,  as  the 
case  may  be. 

Influence  of  the  Nerve  System. — The  activity  of  every  gland  is  con- 
trolled by  nerve-centers  situated  in  the  central  nerve  system.  These  centers 
may  be  excited  to  activity  either  by  impressions  made  on  the  peripheral 
terminations  of  afferent  nerves  or  by  emotional  states,  or,  possibly,  by  changes 
in  the  composition  of  the  blood  itself.  As  a  rule,  all  normal  secretion  is  a 
reflex  act  involving  the  usual  mechanism:  viz.,  a  receptive  surface  (skin, 
mucous  membrane,  or  sense-organ),  an  afferent  nerve,  an  emissive  cell  from 
which  emerges  an  efferent  nerve  to  be  distributed  to  a  responsive  organ,  the 
gland  epithelium. 

For  the  production  of  the  secretion  by  the  epithelial  cell  it  is  believed 
by  some  experimenters  that  two  physiologically  distinct,  efferent  nerve- 
fibers  are  involved — one  stimulating  the  production  of  the  organic  constituents 
{trophic  nerves),  the  other  stimulating  the  secretion  of  water  and  inorganic 
salts  {secretor  nerves).  The  evidence  for  the  influence  of  the  nerve  system 
on  secretion  and  the  mode  of  connection  of  the  nerv^e-fibers  with  the  gland- 
cells  have  been  alluded  to  (page  94)  and  will  again  be  in  subsequent  chapters. 

The  reticular  and  vascular  glands,  though  not  possessing  any  common 
histologic  features,  are  grouped  together  merely  for  convenience,  and  will 
be  considered  in  a  separate  chapter  in  connection  with  the  problems  of 
internal  secretion. 

MAMMARY  GLANDS. 


The  mammary  glands,  which  secrete  the  milk,  arc  two  more  or  less 
hemispheric  organs  situated  in  the  human  female  on  the  anterior  surface  of 

the  thorax.  Though  rudimentary  in 
childhood,  they  gradually  increase  in 
size  as  puberty  approaches.  The  gland 
presents  at  its  convexity  a  small  conical 
eminence  termed  the  mammilla  or  nipple, 
surrounded  by  a  circular  area  of  pig- 
mented skin,  the  areola.  The  gland 
proper  is  covered  by  a  layer  of  adipose 


Fig.  204. — Mammary  Gland. 
Lactiferous  ducts.  2.  Lobuli  of 
mammary  gland. 


the 


Fig.  205. — Acini  of  the  Mammary 
Gland  of  a  Sheep  During  Lactation. 
a.  Membrana  propria.  b.  Secretory 
epithelium. 


tissue  anteriorly  and  is  attached  posteriorly  to  the  pectoral  muscles  by  a  net- 
work of  fibrous  tissue. 


442  TEXT-BOOK  OF  PHYSIOLOGY. 

During  utero-gestation  the  mammary  glands  become  larger,  firmer, 
and  more  lobulated;  the  areola  darkens  and  the  blood-vessels,  especially 
the  veins,  become  more  prominent.  At  the  period  of  lactation  the 
gland  is  the  seat  of  active  histologic  and  physiologic  changes  correlated 
with  the  production  of  milk.  At  the  close  of  lactation  these  activities  cease, 
the  glands  diminish  in  size,  undergo  involution,  and  gradually  return  to  their 
former  non-secreting  condition. 

Structure  of  the  Mammary  Gland. — Each  mammary  gland  consists 
of  an  aggregation  of  some  15  or  20  irregular  pyramidal  lobes,  each  of  which 
is  surrounded  by  a  framework  of  fibrous  tissue.  This  tissue  affords  support 
for  blood-vessels,  lymph-vessels,  and  nerves.  Each  lobe  is  provided  with  a 
single  excretory  duct,  the  lactiferous  duct,  which  as  it  approaches  the  areola 
expands  into  a  fusiform  ampulla  or  reservoir.  At  the  base  of  the  nipple 
the  ampullae  contract  to  form  some  15  or  20  narrow  ducts,  which,  ascending 
the  nipple,  open  by  constricted  orifices  0.5  mm.  in  diameter  on  its  apex 
(Fig.  204).  _ 

On  tracing  the  lactiferous  duct  into  a  lobe,  it  is  found  to  divide  and 
subdivide  into  a  number  of  branches,  which  pass  into  smaller  masses — the 
lobules.  The  lobule  in  turn  is  composed  of  a  large  number  of  tubular  acini 
or  alveoli,  the  final  terminations  of  the  lobular  ducts.  Each  acinus  consists 
of  a  basement  membrane  lined  by  a  single  layer  of  low  cuboidal  epithelial 
cells  (Fig.  205).  Externally  the  acinus  is  surrounded  by  blood-vessels, 
nerves,  and  lymphatics. 

MILK. 

Milk  as  obtained  during  active  lactation  is  an  opaque  bluish- white  fluid, 
almost  inodorous,  with  a  sweet  taste,  an  alkaline  reaction,  and  a  specific 
gravity  of  from  1.025  to  1.040.  Examined  microscopically,  it  is  seen  to 
consist  of  a  clear  fluid,  the  milk  plasma,  holding  in  suspension  an  enormous 
number  of  small,  highly  refractive  oil-globules,  which  measure  on  the  average 
about  Tiro'o'o"  of  an  inch  in  diameter.  It  has  been  asserted  by  some  observers 
that  each  globule  is  surrounded  by  a  thin  proteid  envelope  which  enables  it 
to  maintain  the  discrete  form.     This,  however,  is  at  present  disbelieved. 

The  quantity  of  milk  secreted  daily  by  the  human  female  averages  about 
1200  c.c. 

Chemic  analysis  has  shown  that  the  milk  of  all  the  mammalia  consists 
of  all  the  different  classes  of  nutritive  principles,  though  in  different  propor- 
tions, which  are  necessary  to  the  growth  and  development  of  the  body.  The 
only  exception  appears  to  be  an  insufficient  amount  of  iron  for  the  formation 
of  the  coloring-matter  of  the  blood,  the  hemoglobin. 

Caseinogen  is  the  chief  protein  constituent  of  milk.  Associated  with  it, 
however,  are  two  other  proteins,  lactalbumin  and  lactoglobulin,  both  of 
which  are  present  in  but  small  quantity.  When  milk  is  treated  with  acetic 
acid,  sodium  chlorid,  or  magnesium  sulphate  to  saturation,  the  caseinogen 
is  precipitated  as  such,  and  after  the  removal  of  the  fat  with  which  it  is  entan- 
gled may  be  collected  by  appropriate  chemic  methods.  On  the  addition  of 
rennet,  an  alcoholic  extract  of  the  mucous  membrane  of  the  calf's  stomach, 
which  contains  the  enzyme  rennin  or  pexin,  the  caseinogen  undergoes  a 


SECRETION.  443 

conversion  into  an  insoluble  protein,  casein  or  tyrein.  To  this  process  the 
term  coagulation  has  been  given.  The  presence  of  calcium  phosphate 
appears  to  be  essential  to  this  process,  inasmuch  as  it  does  not  take  place  if 
the  milk  be  completely  decalcified  by  the  addition  of  potassium  oxalate. 
After  coagulation,  the  more  or  less  solid  mass  of  milk  separates  into  a  liquid 
portion,  the  serum,  and  a  solid  portion,  the  coagulum.  The  former,  gener- 
ally termed  whey,  consists  of  water,  salts,  lactalbumin,  sugar;  the  latter,  the 
curd,  consists  of  the  casein  and  entangled  fat.  Boiling  the  milk  retards  and 
even  prevents  the  coagulation  by  rennet,  owing  to  the  precipitation  of  the 
calcium  phosphate.  When  milk  is  taken  into  the  stomach,  it  is  probable 
that  the  rennin  coagules  the  caseinogen  in  a  manner  similar  to,  if  not  identical 
with,  this  process,  which  appears  to  be  essential  to  the  normal  digestion  of 
the  milk. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures.  It  is  a 
compound  of  olein,  palmitin,  and  stearin  with  small  quantities  of  butyrin 
and  caproin.  The  melting-point  of  butter  varies  between  31°  and  34°  C. 
When  milk  is  allowed  to  stand  for  some  time,  the  fat-globules  rise  to  the 
surface  and  form  a  thick  layer  known  as  cream.  Churning  the  milk  or  cream 
causes  the  fat-globules  to  run  together  and  form  a  coherent  mass  termed 
butter. 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk.  It  belongs 
to  the  saccharose  group  and  has  the  follomng  composition:  Cj2H220jj. 
Though  incapable  of  undergoing  fermentation  by  the  action  of  the  yeast  plant 
it  is  readily  reduced  by  the  Bacillus  acidi  lactici  to  lactic  acid  and  carbon 
dioxid,  the  former  of  which  imparts  to  milk  an  acid  reaction  and  a  sour 
taste.  With  the  accumulation  of  the  lactic  acid  the  caseinogen  is  precipi- 
tated as  a  more  or  less  consistent  mass. 

The  inorganic  salts  of  milk  are  chiefly  potassium,  sodium,  calcium, 
and  magnesium  phosphates  and  chlorids.  Iron  is  also  present  in  small 
amount.  The  following  table  of  Bunge  gives  the  quantitative  amounts  of 
these  constituents  in  both  human  and  cow's  milk: 


T  T.     ^  Potas-      c  J-  r-  1  •       I  Masrne-  Iron  ,      .' 

In  1000  Parts  ,      •  ™      ,  Sodium.  ,  Calcium       .  °„      ,      r\  -a  phone 

cmm        I  mum  I  )viri  ,     r_     _  _ 


Oxid. 


Chlorin. 


Acid. 


Human  milk 0.78  0.25  0.33  0.06  0.0036  0.47  0.43 

Cow's  milk 1.76     1     i.ii  i-59  0.21  0.0030  i-97  1.69 

Mechanism  of  Milk  Secretion. — During  the  time  of  lactation  the 
mammary  gland  exhibits  periods  of  secretory  activity  which  alternate  with 
periods  of  repose.  Coincidently  with  these  periods  certain  histologic  changes 
take  place  in  the  secreting  epithelium.  At  the  close  of  a  period  of  active 
secretion  and  after  the  discharge  of  the  milk  each  acinus  presents  the  follow- 
ing features:  The  epithelial  cells  are  short,  cubical,  nucleated,  and  border  a 
relatively  wide  lumen,  in  which  is  found  a  variable  quantity  of  milk.  After 
the  gland  has  rested  for  some  time  active  metabolism  again  begins.  The 
cells  grow  and  elongate;  the  nucleus  divides  into  two  or  three  new  nuclei; 
constriction  takes  place  and  the  inner  portion  is  detached  and  discharged 


444 


TEXT-BOOK  OF  PHYSIOLOGY. 


into  the  lumen  of  the  acinus.  During  the  time  these  changes  are  taking 
place  oil-globules  make  their  appearance  in  the  cell  protoplasm,  some  of 
which  are  discharged  separately  into  the  lumen,  while  others  remain  for  a 
time  associated  with  the  detached  portion  of  the  cell  (Fig.  299).  From 
these  histologic  changes  it  is  inferred  that  the  caseinogen  and  fat  are  products 
of  the  metabolism  of  the  cell  protoplasm  and  not  derived  directly  from 
the  lymph  from  the  blood.  The  lactose  apparently  has  a  similar  origin,  as 
appears  from  the  fact  that  it  is  not  found  either  in  the  blood  or  any  other 
tissue,  and  that  it  is  formed  independently  of  carbohydrate  food.  The  water, 
and  especially  the  inorgani  c  salts,  are  the  result  of  secretor  activity  rather 

than  of  diffusion  and  filtration.  This  is  ren- 
dered probable  from  the  fact  that  the  propor- 
tions of  the  inorganic  salts  of  milk  are  more 
closely  allied  to  those  of  the  tissues  of  the  new- 
born child  than  to  blood.  With  the  passage  of 
the  water  and  salts  into  the  lumen  of  the  acinus 
the  proteids  undergo  disintegration  and  solution 
and  the  liquid  assumes  the  characteristics  of 
milk. 

The  discharge  of  milk  is  occasioned  by  the 
suction  efforts  on  the  part  of  the  child,  aided  by 
atmospheric  pressure  and  the  contractions  of 
the  non-striated  muscle-fibers  of  the  lactiferous 
ducts. 

Influence  of  the  Nerve  System. — Judging 
from  analogy,  it  is  probable  that  the  secretion  of 
milk  is  regulated  by  impulses  emanating  from  the 
nerve  system,  though  the  exact  nerve-channels  for  the  transmission  of  such 
impulses  have  not  been  determined  experimentally.  Various  attempts 
have  been  made  to  isolate  and  study  these  nerves,  but  the  results  are  incon- 
clusive. It  is  well  known  that  emotional  states  on  the  part  of  the  mother 
modify  the  quantity  as  well  as  quality  of  milk,  indicating  a  connection 
between  the  gland-cells  and  the  central  organs  of  the  ner\'e  system.  Nerve 
terminals  have  been  discovered  in  and  around  the  epithelial  cells — a  fact 
which  supports  this  view. 

Colostrum. — Within  a  day  or  two  after  parturition  the  alveoli  become 
filled  with  a  fluid  which  in  some  respects  resembles  milk  and  which  has  been 
termed  colostrum.  This  is  a  watery  fluid  containing  disintegrated  epithelial 
cells  and  fat-globules,  as  well  as  a  colostrum  corpuscles,  which  are  probably 
emigrated  leukocytes.  Colostrum  is  distinguished  from  milk  in  being 
richer  in  sugar  and  inorganic  salts.  It  is  said  to  possess  constituents  which 
act  as  a  laxative  to  the  young  child. 

THE   LIVER. 

The  liver  is  a  large  gland  situated  in  the  upper  and  right  side  of  the 
abdominal  cavity,  where  it  is  held  in  position  largely  by  ligaments  formed  by 
reduplications  of  the  peritoneal  investment.  In  the  adult  it  weighs,  freed 
of  blood,  from  1300  to  1700  grams.  The  liver  is  connected  with  the  duo- 
denal portion  of  the  intestine  by  the  hepatic  duct.     It  receives  blood  both 


Fig.  206. — Section  of  the 
Mammary  Gland  of  a  Cat 
IN  the  Early  Stages  of  Lac- 
tation. A.  Cavity  of  alveoli 
filled  with  granules  and  glob- 
ules of  fat.  I,  2,  3.  Epithe- 
lium in  various  stages  of  milk- 
formation. — (Yeo.) 


SECRETION. 


445 


from  the  hepatic  artery  and  from  the  portal  vein,  and  in  this  respect  differs 
from  all  other  glands  in  the  body.  The  epithelial  structures  of  the  liver  are 
inclosed  by  a  firm  tibrous  membrane,  known  as  Glisson's  capsule.  At  the 
transverse  fissure  it  invests  and  follows  the  blood-vessels,  which  there  enter, 
in  all  their  ramifications  through  the  gland. 

Structure  of  the  Liver. — The  liver  is  composed  of  an  enormous  num- 
ber of  small  masses,  rounded,  ovoid,  or  polygonal  in  shape,  called  lobules, 
measuring  about  one  millimeter  in  diameter  and  separated  from  one  another 
by  a  narrow  space  in  which  are  to  be  found  blood-vessels,  lymphatics,  and 
hepatic  ducts,  supported  by  connective  tissue.  In  the  pig  this  space  and  its 
contained  elements  is  quite  distinct,  sharply  marking  out  the  border  of 
the  lobule  (Fig.  207).  This  is  not  so  ap- 
parent in  man.  Each  lobule  is  made  up 
of  irregular  or  polygonal  shaped  cells 
measuring  about  30  to  40  micromilli- 
meters  in  diameter.  These  cells  are  ar- 
ranged in  a  radial  manner  from  the  center  to 


^  Trabeculae  of 
hepatic  cells. 


Central  vein. 


Fig.  207. — Section  of  Liver  of  Pig,  showing 
VERY  D1.4.GRAMMATICALLY  THE  Lobules,  a.  Interlobu- 
lar connective  tissue,  h,  c.  Branches  of  portal  vein 
and  of  hepatic  artery,  d.  Bile-ducts,  e.  Intralobular 
vein. — {Pier  sol.) 


Interlobular  vein.    Hepatic  duct. 

Fig.  208. — Scheme  of  a  Hepatic 
Lobule,  represented  in  Transverse 
Section  below  and,  by  Partial  Lev- 
eling, in  Longitudinal  Section 
Above.  In  the  left  half  the  blood- 
vessels are  drawn;  in  the  right  half 
onlv  the  cords  of  hepatic  cells.  X  20. 
—(Stohr.) 


the  circumference  of  the  lobule  (Fig.  208).  Each  cell  possesses  one  and 
at  times  tw^o  nuclei.  There  is  no  evidence  for  the  existence  of  a  distinct 
cell-wall.  The  cell  protoplasm  frequently  contains  globules  of  fat,  gran- 
ules of  a  protein  nature,  granules  of  glycogen,  pigment  material,  etc. 
The  appearance  presented  by  the  cell  will  vary  considerably,  according 
to  the  time  it  is  observed.  Thus  there  may  be  a  complete  absenc'e  of  these 
constituents,  when  the  cell  may  present  a  series  of  vacuoles  separated  by 
bands  of  protoplasm.  The  cells  are  the  secreting  structures  of  the  liver, 
and  hence  are  in  close  relation  to  capillary  blood-vessels,  lymphatic  spaces, 
nerves,  and  irregular  channels  or  passageways.  The  latter  running  between 
the  epithelial  cells  may  be  compared  to  the  lumen  of  other  secreting  glands. 
Blood-vessels  and  Their  Distribution. — The  blood-vessels  which  are 
in  relation  with  the  liver  are: 


446  TEXT-BOOK  OF  PHYSIOLOGY. 

1.  The  portal  vein. 

2.  The  hepatic  artery. 

3.  The  hepatic  vein. 

The  portal  vein  and  the  hepatic  artery  enter  the  liver  at  the  transverse 
fissure.  After  penetrating  its  substance  they  divide  and  subdivide  into 
smaller  and  smaller  branches,  which  ulimately  occupy  the  space  between  the 
lobules,  completely  surrounding  and  limiting  them.  From  their  situation  they 
are  termed  interlobular  veins  and  arteries. 

The  interlobular  veins  give  off  small  capillary  vessels  which  penetrate  the 
lobule  at  all  points  of  its  surface.  These  capillaries,  though  frequently 
anastomosing,  form  a  radial  meshwork  which  converges  toward  the  center 
of  the  lobule.  In  the  meshes  of  this  plexus  are  found,  arranged  in  a  corres- 
ponding radial  manner,  the  liver  cells.  The  interlobular  arteries  are  distrib- 
uted to  the  walls  of  the  portal  vein,  to  the  connective  tissue,  and  finally 
terminate  in  the  portal  vein  capillaries.  The  intralobular  capillaries  thus 
receive  and  transmit  blood  which  is  an  admixture  of  both  arterial  and  venous 


Fig.  209. — Tr^^nsverse  Section  of  a  Single  Hepatic  Lobule,  i.  Intralobular  vein,  cut 
across.  2,  2,  2,  2.  Afferent  branches  of  the  intralobular  vein.  3,  3,  3,  3,  3,  3,  3,  3,  3.  Interlobular 
branches  of  the  portal  vein,  with  its  capillary  branches,  forming  the  lobular  plexus,  extending  to 
the  radicles  of  the  intralobular  vein. — (Sappey.) 

blood.  In  the  center  of  each  lobule  there  is  a  large  vein,  formed  by  the  union 
of  the  intralobular  capillaries,  known  as  the  intralobular  vein,  which  collects 
all  the  blood  of  the  lobule  and  transmits  it  through  the  lobule  to  an  underly- 
ing or  sublobular  vein  (Fig.  209).  These  latter  vessels,  uniting  and  reuniting, 
ultimately,  form  the  hepatic  vein,  which  empties  the  blood  into  the  inferior 
vena  cava. 

Bile  Capillaries  and  Hepatic  Ducts. — The  bile  capillaries  are  narrow 
channels  which  penetrate  the  lobule  in  all  directions  and  are  generally  found 
running  along  the  sides  of  the  cells.  These  channels,  which  are  devoid  of 
walls,  receive  from  the  cells  some  of  the  products  of  their  secretor  activity, 
and  hence  are  comparable  to  the  lumen  of  the  alveoli  of  other  secreting 
glands.  At  the  periphery  of  the  lobules  the  bile  capillaries  communicate 
with  larger  channels  which  are  the  beginnings  of  the  hepatic  or  bile-ducts 


SECRETION.  44^ 

lying  in  the  interlobular  spaces.  The  interlobular  bile-ducts  possess  a  dis- 
tinct wall  lined  by  flattened  epithelium.  There  is,  however,  no  distinct  line 
of  demarcation  between  the  cells  of  the  interlobular  ducts  and  the  secreting 
cells  of  the  liver  proper,  as  the  two  blend  insensibly,  the  one  into  the 
other.  As  the  hepatic  ducts  increase  in  size  they  gradually  acquire  the 
structure  characteristic  of  the  main  hepatic  duct:  viz.,  a  mucous,  a  muscle, 
and  a  fibrous  coat. 

Influence  of  the  Nerve  System. — Experimental  investigations  have 
demonstrated  that  the  liver  is  supplied  with  nerves  derived  from  the  central 
nerve  system.  The  route  of  these  nerves  is  probably  by  way  of  the  sptanch- 
nics  and  the  vagi.  Many  of  the  nerves  which  enter  the  liver  are  vaso-motor 
in  function;  as  to  whether  others  are  secretor  in  character  is  yet  a  subject 
of  investigation.  It  has  been  asserted  that  ner^^e  filaments  have  been  demon- 
strated running  between  the  cells  and  even  penetrating  their  substance. 
This  fact  would  indicate  that  the  metabolic  processes  of  the  liver  are  under 
the  control  of  the  central  nerve  system.  , 

Functions  of  the  Liver. — The  anatomic  and  histologic  peculiarities  of 
the  liver  would  indicate  that  it  has  a  variety  of  relations  to  the  general  proc- 
esses of  the  body.  Experimental  investigation  has  brought  some  of  these 
relations  to  light.  Though  its  physiologic  actions  are  not  yet  wholly  under- 
stood, it  may  be  said  that  it  is  engaged  in: 

1.  The  elaboration  and  excretion  of  bile. 

2.  The  production  of  starch  (glycogen)  and  sugar  (glucose). 

3.  The  formation  of  urea. 

4.  The  conjugation  of  products  of  protein  putrefaction. 

The  Elaboration  of  Bile. — The  physical  properties  and  chemical  com- 
position of  the  bile  have  already  been  considered  (page  191).  The  character- 
istic salts  of  the  bile,  sodium  glycocholate  and  taurocholate,  do  not  pre-exist 
in  the  blood,  and  therefore  must  be  formed  by  the  liver  cells  out  of  materials 
derived  from  the  blood  of  the  intralobular  capillaries.  The  antecedents  of 
the  bile  salts,  glycocoll  and  taurin,  are  crystallizable  nitrogenized  compounds, 
and  known  chemically  as  amido-acetic  and  amido-ethylsulphonic  acids. 
Their  chemic  composition  indicates  that  they  are  derivatives  of  the  proteins, 
though  the  intermediate  stages  in  their  production  are  unknown.  The 
origin  of  the  cholalic  acid  with  which  they  are  combined  is  equally  obscure. 
The  bile  salts  as  they  are  found  in  the  bile  are  produced  however  in  the  liver 
cells  by  metabolic  activity. 

The  primary  coloring-matter  of  the  bile,  bilirubin,  has  been  shown  to 
be  a  derivative  of  hematin,  a  product  of  the  disintegration  of  hemoglobin. 
It  is  supposed  that  the  liver  cells  bring  about  this  change  by  combining  water 
with  hematin,  with  the  abstraction  of  iron.  The  product  thus  formed  is  bili- 
rubin, which  is  excreted,  while  the  iron  is  for  the  most  part  retained  in  the 
liver  cells. 

Cholesterin  is  a  waste  product  derived  largely  from  the  nerve-tissue. 
It  is  brought  to  the  liver  and  simply  excreted  by  the  cells.  The  remaining 
constituents  of  the  bile,  water  and  inorganic  salts,  are  secreted  here  in  the 
same  way  as  in  all  other  glands. 

When  once  formed,  the  liver  cells  discharge  these  various  compounds 
into  the  channels  by  which  they  are  surrounded;  they  then  pass  into  the  open 


448  TEXT-BOOK  OF  PHYSIOLOGY. 

mouths  of  the  bile-ducts  at  the  periphery  of  the  lobules.  Under  the  increas- 
ing pressure  which  arises  from  the  secretion  and  accumulation  of  bile,  this 
fluid  flows  from  the  smaller  into  the  larger  bile-ducts,  and  finally  is  emptied 
either  directly  into  the  intestine  or  into  the  gall-bladder,  where  it  is  stored 
until  required  for  digestive  purposes.  The  secretion  of  bile,  as  observed 
by  means  of  a  biliary  fistula,  is  continuous  and  not  intermittent,  though  the 
rate  of  flow  is  subject  to  considerable  variation. 

The  liver  cells,  as  far  as  the  secretion  of  bile  is  concerned,  appear  to  be 
independent  of  the  nerve  system.  Their  activity,  however,  is  stimulated  by 
the  increased  blood-supply  which  arises  during  digestion  in  consequence  of 
the  dilatation  of  the  intestinal  vessels,  since  it  is  at  this  period  that  the  rate 
of  discharge  is  the  greatest.  The  same  results  have  been  shown  by  experi- 
ment. Thus,  division  of  the  splanchnic  nen^es  is  followed  by  an  increased 
discharge  of  bile,  apparently  due  to  the  dilatation  of  the  portal  vessels;  stimu- 
lation of  their  peripheral  ends  is  followed  by  a  decreased  discharge  of  bile 
in  cojisequence  of  the  contraction  of  the  portal  vessels.  The  bile  salts  appear 
to  be  the  most  eflicient  stimulants  to  the  activity  of  the  liver  cells,  for  their 
administration  and  absorption  is  followed  by  an  increase  not  only  in  the 
amount  of  water,  but  of  the  inorganic  salts  and  other  solid  constituents  as 
well. 

The  flow  of  bile  from  the  bile  capillaries  to  the  main  hepatic  duct,  though 
primarily  dependent  on  differences  of  pressure,  is  aided  by  the  contraction 
of  the  muscular  walls  of  the  bfle-ducts  and  the  inspiratory  movements  of  the 
diaphragm.  Any  obstacle  to  the  discharge  of  bile  leads  to  its  accumulation, 
a  rise  of  pressure  beyond  that  of  the  capillary  blood-vessels,  and  a  reabsorp- 
tion  by  the  lymph-vessels  of  the  bile  constituents.  After  their  discharge  into 
the  blood  from  the  thoracic  duct  these  constituents  are  deposited  in  part 
in  various  tissues,  giving  rise  to  the  phenomena  of  jaundice,  and  in  part  are 
eliminated  in  the  urine. 

The  Production  of  Starch  (Glycogen)  and  Sugar  (Glycose  or  Glucose). 
— In  1857  Bernard  discovered  the  fact  that  the  liver  normally  during  life 
produces  a  substance,  analogous  in  its  chemic  composition  to  starch  and 
known  as  liver  starch  or  animal  starch.  This  substance  can  be  obtained  by 
the  following  method:  Small  pieces  of  the  liver  of  an  animal  recently  killed, 
preferably  after  a  meal  rich  in  carbohydrates,  are  placed  in  acidulated  boiling 
water  for  a  few  minutes;  then  rubbed  up  in  a  mortar  with  sand,  again  boiled, 
after  which  the  proteins  are  removed  by  filtration.  The  filtrate  thus  obtained 
is  opalescent  and  resembles  a  solution  of  starch.  The  starch  may  be 
precipitated  from  this  solution  with  alcohol.  It  may  subsequently  be  ob- 
tained free  by  drying,  when  it  presents  itself  as  a  white  amorphous  powder, 
soluble  in  hot  or  cold  water.  Chemic  analysis  shows  that  it  consists  of 
CgH^oOj,  or  a  multiple  of  it. 

When  either  the  original  solution  obtained  by  boiling  or  a  solution  of 
this  amorphous  powder  is  treated  with  iodin,  it  strikes  a  port-wine  color. 
When  digested  with  saliva,  pancreatic  juice,  or  boiled  with  dilute  acids,  the 
solution  becomes  clear,  and  testing  with  Fehling's  solution  reveals  the  pres- 
ence of  sugar. 

For  the  reason  that  this  starch  is  capable  of  being  transformed  into  or  of 
generating  glucose  it  received  the  name  of  glycogen;  and  inasmuch  as  the 


SECRETION.  449 

liver  continually  produces  glycogen  it  is  said  to  have  a  starch-forming  or  a 
glycogenic  or  an  amylogenic  function. 

If  the  liver  be  allovv^ed  to  remain  in  the  body  of  an  animal  for  a  period 
of  twenty-four  hours  before  the  decoction  is  made  as  above  described,  it  v^dll 
be  found  that  the  solution  contains  only  a  small  amount  of  starch  but  a 
relatively  large  amount  of  sugar.  The  inference  drawn  is  that  after  death 
the  starch  is  transformed  by  some  agent,  possibly  a  ferment,  into  sugar 
(glucose).  From  this  fact  as  well  as  from  the  results  of  different  lines  of 
investigation,  it  is  the  generally  received  opinion  that  the  same  change  is 
constantly  taking  place  in  the  living  condition  and  therefore  the  liver  is  said 
to  have  a  sugar-forming  or  a  glyco-genetic  junction. 

The  presence  of  glycogen  in  the  liver  cells  can  be  show^n  microscopically 
in  the  form  of  discrete  hyaline  and  refractive  granules.  As  they  are  soluble  in 
water  they  can  be  readily  dissolved  out  from  the  cells,  leaving  small  vacuoles 
separated  from  one  another  by  strands  of  cell  substance.  The  amount  of 
glycogen  in  a  well-fed  animal  varies  from  1.5  to  4  per  cent,  of  the  total  weight 
of  the  liver.  By  experimental  methods  it  has  been  shown  that  the  produc- 
tion of  glycogen  is  dependent  very  largely  on  the  consumption  of  carbohy- 
drates, the  greater  the  amount  of  sugar  and  starch  in  the  food,  the  greater 
being  the  production  of  glycogen.  Nevertheless  it  is  also  certain  that  gly- 
cogen can  be  derived  from  proteins,  for  if  the  carbohydrates  are  excluded 
from  the  food  and  the  animal  fed  on  a  pure  protein  diet,  glycogen  will  con- 
tinue to  be  formed  in  the  liver  though  in  far  less  amounts. 

The  facts  connected  with  the  formation  of  glycogen,  as  wxll  as  with  its  de- 
struction as  at  present  generally  accepted,  may  be  stated  as  follows:  The 
dextrose  into  which  the  carbohydrates  are  mainly  converted  by  the  action 
of  the  digestive  fluids  is  absorbed  into  the  blood  of  the  portal  vein  and  carried 
directly  to  the  liver,  where  a  certain  portion  of  it  diffuses  through  the  cap- 
illary walls  into  the  surrounding  lymph  spaces;  by  the  action  of  the  cells 
it  is  then  dehydrated,  and  temporarily  deposited  under  the  form  of  the  non- 
dift'usible  body  glycogen.  At  a  subsequent  period  and  in  proportion  to  the 
needs  of  the  system  the  liver  cells,  through  the  agency  of  a  ferment,  trans- 
form the  glycogen  into  glucose  or  dextrose,  return  it  to  the  blood,  by  which 
it  is  transported  to  the  systemic  capillaries,  where  it  disappears  again,  diffus- 
ing through  the  walls  of  the  capillaries  into  the  surrounding  lymph  spaces 
to  play  a  part  in  the  general  nutritive  process.  Though  the  final  disposition 
of  the  sugar  is  uncertain  it  is  highly  probable  that  aften  its  delivery  to  the 
muscles,  for  example,  it  may  be  directly  oxidized  or  temporarily  stored 
as  glycogen  or  possibly  be  used  in  the  formation  of  living  material.  Ulti- 
mately, however,  through  oxidation  it  yields  heat  and  contributes  to  the 
production  of  muscle  energy.  Should  there  be  a  failure  on  the  part  of  the 
liver  cells  to  store  up  its  usual  percentage  of  the  absorbed  sugar,  10  to  20 
per  cent,  by  reason  of  impaired  nutrition,  disturbance  of  the  portal  circu- 
lation, or  a  larger  excess  of  sugar  in  the  blood  of  the  portal  vein  it  would 
pass  through  the  liver  into  the  blood  of  the  general  circulation  and  increase 
the  percentage  amount  of  sugar  above  the  normal  (o.i  to  0.2  per  cent.) 
establishing  the  condition  of  hyperglycemia.  This  would  soon  be  followed 
by  its  elimination  from  the  blood  by  the  kidneys  and  its  appearance  in 
the  urine,  giving  rise  to  a  glycosuria. 
29 


450  TEXT-BOOK  OF  PHYSIOLOGY. 

In  opposition  to  this  view,  Dr.  Pavy,  after  years  of  accurate  experimenta- 
tion, states  that  the  blood  on  the  cardiac  side  of  the  liver  never  under  nor- 
mal circumstances  contains  a  larger  percentage  of  sugar  than  is  to  be  found  in 
any  part  of  the  circulation,  except  in  the  portal  vein.  He  states  that  glycogen 
is  never  reconverted  into  sugar,  and  denies  that  the  liver  produces  sugar,  to 
be  discharged  into  the  blood;  the  function  of  the  liver  is  merely  to  arrest 
the  passage  of  sugar,  and  so  to  shield  the  general  circulation  from  an 
excess;  the  sugar  which  arises  in  the  liver  after  death  is  a  post-mortem 
product  and  not  an  illustration  of  what  takes  place  during  life.  Dr.  Pavy, 
having  apparently  demonstrated  the  glucosid  constitution  of  protein  mate- 
rial in  general,  accounts  for  the  presence  of  glycogen  in  muscles  and  other 
tissues  on  the  assumption  that  during  the  cleavage  of  the  protein  molecule 
the  carbohydrate  element  is  set  free  and  temporarily  stored  as  glycogen. 
He  thus  accounts  for  the  production  of  sugar  in  the  body,  even  in  the  absence 
of  all  sugar  and  starch  from  the  food.  Pavy  believes  that  the  glycogen  pro- 
duced in  the  liver  is  utilized  in  the  formation  of  fat  and  the  synthesis  of 
complex  proteins  necessary  to  the  construction  of  the  tissues. 

The  Influence  of  the  Nerve  System.-^The  results  of  various  experi- 
mental investigations  indicate  that  the  production  of  sugar  from  the  glycogen 
in  the  liver  is  influenced  by  the  activities  of  the  nerve  system.  It  was  dis- 
covered by  Bernard  that  puncture  of  the  floor  of  the  fourth  ventricle,  at  a 
point  between  the  acoustic  and  vagus  nerves,  near  the  middle  line,  is  followed 
within  an  hour  or  two  by  the  appearance  of  sugar  in  the  urine,  which  lasts 
for  from  five  to  six  hours  in  the  rabbit  and  from  two  to  three  or  even  seven  in 
the  dog.  For  this  reason  Bernard  gave  to  this  area  the  name  of  "diabetic 
area." 

Coincident  with  the  appearance  of  sugar  in  the  urine  (glycosuria)  there 
is  an  increase  in  the  percentage  of  sugar  in  the  blood  (hyperglycemia).  The 
liver  at  the  same  time  contains  a  higher  percentage  of  sugar  than  normally. 
Apparently  the  initial  step  in  this  series  of  phenomena  is  an  increased  con- 
version of  glycogen  into  sugar.  This  supposition  receives  support  from  the 
fact  that  the  degree  of  the  hyperglycemia,  and  the  subsequent  glycosuria, 
will  depend  on  the  amount  of  glycogen  previously  in  the  liver.  If  the  animal 
has  been  well  fed  on  carbohydrates,  the  resulting  glycosuria  will  be  pro- 
nounced; if,  on  the  contrary,  it  has  been  allowed  to  fast  for  several  days, 
the  glycosuria  will  be  slight. 

Assuming  that  the  nerve-cells  which  constitute  the  diabetic  area  influence 
the  conversion  of  glycogen  into  sugar,  the  question  arises  as  to  whether  the 
puncture  destroys  the  nerve-cells,  or  whether  it  stimulates  them  to  increased 
activity.  The  results  of  experiment  lead  to  the  latter  supposition.  Thus 
if  the  vagus  nerve  is  divided  in  the  neck  and  its  central  end  stimulated  there 
is  developed  a  glycosuria.  Stimulation  of  other  sensor  nerves  has  a  similar 
effect.  As  stimulation  of  the  vagus  has  the  same  effect  as  the  puncture,  the 
inference  is  that  the  center  is  normally  excited  to  physiologic  activity  by 
impulses  reflected  from  some  surface  or  organ  in  the  peripheral  distribution 
of  this  nerve. 

If  the  nerve-cells  in  the  diabetic  area  regulate  the  production  of  sugar  in 
the  liver,  the  further  question  arises  as  to  the  pathway  through  which  the 
nerve  impulses  emanating  from  them  reach  the  liver,  whether  by  way  of  the 


SECRETION.  451 

vagi  or  by  v/ay  of  the  spinal  cord  and  splanchnic  nerves.  That  it  is  not  by 
way  of  the  vagi  is  shown  by  the  fact  that  the  glycosuria  established  by  the 
puncture  does  not  disappear  when  they  are  divided;  that  it  is  by  way  of  the 
spinal  cord,  as  far  at  least  as  the  first  dorsal  nerve,  and  subsequently  the 
splanchnic  nerves,  is  indicated  by  the  fact  that  a  cross-section  of  the  spinal 
cord  above  this  level,  destruction  of  the  upper  three  dorsal  roots  as  well  as 
division  of  the  spanchnic  nerves  prevents  the  development  of  the  glycosuria 
which  follows  puncture  of  the  medulla.  Though  stimulation  of  the  upper 
dorsal  (pre-ganglionic)  nerve-fibers  gives  rise  to  glycosuria,  yet,  contrary  to 
expectation,  stimulation  of  the  splanchnic  (post-ganglionic)  nerve-fibers 
does  not  have  the  same  effect.  This  may  be  due,  however,  to  changes  in 
the  relation  of  the  capillary  blood-vessels  to  the  liver  cells  or  to  the  character 
of  the  stimulus  employed. 

A  further  question  arises  as  to  whether  the  nerve  impulses  which  pass 
from  the  diabetic  center  to  the  liver  are  vaso-motor  in  character,  exerting 
their  effect  on  the  blood-vessels,  or  whether  they  are  secretor  in  character 
and  exerting  their  effect  on  the  liver  cells.  Bernard  was  of  the  opinion  that 
they  are  vaso-motor  in  character  and  that  the  diabetic  area  was  a  part  of  the 
general  vaso-motor  center.  More  recent  investigators  are  of  the  opinion 
that  they  are  secretor  in  character,  for  the  reason  that  whether  the  blood- 
pressure  rises  from  a  stimulation  of  the  central  end  of  the  divided  vagus,  or 
falls  from  a  stimulation  of  the  depressor  ner\^e,  in  each  instance  there  follows 
a  glycosuria. 

If  the  production  of  sugar  in  the  liver  is  a  reflex  act  as  Bernard  supposed, 
taking  place  through  a  mechanism  consisting  of  an  afferent  pathway,  the 
vagus  ners'e,  and  an  efferent  pathway  consisting  of  the  spinal  cord  and 
splanchnic  nerves,  the  question  arises  as  to  the  seat  of  action  of  the  stimulus. 
This  Bernard  located  in  the  lungs,  for  the  reason  that  though  division  of  the 
vagus  in  the  neck  checks  the  production  of  the  sugar,  division  below  the 
origin  of  the  pulmonary  branches  had  no  such  effect. 

Muscle  Glycogen. — Glycogen  is  also  found  in  muscles  and  to  some 
extent  in  the  placenta,  and  embryonic  tissues  generally.  Chemic  analysis 
has  shown  that  muscles  contain  from  0.5  per  cent,  to  i  per  cent,  and  as 
these  organs  amount  to  about  40  per  cent.  (28  kgm.)  of  weight  of  the  body, 
70  kgm.,  they  generally  contain  from  140  to  280  grams  of  glycogen.  Inas- 
much as  chemic  analysis  has  failed  to  demonstrate  the  presence  of  glycogen 
in  the  blood,  the  inference  is  that  it  arises  in  the  muscle-cell  in  a  manner 
similar  to  that  observed  in  the  liver-cell,  viz.,  by  a  transformation,  through 
hydration,  of  the  sugar  of  the  blood.  By  reason  of  this  fact  it  may  be  said 
that  the  muscle  also  possesses  a  glycogenic  function.  If  it  is  a  fact  that  of 
the  sugar  absorbed  only  from  12  to  20  per  cent,  is  temporarily  arrested  by  the 
liver,  the  remainder  passing  on  into  the  blood  of  the  general  circulation,  it  is 
readily  conceivable  that  the  storage  of  the  sugar  under  the  form  of  glycogen 
by  the  muscle-cells  is  necessary  not  only  for  the  activity  of  the  muscle  itself, 
but  as  a  means  of  preventing  an  abnormal  percentage  of  sugar  in  the 
circulating  blood.  It  is  generally  admitted  that  though  the  glycogen  is  the 
source  of  the  energy  expended  by  the  muscle,  it  cannot  be  disrupted  and 
oxidized  as  such,  but  that  it  must  first  be  transformed  into  sugar  (glucose) ; 
and  for  this  purpose  the  assumption  is  made  that  a  special  enzyme  is  present 


452  TEXT-BOOK  OF  PHYSIOLOGY. 

and  active.  The  muscle  is  therefore  said  to  possess  or  exhibit  a  glyco-gen- 
etic  function.  During  the  periods  of  prolonged  activity  of  the  muscles  the 
percentage  of  glycogen  rapidly  diminishes,  a  fact  that  leads  to  the  inference 
that  it  is  the  source  in  large  part  of  the  energy  expended  by  the  muscle. 
During  the  period  of  rest  the  percentage  of  glycogen  rapidly  increases  un- 
until  the  normal  is  regained. 

The  metabolism  of  the  carbohydrates  or  the  manner  in  which  they  are 
stored,  transformed  and  iinally  oxidized  is  a  subject  about  which  there  is 
much  obscurity  and  uncertainty.  Some  light  is  thrown  on  the  problem  by  a 
consideration  of  the  pathologic  state  known  as 

Diabetes. — Diabetes  is  a  chronic  disease  characterized  by  the  appear- 
ance of  sugar  in  the  urine  in  variable  amounts.  Under  normal  circumstances 
all  the  sugar  consumed  as  food  undergoes  oxidation  to  carbon  dioxid 
and  water,  none  appearing  in  the  urine  except  a  mere  trace.  In  some  dis- 
ordered states  of  nutrition  this  oxidation  is  imperfect  or  entirely  lacking. 
The  sugar  therefore  accumulates  in  the  blood  after  which  it  is  eliminated  in 
the  urine  in  large,  though  variable  amounts,  a  condition  which  may  endure 
for  months  or  years  though  eventually  leading  to  the  death  of  the  individual. 
The  pathologic  condition  underlying  this  incomplete  oxidation  is  imper- 
fectly understood  and  has  usually  been  associated  with  derangements  of  the 
glycogenic  function  of  the  liver,  though  doubtless  derangements  of  other 
organic  functions  will  produce  the  same  condition.  At  the  present  time  it 
is  believed  that  the  persistent  excretion  of  sugar  by  the  kidneys  depends  on 
several  causes:  (i)  An  ineffectual  abstraction  and  storage  of  sugar  due  to 
some  impairment  in  the  activity  of  the  liver  cells;  (2)  an  imperfect  oxidation 
in  the  muscles  by  reason  of  the  absence  of  the  necessary  enzymes;  (3)  a 
cleavage  of  the  protein  constituents  of  the  tissues,  in  consequence  of  some 
profound  alteration  in  the  nutritive  process,  whereby  their  glucose  radicals 
are  liberated  in  unusual  amounts. 

The  physiologic  mechanism  by  which  the  normal  metabolism  of  the 
carbohydrates  is  regulated  is  unknown.  That  it  is  complex  in  character  is 
shown  by  the  phenomena  which  follow  not  only  puncture  of  the  medulla, 
and  other  injuries  to  and  disturbances  of  the  nerve  system,  but  also  removal 
of  the  pancreas  and  the  administration  of  various  toxic  agents. 

Nerve  influences,  whether  the  result  of  injuries  to  the  nerve  system  or  of 
various  pathologic  states  occasionally  apparently  precede  and  cause  the 
diabetic  state,  though  the  manner  in  which  they  do  so  is  practically  unknown. 
Some  light  is  thrown  on  the  process  by  a  consideration  of  the  facts 
detailed  in  a  previous  paragraph  relating  to  the  influence  of  the  nerve  system 
in  the  production  and  storage  of  glycogen  in  the  liver.  It  is  quite  possible 
that  nerve  impulses  abnormally  developed  may  lead  to  an  incomplete 
storage  of  glycogen  by  the  liver-cells,  the  result  of  an  imperfect  nutrition  or 
a  deranged  circulation. 

Removal  of  the  pancreas  from  the  body  of  a  dog  or  other  animal  is  in  a 
few  days  followed  by  a  rise  in  the  percentage  of  sugar  in  the  blood  and  its 
elimination  by  the  kidneys.  In  a  short  time  acetone,  aceto-acetic  and 
/3-oxybutyric  acids  make  their  appearance,  attended  by  the  usual  symptoms 
characteristic  of  glycosuria  in  man.  Death  usually  occurs  at  the  end  of 
three  or  four  weeks.     The  quantity  of  sugar  excreted  and  the  gravity  of  the 


SECRETION.  453 

attendant  symptoms  may  be  much  diminished  by  allowing  a  portion  of  the 
gland  to  remain  in  situ,  even  though  its  capacity  for  the  production  of  pan- 
creatic juice  is  entirely  abolished.  Transplantation  of  the  pancreas  to  the 
subcutaneous  tissue  or  to  the  abdominal  cavity  will  practically  prevent  the 
glycosuria.  The  explanations  which  have  been  offered  as  to  the  manner  in 
which  the  pancreatic  tissue  prevents  and  its  absence  gives  rise  to  the  excre- 
tion of  sugar  are  purely  hypothetical.  It  has  been  claimed  by  some  in- 
vestigators that  the  pancreas  secretes  a  specific  material,  which  after  its  en- 
trance into  the  blood  and  its  distribution  to  the  tissues,  particulary  the  muscles, 
promotes  oxidation  of  the  sugar.  In  the  absence  of  this  material  the 
oxidizing  power  is  lost  and  hence  the  sugar  accumulates,  and  is  finally 
eliminated  by  the  kidneys.  Since  the  discovery  of  the  islands  of  Langer- 
hans  it  has  been  suggested  by  some  investigators  that  the  production  of  the 
material  which  regul»tes  carbohydrate  metabolism  should  be  attributed  to 
them  rather  than  to  the  pancreas  as  a  whole.  The  presence  however  of  a 
glycolytic  enzyme  in  either  the  pancreas  or  the  muscle  has  not  been  positively 
demonstrated,  but  if  sugar  be  subjected  to  the  action  of  a  mixture  of  pancre- 
atic and  muscle  juices,  it  is  quickly  oxidized,  from  which  it  has  been  inferred 
that  the  secretion  of  the  pancreas  activates  the  enzyme  of  the  muscle.  That 
the  pancreas  is  actively  associated  with  carbohydrate  metabolism  is  indica- 
ted by  the  fact  that  in  a  considerable  percentage  of  cases  lesions  of  the  pan- 
creas, more  or  less  extensive,  have  been  found.  The  sugar  excreted  doubtless 
in  part  comes  from  the  glycogen  of  the  liver,  as  this  disappears  in  a  short 
time.  But  as  sugar  continues  to  be  excreted,  even  though  all  carbohydrates 
be  withdrawn  from  the  food,  the  conclusion  is  justifiable  that  it  arises  in 
consequence  of  increased  protein  metabolism.  This  supposition  is  strength- 
ened by  the  fact  that  the  quantity  of  nitrogen  excreted  rises  and  falls  with  the 
quantity  of  sugar  excreted. 

Phlorizin,  a  glucoside  obtained  from  the  root  bark  of  the  cherry  and 
plum  tree,  gives  rise  to  the  appearance  of  sugar  in  the  urine,  in  amounts 
beyond  that  which  might  come  from  the  glucose  normally  present  in  the 
blood  or  from  the  glycogen  of  the  liver.  As  there  is  a  concomitant  increase 
in  the  amount  of  urea  excreted,  the  supposition  is  that  phloridzin  increases 
protein  metabolism. 

Ciirara,  in  doses  sufficient  to  paralyze  the  muscles,  also  gives  rise  to  the 
appearance  of  sugar  in  the  urine.  This  is  not  due,  however,  to  an  increased 
production  on  the  part  of  the  liver,  but  rather  to  a  want  of  consumption  on 
the  part  of  the  muscles,  due  to  their  inactivity.  The  accumulation  of  the 
sugar  in  the  blood  which  takes  place  for  this  reason  leads  very  promptly  to 
its  removal  by  the  kidneys. 

The  Formation  of  Urea. — It  is  now  generally  believed  that  the  liver 
is  the  most  active  of  all  the  organs  which  may  be  engaged  in  the  production 
of  urea.  This  belief  is  based  on  numerous  physiologic  and  pathologic 
data.  The  compounds  out  of  which  the  hepatic  cells  construct  urea  have 
been  for  chemic  reasons  asserted  to  be  the  ammonium  salts,  e.g.,  the  car- 
bonate, lactate,  and  carbamate,  which  are  constantly  present  in  the  blood. 
These  salts,  which  result  from  protein  metabolism,  may  be  absorbed  from 
the  tissues  or  from  the  intestines,  carried  to  the  liver,  and  there  synthesized  to 
urea.     This  supposition  is  supported  by  an  experiment  as  follows:     The 


454  TEXT-BOOK  OF  PHYSIOLOGY. 

liver  of  an  animal  recently  living  is  removed  from  the  body  and  its  vessels 
perfused  continuously  with  blood  (the  urea  content  of  which  is  known) 
containing  the  ammonium  salts.  An  analysis  of  this  blood  shows,  after  a 
time,  a  diminution  of  these  salts,  and  a  large  increase  in  the  amount  of  the 
urea.  After  the  establishment  of  an  Eck  fistula  (the  union  of  the  portal 
vein  with  the  ascending  vena  cava  whereby  the  liver  is  largely  excluded  from 
acting  on  products  absorbed  from  the  intestines)  there  is  a  marked  diminu- 
tion in  the  production  of  urea  while  the  ammonia  content  of  the  urine 
largely  increases.  One  large  source  for  the  ammonium  which  is  trans- 
formed into  urea  by  the  liver-cells,  is  the  amino-acid  compounds  in  the  in- 
testine which  are  not  needed  for  the  reconstruction  of  the  protein  molecule. 
These  compounds  are  absorbed  by  the  epithelial  cells  of  the  villi  and 
mucous  membrane  generally,  deamidized  or  deprived  of  their  amino-acid 
nitrogen  (NHj)  which  is  at  once  converted  into  ammonia.  The  ammonia 
in  turn  combines  with  carbon  dioxid  with  formation  of  ammonium 
carbonate.  When  this  compound  is  transported  to  the  liver  by  the  portal 
blood,  the  cells  convert  it  into  urea  in  a  manner  shown  in  the  following 
formula : 

(NH  J  3CO3  -  2H2O  =  CON^H,. 

Destructive  diseases  of  the  liver — e.g.,  acute  yellow^  atrophy,  suppuration, 
cirrhosis — largely  diminish  the  production  of  urea,  but  increase  the  quanti- 
ties of  the  ammonium  salts  in  the  urine.  The  same  is  true  when  the  liver 
cells  are  destroyed  during  acute  phosphorus  poisoning. 

The  Conjugation  of  Products  of  Protein  Putrefaction. — One  of  the 
important  functions  of  the  liver  is  the  conversion  of  toxic  compounds,  the 
products  of  the  putrefaction  of  proteins,  into  non-toxic  compounds.  These 
compounds  are  formed  in  the  intestine,  are  absorbed  and  carried  by  the  blood 
of  the  portal  vein  to  the  liver.  In  their  passage  through  the  capillaries  of 
the  liver  they  are  conjugated  for  the  most  part  with  potassium  sulphate  by 
the  action  of  the  liver  cells  and  thus  deprived  of  their  toxicity.  Among  the 
substances  thus  conjugated  are  indol,  skatol,  phenol,  and  cresol.  After 
absorption  indol  and  skatol  are  oxidized  to  indoxyl  and  skatoxyl  and  then 
combined  with  potassium  sulphate  giving  rise  to  potassium  indoxyl  sulph- 
ate and  potassium  skatoxyl  sulphate.  Phenol  and  cresol  are  apparently 
directly  combined  with  potassium  sulphate.  All  of  these  compounds  then 
pass  into  the  blood  of  the  general  circulation  and  finally  are  eliminated  by 
the  kidneys.  Potassium  indoxyl  sulphate  or  indican  is  the  source  of  the 
indigo-forming  substance  found  in  the  urine.  Other  compounds  are  like- 
wise reduced  in  toxicity  by  the  liver  cells  though  the  methods  by  which  this 
is  accomplished  vary  with  the  nature  of  the  compound.  The  liver  thus 
presents  a  chemic  defense  against  the  entrance  of  more  or  less  toxic  agents 
into  the  blood  of  the  general  circulation. 

VASCULAR  OR  DUCTLESS  GLANDS. 

INTERNAL  SECRETIONS. 

The  metabolism  of  the  body  generally,  as  well  as  that  of  individual 
organs,  has  been  shown  to  be  related  not  only  to  the  physiologic  activity 
of  such  organs  as  the  liver  and  pancreas,  but  also  to  the  activity  of  the  so- 


SECRETION. 


455 


called  vascular  or  ductless  glands.  The  influence  of  the  pancreas  in  regulat- 
ing the  oxidation  of  sugar  and  the  influence  of  the  liver  in  the  maintenance 
of  the  general  metabolism  through  the  production  of  glycogen  and  the 
formation  of  urea,  are  now  established  facts.  That  the  vascular  or  ductless 
glands  to  an  equal  extent,  though  perhaps  in  a  different  way,  assist  in  the 
maintenance  of  physiologic  processes,  appears  certain  from  the  results  of 
animal  experimentation.  The  explanation  given  for  the  influence  of  these 
glands  is  that  they  produce  specific  substances,  which  are  poured  into  the 
blood  or  lymph  and  carried  direct 
to  the  tissues,  to  the  activities  of 
which  they  appear  to  be  essential; 
for  without  these  substances  the 
nutrition  of  the  tissues  declines  and 
in  a  short  time  a  fatal  termination 
ensues. 

Inasmuch  as  these  partly  un- 
known, substances  are  formed  by 
cell  activity  and  are  poured  into 
the  circulating  blood,  they  have 
been  termed  'internal  secretions." 
Though  the  term  internal  secre- 
tions is  applicable  to  all  sub- 
stances which  arise  in  consequence 
of  tissue  metabolism,  and  which, 
after  being  poured  into  the  blood, 
influence  in  varying  degrees  and 
ways  physiologic  processes,  yet  the  term  in  this  connection  will  be  applied 
only  to  the  secretions  of  the  thyroid  and  parathyroid  glands,  pituitary  body 
or  hypophysis  cerebri,  and  adrenal  bodies. 

Thyroid  Gland. — The  thyroid  gland  or  body  consists  of  two  lobes  situated 
on  the  lateral  aspect  of  the  upper  part  of  the  trachea  (Fig.  210).  Each  lobe 
is  pyriform  in  shape,  the  base  being  directed  downward  and  on  a  level 
with  the  fifth  or  sixth  tracheal  ring.  The  lobe  is  about  50  mm.  in  length, 
20  mm.  in  breadth,  and  25  mm.  in  thickness.  As  a  rule,  the  lobes  are  united 
by  a  narrow  band  or  isthmus  of  the  same  tissue.  The  gland  is  reddish  in 
color,  and  abundantly  supplied  with  blood-vessels  and  lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an  enormous 
number  of  closed  sacs  or  vesicles,  variable  in  size,  the  largest  not  measuring 
more  than  o.i  mm.  in  diameter  (Fig.  211).  Each  sac  is  composed  of  a  thin 
homogenous  membrane  lined  by  cuboid  epithelium.  The  interior  of  the 
sac  in  adult  life  contains  a  transparent,  viscid  fluid  containing  albumin 
and  termed  "colloid"  substance.  Externally,  the  sacs  are  surrounded  by  a 
plexus  of  capillary  blood-vessels  and  lymphatics.  The  individual  sacs  are 
united  and  supported  by  connective  tissue,  which  forms,  in  addition,  a  cov- 
ering for  the  entire  gland. 

Effects  of  Removal  of  the  Thyroid. — The  knowledge  at  present  pos- 
sessed as  to  the  function  of  the  thyroid  gland,  especially  in  mammals,  is  the 
outcome  of  a  study  of  the  effects  which  follow  its  arrested  development  in 
the  child,  its  degeneration  in  the  adult,  and  its  extirpation  in  the  human 


Fig.  210. — View  of  Thyroid  Body.  i. 
Thyroid  isthmus.  2.  Median  portion  of  crico- 
thyroid membrane.  3.  Crico-thyroid  muscle. 
4.   Lateral  lobe  of  thyroid  body. — {After  Morris) 


456 


TEXT-BOOK  OF  PHYSIOLOGY. 


being  as  well  as  in  animals.  The  results,  however,  which  follow  its  extirpation 
are  not  always  uniform  in  all  animals,  though  sufficient  reasons  for  the 
lack  of  uniformity  cannot  always  be  assigned. 

Cretinism,  a  condition  characterized  by  a  want  of  physical  and  mental 
development,  is  associated  with,  if  not  directly  dependent  on,  a  congenital 
absence  of  the  thyroid,  or  its  arrested  development  during  the  early  years 
of  childhood. 

Myxedema,  a  condition  of  the  skin  in  which  there  is  a  hyperplasia  of 
the  connective  tissue,  of  an  embryonic  type,  rich  in  mucin,  is  generally 

regarded  as  one  of  the 
effects  of  degenerative  pro- 
cesses in  the  thyroid  in  the 
adult.  Partly  in  conse- 
quence of  this  change  in 
the  skin  the  face  becomes 
broader,  swollen,  and  flat- 
tened, giving  rise  to  a  loss 
of  expression.  At  the  same 
time  the  mind  becomes 
dull,  clouded,  even  approxi- 
mating the  idiotic  type. 
This  supposed  infiltration 
of  the  skin  with  mucin  was 
termed  myxedema  by  Ord, 
who  at  the  same  time  asso- 
ciated it  with  a  change  in 
the  structure  of  the  thyroid 
as  a  result  of  which  it  be- 
came functionally  useless. 

Extirpation  of  the  thy- 
roid, for  relief  from  symp- 
toms due  to  grave  patho- 
logic changes,  has  been  followed  in  human  beings  by  symptoms  similar  to 
those  of  myxedema.  To  this  condition  the  terms  operative  myxedema  and 
cachexia  strumipriva  have  been  applied. 

After  the  publication  of  the  history  of  the  myxedema  which  followed 
surgical  removal  of  the  thyroid,  Schiff,  in  1887,  repeated  his  earlier  experi- 
ments on  dogs,  and  found  again  that  removal  of  the  thyroid  was  speedily 
followed  by  tremors,  convulsions,  and  death.  Similar  experiments  were 
made  by  Horsley  on  monkeys,  with  results  which  resembled  those  character- 
istic of  myxedema.  Among  the  symptoms  which  developed  within  a  few 
days  after  the  removal  of  the  gland  may  be  mentioned  loss  of  appetite; 
fibrillar  contractions  of  muscles;  tremors  and  spasms;  mucinoid  degeneration 
of  the  skin,  giving  rise  to  puffiness  of  the  eyelids  and  face  and  to  a  swollen 
condition  of  the  abdomen;  hebetude  of  mind,  frequently  terminating  in 
idiocy;  fall  of  blood-pressure;  dyspnea;  albuminuria;  atrophy  of  the  tissues, 
followed  by  death  of  the  animal  in  the  course  of  from  five  to  eight  weeks. 
The  complexus  of  symptoms  observed  in  monkeys  was  divided  by  Horsley 
into  three  stages:  viz.,  the  neurotic,  the  mucinoid,  and  the  atrophic. 


Fig  211. — A  Lobule  from  a  Thin  Section  of  the 
Thyroid  Gland  of  an  Adult  Man.  i.  Colloid  sub- 
stance. 2.  Epithelium.  3.  Tangential  section  of  a  tubule, 
the  epithelium  viewed  from  the  surface.  4.  Tubule  in 
transverse  section.     5.   Connective  tissue. — (Stohr.) 


SECRETION.  457 

It  is  evident  that  the  presence  of  the  thyroid  is  essential  to  the  normal 
activity  of  the  tissues  generally.  As  to  the  manner  in  which  it  exerts  its 
favorable  influence,  there  is  some  difference  of  opinion.  The  view  that  the 
gland  removes  from  the  blood  certain  toxic  bodies,  rendering  them  innocuous 
and  thus  preserving  the  body  from  a  species  of  auto-intoxication,  is  gradually 
yielding  to  the  more  probable  view  that  the  epithelium  is  engaged  in  the 
secretion  of  a  specific  material,  which  finds  its  way  into  the  blood  or  lymph 
and  in  some  unknown  way  influences  favorably  tissue  metabolism.  This 
view  of  the  function  of  the  thyroid  is  supported  by  the  fact  that  successful 
grafting  of  a  portion  of  the  thyroid  beneath  the  skin  or  in  the  abdominal 
cavity  will  prevent  the  usual  symptoms  which  follow  thyroidectomy.  The 
same  result  is  obtained  by  the  intravenous  injection  of  thyroid  juice  or  by 
the  administration  of  the  raw  gland.  It  was  shown  by  Murray  that  myxe- 
dematous patients  could  be  benefited,  and  even  cured,  by  feeding  them  with 
fresh  thyroids  or  with  the  dry  extract. 

The  chemic  features  of  the  material  secreted  and  obtained  from  the 
structures  of  the  thyroid  indicate  that  it  is  a  complex  protein  containing 
iodin,  which,  under  the  influence  of  various  reagents,  undergoes  cleavage, 
giving  rise  to  a  non-protein  residue,  which  carries  with  it  the  iodin  and 
phosphorus.  The  amount  of  iodin  in  the  thyroid  varies  from  0.33  to  i 
milligram  for  each  gram  of  tissue.  To  this  compound  the  term  thyro-iodin 
has  been  given.  The  administration  of  this  compound  produces  effects 
similar  to  those  which  follow  the  therapeutic  administration  of  the  fresh 
thyroid  itself;  viz.,  a  diminution  of  all  myxedematous  symptoms.  In  normal 
states  of  the  body,  thyro-iodin  influences  very  actively  the  general  metabolism. 
It  gives  rise  to  a  decomposition  of  fats  and  proteins  and  to  a  decline  in 
body-weight.  In  large  doses  it  may  produce  toxic  symptoms,  e.g.,  increased 
cardiac  action,  vertigo,  and  glycosuria. 

It  has  also  been  suggested  from  the  clinical  side  that  the  symptoms 
comprised  under  the  term  exophthalmic  goiter,  viz.,  enlargement  (hyper- 
trophy) of  the  thyroid,  rapid  action  of  the  heart,  pulsation  of  the  large 
arteries  at  the  base  of  the  neck,  protrusion  of  the  eye-balls  and  fine  tremors 
of  the  hands  are  due  to  a  hypersecretion  of  the  thyroid  cells,  a  condition 
spoken  of  as  hyperthyroidism. 

The  conclusions  as  to  the  functions  of  the  thyroid  gland  which  have  been 
drawn  from  the  results  that  have  followed  its  removal  from  animals  by 
surgical  procedures,  have  been  made  questionable,  since  the  discovery  of 
the  parathyroid  glands  and  a  study  of  the  phenomena  which  follow  when 
they  alone  are  removed.  From  their  situation  and  close  relationship  to  the 
thyroid  gland  it  is  generally  accepted,  that  in  the  earlier  experiments,  espe- 
cially those  made  on  cats  and  dogs,  and  some  other  carnivorous  animals,  both 
sets  of  glands  were  removed  and  hence  some  of  the  symptoms  which  devel- 
oped after  the  removal  of  the  thyroids  were  due  to  the  loss  of  function  not 
of  the  thyroid  but  of  the  parathyroids. 

This  is  especially  true  of  the  fibrillar  contractions,  tremors  and  spasms. 
These  it  is  now  more  generally  believed  arise  only  in  consequence  of  the 
simultaneous  removal  of  the  parathyroids.  The  myxedema  and  the  failure 
of  the  mental  powers  are  attributed  to  the  loss  or  degeneration  of  the  thyroid, 
and  cretinism  to  the  arrest  of  its  development. 


458 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Parathyroids. — The  parathyroids  are  small  bodies,  usually  four  in 
number,  two  on  each  side.  They  are  divided  into  superior  and  inferior. 
The  superior  are  situated  internally  and  on  the  posterior  surface  in  close 
relation  to,  and  frequently  imbedded  in,  the  substance  of  the  thyroid;  the 
inferior  are  situated  externally,  sometimes  in  contact  with,  and  at  other 
times  removed  a  variable  distance  from  the  thyroid  (Fig.  212).  Micro- 
scopically the  parathyroids  consist  of  thick  cords  of  epithelial  cells  separated 
by^septa  of  fine  connective  tissue  and  surrounded  by  capillary  blood-vessels. 
Effects  of  Parathyroid  Removal. — The  surgical  removal  of  the  para- 
thyroids is  followed  in  the  course  of  from  two  to  hve  days  by  the  death  of  the 
animal  preceded  in  most  instances  by  a  series  of  symptoms  which  are  em- 
braced under  the  general  term  "tetany."  These  symptoms  are  fibrillary 
contractions  of  muscles,  tremors,  spasmodic  contractions  and  paralyses  of 

groups  of  muscles  and  not  infrequently 
convulsive  seizures  and  coma.  During  the 
convulsion  there  is  an  acceleration  of  the 
heart-beat,  and  increase  in  the  respiratory 
movements  which  frequently,  become  dysp- 
neic  in  character.  There  is  also  a  loss 
of  appetite,  nausea,  mucous  vomiting,  and 
diarrhea.  Death  may  occur  during  a 
convulsion  or  from  coma.  (Morat  and 
Doyon.) 

These  results  for  the  most  part  occur 
only  when  all  the  parathyroids  are  removed. 
It  is  asserted  that  even  if  one  gland  is  re- 
tained the  animal  does  not  die.  The  above 
described  symptoms  may  manifest  them- 
selves, however,  but  they  are  slight  in 
degree. 

Vincent  and  Jolly  have  recently  pub- 
lished the  results  of  a  series  of  experiments 
which  seem  to  negative  to  some  extent  the 
preceding  statements.  These  experiment- 
ers state  that  while  it  is  true,  that,  as  a 
rule,  the  removal  of  both  thyroids  and  para- 
thyroids in  the  carnivora  is  a  fatal  opera- 
tion, there  are  nevertheless  many  excep- 
tions; and  in  the  mammalia  generally,  e.g., 
cats,  dogs,  foxes,  guinea-pigs,  rats,  and  monkeys,  the  exception  becomes  the 
rule  as  more  than  51  per  cent,  of  animals  survived  the  operation  for  a  pro- 
longed period  and  of  these  68  per  cent,  showed  no  specific  symptoms  of  any 
kind.  From  the  contradictory  observations  it  is  evident  that  the  subject 
needs  further  investigation. 

The  Pituitary  Body. — The  pituitary  is  a  small  body  lodged  in  the 
sella  turcica  of  the  sphenoid  bone.  It  measures  14  mm.  from  side  to 
side,  8  mm.  from  befcre  backward,  and  6  mm.  from  above  down,  and  con- 
sists of  an  anterior  lobe  somewhat  pink  in  color,  and  a  posterior  lobe  yellow- 
ish-gray in  color.    The  anterior  lobe  is  much  the  larger  and  partly  embraces 


Fig.    212. — The  position  of  the  para 
thjToid  glands. — {Zuckerkandl.) 


SECRETION.  459 

the  posterior  lobe.  (Fig.  213)  The  anterior  lobe  is  developed  from  an  in- 
vagination of  the  ectoderm  of  the  buccal  cavity  and  consists  of  gland  tissue 
surrounded  by  a  thin  envelope  of  connective  tissue.  It  becomes  separated  from 
the  mouth  by  the  fusion  of  the  sphenoid  cartilages.  The  posterior  lobe  is  an 
outgrowth  from  the  mid-brain  and  is  connected  with  the  infundibulum  of 
the  third  ventricle  by  a  short  stalk.  In  the  early  stages  of  its  development 
it  presents  a  central  cavity  which  is,  however,  soon  obliterated  by  the  growth 
of  special  tissues.  It  persists  in  the  cat.  It  has  been 
suggested  that  the  term  hypophysis  cerebri  be  reserved 
for  the  anterior  lobe  and  the  term  infundibular  body 
for  the  posterior  lobe.  This  distinction  appears  to  be 
desirable  inasmuch  as  in  their  origin,  structure  and  func- 
tions they  are  separate  and  distinct  bodies. 

Histology  of  the  Pituitary  Body. — If  a  mesial 
sagittal  section  be  made  through  the  pituitary  it  will 
present  an  appearance  which  in  a  general  wav  is  the 

■*■  *■  *■  ^-^  *■  T*  Tp     T  T  '2 Sac ttt-x l 

same  in  many  animals  though  the  details  vary  some-  section  of  the  Pitu- 
what  in  each  animal.  In  the  monkey  the  arrangement  itary  Body  and  Infux- 
of  the  anatomic  parts  (Fig.  214)  is  similar  to  the  ar-  ^JcpiRToTSmSvEN- 
rangement  in  man.  It  will  be  observed  that  the  posterior  tricle.  a.  Anterior 
lobe  is  solid  and  that  there  is  no  open  connection  with  !obe.  a',  a  projection 
the  cavity  of  the  third  ventricle;  that  it  is  invested,  over  oT?he' Sdlbulum?? 
a  large  part  of  its  surface,  by  a  thin  layer  of  epithelium.  Posterior  lobe  connected 
The  anterior  lobe,  which  lies  in  front  of  it  is  separated  p'  ^^^^f^  '^^^  the  in- 
by  a  cleft  which  is  the  remnant  of  the  cavity  of  the  ^^^^  ^f  ^^^^^  chiasm.— 
buccal  pouch.  Though  the  appearance  of  the  ante-  {Schwalbe  from  Quain.) 
rior  lobe,  and  the  epithelial  investment  of  the  posterior 
lobe  is  somewhat  different,  the  latter  is  but  a  differentiation  of  the  former,  a 
procedure  that  takes  place  in  fetal  life.  The  epithelial  investment  is 
usually  spoken  of  as  the  pars  intermedia,  and  regarded  histologically  and 
physiologically  as  a  part  of  the  posterior  lobe.  Superiorly  the  anterior  lobe 
and  the  pars  intermedia  are  united,  though  a  portion  of  the  latter  passes 
upward  and  embraces,  if  it  does  not  entirely  surround,  the  infundibular 
stalk;  inferiorly  and  posteriorly  the  two  bodies  also  unite.  The  posterior 
surface  of  the  posterior  lobe  is  free  from  epithelial  investment  in  the  mid-line. 
The  extent  to  which  the  epithelium  invests  the  posterior  lobe  varies  in  dif- 
ferent animals.     In  the  cat  and  dog  it  is  almost  complete. 

Microscopic  examination  of  the  anterior  lobe  shows  the  presence  of 
granular  epithelial  cells,  the  descendents  of  the  original  buccal  epithelium, 
arranged  in  columns  between  which  pass  large  thin-walled  blood-vessels. 
In  view  of  the  physiologic  importance  of  this  lobe  it  is  believed  that  the 
granules  of  the  cell  represent  an  internal  secretion,  which  passes  into  the 
blood-stream  and  is  thus  distributed  to  various  regions  of  the  body. 

The  pars  intermedia  consists  of  several  layers  of  finely  granular  epithelial 
cells  which  develop  a  colloid  material  that  subsequently  passes  into  the  pos- 
terior lobe  where  it  becomes  hyaline  in  character.  The  epithelial  investment 
is  separated  from  the  posterior  lobe  by  a  layer  of  blood-vessels  though 
columns  of  cells  penetrate  it. 

The  posterior  lobe  consists  of   neuroglia  cells  and  fibers.     True  nerve- 


460 


TEXT-BOOK  OF  PHYSIOLOGY. 


cells  are  apparently  wanting.  Throughout  the  lobe  there  are  numerous  small 
hyaline  bodies  which  are  apparently  streaming  upward  to  the  ventricular 
cavity.  In  view  of  the  physiologic  importance  of  this  infundibular  body  or 
pars  nervosa ,  these  hyaline  masses  are  believed  to  represent  an  internal  secretion 
which  passes  upward  through  loose  tissue  channels  toward  the  infundibulum 
to  be  discharged  into  the  fluid  of  the  third  ventricle.  If  the  stalk  be 
divided  there  is  an  accumulation  of  these  bodies  in  the  posterior  lobe. 
Both  parts  of  the  pituitary  are  well  supplied  with  blood  though  from  different 
sources. 

Effects  of  Total  Removal. — The  effects  which  were  observed  by  the 
earlier  investigators  to  follow  total  removal  of  the  hypophysis  were  not 
always  in  accord  by  reason  of  the  difference  in  the  operative  methods 

pursued,  injuries  to  the  brain, 
infections,  imperfect  removals 
as  shown  by  post-mortem  ex- 
amination, etc.  Some  inves- 
tigators claimed  that  after  total 
removal  animals  lived  for  long 
periods  and  that  therefore  the 
gland  was  not  essential  to  life; 
others  claimed,  however,  its 
total  removal  was  followed  very 
shortly  by  death  preceded  by  a 
series  of  characteristic  symp- 
toms and  that  therefore  it  was 
absolutely  essential  to  life. 
The    introduction    of    a    new 

^  method  of  procedure  for  the  re- 

FiG.  214. — Mesial  Saggital  Section  OF  THE  Pit-  _        ,       r  \u       i,  u      •      u 

uiTARY  Body  of  the  Monkey,     a,  Optic  chiasm;  &,  "^O^al     of    the  _  hypophysis    by 

process  of  the  pars    intermedia;  c,  third    ventricle;  d,  Paulesco    and    its    employment 

anterior  lobe  proper;/,  posterior  lobe  or  pars  nervosa;  jjy  Cushing  and  his  CO-WOrkers 

?,  epithelium  mvestment  of  the  posterior  lobe;  ^,epTthe-  \.'      \    a   .  1,         U"   V.  ( 

Hum  of  the  pars  intermedia  passing  over  the  neighbor-  ^^^  ^^^  tO  reSUltS  WxUCll  are  lOr 

ing  brain  mass;  e,  cMi.— {After  Herring.)  the  most  part  in  general  agree- 

ment. This  method  involves 
an  approach  to  the  gland  through  the  temporal  bone  instead  of  through  the 
buccal  cavity  as  was  formerly  the  case.  The  temporal  muscles  are  first 
dissected  away  from  the  skull  on  both  sides  and  reflected  downward. 
Large  openings  are  made  in  the  bone  and  dura  of  both  sides.  The  temporal 
lobe  on  on.e  side  is  lifted  up  with  a  spoon-shaped  spatula  sufficiently  large 
to  expose  the  hypophysis,  hanging  from  the  infundibulum.  Owing  to  the 
opposite  opening  in  the  skull,  the  opposite  half  of  the  cerebrum  is  displaced 
and  protruded  so  that  injury  to  the  brain  from  compression  is  prevented. 
The  gland  can  then  be  picked  up  with  forceps  and  removed.  Paulesco 
reported  that  the  total  removal  of  the  gland  in  24  dogs  resulted  in  death  in 
24  hours.  In  seven  other  animals  the  fatal  result  was  postponed  for  variable 
periods.  One  animal  survived  for  five  months  and  another  for  a  year 
without  exhibiting  any  very  characteristic  symptoms.  As  a  post-mortem 
examination  showed  that  the  gland  was  only  partially  removed  it  was 
assumed  that  the  remaining  portion  had  been  sufficient  to  maintain  life. 


SECRETIOX.  461 

Removal  of  the  anterior  lobe  alone  was  followed  by  death  as  certainly  as  when 
the  entire  gland  was  removed.  Removal  of  the  posterior  lobe  alone  was  not 
followed  by  noticeable  effects.  From  these  facts  Paulesco  asserted  that  the 
hypophysis  is  an  organ  indispensable  to  life  as  its  removal  rapidly  eventuates  in 
death,  and  that  of  its  different  parts  the  anterior  lobe  is  the  more  important. 
Crowe,  Gushing  and  Homans  have  more  recently  reported  a  series  of  100 
operations  for  the  removal  of  the  hypophysis,  the  results  of  which  are  corrob- 
orative in  many  respects  of  the  results  of  Paulesco.  It  was  found  that  the 
duration  of  life  in  adult  dogs  was  from  two  to  three  days  and  in  young  dogs 
about  eleven  days.  In  a  few  cases  the  animals  survived  for  several  weeks 
but  a  post-mortem  examination  showed  that  small  viable  portions  of  the 
gland  had  escaped  removal.  Among  the  many  symptoms  that  followed  to- 
tal hypophysectomy  according  to  these  experimenters  the  more  striking  after 
24  hours  were  a  lowering  of  body-temperature,  unsteadiness  of  gait  and  stiff- 
ness of  movement,  a  fall  of  blood-pressure,  feeble  and  slow  respiration, 
muscle  twitchings,  lethargy,  coma,  and  death.  In  old  animals  there  was 
occasional  glycosuria;  in  young  animals  polyuria.  Total  removal  of  the 
anterior  lobe  alone  in  this  series  of  experiments  was  also  as  fatal  as  removal 
of  the  entire  gland. 

The  Effects  of  Partial  Removal  of  the  Anterior  Lobe. — When  only  a  portion 
of  the  anterior  lobe  was  removed  the  animal  survived  for  a  much  longer 
period  than  when  the  removal  was  complete.  The  duration  of  life  appar- 
ently depended  on  the  amount  and  the  cellular  activity  of  the  parts  left  be- 
hind. As  a  result  of  the  partial  removal  only  there  developed  a  series  of 
phenomena  to  which  the  term  cachexia  'hypophyseopriva  has  been  given. 
These  phenomena  varied  somewhat  in  accordance  with  the  age  of  the 
animal.  Adult  animals  became  adipose  and  degenerated  sexually,  young 
animals  likewise  became  adipose  but  they  remained  undersized  and  failed 
to  develop  sexual  characteristics  and  hence  sexual  infantilism  persisted. 
The  organs  of  reproduction  in  both  sexes  remained  rudimentary.  The 
temperature  was  subnormal  and  nutritive  disorders  of  the  skin  developed. 
These  various  symptoms  were  attributed  at  the  time  to  the  partial  removal 
of  the  anterior  lobe  alone  and  hence  a  deficiency  of  secretion,  but  the  results 
of  a  series  of  experiments,  subsec{uently  published  by  Gushing,  led  to  the 
belief  that  some  of  these  phenomena  were  due  to  injury  or  impairment  of  the 
normal  function  of  the  posterior  lobe  at,  or  subsequent  to,  the  time  of  the 
operation.  Just  which  of  these  phenomena  are  due  to  a  diminished  secre- 
tion of  the  anterior  lobe  and  which  to  a  diminished  secretion  of  the  posterior 
lobe  future  investigations  only  will  determine. 

The  Effects  of  Pathologic  Conditions. — In  recent  years  the  idea  has  gradu- 
ally developed  that  certain  pathologic  states  of  the  body  are  associated  in 
some  way  with  pathologic  states  of  the  pituitary  body.  Thus  the  condition 
of  gigantism  which  begins  in  youth  and  the  condition  of  acromegaly  which 
appears  in  adult  life  are  believed  to  be  the  result  of  a  hypersecretion  of 
the  anterior  lobe,  which  in  turn  may  be  due  to  a  hyperplasia  of  the  gland 
elements  excited  by  a  variety  of  causes.  In  both  gigantism  and  acromegaly 
there  is  an  increased  activity  in  the  nutritive  process  leading  .to  an  over- 
growth of  osseous  tissue  and  the  overlying  structures.  In  the  former  con- 
dition the  overgrowth  is  general;  in  the  latter  it  is  confined  to  the  face  and 


462  TEXT-BOOK  OF  PHYSIOLOGY. 

the  extremities,  hands  and  feet.     To  this  phase  of  pituitary  activity  the  term 
hyperpituitarism  has  been  given. 

The  opposite  condition,  infantilism  and  adiposity,  have  also  been  shown 
to  be  associated  with  pathologic  changes  in  the  pituitary.  In  these  cases 
not  only  is  the  individual  of  small  size  but  the  genital  organs  are  undeveloped. 
In  addition  there  may  be  a  subnormal  temperature,  loss  of  hair,  etc.  These 
phenomena  are  believed  to  be  due  to  a  diminished  or  defective  secretion 
partly  of  the  anterior  lobe  and  partly  of  the  posterior  lobe.  To  this  condi- 
tion the  term  hypopituitarism  has  been  given. 

The  Effects  of  Removal  of  the  Posterior  Lohe. — Gushing  in  a  series  of  ex- 
periments (191 1)  has  demonstrated  that  the  posterior  lobe  with  its  epithelial 
investment  exerts,  contrary  to  general  opinion,  a  profound  influence  on 
metabolism  and  more  especially  on  the  metabolism  of  the  carbohydrates, 
either  alone  or  in  conjunction  with  other  glands  having  internal  secretions, 
as  will  be  explained  more  fully  in  a  following  paragraph.  These  experi- 
ments also  led  to  the  belief  that  some  of  the  phenomena  detailed  in  the 
foregoing  paragraph,  especially  the  deposition  of  fat,  the  subnormal  tem- 
perature and  perhaps  the  imperfect  development  of  the  sexual  organs  are 
due  rather  to  a  deficiency  or  absence  of  the  secretion  of  the  posterior  lobe 
than  to  a  deficiency  or  absence  of  the  secretion  of  the  anterior  lobe. 

It  has  apparently  been  demonstrated  by  Gushing  that  the  hyaline  bodies 
found  in  the  posterior  lobe  represent  an  internal  secretion;  that  they  are 
discharged  into  the  cavity  of  the  third  ventricle  where  they  undergo  solution 
in  the  cerebro-spinal  fluid,  by  means  of  which  the  dissolved  material 
enters  the  blood-stream.  The  presence  in  the  cerebro-spinal  fluid  of  an 
agent  that  produces  the  same  physiologic  effects  when  intravenously  in- 
jected, as  injections  of  extracts  of  the  posterior  lobe  do,  has  also  been 
established. 

In  the  various  operative  procedures  incident  to  the  removal  of  the  entire 
hypophysis  or  of  the  anterior  lobe  a  transient  glycosuria  is  frequently  ob- 
served, a  phenomenon  attributed  to  the  discharge  under  the  circumstances  of 
an  excessively  large  amount  of  the  reserve  hyaline  substance  or  of  the 
posterior  lobe  secretion  into  the  third  ventricle.  This  secretion  in  some 
unknown  way  leads  to  a  hyperglycemia  and  glycosuria.  At  the  same  time 
the  animal  becomes  unable  to  tolerate  or  assimilate  the  usual  amount  of 
sugar  experimentally  ingested  without  increasing  the  glycosuria,  which  is 
assumed  therefore  to  be  of  alimentary  origin. 

If  the  posterior  lobe  with  its  epithelial  investment  is  totally  removed 
or  if  the  infundibular  stalk  is  compressed  by  a  clip  so  as  to  prevent  the  dis- 
charge of  the  secretion  into  the  ventricle  the  animal  becomes  very  tolerant  of 
sugar  and  is  enabled  to  assimilate  larger  quantities  than  formerly  without 
the  development  of  alimentary  glycosuria.  As  a  probable  result  of  the  in- 
creased carbohydrate  assimilation,  a  condition  of  nutrition  is  established, 
characterized  by  a  general  deposition  of  fat  suggesting  a  conversion  of  the 
sugar  into  fat.  There  is  probably  at  the  same  time  an  imperfect  oxidation 
of  the  carbohydrates  as  indicated  by  the  lowered  temperature. 

That  the  condition  of  generalized  adiposity  is  probably  due  to  deficient 
posterior  lobe  secretion  is  shown  by  the  fact  that  the  increased  tolerance  for 


SECRETION.  463 

sugar  can  be  lowered  very  promptly  by  the  coincident  intravenous  or  sub- 
cutaneous injection  of  extracts  of  the  posterior  lobe. 

From  the  foregoing  facts  it  may  be  assumed  that  the  secretion  of  the 
posterior  lobe  in  some  unknown  way  influences  the  metabolism  of  sugar. 
From  the  facts  at  hand  it  may  be  assumed  that  a  hypersecretion  from  any 
cause  whatever,  leads  to  a  diminished  tolerance  for  or  assimilation  of  sugar, 
as  shown  by  the  hyperglycemia  and  glycosuria,  though  the  manner  in  which 
the  hyperglycemia  is  developed,  whether  by  a  more  rapid  conversion  of 
glycogen  to  sugar  or  by  an  inefficient  storage  of  sugar  as  glycogeh  is  unknown. 
A  hyposecretion  from  any  cause  leads  to  an  increased  tolerance  for  or 
assimilation  of  sugar  which  eventually  contributes  to  the  formation  and 
deposition  of  fat.  In  the  complexus  of  symptoms  that  accompany  patho- 
logic changes  in  the  hypophysis  either  in  the  anterior  or  posterior  lobe  it  is 
difficult  to  indicate  those  which  are  to  be  attributed  to  increased  or  decreased 
secretion  of  either  the  anterior  or  posterior  lobe  by  reason  of  their  close 
juxtaposition  and  their  possible  simultaneous  involvement;  again  it  is  also 
uncertain  as  to  whether  the  secretions  produce  their  effects  alone  or  through 
the  cooperation  of  the  secretions  of  other  organs  having  more  or  less  influ- 
ence in  the  metabolism  of  the  carbohydrates. 

The  Effects  of  Injections  of  Extracts. — The  extracts  of  the  anterior  lobe 
when  intravenously  injected  appear  to  be  without  any  appreciable  effect 
on  any  of  the  physiologic  mechanisms.  Injections  of  the  extracts  of  the 
posterior  lobe,  however,  give  rise  very  promptly,  as  shown  by  Howell,  to  an 
increase  in  the  blood-pressure  which  appears  to  be  due  to  an  increased 
contraction  of  the  arteriole  muscle  rather  than  to  a  stimulation  of  the  vaso- 
motor centers,  as  the  contraction  takes  place  even  after  destruction  of  the 
spinal  cord  and  medulla  oblongata.  The  action  of  the  active  constituent 
of  the  extract  appears  to  be  very  general  as  there  is  a  simultaneous  diminution, 
as  shown  by  plethysmographic  investigations,  in  the  volume  of  various 
organs.  On  the  heart  the  extract  has  an  inhibitor  action  which  takes  place 
concomitantly  with  the  contraction  of  the  arterioles  and  the  rise  of  the 
pressure  as  shown  by  Howell.  This  is  attributed  to  a  direct  stimulation  of 
the  cardio-inhibitor  center  as  the  retardation  is  partly  prevented  at  least 
when  the  vagus  is  divided  or  its  function  suspended  by  atropin.  Even  after 
this  has  been  done,  however,  a  slowing  of  the  heart  may  still  be  induced,  a 
fact  which  suggests  that  the  extract  acts  directly  on  the  heart  muscle  as  well. 
Schafer  and  his  co-workers  have  also  demonstrated  that  pituitary  extracts 
cause  dilatation  of  the  renal  vessels  and  stimulate  specifically  the  renal  cells 
to  activity,  thus  causing  a  marked  diuresis.  The  extract  also  stimulates  the 
non-striated  muscles  of  the  intestines,  bladder,  uterus,  as  well  as  the  dilatator 
muscle  of  the  iris. 

Adrenal  Glands,  or  Suprarenal  Capsules. — These  are  two  flattened 
bodies,  somewhat  crescentic  or  triangular  in  shape,  situated  each  upon  the 
upper  extremity  of  the  corresponding  kidney,  and  held  in  place  by  connective 
tissue.  They  measure  about  40  mm.  in  height,  30  mm.  in  breadth,  and 
from  6  to  8  mm.  in  thickness.  The  weight  of  each  is  about  4  gm.  Acces- 
sory glands  are  sometimes  found  in  the  surrounding  connective  tissue  along 
the  abdominal  sympathetic  and  in  the  neighborhood  of  the  genital  organs. 

In  some  animals  such  as  the  dog,  cat  and  rabbit,  these  glands  have  no 


464 


TEXT-BOOK  OF  PHYSIOLOGY. 


( 


anatomic  connection   with   the  kidneys,   but   are  situated  at  varying  dis- 
tances from  them. 

Histology. — The  gland  is  covered  externally  by  a  fibrous  tissue  from 
which  septa  pass  into  the  more  central  portions  thus  forming  a  framework 
for  the  support  of  blood-vessels  and  cells. 

A  section  of  the  gland  shows  just  beneath  the  capsule  an  outer  portion 
termed  the  cortex  and  an  inner  portion  termed  the  medulla  (Fig.  215).     The 
cortex  consists  mainly  of  cuboid  cells  arranged  in  cylindric  columns.     The 
outer  layers  of  cells  are  arranged  in  irregular  masses  forming  what  has  been 
called  the  zona  glomerulosa.     The  medulla  consists  of  uniting  and  interlacing 
^^^^s^;^=-ss^^5s^^^       cords  of  polyhedral  cells,  the  cytoplasm  of  which 
"^  ^--^'-    :'  ':  ■   —        ya     contains  granular  matter  and  a  distinct  nucleus. 
^6     When  treated  with  chromic  acid  or  chromium 
3.      salts  the  cytoplasm  stains  a  dull  brown  or  yellow 
%      color.     For  this  reason  they  are  termed  chro- 
I      maffin  cells.     Similar  cells  are  found  in  sym- 
pathetic ganglia. 

The  gland  is  abundantly  supplied  with 
blood-vessels  and  nerves.  The  arteries  are 
branches  of  the  aorta,  the  phrenic,  and  renal 
arteries.  After  penetrating  the  gland  they 
divide  into  smaller  branches  and  capillaries 
which  ultimately  come  into  close  relation  with 
the  cells  of  both  the  cortex  and  medulla. 
The  veins  emerge  from  the  gland  at  the  hilum 
and  empty  on  the  right  side  into  the  vena  cava 
and  on  the  left  side  into  the  renal  vein.  The 
nerves  passing  to  the  gland  are  derived  for  the 
most  part  from  the  autonomic  system.  The 
pre-ganglionic  fibers  pass  from  the  cord  by  way 
of  the  splanchnics  to  the  semilunar  ganglion. 
The  post-ganglionic  pass  from  the  semilunar 
Fig.  215— Section  or  Human  ganglia  through  its  branches  direct  to  the  gland. 
Suprarenal   Body,     a,   fibrous  According  to  Bergmann  nerves  come  from  the 

phrenic  and  vagus  also. 

The  Effects  of  Disease  and  Removal  of 
the  Adrenal  Glands. — It  was  observ^ed  by  Addi- 
son that  a  profound  disturbance  of  the  nutrition,  characterized  by  a 
bronze-like  discoloration  of  the  skin  and  of  the  mucous  membranes 
of  the  mouth,  extreme  muscular  weakness,  and  profound  anemia,  were  asso- 
ciated with,  if  not  dependent  on,  pathologic  conditions  of  the  suprarenal 
glands.  In  the  progress  of  the  disease  the  asthenia  gradually  increases,  the 
heart  becomes  weak,  the  pulse  small,  soft,  and  feeble,  indicating  a  general 
loss  of  tone  of  the  muscular  and  vascular  apparatus.  Death  ensues  from 
paralysis  of  the  respiratory  muscles.  The  essential  nature  of  the  lesion 
which  gives  rise  to  these  symptoms  has  not  in  all  instances  been  determined. 
A  very  common  lesion  is  a  tuberculous  degeneration.  The  symptoms  were 
attributed  by  Addison  to  a  loss  of  the  function  of  the  glands. 

Removal  of  these  bodies  from  various  animals  is  invariablv  and  in  a 


^•^iifw'^''^ 


capsule;  b,  zona  glomerulosa 
zona  f  asciculata ; V,  zona  reticularis 
e,  medullary  cords;/,  venous  chan 
nel;  ^,  ganglion-cefls.      (Piersol). 


SECRETION.  465 

short  time  followed  by  death,  preceded  by  some  of  the  symptoms  characteristic 
of  Addison's  disease.  Thus,  shortly  after  their  removal  the  animal  becomes 
tranquil  and  apathetic;  the  respiration  soon  becomes  feeble  and  difficult; 
prostration  supervenes  and  the  animal  appears  as  though  paralyzed,  but  the 
irritability  of  the  skeletal  muscles  and  nerves  is  normal;  the  heart  becomes 
slow,  feeble  and  irregular;  the  blood-pressure  falls  promptly  20  to  30  mm. 
of  mercury,  after  which  it  steadily  falls  to  a  low  level;  the  appetite  fails,  the 
temperature  declines  and  death  occurs  in  from  twelve  to  forty-eight  hours. 
In  some  instances  a  pigmentation  of  the  skin  similar  to  that  seen  in  Addison's 
disease  has  been  observed.  From  the  fact  that  animals  so  promptly  die  after 
extirpation  of  these  bodies,  and  the  further  fact  that  the  blood  of  such 
animals  is  toxic  to  the  subjects  of  recent  extirpation,  but  not  to  normal 
animals,  the  conclusion  was  drawn  that  the  function  of  the  adrenal  bodies 
is  to  remove  from  the  blood  some  toxic  product  of  muscle  metabolism.  Its 
accumulation  after  extirpation  gives  rise  to  death  through  auto-intoxication. 
This  view^  is,  however,  not  generally  accepted. 

The  Effects  of  the  Injection  of  Gland  Extracts. — On  the  supposition 
that  the  adrenals  might  secrete  and  pour  into  the  blood  a  specific  material 
that  favorably  influences  general  metabolism,  Schafer  and  Oliver  injected 
hypodermatically  glycerin  and  water  extracts  of  the  medulla  into  the  bodies 
of  various  animals  and  observed  at  once  an  increased  rate  of  the  heart- 
beats and  of  the  respiratory  movements.  The  effects  however  were  only 
transitory.  When  these  extracts  were  injected  into  the  veins  directly,  there 
foUow^d  in  a  short  time  a  cessation  of  the  auricular  contraction  though  the 
ventricular  contraction  continued  vigorously  but  with  a  slower  rhythm. 
The  blood-pressure  at  the  same  time  was  markedly  increased.  If  the  vagi 
were  cut  previous  to  the  injection  or  if  the  inhibitor  influence  of  the  vagi 
was  removed  by  an  injection  of  atropin  the  reverse  effects  were  produced, 
viz.,  an  increase  in  the  rapidity  and  vigor  of  both  the  auricular  and  ventricular 
contraction  accompanied  by  a  still  more  marked  rise  of  blood-pressure.  This 
latter  effect  is  the  result  partly  of  the  increased  action  of  the  heart  but  very 
largely  the  result  of  a  vigorous  contraction  of  the  muscle-fibers  in  the  walls  of  the 
arterioles.  This  is  attributed  to  a  direct  stimulation  of  the  arterioles  and  not 
to  a  stimulation  of  the  vasoconstrictor  center.  The  contraction  of  the 
arterioles  is  quite  general  as  shown  by  plethysmographic  studies  of  the  limbs, 
the  spleen,  kidney,  etc.  The  arterioles  of  the  lungs  and  brain  do  not  con- 
tract under  its  influence  to  the  same  extent  as  do  the  arterioles  in  other 
regions  of  the  body.  Applied  locally  to  the  mucous  membranes,  adrenalin 
extract  produces  contraction  of  the  blood-vessels  as  shown  by  the  pallor 
which  follows.  The  skeletal  muscles  are  affected  by  the  extract  very  much  as 
they  are  by  veratrin.  The  duration  of  a  single  contraction  is  very  much 
prolonged,  especially  in  the  phase  of  relaxation  or  of  decreasing  energy. 
In  the  foregoing  instances  the  extract  apparently  produces  its  effects  by 
an  augmentation  of  the  normal  tonus  of  the  arteriole  muscle. 

It  is  evident  from  these  experiments  that  the  adrenal  bodies  are  engaged 
in  elaborating  and  pouring  into  the  blood  a  specific  material  which- stimu- 
lates to  increased  activity  the  muscle-fibers  of  the  heart  and  arteries,  and 
thus  assists  in  maintaining  the  normal  blood-pressure  as  well  as  the  tonicity 
of  the  skeletal  muscles.     An  alkaloidal  substance  was  isolated  by  Abel  from 


466  TEXT-BOOK  OF  PHYSIOLOGY. 

extracts  of  this  gland,  to  which  the  term  epinephrin  was  given.  A  crystalli- 
zable  substance  was  isolated  lirst  by  Takamine  and  later  by  Aldrich,  to  which 
the  term  adrenalin  was  given.  Both  substances  are  apparently  equally 
efficacious  in  causing  contraction  of  the  blood-vessels  and  in  raising  the  blood- 
pressure.  The  question  as  to  which  of  these  two  substances  represents  the 
active  principle  of  the  gland  is  as  yet  a  subject  of  discussion. 

The  action  of  adrenal  extract  however  is  not  limited  to  the  non-striated 
muscle-fibers  of  the  arterioles  but  extends  itself  to  the  non-striated  fibers 
found  in  the  the  walls  of  the  viscera,  e.g.,  stomach  and  intestines,  gall 
bladder,  urinary  bladder,  uterus,  etc.  The  administration  of  this  secretion 
is  followed  however,  in  these  regions,  by  an  inhibition  of  the  contraction  and 
a  subsequent  relaxation  of  the  visceral  walls.  In  these  instances  the  extract 
or  the  active  principle  produces  its  effects  apparently  by  an  inhibition  of 
the  tonus  of  the  visceral  muscle. 

It  has  been  a  subject  of  discussion  as  to  whether  adrenalin  acts  on  the 
muscle-fiber  directly  or  upon  the  endings  of  the  sympatheticnerves  with  which 
they  are  functionally  associated.  By  reason  of  the  fact  that  non-striated 
muscles  that  have  no  nerve  connections  with  the  sympathetic  system  are 
not  influenced  by  adrenalin  and  the  further  fact  that  non-striated ' 
muscles  that  have  been  deprived  of  their  nerve  connections  through  degenera- 
tive changes  following  division  of  the  nerves,  are  influenced  by  adrenalin, 
have  led  to  the  assumption  that  it  acts  neither  on  muscle  nor  nerve,  but  on 
some  material  which  intervenes  between  the  nerve  endings  and  the  muscle 
but  which  is  intimately  related  to  the  muscle.  To  this  material  Langley 
has  applied  the  term  "receptive  substance." 

Influence  of  the  Nerve  System. — The  secretory  activity  of  the  adrenals 
is  regulated  by  the  nerve  system.  Thus  Dreyer  found  that  the  blood  of  the 
adrenal  vein  after  stimulation  of  the  splanchnics  was  capable  of  causing  to  a 
much  greater  extent  the  usual  physiologic  effects  when  injected  into  an 
animal  than  blood  of  the  adrenal  vein  before  stimulation  and  this  indepen- 
dent of  the  vascular  changes  that  were  simultaneously  provoked.  It  has  also 
been  shown  by  Ascher  that  a  high  blood-pressure  can  be  maintained  by 
prolonged  stimulation  of  the  splanchnics.  Cannon  has  reported  that  major 
emotional  disturbances  such  as  fright  lead  to  an  increase  in  the  secretion 
of  the  adrenals  as  shown  by  the  fact  that  the  blood  taken  from  the  vena  cava 
above  the  level  of  the  adrenal  veins  will  promptly  produce  an  inhibition 
of  a  contracting  intestinal  strip,  while  blood  taken  from  the  animal  previous 
to  the  fright,  had  no  such  effect.  After  ligation  of  the  veins  and  removal' 
of  the  adrenals  there  was  a  failure  of  this  effect  upon  excitement. 

Emotional  excitement  in  cats  at  least  is  also  attended  with  hypergly- 
cemia and  glycosuria  which  is  probably  due  to  an  increase  of  the  adrenal 
secretion  in  the  blood  inasmuch  as  a  similar  effect  follows  the  injection  of  the 
extract  into  the  blood.  The  hyperglycemia  and  glycosuria  caused  either 
by  the  intraveneous  injection  of  the  extract  or  by  an  increased  activity  of  the 
adrenals  following  emotional  excitement,  fear  or  rage,  is  difficult  of  explana- 
tion. It  may  be  the  result  of  a  direct  action  or  an  indirect  action  through 
secretor  nerves  on  the  liver  cells,  in  consequence  of  which  the  stored  glycogen 
is  rapidly  transformed  into  sugar  and  discharged  into  the  blood. 

An  advantage  that  would  accrue  to  the  animal  from  the  accumulation 


SECRETION.  467 

of  sugar  in  the  blood  under  these  circumstances,  would  be  a  quickly  avail- 
able source,  of  energy-yielding  material  for  the  continued  muscle  activity 
that  would  attend  either  flight  or  defense. 

The  Spleen. — The  spleen  is  a  soft  bluish-red  organ,  oval  in  shape,  from 
twelve  to  lifteen  centimeters  long  by  eight  broad  and  four  thick.  It  is  situ- 
ated in  the  left  hypochondrium  between  the  stomach  and  the  diaphragm. 
In  this  situation  it  is  held  in  position  by  a  fold  of  the  peritoneum  which  passes 
from  the  upper  border  to  the  diaphragm. 

Structure. — A  section  of  the  spleen  shows  that  it  consists  of  connective 
tissue,  blood-vessels,  lymph-corpuscles,  and  lymphoid  tissue.  The  surface 
of  the  spleen  is  covered  by  a  capsule  composed  of  dense  fibrous  tissue, 
from  the  inner  surface  of  which  septa  or  trabeculae  pass  inward  toward  the 
center  of  the  organ.  In  their  course  they  give  off  a  series  of  processes  which 
unite  freely,  forming  a  spongy  connective-tissue  framework.  The  capsule 
and  the  main  trabeculae  in  some  animals  contain  numerous  non-striated 
muscle-fibers.  In  man  they  are  relatively  few  in  number.  The  blood-ves- 
sels which  enter  the  spleen  are  supported  by  the  connective-tissue  septa. 
As  they  pass  toward  the  center  of  the  organ  they  divide  very  rapidly  and 
soon  diminish  in  size.  In  their  course  small  branches  are  given  off,  which 
penetrate  the  inter-trabecular  tissue  and  become  encased  with  spheric  or 
'cylindric  masses  of  adenoid  tissue  known  as  Malpighian  corpuscles.  These 
corpuscles  are  composed  largely  of  leukocytes.  In  some  animals  the  leuko- 
cytes, instead  of  being  arranged  in  masses,  are  distributed  along  the  walls 
of  the  artery  as  a  continuous  layer.  Within  the  corpuscles  the  arteries  pass 
into  capillaries;  whether  the  artery  passes  directly  to  the  splenic  pulp  or  indi- 
rectly by-way  of  the  corpuscles,  its  ultimate  branches  terminate  in  capillaries 
which  open  into  the  spaces  of  the  splenic  pulp.  From  these  spaces  a  net- 
work of  venules  gathers  the  blood  and  transmits  it  to  the  veins.  It  is  a  dis- 
puted cjuestion  as  to  whether  the  spaces  are  lined  by  epithelium,  thus  form- 
ing a  continuous  blood  channel,  or  w^hether  they  are  wanting  in  this  histologic 
element. 

The  Splenic  Pulp. — The  spaces  of  the  connective-tissue  framework 
are  filled  with  a  dark  red  semifluid  mass  known  as  the  splenic  pulp.  When 
microscopically  examined,  the  pulp  presents  a  "fine  loose  network  of  adenoid 
tissue,  large  numbers  of  leukocytes  or  lymph-corpuscles,  red  corpuscles  in 
various  stages  of  disintegration,  and  pigment  granules.  Chemic  analysis 
reveals  the  presence  of  a  number  of  nitrogen-holding  bodies,  e.g.,  leucin, 
tyrosin,  xanthin,  uric  acid;  organic  acids,  e.g.,  acetic,  lactic,  succinic  acids;  pig- 
ments containing  iron,  and  inorganic  salts. 

The  Functions  of  the  Spleen. — Notwithstanding  all  the  experiments 
which  have  been  made  to  determine  the  functions  of  the  spleen,  it  can  not  be 
said  that  any  very  definite  results  have  been  obtained.  The  fact  that  the 
spleen  can  be  removed  from  the  body  of  an  animal  without  appreciably  inter- 
fering with  the  normal  metabolism  would  indicate  that  its  function  is  not  very 
important.  The  chief  changes  observed  after  such  a  procedure  are  an  en- 
largement of  the  lymphatic  glands  and  an  increase  in  the  activity  of  the  red 
marrow  of  the  bones.  The  presence  of  large  numbers  of  leukocytes  in  the 
splenic  pulp  and  in  the  blood  of  the  splenic  vein  suggested  the  idea  that  the 
spleen  is  engaged  in  the  production  of  leukocytes,  and  to  this  extent  contrib- 


468  TEXT-BOOK  OF  PHYSIOLOGY. 

utes  to  the  formation  of  blood.  The  presence  of  disintegrated  red  blood- 
corpuscles  has  suggested  the  view  that  the  spleen  exerts  a  destructive  action 
on  functionally  useless  red  corpuscles.  These  and  other  theories  as  to 
splenic  functions  have  been  offered  by  different  observers,  but  all  are  lack- 
ing positive  conl'irmation. 

Volume  Variations  of  the  Spleen. — It  was  shown  some  years  since  by 
Roy,  with  the  aid  of  the  plethysmograph,  that  the  spleen  undergoes  rhyth- 
mic variations  in  volume  from  moment  to  moment.  In  the  cat  and  in  the 
dog  the  diminution  in  the  volume  (the  systole)  and  the  increase  in  volume 
(the  diastole)  together  occupied  about  one  minute. 

This  fact  was  determined  by  withdrawing  the  spleen  through  an  opening 
in  the  abdominal  wall  and  enclosing  it  in  a  box  with  rigid  walls,  the  interior 
of  which  was  connected  with  a  piston  recording  apparatus.  The  system 
being  filled  with  oil,  each  variation  in  volume  was  attended  by  a  to-and-fro 
displacement  and  a  corresponding  movement  of  the  recording  lever.  The 
special  form  of  plethysmograph  used  for  this  purpose  is  known  as  the  on- 
cometer or  bulk  measurer,  and  the  recording  apparatus  as  the  oncograph. 

The  cause  of  these  variations  in  volume  Roy  attributed  to  a  rhythmic 
contractility  of  the  non-striated  muscle-fibres  in  the  capsule  and  trabeculae, 
and  not  to  changes  in  the  arterial  blood-pressure,  as  the  curve  of  the  pressure 
taken  simultaneously  remained  practically  uniform.  The  effect  of  the 
rhythmic  contractions  of  the  splenic  muscle  tissue  is  to  force  the  blood 
through  the  organ,  a  condition  necessitated  perhaps  by  the  pressure  relations 
within,  though  what  function  is  thereby  fulfilled  is  not  apparent. 

It  was  subsequently  shown  by  Schafer  and  Moore  that  the  splenic 
volume  is  extremely  responsive  to  all  fluctuations  of  the  arterial  blood-pressure ; 
that  though  the  spleen  may  passively  expand  and  recoil  in  response  to  the 
rise  and  fall  of  the  blood-pressure,  nevertheless  the  reverse  conditions  may 
obtain:  viz.,  that  the  splenic  volume  may  diminish  as  the  pressure  rises,  if 
the  splenic  arterioles  contract  simultaneously  with  the  contraction  of  the 
arterioles  generally.  On  the  contrary,  the  splenic  volume  increase  is  coinci- 
dent with  a  dilatation  of  the  splenic  and  systemic  arterioles.  In  addition  to 
the  rhythmic  variations,  the  splee'n  steadily  increases  in  volume  for  a  period 
of  five  hours  after  digestion,  and  then  gradually  returns  to  its  former 
condition. 

Influence  of  the  Nerve  System. — The  nerves  which  supply  the  vascu- 
lar and  visceral  muscles  in  the  spleen  are  derived  directly  from  the  semilunar 
ganglion  (post-ganglionic  fibres)  and  pass  to  it  in  company  with  the  splenic 
artery.  The  nerve-cells  from  which  they  arise  are  in  physiologic  relation 
with  nerve-fibres  (pre-ganglionic  fibers)  which  emerge  from  the  spinal  cord 
in  the  anterior  roots  of  the  third  thoracic  to  the  first  lumbar  nerves  inclusive, 
though  they  are  found  most  abundantly  in  the  sixth,  seventh,  and  eighth 
thoracic  nerves.     Their  center  of  origin  is  in  the  medulla  oblongata. 

Stimulation  of  the  nerves  in  any  part  of  their  course  gives  rise  to  a 
diminution  in  splenic  volume;  division  of  the  nerves  is  followed  by  an  increase 
in  the  volume.  In  asphyxia  the  spleen  is  small  and  contracted,  a  condition 
attributed  to  a  stimulation  of  the  centers  in  the  medulla  by  the  venosity  of 
the  blood. 

The  musculature  of  the  spleen  may  also  be  excited  to  contraction  by 


SECRETION.  469 

reflex  influences,  as  shown  by  the  fact  that  stimulation  of  the  central  end  of  a 
nerve  is  attended  by  a  diminution  of  volume. 

Inasmuch  as  the  excised  spleen  will  continue  to  exhibit  variations  in 
volume  when  perfused  with  blood,  it  would  appear  that  it  possesses  some 
contractile  mechanism  independent  to  some  extent  of  the  nerve  system. 


CHAPTER  XVIII. 
EXCRETION. 

As  stated  in  the  preceding  chapter,  the  term  excretion  is  limited  to  the 
process  by  which  the  end-products  of  tissue  metaboHsm  are  removed  from 
the  body,  the  nature  of  the  process,  however,  differing  in  no  essential  particu- 
lars from  that  underlying  the  process  of  secretion.  The  histologic  structures 
involved  and  the  forces  at  work  being  of  the  same  general  character,  it  is 
impossible  to  draw  any  sharp  line  of  distinction  between  them.  As  a  general 
fact  it  may  be  stated  that  in  their  composition  all  the  characteristic  ingredi- 
ents of  the  excretions  are  incapable  either  of  entering  into  the  formation  of 
tissue  or  of  undergoing  oxidation  for  the  purpose  of  heat-production.  As 
the  retention  of  these  end-products  in  the  body  would  exert  a  deleterious 
influence  on  normal  metabolism,  their  prompt  removal  becomes  essential  to 
the  maintenance  of  physiologic  activity.  The  principal  excretions  of  the 
body — urine,  perspiration,  and  bile — are,  with  the  exception  of  those  given 
off  in  the  lungs,  complex  fluids  in  which  are  to  be  found  in  varying  propor- 
tions the  chief  end-products  of  metabolism. 

THE  URINE. 

Normal  urine  has  a  pale  yellow  or  amber  color,  an  aromatic  odor,  an 
acid  reaction,  and  a  specific  gravity  of  1.020.  As  a  rule,  it  is  perfectly 
transparent,  though  its  transparency  may  be  diminished  from  the  presence 
of  mucus,  calcium  and  magnesium  phosphates,  and  mixed  urates. 

The  color,  which  varies  within  physiologic  limits  from  a  pale  yellow  to 
a  reddish-brown,  is  due  to  the  presence  of  the  coloring-matters  urobilin, 
urochrome,  and  iiroerythrin ,  all  of  w^hich  are  derivatives  of  the  bile  pig- 
ments absorbed  from  the  liver  or  the  alimentary  canal. 

The  reaction  of  the  urine  is  acid,  owing  to  the  presence  of  the  acid  phos- 
phates of  sodium  and  calcium.  The  degree  of  acidity,  however,  varies  at 
different  periods  of  the  day.  Urine  passed  in  the  morning  is  strongly  acid, 
while  that  passed  during  and  after  digestion,  especially  if  the  food  be  largely 
vegetable  in  character  and  rich  in  alkaline  salts,  is  either  neutral  or  alkaline  in 
reaction.  The  diminished  acidity  after  meals  is  attributed  to  the  formation 
of  hydrochloric  acid  by  the  gastric  glands  and  the  consequent  liberation  of 
bases  which  are  excreted  in  the  urine.  The  phosphoric  acid  which  enters 
into  combination  with  sodium  and  potassium  bases  is  a  product  of  tissue 
metabolism. 

The  specific  gravity  is  about  1.020,  though  it  varies  from  1.015  to  1.025. 
It  will  diminish,  other  things  being  equal,  with  increased  consumption  of 
water  and  diminished  activity  of  the  skin;  it  w^ill  be  increased  of  course  by 
the  opposite  conditions. 

The  quantity  of  urine  excreted  in  twenty-four  hours  varies  from  1200  to 

470 


EXCRETION.  471 

1700  c.c.     Amounts  both  above  and  below  these  are  frequently  passed  from 
a  variety  of  causes. 

The  odor  of  the  urine  is  characteristic  and  due  to  the  presence  of  aromatic 
compounds. 

COMPOSITION  OF  URINE. 

Water 1500.00  c.c. 

Total  solids 72  .00  grams. 

Urea 33  -iS  grams. 

Uric  acid  (urates) o  ■  55  grams. 

Hippuric  acid  (hippurates) o  .40  grams. 

•Kreatinin,    .xanthin,    hypoxanthin,    guanin,    ammonium 

,,       ■    '      ^     ^            •'  ^  ,11.21  grams, 

salts,  pigment,  etc ;  ° 

Inorganic  salts;  sodium  and  potassium  sulphates,  phos-  \ 
phates,  and  chlorids;  magnesium  and  calcium  phos-  | 
phates )■  27  .00  grams. 

Organic    salts:     lactates,    acetates,    formates    in    small 
amounts J 

Sugar a      trace 

Gases,  nitrogen,  and  carbonic  acid. 

The  estimation  of  total  urinary  solids  in  any  given  sample  of  urine  is 
frequently  a  matter  of  clinical  interest.  This  may  approximately  be  attained 
by  multiplying  the  last  two  figures  of  the  specific  gravity  by  the  coefficient 
of  Haeser  or  Christison,  2.33.  The  result  expresses  the  total  solids  in  looo 
parts:  e.g.,  urine  with  a  specific  gravity  of  1.020  w^ould  contain  20X2.33,  or 
46.60  grams  of  solid  matter  per  1000  c.c.  If  the  amount  passed  in  twenty- 
four  hours  be  1500  c.c,  the  total  solids  would  amount  to  69.9  grams  daily. 

The  "Water  of  the  Urine. — The  amount  of  urinary  water  and  its  ratio 
to  the  solid  constituents  will  vary  with  the  amount  consumed  and  the  activity 
of  the  skin  and  lungs.  In  summer  the  foods,  liquid  and  solid,  remaining  the 
same,  the  quantity  of  water  in  the  urine  is  diminished  in  consec^uence  of 
increased  activity  of  skin  and  lungs  and  the  ratio  of  water  to  solids  decreased. 
In  winter  the  reverse  conditions  obtain.  The  food  remaining  the  same,  the 
consumption  of  large  quantities  of  water  hastens  at  least  the  removal  of  end- 
products  from  the  tissues  and  thus  increases  the  urinary  solids. 

Urea. — Urea  is  the  most  abundant  of  the  organic  constituents  of  the 
urine  and  is  present  to  the  extent  of  from  2  to  3  per  cent.  It  is  a  colorless 
neutral  substance,  crystallizing  under  varying  conditions  in  long  silky  needles 
or  in  rhombic  prisms.  It  is  soluble  in  water  and  alcohol."  It  is  composed  of 
CON2H^.  When  subjected  to  prolonged  boiling,  it  combines  with  water, 
giving  rise  to  ammonium  carbonate.  The  presence  of  Micrococcus  urece 
in  urine  will  also  convert  the  urea,  by  combining  it  with  two  molecules  of 
water,  into  ammonium  carbonate,  C0N2H^+ 211,0  =  (NHJ^COj. 

The  amount  of  urea  excreted  each  day  varies  from  30  to  40  grams  the 
average  being  about  34  grams  and  therefore  represents  an  amount  of 
protein  metabolized  equivalent  to  from  90  to  120  grams  or  an  average 
of  about  100  grams.  The  remaining  nitrogen-holding  compounds  in  the 
urine  represent  as  shown  by  their  nitrogen  content  a  protein  metabolism 
of  about  12  grams.  As  to  how  much  of  the  urea  or  of  the  total  nitrogen  is 
derived  from  the  metabolism  of  tissue  protein  and  how  much  from  the 
metabolism  of  the  food  protein  that  is  not  elaborated  into  tissue  protein, 
is  difhcult  to  state.     It  has  been  observed  however  in  human  beings  in  the 


472  TEXT-BOOK  OF  PHYSIOLOGY. 

fasting  condition  that  for  a  period  of  lo  days,  there  is  a  daily  excretion 
of  about  21  grams  of  urea  equivalent  to  about  63  grams  of  protein  metabo- 
lized. If  it  be  accepted  that  approximately  63  grams  of  tissue  protein  are 
metabolized  each  day  then  of  the  34  grams  of  urea  excreted,  13  grams  must 
come  from  about  40  grams  of  metabolized  food  protein.  That  the  urea  that 
comes  from  the  tissue  protein  is  a  rather  constant  factor  and  that  the  urea 
that  comes  from  the  food  protein  is  a  variable  factor  is  shown  by  the  fact  that 
the  amount  of  urea  excreted  rises  and  falls  proportionately  to  the  protein  con- 
sumed. As  to  the  particular  tissues  that  are  undergoing  protein  metabolism 
there  is  much  obscurity.  Contrary  to  what  might  be  expected  there  is  ap- 
parently but  little  protein  metabolism  in  muscle  tissue  for  there  is  no  parallel- 
ism between  urea  production  and  muscle  work.  Even  after  severe  labor 
extending  over  a  period  of  some  hours  there  is  no  noticeable  increase  in  the 
urea  excreted. 

Seat  of  Formation  and  Antecedents  of  Urea. — It  has  been  stated  in  a 
foregoing  paragraph  that  the  excretory  organs  are  engaged  in  the  process  of 
eliminating  from  the  blood,  rather  than  in  elaborating,  the  end  products  of 
metabolism.  Therefore  the  supposition  is  that  the  kidneys  are  not  the  seat 
of  urea  formation  but  only  the  means  by  which  it  is  eliminated  from  the 
blood.  This  supposition  is  rendered  highly  probable  from  the  following 
facts:  the  blood  of  the  renal  artery  contains  from  one-third  to  one-half 
more  urea  than  the  blood  of  the  renal  vein;  ligation  of  the  renal  arteries 
or  removal  of  the  kidneys  leads  to  an  accumulation  of  urea  in  the  blood  to  an 
extent  four  times  the  normal  amount  in  24  hours;  perfusion  of  the  excised 
kidney,  which  still  retains  its  physiologic  activity,  with  blood  containing 
known  antecedents  of  urea  is  unattended  with  urea  formation.  These  and 
other  facts  of  a  similar  character  confirm  the  view  that  the  kidney  does  not 
manufacture  but  simply  excretes  urea  brought  to  it  by  the  blood.  Since 
urea  is  always  present  in  the  blood  to  an  extent  of  from  0.04  per  cent,  to 
0.06  per  cent.,  i.e.,  from  4  to6  grams  per  10,000  grams,  and  that  it  is  being  ex- 
creted at  the  rate  of  about  1.5  grams  per  hour,  it  is  evident  that  it  is  being  as 
constantly  formed  in  some  one  or  more  organs,  and  discharged  into  the 
blood. 

The  experimental  evidence  now  at  hand  indicates  the  liver  as  the  chief 
organ  engaged  in  this  process.  The  following  facts  support  this  view,  viz.: 
destructive  diseases  of  the  liver,  e.g.,  acute  yellow  atrophy,  interstitial  hepa- 
titis, and  suppuration,  largely  diminish  the  production  of  urea  but  increase  the 
amount  of  the  ammonium  salts  in  the  urine;  the  establishment  of  an  Eck 
fistula  (the  union  of  the  portal  vein  with  the  ascending  vena  cava  whereby 
the  liver  is  almost  entirely  excluded  from  receiving  compounds  absorbed 
from  the  intestine)  is  followed  by  a  decrease  in  the  production  of  urea  and 
an  increase  in  the  ammonium  content  of  the  urine;  the  perfusion  of  the 
liver  of  a  recently  killed  animal  with  a  given  amount  of  blood  containing 
ammonium  salts  will  be  followed  after  the  lapse  of  several  hours  by  an  amount 
of  urea  in  the  blood  two  or  three  times  the  normal  quantity.  These  and 
other  facts  indicate  that  the  chief  seat  of  urea  formation  is  to  be  found  in 
the  liver  cells. 

The  antecedents  of  urea,  out  of  which  the  hepatic  cells  construct  urea 
have,  for  chemic  reasons  as  well  as  from  the  foregoing  experimental  results, 


EXCRETION.  473 

been  shown  to  be  the  saUs  of  ammonia  the  carbonate,  carbamate,  and 
lactate.  The  increase  in  the  ammonia  of  the  urine  simultaneously  with 
the  decrease  in  the  urea  renders  it  extremely  probable  that  these  salts  are 
antecedents  of  urea  and  that  the  transformation  takes  place  in  the  liver 
cells.  The  chemic  change  that  takes  place  is  simply  the  abstraction  of  two 
molecules  of  water  as  shown  in  the  following  formula: 

(NHJX03-2H20  =  CON2H,. 

The  source  of  the  ammonia  is  probably  in  part  the  intestine  as  this 
compound  is  one  of  the  products  of  the  hydrolysis  and  cleavage  of  the  proteins 
during  digestion.  That  this  is  the  case  is  apparent  from  the  fact  that  the 
blood  of  the  portal  vein  always  contains  more  ammonia  that  the  blood  of 
any  other  region  of  the  vascular  apparatus.  The  advantage  to  the  body 
that  results  from  the  conversion  of  ammonia  to  urea  is  that  it  prevents  an 
ammonia  intoxication  with  its  attendant  evils  that  would  otherwise  arise. 

The  amino-acids,  as  tyrosin,  leucin,  glutamic,  and  aspartic  acids,  diamino- 
acids  and  bases,  as  lysin,  arginin,  histidin  which  are  also  products  of  the  hy- 
drolysis of  proteins  during  digestion  are  capable  of  being  absorbed  as  such 
by  the  epithelial  cells  of  the  villi  and  mucous  membrane,  in  which  they 
undergo  a  cleavage  into  an  NH,  portion  and  an  organic  portion;  the  former 
is  then  converted  to  ammonia  and  subsequently  to  urea  by  the  liver  cells, 
the  latter  the  organic  portion  contributes  to  the  production  of  fat  or  sugar, 
which  are  in  due  time  oxidized  and  thus  contribute  to  the  store  of  body  heat. 
A  portion  of  the  amino-acids,  such  as  is  not  used  in  the  formation  of  tissue 
protein  is  apparently  disposed  of  in  this  way. 

From  the  foregoing  facts  it  is  evident  that  given  the  presence  of  am- 
monia salts  and  amino-acids  in  the  blood  of  the  portal  vein  the  appearance  of 
urea  in  the  urine  is  readily  accounted  for.  How^ever  it  must  be  remembered 
that  though  the  intestine  is  a  source  of  the  ammonia  and  the  amino-acids  it  is 
probably  not  the  only  source  for  there  is  evidence  that  the  proteins  that 
enter  into  the  composition  of  all  tissues  and  tissue  fluids,  are  undergoing  at 
all  times  a  hydrolysis  under  the  influence  of  enzymes  whereby  products 
are  produced  similar  to  if  not  identical  with  those  produced  in  the  intestine. 
These  after  their  discharge  into  the  blood  stream  are  carried  to  the  liver 
where  they  undergo  the  same  change  as  though  derived  from  the  intestine. 

The  question  arises  however  as  to  what  percentage  of  the  urea  is  derived 
from  the  products  of  the  metabolism  of  the  tissue  proteins  (endogenous 
urea)  and  what  percentage  is  derived  from  the  products  of  the  metabolism 
of  the  food  proteins  in  the  alimentary  canal  (exogenous  urea).  The  answer 
to  this  question  is  connected  with  the  further  question  as  to  the  amount  of 
protein  necessary  to  keep  the  body  in  nitrogen  equilibrium  at  its  lowest 
level  compatible  wuth  health  and  efiiciency.  If  this  amount  be  from  30  to 
50  grams  as  recent  experiments  would  seem  to  show  then  the  endogenous 
urea  would  be  approximately  from  10  to  17  grams.  Accordingly  as  this 
lower  level  is  raised  will  the  amount  of  the  endogenous. urea  be  increased. 

Uric  Acid. — Uric  acid  is  one  of  the  constant  ingredients  of  the  urine. 
It  is  a  crystalline  nitrogen-holding  body  closely  resembling  urea,  its  formula 
being  CsH^NjOg.  The  total  quantity  excreted  daily  varies  from  0.2  to  i 
gram.     It  is  doubtful  if  uric  acid  exists  in  a  free  state  in  the  urine,  the  indi- 


474  TEXT-BOOK  OF  PHYSIOLOGY. 

cations  being  that  it  is  combined  with  sodium  and  potassium  in  the  form  of  a 
quadriurate.  The  urates  when  in  excess  are  frequently  deposited  from  the 
urine  as  a  brick-red  sediment,  the  color  being  due  to  their  combination  with 
the  coloring-matter  uroerythrin.  When  pure,  uric  acid  crystallizes  in  the 
rhombic  form,  though  is  assumes  a  variety  of  forms.  Uric  acid  was  long 
regarded  as  a  product  of  general  protein  metabolism.  This  view  has  been 
abandoned.  At  present  it  is  believed  that  it  is  a  cleavage  product  of  nuclein,  a 
constituent  of  all  cell  nuclei.  In  the  metabolism  of  nuclein  a  protein  and 
nucleic  acid  are  formed,  from  the  latter  of  which  uric  acid  is  derived.  Nu- 
cleic acid  when  decomposed  yields  a  series  of  bases,  such  as  xanthin,  hypo- 
xanthin,  adenin,  guanin,  etc.  Because  of  the  fact  that  these  bodies  can  also 
be  obtained  from  a  synthesized  body  termed  purin  they  are  known  collect- 
ively as  the  purin  bases.  Though  there  is  a  close  relationship  between  uric 
acid  and  the  purin  bases,  it  has  been  impossible  experimentally  to  derive  one 
from  the  other.  When  hypoxanthin,  however,  is  given  internally  it  is 
oxidized  and  converted  into  uric  acid.  It  is  extremely  probable,  therefore, 
that  uric  acid  is  an  oxidation  product  of  one  or  more  of  the  purin  bases. 

It  is  probable,  however,  that  not  all  of  the  uric  acid  eliminated  is  derived 
from  the  nuclein  of  tissue-cells  and  their  decomposition  products,  the 
purin  bases.  Some  of  it  is  undoubtedly  derived  from  the  nucleins  contained 
in  foods.  The  uric  acid  eliminated  is  therefore  partly  endogenous  and 
partly  exogenous  in  origin. 

There  is  some  evidence  that  not  all  the  uric  acid  produced  in  the  body  is 
excreted  as  such,  but  that  a  portion  perhaps  one-half  is  changed  to  urea. 

Xanthin,  hypoxanthin,  guanin,  etc.,  are  also  found  in  urine  in  small 
but  variable  amounts.  They  are  nitrogenized  compounds  derived  mainly 
from  the  metabolism  of  the  nuclein  bodies. 

Kreatinin  is  a  crystalline  nitrogenous  compound  closely  resembling 
kreatin,  one  of  the  constituents  of  muscle  tissue.  The  amount  excreted 
daily  is  about  i  gram.  The  origin  of  kreatinin  is  not  very  clear.  It  is 
probable,  however,  as  kreatin  is  capable  of  transformation  into  kreatinin 
that  a  certain  portion  is  derived  from  the  kreatin  contained  in  the  meat  con- 
sumed as  food.  But  as  kreatinin  is  steadily  excreted  though  in  less  amounts 
on  a  diet  from  which  meat  is  excluded  it  is  certain  that  this  portion  at  least 
must  have  some  other  source  containing  nitrogen,  and  the  inference  is  that 
it  is  one  of  the  end-products  of  the  protein  metabolism  that  is  taking 
places  in  tissues  generally  and  more  particularly  in  muscle  tissue. 

Hippuric  acid  in  combination  with  sodium  and  potassium  is  very  gener- 
ally present  in  urine,  though  in  small  amounts.  It  is  more  abundant  in  the 
urine  of  the  herbivora  than  the  carnivora.  In  man  the  amount  excreted 
daily  is  about  0.7  gram,  though  the  amount  may  be  raised  by  a  diet  of 
asparagus,  plums,  cranberries,  etc.,  and  by  the  administration  of  benzoic  and 
cinnamic  acids.  There  is  evidence  that  hippuric  acid  is  formed  in  the 
kidney  from  benzoic  acid,  its  precursors,  or  related  bodies.  Various  com- 
pounds of  this  class  are  found  in  vegetable  foods,  a  fact  which  may  account 
for  the  increase  in  the  excretion  of  hippuric  acid  on  a  vegetable  diet. 

Indol,  skatol,  phenol,  cresol,  products  of  the  putrefactive  changes  in  the 
derivatives  of  protein  are  present  in  variable  amounts,  associated  with 
potassium  sulphate    (see  page  454).     These  compounds  are  known  as  the 


EXCRETION.  475 

ethereal  sulphates.  The  extent  to  which  they  are  present  is  taken  as  a  meas- 
ure of  the  extent  of  intestinal  putrefaction;  their  presence  can  be  deter- 
mined by  various  tests.  Of  these  compounds  the  one  generally  tested  for  is 
potassium  indoxyl  sulphate  or  indican.  If  hydrochloric  acid  and  a  small 
quantity  of  potassium  chlorate  be  added  to  suspected  urine,  the  indican  if 
present  will  be  separated  into  indoxyl  and  potassium  sulphate.  The  former 
compound  will  then  be  oxidized  and  form  indigo  blue.  The  depth  of  the 
color  is  indicative  of  the  quantity  present  and  the  extent  of  the  intestinal 
putrefaction. 

Inorganic  Salts. — Sodium  and  potassium  phosphates,  known  as  the 
alkaline  phosphates,  are  found  in  both  blood  and  urine.  The  total  quantity 
excreted  daily  is  about  4  grams.  Calcium  and  magnesium  phosphates, 
known  as  the  earthy  phosphates,  are  present  to  the  extent  of  i  gram.  Though 
insoluble  in  water,  they  are  held  in  solution  in  the  urine  by  its  acid  constitu- 
ents. If  the  urine  be  rendered  alkaline,  they  are  at  once  precipitated. 
Sodium  and  potassium  sulphates  are  also  present  to  the  extent  of  about  2 
grams.  The  phosphoric  and  sulphuric  acids  which  are  combined  with  these 
bases  enter  the  body  for  the  most  part  in  the  foods,  though  there  is  evidence 
that  they  also  arise  by  oxidation  in  consequence  of  the  metabolism  of  proteins 
which  contain  phosphorus  and  sulphur.  Sodium  chlorid  is  the  most 
abundant  of  the  inorganic  salts.  It  is  derived  mainly  from  the  food.  The 
amount  excreted  is  about  15  grams  in  twenty-four  hours. 

THE  KIDNEYS. 

The  kidneys  are  the  organs  engaged  in  the  excretion  of  the  urinary 
constituents  from  the  blood.  Each  resembles  a  bean  in  shape,  is  from  10 
to  12  centimeters  in  length,  2  in  breath,  and  weigh  from  144  to  170  grams. 
They  are  situated  in  the  lumbar  region,  one  on  each  side  of  the  vertebral 
column  behind  the  peritoneum,  and  extend  from  the  eleventh  rib  to  the  crest 
of  the  ilium.  The  anterior  surface  is  convex,  the  posterior  surface  concave. 
The  latter  presents  a  deep  notch — the  hilum.  The  kidney  is  surrounded  by 
a  thin  smooth  membrane  composed  of  white  fibrous  and  yellow  elastic 
tissue;  through  it  is  attached  to  the  surface  of  the  kidney  by  minute  processes 
of  connective  tissue,  it  can  very  readily  be  torn  away.  The  substance  of  the 
kidney  is  dense  but  friable. 

Upon  making  a  longitudinal  section  of  the  kidney  it  will  be  observed  that 
the  hilum  extends  into  the  interior  of  the  organ  and  expands  to  form  a 
cavity  known  as  the  sinus,  in  which  are  found  the  blood-vessels,  nerves,  and 
duct  (Fig.  216).  This  cavity  is  mainly  occupied  by  the  upper  part  of  the 
renal  duct,  the  ureter,  the  interior  of  which  is  termed  the  pelvis.  The  ureter 
divides  into  several  portions  which  terminate  in  small  caps  or  calyces  which 
receive  the  apices  of  the  pyramids.  The  parenchyma  of  the  kidney  consists 
of  two  portipns :  viz : 

1.  An  internal  or  medullary  portion,  consisting  of  a  series  of  pyramids  or 

cones,  some  twelve  or  fifteen  in  number,  which  present  a  distinctly 
striated  appearance. 

2.  An  external  or  cortical  portion,  half  an  inch  in  thickness  and  distinctly 

friable  in  character. 


476 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Histology  of  the  Kidney. — The  kidney  is  composed  of  a  connective- 
tissue  framework  supporting  secreting  tubules,  blood-vessels,  lymphatics, 
and  nerves,  all  of  which  are  directly  connected  with  the  removal  of  the  urinary 
constituents  from  the  blood.  The  kidney  is  structurally  a  compound  tubular 
gland.  If  the  apex  of  each  pyramid  be  examined  with  a  lens,  it  will  present 
a  number  of  small  orifices  which  may  be  regarded  as  the  beginnings  of  the 
uriniferous  tubules.     From  this  point  the  tubules  pass  outward  in  a  straight 

but  somewhat  diverging 
manner  toward  the  cortex, 
giving  off  at  acute  angles  a 
number  of  branches  (Fig. 
218).  From  the  apex  to  the 
base  of  the  pyramids  they 
are  known  as  the  tubules  of 
Bellini.  In  the  cortical  por- 
tion of  the  kidney  the  tubule 
becomes  enlarged  and  twist- 
ed, and,  after  pursuing  an 
extremely  convoluted  course, 
turns  backward  into  the 
medullary  portion  for  some 
distance,  forming  the  ascend- 
ing limb  of  Henle's  loop;  it 
then  turns  upon  itself,  form- 
ing the  descending  limb  of 
the  loop,  reenters  the  cortex, 
again  expands  and  becomes 
convoluted,  and  finally  ter- 
minates in  an  ovoid  enlarge- 
ment known  as  Muller's  or 
Bowman's  capsule,  in  which 
is  contained  a  small  tuft  of 
blood-vessels — the  glomeru- 
lus. Each  tubule  consists  of 
a  basement  membrane  lined 
throughout  its  entire  extent 
by  epithelial  cells.  The  epi- 
thelium as  well  as  the  tubule 
vary   in   shape   and   size  in 


Fig.  216.  —  Longitudinal  Section  through  the 
Kidney,  the  Pelvis  ov  the  Kidney,  and  a  Number 
of  Renal  Calyces.  A.  Branch  of  the  renal  artery. 
U.  Ureter.  C.  Renal  calyx,  i.  Cortex,  i'.  Medullary 
rays.  i".  Labyrinth,  or  cortex  proper.  2.  Medulla. 
2'.  Papillary  portion  of  medulla,  or  medulla  proper. 
2".  Border  layer  of  the  medulla.  3,  3.  Transverse  sec- 
tion through  the  axes  of  the  tubules  of  the  border  layer. 
4.  Fat  of  the  renal  sinus.  5,  5.  Arterial  branches. 
*.  Transversely  coursing  medulla  rays, 
Henle.) 


different  parts  of  its  course. 
{Tyson,  ajter  ^^  ^^^  capsule  the  epithelium 

is  flattened,  lining  not  only 
the  inner  surface  of  the  capsule  but  reflected  over  the  blood-vessels  as  well. 
This  is  known  as  the  glomerular  epithelium.  In  the  convoluted  portions  of 
the  tubules  the  epithelium  is  cuboid,  granular,  and  somewhat  striated;  in 
Henle's  loop  it  is  more  or  less  flattened. 

The  Blood-vessels  of  the  Kidney. — The  renal  artery  enters  the  kidney 
at  the  hilum  behind  the  ureter;  it  soon  divides  into  several  large  branches 
which  penetrate  the  substance  of  the  kidney  between  the  pyramids  and  pass 


EXCRETION. 


477 


outward  into  the  cortex.  At  the  base  of  the  pyramids  branches  of  the 
arteries  form  an  anastomosing  plexus.  From  this  plexus  vessels  are  given 
off,  some  of  which  follow  the  straight  tubules  torwad  th  apexeof  the  pyramids, 


Lobule. 


Lobule. 


Papillary  duct 


Tunica  albuginea. 


Stellate  vein. 


Interlobular 

artery. 

Interlobulai* 

vein. 


Arciform  artery 


Arcitorm  vein. 


Interlobar  artery. 
Interlobar  vein. 


Fig.  217  —Scheme  of  the  Course  of  the  Uriniferous  Tubules  and  the  Renal 

^'ESSELS. 

vasa  recta,  while  others  enter  the  cortex  and  pass  to  its  surface  (Fig.  218). 
In  the  course  of  the  latter  small  branches  are  given  off,  each  of  which  soon 
divides  and  subdivides  to  form  a  ball  of  capillary  vessels  known  as  the  glomer- 


478 


TEXT-BOOK  OF  PHYSIOLOGY. 


ulus.  These  capillaries,  however,  do  not  anastomose,  but  soon  reunite  to 
form  an  efferent  vessel  the  caliber  of  which  is  less  than  that  of  the  afferent 
artery.  In  consequence  of  this,  there  is  a  greater  resistance  to  the  outflow 
of  blood  than  to  the  inflow,  and  therefore  a  higher  blood-pressure  in  the 
glomerulus  than  in  capillaries  generally.  The  relation  of  the  glomerulus  to 
the  tubule  is  important  from  a  physiologic  point  of  view.  As  stated  above, 
the  glomerulus  is  received  into  and  surrounded  by  the  terminal  expansion 
or  capsule  of  the  tubule.  This  capsule,  formed  by  an  indentation  of  the 
terminal  portion  of  the  tubule,  consists  of  two  walls,  an  outer  one  consisting 
of  an  extremely  thin  basement  membrane,  covered  by  flattened  epithelial 
cells,  and  an  inner  one  consisting  apparently  only  of  flattened  epithelium 
which  is  reflected  over  and  closely  invests  the  glomerular  blood-vessels 
(Fig.  218).  The  blood  is  thus  separated  from  the  interior  of  the  capsule 
by  the  epithehal  wall  of  the  capillary  and  the  epithelium  of  the  reflected  wall 

of  the  capsule.  During  the  periods 
of  secretor  activity  the  blood-vessels 
of  the  glomerulus  are  filled  with  blood 
to  such  an  extent  that  the  sac  cavity 
is  almost  obliterated.  After  its  exit 
from  the  capsule  the  efferent  vessel 
of  the  glomerulus  soon  again  divides 
and  subdivides  to  form  an  elaborate 
capillary  plexus  which  surrounds  and 
closely  invests  the  convoluted  tubules. 
From  this  plexus  as  well  as  from  the 
plexus  which  surrounds  the  straight 
tubules  veins  arise  which  pass  toward 
and  empty  into  veins  at  the  base 
of  the  pyramids.  The  renal  vein 
formed  by  the  union  of  these  latter 
veins  emerges  from  the  kidney  at  the 
hilum  and  finally  empties  into  the 
vena  cava  inferior. 

The  nerves  of  the  kidney  are  de- 
rived directly  from  small  ganglia  near 
From  this  origin  they  pass  through  the  renal  plexus 
of  the  blood-vessels  to  their  termination.     Experi- 
these  nerves  are  both  vaso-constrictors  and  vaso- 


FiG.  218. — Scheme  of  the  Rexal  orMal- 
PiGHiAN^  Corpuscle,  r.  Interlobular  artery. 
2.  Afferent  vessel.  3.  Efferent  vessel.  4.  Outer 
wall.  5.  Inner  wall.  6.  Glomerulus.  7.  Neck 
of  tubule. — (Stohr.) 


the  semi-lunar  ganglion. 
and  follow  the  course 
ment  has  shown  that 
dilatators. 

The  Renal  Duct. — The  excretorv'  duct  of  the  kidney,  the  ureter,  is  a 
musculo-membranous  tube  about  5  mm.  in  diameter  when  distended.  30  cm. 
in  length,  and  extends  from  the  hilum  to  the  base  of  the  bladder  into  which 
it  empties.  The  upper  extremity  is  expanded  and  within  the  renal  sinus 
becomes  irregularly  branched,  giving  rise  to  a  number  of  short  tubes,  called 
calyces,  each  of  which  embraces  the  apex  of  a  Malpighian  pyramid.  The 
interior  of  the  expanded  portion  of  the  ureter  is  known  as  the  pelvis.  The 
wall  of  the  ureter  consists  of  a  mucous  membrane,  a  muscle  coat,  and  an 
external  fibrous  investment. 


EXCRETION.  479 

MECHANISM  OF  URINE  SECRETION. 

The  secretion  of  urine  is  a  complex  process  and  susceptible  of  several 
interpretations.  It  was  originally  inferred  by  Bowman  that,  as  the  kidney 
presents  anatomically  an  apparatus  for  filtration,  the  capsule  with  its  enclosed 
glomerulus,  and  an  apparatus  for  secretion,  the  epithelium  of  the  urinary 
tubules,  therefore  the  elimination  of  the  urinary  constituents  from  the  blood 
is  accomplished  by  the  two  processes  of  filtration  and  secretion;  that  the 
water  and  highly  diffusible  inorganic  salts  simply  pass  by  filtration, 
under  pressure,  through  the  walls  of  the  glomerular  capillaries,  while  the 
organic  constituents  are  removed  by  the  epithelium  lining  the  tubules. 

Influenced  largely  by  the  facts  of  blood-pressure  Ludwig  advanced  the 
view  that  the  factors  concerned  in  the  secretion  of  urine  were  purely  physical ; 
that  in  consequence  of  the  high  pressure  in  the  vessels  of  the  glomeruli, 
due  to  the  high  pressure  in  the  renal  artery  on  the  one  hand  and  to  the  resis- 
tance offered  by  the  smaller  efferent  vessel  on  the  other  hand  all  the  urinary 
constituents  were  filtered  off  in  a  state  of  extreme  dilution.  In  order  to  ac- 
count for  the  higher  percentage  of  the  organic  constituents  in  the  urine,  it 
was  assumed  that  as  the  dilute  urine  passed  through  the  tubules  the  water 
and  possibly  other  substances  as  well  were  partly  reabsorbed,  passing  by 
diffusion  into  the  lymph  and  blood  until  the  urine  acquired  its  normal 
characteristics  and  degree  of  concentration.  In  support  of  this  view,  a  large 
number  of  facts  relating  to  the  influence  of  an  increase  and  a  decrease  of 
pressure  in  the  blood-vessels  of  the  glomeruli,  the  velocity  of  the  blood-stream, 
etc.,  in  determining  the  rate  of  urinary  flow  were  adduced,  all  of  which 
apparently  indicated  that  the  former  stood  to  the  latter  in  the  relation  of 
cause  and  effect,  and  that  the  formation  of  urine  was  accomplished  ent-rely 
by  physical  forces. 

The  progress  of  physiologic  investigation,  however,  has  thrown  some 
doubt  on  the  validity  of  this  physical  interpretation,  and  has  rather  served 
to  support  the  view  of  Bowman  that  the  organic  constituents  at  least  are 
removed  from  the  blood  by  a  process  of  selection  on  the  part  of  the  epithelium 
of  the  convoluted  urinary  tubules;  in  other  words,  that  the  secretion 
of  urine  is  physiologic  rather  than  physical.  Heidenhain  has  brought 
forward  a  series  of  facts  which  support  this  view.  As  e\idence  that  the  cells 
possess  a  selective  power,  he  presented  the  following  experiment:  The 
spinal  cord  of  an  animal  is  divided  in  the  neck  for  the  purpose  of  lowering  the 
blood-pressure  in  the  kidney  below  the  pressure  at  which  the  urine  is  secreted. 
Five  to  twenty  c.c.  of  a  saturated  solution  of  indigo-carmine  is  injected  into 
the  blood-vessels;  after  varying  intervals  of  ten  minutes  the  animal  is  killed, 
the  blood-vessels  washed  out  with  alcohol  for  the  purpose  of  precipitating 
the  indigo-carmine  in  situ.  Section  of  the  kidney  shows  a  uniform  blue 
stain  of  the  cortex  alone.  (Fig.  219.)  Microscopic  examination  reveals  the 
fact  that  the  blue  stain  is  due  to  the  deposition  of  the  pigment  in  the 
lumen  and  in  the  lumen  border  of  the  cells  of  the  convoluted  tubules 
(Fig.  220)  and  the  ascending  limb  of  Henle's  loop;  while  the  epithelium  of 
Bowman's  capsule  as  well  as  the  glomerular  epithelium  present  no  evidence 
of  pigmentation.  The  physiologic  action  of  the  cells  of  the  convoluted 
tubules  in  elimination  of  indigo-carmine,  is  supposed  to  indicate  their  action 


48o  TEXT-BOOK  OF  PHYSIOLOGY. 

in  the  elimination  of  urea  and  other  nitrogen-holding  compounds.  The 
absence  of  the  pigment  from  the  glomerular  epithelium  lends  support  to  the 
view,  that  its  function  is  the  elimination  of  water  and  highly  diffusible 
inorganic  salts. 

Nussbaum  attempted  to  establish  the  secretory  power  of  the  epithelium 
in  another  way.  In  the  frog  the  kidney  receives  blood  from  two  sources :  the 
glomeruli  receive  their  blood  from  the  renal  artery,  the  tubules  from  the 
capillaries  formed  by  the  anastomosis  of  branches  of  the  efferent  vessel 
of  the  glomerulus  and  the  branches  of  the  renal  portal  vein.  Nussbaum 
believed  that  by  ligating  the  renal  artery  all  glomerular  activity  could  be 
abolished  and  the  part  played  by  the  epithelium  could  be  determined.  After 
so  doing  the  flow  of  urine  w^as  at  once  checked;  the  injection  of  urea  at 
once  reestablished  it.  This  fact  was  taken  as  a  proof  that  the  tubular 
epithelium  not  only  excreted  urea,  but  water  and  perhaps  other  constituents 


M 


Fig.  219. — Kidney  of  a  R.ab-  Fig.  220. — Microscopic  Appearance  of 

BIT.     Cortex    alone  stained  with  the  Lumen  of  the  Convoluted  Tubules 

the  indigo-carmine  at  the  end  of  Containing    the    Indigo-carmine.- — {Hei- 

one  hour. — {Heidenhain.)  denhain.) 

as  well.  It  was  also  found  that  sugar,  peptones,  carmine,  etc.,  which  are 
always  eliminated  from  the  blood  under  normal  conditions,  are  not  removed 
after  ligation  of  the  renal  artery.  It  was  concluded  from  these  experiments 
that  the  secreting  structures  of  the  kidney  consist  of  two  distinct  systems, 
the  glomerular  and  the  tubular;  the  former  secreting  water,  salts,  sugar, 
peptone,  etc.;  the  latter  urea,  uric  acid,  etc.  These  and  similar  facts  in- 
dicate that  the  renal  epithelium  possesses  a  secretor  rather  than  an  absorptive 
function.  Heidenhain  and  those  who  agree  with  him  assert  that  even  the 
water  and  inorganic  salts  which  pass  through  the  glomerular  epithelium 
do  so  in  consequence  of  cell  selection  and  cell  activity;  that  the  entire  process 
is  one  of  secretion,  though  conditioned  by  blood-pressure,  blood  velocity,  etc. 
Influence  of  Blood-pressure. — Whether  the  elimination  of  the  urinary 
constituents  is  entirely  secretor  (physiologic)  in  character  or  not  there  can 
be  no  doubt  that  the  whole  process  is  largely  determined  by  the  pressure  and 
velocity  of  the  blood  in  the  glomerular  capillaries,  or,  to  state  it  more  accu- 
rately, on  the  difference  of  pressure  between  the  blood  in  the  capillaries  and 
the  urine  in  the  capsules.     As  a  rule,  this  latter  pressure  is  at  a  minimum.     If 


EXCRETION. 


481 


the  urine  should  accumulate  in  the  ureter  and  tubules  either  from  ligation  or 
mechanical  obstruction  until  its  pressure  approximated  that  of  the  blood,  the 
secretion  should  be  diminished  if  not  abolished.  It  is  difficult  to  determine 
the  average  pressure  or  velocity  of  the  blood  in  the  glomerular  capillaries, 
though  they  both  must  be  greater  than  in  capillaries  in  other  parts  of  the 
body,  from  the  fact  that  the  efferent  vessel  is  narrower  than  the  afferent, 
and  therefore  offers  great  resistance  to  the  outflow  of  blood,  a  condition  most 
favorable  to  the  production  of  a  high  pressure  in  the  glomerulus. 

The  pressure  of  the  blood  in  the  glomeruli  is  the  resultant  of  the  pressure 
in  the  renal  artery  and  the  resistance  to  the  outflow  of  blood  through  the  effer- 
ent vessel  and  the  capillaries  beyond. 

The  pressure  of  blood  in  the  renal  artery  may  be  augmented  and 
the  velocity  of  the  blood  stream  increased : 

1.  By  an  increase  in  blood-pressure  generally. 

2.  By  an  increase  in  the  blood-pressure 
of  the  renal  artery  alone. 

The  first  condition  may  be  caused 
by  an  increase  in  either  the  force  or 
frequency  of  the  heart's  action  or  by 
a  contraction  of  the  arterioles  of  the 
vascular  areas  in  any  or  all  parts  of  the 
body,  excepting,  of  course,  the  renal  vas- 
cular area.  Should  this  condition  arise, 
the  blood  would  be  forced  into  the  renal 
artery  in  larger  volumes  and  in  conse- 
quence its  pressure  would  be  increased. 
The  second  condition  is  brought  about 
by  a  dilatation  of  the  renal  artery  alone 
and  possibly  by  a  contraction  of  the 
efferent  vessels  of  the  glomeruli. 

The  pressure  of  the  blood  in  the 
renal  artery  and  therefore  in  the  glomer- 
uli may  be  diminished  and  the  velocity 
decreased: 


Fig.  221. — To  Illustrate  the  Effect 
OF  Active  Changes  in  the  V.asa  Affer- 

ENTIA    AND    EfFERENTIA    ON  THE    PRESSURE 

IN  THE  Glomerular  Capillaries.  A.  Re- 
nal arteries.  G.  Glomerular  capillaries. 
C.  Tubular  capillaries.  V.  Vein.  The  short 
thick  lines  represent  the  vasa  afferentia  and 
efferentia.  The  continuous  heavy  line  repre- 
sents the  mean  average  pressure.  If  the  vas 
afferens  dilates  and  the  vas  efferens  contracts 
separately  or  conjointly,  the  pressure  will  rise, 
as  indicated  by  the  upper  dotted  line.    If  the 

1.  By  a   decrease  in  the  blood-pressure    vas  afferens  contracts  and  the  vas  efferens 

^11  dilates  separate)  v  or  con  joint!  v,  the  pressure 

generally.  ^  ^,jjj    ^^jj^  ^^   indicated  bv  the  lower  dotted 

2.  By  a  decrease  in  the  blood-pressure  line.— (After  Mora t.) 
of  the  renal  artery  alone. 

The  first  condition  may  be  caused  by  a  decrease  in  either  the  force  or 
frequency  of  the  heart's  action  or  by  a  dilatation  of  the  arterioles  of  large 
vascular  areas  in  any  or  all  parts  of  the  body.  Should  this  condition  arise, 
the  volume  of  blood  delivered  to  the  kidney  in  the  unit  of  time  would  be 
diminished  and  hence  its  pressure  would  fall.  The  dilatation  of  the  cutane- 
ous vessels  in  summer,  the  result  of  the  high  temperature  leads  to  a  di- 
minished blood  supply  to  the  kidney  and  a  diminution  in  the  amount  of  urine 
secreted.  The  second  condition  is  brought  about  by  contraction  of  the 
renal  artery  alone  and  possibly  by  a  dilatation  of  the  efferent  vessels  of  the 
glomeruli.  IVLoreover  the  pressure  in  the  vessels  of  the  glomeruli  may  be 
varied  according  to  the  degree  of  contraction  or  relaxation  of  the  muscle 
31 


482 


TEXT-BOOK  OF  PHYSIOLOGY. 


coat  of  the  afferent  and  efferent  vessels.  See  Fig.  220  and  the  accompanying 
explanation. 

Coincident  with  the  rise  and  fall  of  pressure  in  the  glomerular  capillaries 
there  is  a  rise  and  fall  in  the  rate  of  urinary  flow.  Thus  it  has  been  found  that 
an  increase  in  the  aortic  pressure  from  127  to  142  mm.  of  mercury,  from 
ligation  of  the  carotid,  femoral,  and  vertebral  arteries,  increased  the  rate  of 
urinary  flow  from  8.7  grams  in  thirty  minutes  to  21.2  grams.  On  the 
contrary,  a  decrease  in  aortic  pressure  below  40  mm.  of  mercury  caused  by 
division  of  the  spinal  cord  is  followed  by  a  total  abolition  of  the  urinary  flow. 
These  facts  serve  to  indicate  the  dependence  of  the  secretion  on  blood- 
pressure. 

The  period  of  functional  activity  of  the  kidney  is  accompanied  by  an 
increase  in  the  volume  of  blood  flowing  through  it  as  is  evident  from  an  in- 
spection of  the  organ.     At  this  time  it  is  enlarged,  swollen,  and  red  in  color. 


Fig.  222.  Fig.  223. 

Fig.  222. — Oncometer.  -K.  Kidney;  the  thick  line  is  the  metaUic  capsule,  h.  Hinge.  I. 
Tube  for  filUng  apparatus.  T.  Tube  to  connect  with  T,  a,  v,  u.  Artery,  vein,  ureter. — {Stirling, 
after  Roy.) 

Fig.  223. — Oncograph.  C.  Chamber  filled  with  oil,  communicating  by  T,  with  T.  p. 
Piston.     /.  Writing-lever. — (Stirling,  after  Roy.) 


The  blood  in  the  renal  vein  is  bright  red  in  color  and  contains  more  oxygen 
and  less  carbon  dioxid  than  venous  blood  generally.  During  the  intervals 
of  activity  the  kidney  is  supplied  with  a  less  amount  of  blood  and  hence  it 
diminishes  in  size,  becomes  pale  in  color  and  the  blood  of  the  renal  vein 
becomes  dark  and  venous  in  character.  These  variations  in  the  volume  of 
the  kidney  have  also  been  experimentally  determined  and  registered  by 
means  of  the  oncometer  and  oncograph  devised  by  Roy. 

The  oncometer  consists  of  a  metallic  box  (Fig.  222)  composed  of  halves 
which  open  and  close  by  means  of  a  hinge.  It  is  connected  with  a  recording 
apparatus,  the  oncograph  (Fig.  223),  through  the  tube  T.  The  kidney, 
withdrawn  from  the  body,  is  placed  within  the  oncometer.  Through  an 
opening  in  the  side  pass  the  artery,  vein,  and  ureter.  Between  the  kidney 
and  the  wall  of  the  capsule  there  is  placed  a  thin  membrane.  Oil  is  then 
poured  through  the  side  tube  I  until  the  space  between  the  capsule  and  the 
kidney,  as  well  as  the  tube  leading  to  the  chamber  of  the  oncograph,  are 
completely  filled.     When  the  tube  I  is  closed,  the  conditions  are  such  that  all 


EXCRETION.  4&3 

variations  in  the  volume  of  the  kidney  are  taken  up  and  reproduced  by  the 
recording  lever  attached  to  the  piston  of  the  oncograph.  A  curve  of  the 
variations  in  the  volume  of  the  kidney  is  shown  in  Figure  223  taken  simul- 
taneously with  the  curve  of  the  blood-pressure.  An  examination  of  this 
curve  shows  that  the  volume-changes  coincide  with  changes  in  the  blood- 
pressure,  exhibiting  not  only  the  respiratory  but  also  the  cardiac  undulations. 
Influence  of  the  Nerve  System. — The  influence  of  the  nerve  system  in 
regulating  the  blood-supply  to  the  kidney  is  evident  from  the  results  of  ex- 
perimentation. If  the  nerves  which  accompany  the  renal  artery  into  the 
kidney  are  divided,  the  artery  at  once  dilates,  the  kidney  enlarges,  and  a 
copious  flow  of  urine  takes  place.  If  the  peripheral  ends  of  these  nerves  be 
stimulated  with  induced  electric  currents  the  artery  contracts,  the  kidney 
diminishes  in  size,  and  the  flow  of  urine  ceases.  In  addition  to  these  vaso- 
constrictor nerves,  there  is  evidence  that  the  kidney  also  receives  vaso-dilata- 
tor  nerves  w^hich  emerge  from  the  spinal  cord  and  are  found  in  the  anterior 


B.P. 


BLOOD     PR£S SURE    CURVE 


KIDNEY     CURVE 


A^VWVV'V\A,VVW\AA^A 


Fig.   224. — B.P.  Blood-pressure  curve.     K.  Curve  of  the  volume  of  the  kidney.     T.  Time  curve; 
intervals  indicate  a  quarter  of  a  minute.     A.  Abcissa. — {Stirling,  after  Roy.) 

roots  of  the  eleventh,  twelfth,  and  thirteenth  dorsal  nerves,  in  the  dog. 
Direct  and  reflex  stimulation  of  these  nerves  gives  rise  to  a  dilatation  of  the 
artery,  a  swelling  of  the  kidney,  and  an  increase  in  secretion,  independent 
of  any  variation  in  general  blood-pressure. 

The  route  of  the  vaso-constrictor  nerves  is,  in  the  dog  at  least,  through 
the  lesser  splanchnics,  the  terminal  branches  of  which  arborize  around  the  cells 
of  the  renal  ganglia;  from  these  ganglia  new  fibers  arise  which  pass  through 
the  renal  plexus  into  the  kidney  to  be  distributed  to  the  muscle  coat  of  the 
renal  artery  branches.  Section  of  these  nerves  is  followed  by  a  dilatation 
of  the  renal  vessels  and  an  increase  in  the  flow  of  urine.  Stimulation  of  the 
peripheral  ends  is  followed  by  a  constriction  of  the  vessels  and  a  cessation 
of  the  flow  of  urine. 

The  vaso-motor  center  for  the  blood-vessels  of  the  kidney  is  in  all  proba- 
bility situated  in  the  medulla  oblongata  in  close  proximity  to  the  general 
vaso-motor  centers,  though  subordinate  centers  are  doubtless  present  in  the 
spinal  cord.  It  was  found  by  Bernard  that  puncture  of  the  medulla  was 
occasionally  followed  by  a  profuse  secretion  of  urine  without  the  presence 
of  sugar.  The  route  of  the  vaso-motor  impulses  which  influence  the  renal 
blood-supply  is  down  the  cord  to  local  vaso-motor  centers,  thence  through  the 
splanchnics  to  the  renal  ganglia,  thence  through  the  renal  plexus  to  the 
blood  vessels. 

Influence  of  Variations  in  the  Composition  of  the  Blood. — As  it  is 
the  function  of  the  kidneys  to  excrete  water,  inorganic  salts,  and  various 


484  TEXT-BOOK  OF  PHYSIOLOGY. 

end-products  of  metabolism  from  the  blood  and  thus  maintain  a  general 
average  composition,  it  is  highly  probable  that  as  soon  as  they  accumulate 
beyond  a  certain  precentage  they  themselves  act  as  stimuli  to  renal  activity, 
either  by  acting  directly  on  the  renal  epithelium  or  by  increasing  the  glomer- 
ular pressure.  There  is  evidence  at  least  that  urea  acts  in  the  former  manner 
and  that  an  excess  of  water  in  the  blood,  from  copious  drinking  or  from  a  sud- 
den checking  of  the  skin  from  a  fall  of  temperature,  acts  in  the  latter  manner. 
The  introduction  into  the  blood  of  inorganic  salts,  such  as  potassium  nitrate, 
sodium  acetate,  etc.,  will  in  a  short  time  lead  to  increased  activity  of  the 
kidneys,  as  shown  by  an  increase  in  the  quantity  of  urine  excreted.  The 
manner  in  which  these  agents  and  other  members  of  their  class,  the  so-called 
saline  diuretics,  increase  renal  activity  is  yet  a  subject  of  discussion.  On  the 
one  hand,  it  is  stated  that  they  promote  an  absorption  of  water  from  the 
tissues  to  such  an  extent  that  a  condition  of  hydremic  plethora  is  produced, 
which  in  itself  increases  not  only  the  general  blood-pressure  but  the  local 
renal  pressure  as  well,  and  that  it  is  this  factor  which  is  the  cause  of  the 
increased  flow  of  urine.  On  the  other  hand,  it  is  asserted  that  though  the 
salts  increase  the  local  pressure  and  the  volume  of  the  kidney,  they  never- 
theless act  specifically  on  the  renal  epithelium,  and  therefore  may  be  regarded 
as  secreto-motor  agents.  An  increase  in  the  percentage  of  sugar  or  urea  in 
the  blood  has  a  similar  influence  on  the  kidney. 

The  Storage  and  Discharge  of  Urine. — Urination. — The  urinary  con- 
stituents, as  soon  as  they  are  eliminated  from  the  blood,  pass  into  and  through 
the  uriniferous  tubules  and  by  them  are  discharged  into  the  pelvis  of  the 
kidney.  They  then  enter  the  ureter  by  which  they  are  conducted  to  the 
bladder.  The  immediate  cause  of  this  movement  is  undoubtedly  a  difference 
of  pressure  between  the  terminal  portions  of  the  tubules  and  the  terminal 
portion  of  the  ureter,  aided  by  the  peristaltic  contraction  of  the  muscle  wall 
of  the  ureter. 

The  bladder  is  a  reservoir  for  the  temporary  reception  of  the  urine  prior 
to  its  expulsion  from  the  body.  When  distended  it  is  ovoid  in  shape  and  is 
capable  of  holding  from  600  to  800  cu.  cm.  The  bladder  is  composed  of 
four  coats:  viz.,  serous,  muscle,  areolar,  and  mucous.  The  muscle  coat 
consists  of  external  longitudinal  and  internal  circular  and  oblique  layers  of 
fibers  of  the  non-striated  variety  which  collectively  encircle  the  entire  organ. 
As  these  fibers  by  their  contraction  expel  the  urine  from  the  bladder,  they 
are  known  collectively  as  the  detrusor  urincB  muscle.  At  the  exit  of  the  bladder 
the  circular  fibers  are  somewhat  increased  in  number,  giving  rise  to  the 
appearance  of  a  distinct  muscle  band  which  has  been  termed  the  sphincier 
vesiccB  muscle.  The  presence  of  this  muscle  has,  however,  been  denied  and 
the  retention  of  the  urine  has  been  attributed  to  mechanic  conditions  at  the 
neck  of  the  bladder.  The  urethra  just  beyond  the  bladder  is  provided  with 
a  distinct  circular  muscle  composed  of  striated  fibers,  the  sphincter  urethra 
muscle.  At  the  close  of  an  act  of  urination  or  micturition  the  bladder  is 
small,  contracted,  and  its  cavity  is  almost  obliterated,  but  as  urine  is  con- 
tinually descending  the  ureter  and  entering  the  bladder  at  its  base,  the 
detrusor  muscle  gradually  relaxes  or  becomes  sufiiciently  inhibited  from 
moment  to  moment  to  receive  it.  The  escape  of  urine  into  the  urethra  is 
prevented  either  by  mechanic  conditions  or  by  the  contraction  of  the  sphincter 


EXCRETION.  485 

muscle  at  the  vesic  oritice.  When  the  accumulating  urine  reaches  a  certain 
volume,  it  gives  rise  to  an  intra-vesic  pressure.  When  this  pressure  rises  to 
about  80  cm.  of  water  the  detrusor  urince  acquires  a  certain  degree  of  tension 
or  tonus.  This  is  followed  by  rhythmic  contractions  of  the  detrusor  urinse 
which  increase  in  extent  and  vigor  as  the  urine  continues  to  accumulate  until 
finally  a  general  contraction  develops,  the  force  of  which  overcomes  the 
constricting  influences  at  the  bladder  orifice  and  the  fluid  is  discharged. 
This  action  of  the  detrusor  muscle  is  generally  reinforced  by  the  contraction 
of  the  abdominal  muscles.  The  latter  portions  of  the  urine  are  ejected 
through  the  urethra  by  the  rhythmic  action  of  the  accelerator  urinae  muscles. 

The  Nerve  Mechanism  of  Urination. — The  muscle  mechanisms  which 
retain  as  well  as  expel  the  urine  are  under  the  control  of  the  nerve  system. 
The  nerves  involved  in  the  regulation  of  this  mechanism  reach  the  bladder 
by  two  different  routes,  viz:  (I)  by  way  of  the  lumbar  nerves  and  their  con- 
tinuations, the  hypogastrics,  and  (II)  by  way  of  the  sacral  nerves  and  their 
continuations  in  the  pelvic  plexus.  The  centers  of  origin  of  these  special 
nerve  fibers  are  located  in  the  spinal  cord  in  the  neighborhood  of  the  third, 
fourth  and  fifth  lumbar  segments. 

The  lumbar  nerves  from  the  third  to  the  fifth  give  off  branches  (pre- 
ganglionic) which  pass  forward  to  the  inferior  mesenteric  ganglion,'  around 
the  cells  of  v/hich  their  terminal  branches  arborize;  from  the  cells  of  this 
ganglion  new  fibers  arise  (post-ganglionic),  which  descend  through  the  hypo- 
gastric nerves  to  the  muscle  coat  of  the  bladder. 

The  sacral  nerves  from  the  second  to  the  fourth,  give  off  branches  which 
emerge  from  the  sacral  foramina  and  then  pass  forward  in  the  nervi  erigentes 
(pre-ganglionic)  to  small  ganglia  in  the  pelvic  or  vesical  plexus  around  the 
cells  of  which  their  terminal  fibers  arborize;  from  these  ganglia  new  fibers 
arise  (post-ganglionic)  which  are  distributed  also  to  the  muscle  coat  of  the 
bladder.  Afferent  fibers  pass  from  the  mucous  coat  through  the  posterior 
roots  of  the  sacral  nerves  and  reach  the  spinal  cord  centers  from  which 
the  efferent  fibers  for  the  muscle  coat  emanate. 

Though  the  origin,  course  and  distribution  of  the  nerves  composing  this 
mechanism  are  fairly  well  known,  their  mode  of  action  is  somewhat  obscure 
and  the  results  of  experimentation  not  always  in  accord.  According  to  v. 
Zeissl  stimulation  of  the  peripheral  ends  of  the  divided  hypogastric  nerves 
causes  mainly  a  contraction  of  the  sphincter  muscles  and  a  relaxation  of  the 
detrusor  muscle,  while  a  stimulation  of  the  peripheral  ends  of  the  divided 
sacral  nerves  causes  a  vigorous  contraction  of  the  detrusor  muscle  and  a  re- 
laxation of  the  sphincter  muscles.  The  lumbar  centers  would  therefore 
cause  a  reception  and  a  retention  of  the  urine,  and  the  sacral  centers  would 
cause  its  expulsion. 

The  expulsion  of  the  urine  is  primarily  a  reflex  act,  though  in  the  adult  it 
is  subject  to  a  variable  amount  of  volitional  control.  When  the  accumulated 
urine  has  reached  a  certain  volume  it  causes,  as  previously  stated,  an 
intra-vesic  pressure,  a  muscle  tonus,  and  slight  rhythmic  contractions  of  the 
detrusor  muscle.  Coincidently  nerve  impulses  are  developed  in  the  termin- 
als of  the  afferent  nerves  in  the  mucous  coat  which  are  then  transmitted  to 
the  spinal  cord  centers  and  to  the  brain,  where  the  characteristic  sensation 
and  the  desire  to  urinate  arises.     This  desire  is  probably  reinforced  by 


486  TEXT-BOOK  OF  PHYSIOLOGY. 

another  sensation  due  to  the  passage  of  a  small  (quantity  of  urine  into  the 
urethra.  In  a  young  child  the  arrival  of  the  reflex  impulses  in  the  spinal 
cord  is  immediately  followed  by  an  inhibition  of  the  sphincter  center  and 
a  stimulation  of  the  detrusor  center,  as  a  result  of  which  the  sphincter  muscle 
relaxes  and  the  detrusor  muscle  contracts,  thus  expelling  the  urine.  In 
the  adult  if  the  act  of  urination  is  to  be  permitted  volitional  impulses 
descend  the  cord  which  cause  a  contraction  of  the  abdominal  muscles  which 
through  pressure  on  the  bladder  assist  in  the  expulsion  of  the  urine.  If  the 
act  of  urination  is  to  be  suppressed  volitional  impulses  descend  the  cord  and 
cause  a  contraction  of  the  sphincter  urethrae  muscle  and  thus  temporarily 
prevent  the  discharge  of  the  urine.  After  urination  the  entrance  of  urine 
from  the  ureter  brings  about  a  reflex  contraction  of  the  sphincter  muscle 
by  stimulation  of  the  lumbar  sphincter  center  and  an  inhibition  of  the 
detrusor  muscle  by  stimulation  of  the  lumbar  inhibitor  center  in  con- 
sequence of  which  the  urine  is  received  and  retained  until  the  pressure  of  the 
accumulated  urine  again  causes  its  expulsion. 

PERSPIRATION;  SEBUM. 

The  perspiration  or  sweat,  the  chief  secretion  of  the  skin,  is  a  clear  colorless 
fluid,  slightly  acid  in  reaction  and  saline  to  the  taste.  Its  specific  gravity 
varies  from  1.003  to  1.006.  Unless  collected  from  the  soles  of  the  feet  and 
the  palms  of  the  hand,  it  is  apt  to  be  mixed  with  epithelial  cells  and  sebum. 
The  total  cjuantity  of  perspiration  secreted  daily  has  been  variously  estimated 
at  from  700  to  1000  grams;  the  exact  amount,  however,  is  difficult  of  determi- 
nation, for  the  reason  that  the  rate  of  secretion  varies  greatly  with  variations  in 
temperature,  food,  drink,  season  of  the  year,  etc. 

Chemic  analysis  of  the  sweat  shows  that  it  contains  but  from  0.5  to  2.5 
per  cent,  of  solid  constituents,  the  variation  in  the  percentage  depending  on 
the  quantity  of  water  secreted.  The  solids  consist  of  traces  of  urea,  neutral 
fats,  lactic  and  sudoric  acids  in  combination  with  alkaline  bases,  and  inorganic 
salts  (Fovel).  Other  observers,  however,  have  not  been  able  to  detect  the 
presence  of  either  lactic  or  sudoric  acid.  Urea  is  a  constant  ingredient, 
though  its  percentage  is  extremely  small,  possibly  not  more  than  o.i  per 
cent.  The  amount,  however,  may  be  very  much  increased  in  uremic 
conditions,  the  result  of  acute  or  chronic  disease  of  the  kidneys.  The  inor- 
ganic constituents  consist  mainly  of  sodium  chlorid  and  alkaline  and 
earthy  phosphates.  Carbonic  acid  is  also  present  in  the  free  state  as  well 
as  in  combination  with  alkaline  bases. 

The  very  small  quantity  of  the  solid  constituents  in  the  sweat,  taken  in 
connection  with  the  fact  that  it  is  excreted  most  abundantly  when  the  external 
temperature  is  high,  indicates  that  it  is  not  so  important  as  an  excrementi- 
tious  fluid  as  it  is  as  a  means  for  the  regulation  of  the  temperature  of 
the  body. 

The  sweat  is  a  product  of  the  secretory  activity  of  specialized  glands, 
the  sweat-glands,  embedded  in  the  skin,  to  the  histologic  structures  of 
which  they  bear  a  special  relation. 

THE  SKIN. 

The  skin  is  a  complexly  organized  structure  investing  the  entire  external 
surface  of  the  body.     Its  total  area  varies  from  1.17  to  1.35  square  meters  in 


EXCRETION. 


487 


man  and  from  i.i  to  1.17  square  meters  in  woman.  It  varies  in  thickness 
in  different  localities  of  the  body  from  J  to  yifo'  of  an  inch.  The  skin 
consists  of  two  principal  layers:  viz.,  a  deep  layer,  the  derma  or  corium, 
and  a  superficial  layer,  the  epidermis. 

The  derma  or  corium  may  be  subdivided  into  a  reticulated  and  a  pa- 
pillary layer.  The  reticulated  layer  consists  of  white  fibrous  and  yellow 
elastic  tissue,  non-striated  muscle-fibers,  woven  together  in  every  direction 
and  forming  an  areolar  network,  in  the  meshes  of  which  are  deposited 
masses  of  fat  and  a  structureless  amorphous  matter;  the  papillary  layer  con- 
sists mainly  of  club-shaped  elevations  or  projections  of  the  amorphous 
matter  constituting  the  papillas.  The  reticulated  layer  serves  to  connect 
the  skin  with  the  underly- 
ing structures  and  to  afford 
support  for  the  blood-ves- 
sels, nerves,  and  lymphat- 
ics which  are  distributed 
to  the  papillae  (Fig.  225). 

The  epidermis  is  an 
extra-vascular  structure 
consisting  entirely  of  epi- 
thelial cells.  It  may  also 
be  subdivided  into  two 
layers — the  Malpighian  or 
pigmentary  layer,  and  the 
corneous  or  horny  layer. 
The  former  is  closely  ap- 
plied to  the  papillary  layer 
of  the  true  skin  and  is 
composed  of  large  nucle- 
ated cells,  the  lowest  layer 
of  which,  the  "prickle 
cells,"  contains  the  pig- 
ment granules  which  give 
to  the  skin  its  varying 
hues  in  different  individ-  Fig.  225.— Section  Perpendicularly  Through  the 
uals  and  indifferent  races  Healthy  Skin,  a  Epidermis  or  scarfskin  ^--/ete  mu- 
,  cosum,  or  rete    malpighii.     c.    Papillary  layer,     a.    Derma, 

of  men ;  the  corneous  layer  corium,  or  true  skin.  e.  Panniculus  adiposus,  or  fatty  tis- 
is  composed  of  flattened  sue.  /,  g,  h.  Sweat-gland  and  duct,  i,  k.  Hair,  wdth  its 
cells  which  from  their  ex-    foUide  and  papilla.     /.  Sebaceous  gland. 

posure  to  the  atmosphere,  etc.,  are  hard  and  horny  in  texture. 

The  Sweat-glands. — These  glands  are  tubular  in  shape,  the  inner 
extremity  of  each  being  coiled  upon  itself  a  number  of  times,  forming  a 
little  ball  situated  in  the  derma  or  the  subcutaneous  connective  tissue.  From 
this  coil  the  duct  passes  up  in  a  straight  direction  to  the  epidermis,  where 
it  makes  a  few  spiral  turns,  after  which  it  opens  obliquely  on  the  surface. 
The  gland  consists  of  a  basement  membrane  lined  with  epithelial  cells. 
It  is  supplied  abundantly  with  blood-vessels  and  nerves.  The  sweat 
glands  are  extremely  numerous  all  over  the  cutaneous  surface,  though 
they   are   more   thickly   disposed   in   some   situations   than  others.     They 


^  =  : 


<si^:^' 


488  TEXT-BOOK  OF  PHYSIOLOGY. 

probably  average  400  to  the  square  centimeter;  the  total  number  has  been 
estimated  at  from  2,000,000  to  2,500,000. 

The  Influence  of  the  Nerve  System  on  the  Production  of  Sweat. — 
The  secretion  of  sweat,  though  a  product  of  the  activity  of  epithelial  cells 
and  dependent  on  a  variety  of  conditions,  is  regulated  to  a  large  extent  by 
the  nerve  system.  Here  as  in  other  secreting  glands  the  fluid  is  derived 
from  materials  in  the  lymph-spaces,  furnished  by  the  blood.  Generally 
the  two  conditions,  increased  blood-flow  and  increased  glandular  action, 
coexist.  At  times,  however,  a  profuse  clammy  perspiration  is  secreted  with 
diminished  blood-flow.  Two  sets  of  nerves  are  evidently  concerned  in 
this  process:  viz.,  vaso-motor  nerves,  which  regulate  the  blood-supply,  and 
secrelor  nerves,  which  stimulate  the  gland  cells  to  activity. 

The  nerve-centers  which  control  the  sweat-glands  are  situated  in  the 
spinal  cord,  though  the  number  of  such  centers  and  their  exact  location  for 
the  different  regions  of  the  body  have  not  yet  been  satisfactorily  determined. 
From  observation  of  clinic  and  pathologic  conditions  in  human  beings  and 
from  experiments  mado  on  animals  it  may  be  stated  in  a  general  way 
that  the  centers  for  the  head  and  face  lie  in  the  upper  thoracic  region  of  the 
cord;  for  the  upper  extremities,  in  the  upper  two-thirds  of  the  thoracic 
region;  for  the  lower  extremities,  in  the  lower  thoracic  and  upper  lumbar 
region.  The  secretor  nerves  which  emerge  from  these  centers  are  contained 
in  the  ventral  roots  of  the  thoracic  and  upper  lumbar  nerves,  which  they  leave 
by  way  of  the  white  rami  communicantes  asmedullated  (pre-ganglionic)  fibers 
to  enter  the  sympathetic  ganglia,  around  the  cells  of  which  they  arborize. 
From  these  ganglia  non-medullated  (post-ganglionic)  fibers  emerge,  re-enter 
the  spinal  nerves,  with  the  exception  of  those  for  the  head  and  face,  and 
then  pass  to  the  sweat  glands  in  various  regions  of  the  body,  following  a 
course  similar  to  that  pursued  by  the  vaso-constrictor  nerves  for  correspond- 
ing regions.  It  is  probable,  though  it  has  not  been  demonstrated,  that 
there  is  also  in  the  medulla  a  general  dominating  sweat  center. 

The  exact  course  for  the  sweat  nerves  has  been  experimentally  deter- 
mined only  for  the  cat  and  dog.  In  these  animals,  however,  sweat  glands 
are  found  only  in  the  balls  of  the  feet.  According  to  Langley's  observations 
the  sweat  nerves  for  the  fore-feet  leave  the  spinal  cord  in  the  thoracic 
nerves  from  the  fourth  to  the  tenth  inclusive.  After  passing  into  the  sym- 
pathetic chain  they  ascend  to  the  stellate  ganglion,  around  the  cells  of  which 
their  end  branches  arborize.  From  this  ganglion  non-medullated  fibers  pass 
in  the  gray  rami  communicantes  to  the  nerves  composing  the  brachial  plexus 
and  then  to  the  feet.  The  sweat  nerv'es  for  the  hind  feet  leave  the  cord 
mainly  in  the  first  and  second  lumbar  and  terminate  in  sympathetic  ganglia, 
from  which  the  non-medullated  nerves  pass  into  the  nerve-trunks  included 
between  the  sixth  lumbar  and  the  second  sacral  nerv^es,  which  enter  into 
the  formation  of  the  sacral  plexus  and  through  which  they  pass  to  the  feet. 

That  the  sweat-glands  are  stimulated  to  activity  by  nerve  impulses  is 
shown  by  the  fact  that  stimulation  of  the  peripheral  end  of  the  divided 
cervical  sympathetic,  of  the  brachial  plexus,  or  of  the  sciatic  nerve  is  followed 
in  a  few  seconds  by  a  profuse  secretion.  Though  under  physiologic  con- 
ditions there  is  a  simultaneous  dilatation  of  the  blood-vessels  and  an  increased 
supply  of  blood,  this  is  merely  a  condition  and  not  a  cause  of  the  secretion; 


EXCRETION. 


for  the  secretion  can  be  excited  and  the  flow  maintained  for  a  period  of  from 
ten  to  fifteen  minutes  after  Ugation  of  the  blood-vessels  of  the  Umb  or  even 
after  its  amputation,  when  the  corresponding  nerve  is  stimulated. 

The  sweat-glands  may  be  excited  to  activity  by  their  related  nerve-centers, 
either  by  central,  by  reflex,  or  by  local  peripheral  influences.  Among  the  first 
may  be  mentioned  mental  emotions,  venosity  of  the  blood,  increased  tempera- 
ture of  the  blood,  hot  drinks,  violent  muscular  exercise,  etc.  Among  the 
second  may  be  mentioned  powerful  stimulation  of  various  afferent  or  sensor 
nerves,  heightened  external  temperature,  etc.  Among  the  last  may  be 
mentioned  various  drugs.  Pilocarpin  injected  into  the  blood  causes  a  profuse 
secretion  even  when  the  nerves  have  been  divided.  Its  action  is  supposed 
to  be  exerted  on  the  terminal  branches  of  the  nerves  and  possibly  on  the 
cells  themselves.  As  in  the  case  of  the 
salivary  glands  atropin  suspends  the  activity 
of    the    terminal    branches    of   the  secretor  \  WKlia 

nerves. 

Hairs. — Hairs  are  found  in  almost  all 
portions  of  the  body,  and  can  be  divided 
into — 

1.  Long,  soft  hairs,  on  the  head. 

2.  Short,  stiff  hairs,  along  the  edges  of  the 
eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cuta- 
neous surface. 

They  consist  of  a  root  and  a  shaft.  The 
shaft  is  oval  in  shape  and  about  60  micro- 
millimeters  in  diameter;  it  consists  of  fibrous 
tissue,  covered  externally  by  a  layer  of  im- 
bricated cells,  and  internally  by  cells  conta-in- 
ing  granular  and  pigment  material. 

The  root  of  the  hair  is  embedded  in  the 
hair-follicle,  formed  by  a  tubular  depression 
of  the  skin,  extending  nearly  through  to  the 

subcutaneous  tissue;  its  walls  are  formed  by  the  layers  of  the  corium,  covered 
by  epidermic  cells.  At  the  bottom  of  the  follicle  there  is  a  papillary  pro- 
jection of  amorphous  matter,  corresponding  to  a  papilla  of  the  true  skin, 
containing  blood-vessels  and  nerves,  upon  which  the  hair-root  rests. 
The  investments  of  the  hair-roots  are  formed  of  epithelial  cells,  consti- 
tuting the  internal  and  external  root-sheaths. 

The  lower  portion  of  the  hair-follicle  is  connected  with  the  upper  surface 
of  the  derma  by  bundles  of  non-striated  muscle-fibers  which  are  termed 
arrectores  piloriim  muscles.  Their  inclination  and  insertion  are  such  that 
their  contraction  is  followed  by  erection  of  the  hair-follicle  and  hair-shaft. 
These  muscles  are  excited  to  action  by  nerves  termed  pilo-motor  nerves. 

THE  SEBUM. 

The  sebum  or  sebaceous  matter  is  a  peculiar  oily  material  produced 
by  specialized  glands  in  the  skin.  It  consists  of  water,  epithelium,  pro- 
teids,  fat,  cholesterin,  and  inorganic  salts. 


Fig.  226. — Laege  Sebaceous 
Gland,  i.  Hair  in  its  follicle.  2,  3, 
4,  5.  Lobules  of  the  gland.  6.  Ex- 
cretory duct  traversed  by  the  hair. 
—{Sappey.') 


490  TEXT-BOOK  OF  PHYSIOLOGY. 

The  sebaceous  glands  are  simple  and  compound  racemose  glands 
opening  by  a  common  excretory  duct  on  the  surface  of  the  epidermis  or 
into  the  shaft  of  a  hair-follicle  (Fig.  226).  These  glands  are  extremely  numer- 
ous and  found  in  all  portions  of  the  body,  with  the  exception  of  the  palms 
of  the  hands  and  soles  of  the  feet,  and  most  abundantly  in  the  face.  They 
are  formed  by  a  delicate  structureless  membrane  lined  by  polyhedral 
epithelium. 

The  sebum  is  not  produced  by  an  act  of  true  secretion,  but  is  formed  by 
a  proliferation  and  degeneration  of  the  gland  epithelium.  When  first 
poured  on  the  surface,  the  sebum  is  oily  and  semiliquid  in  character,  but 
soon  hardens  and  acquires  a  cheese-like  consistence.  It  serves  to  lubri- 
cate the  hair  and  skin  and  prevent  them  from  becoming  dry  and  harsh. 

The  surface  of  the  fetus  is  generally  covered  with  a  thick  layer  of  seba- 
ceous matter,  the  vernix  caseosa,  which  possibly  keeps  the  skin  in  a  normal 
condition  by  protecting  it  from  the  effects  of  the  long-continued  action  of 
the  amniotic  fluid  in  which  the  fetus  is  suspended. 


CHAPTER  XIX. 

THE  CENTRAL  ORGANS  OF  THE  NERVE  SYSTEM  AND 
THEIR  NERVES. 

The  central  organs  of  the  nerve  system  are  the  encephalon  and  the 
spinal  cord,  lodged  within  the  cavity  of  the  cranium  and  the  cavity  of  the 
spinal  or  vertebral  column  respectively.  The  general  shape  of  these  two 
portions  of  the  nerve  system  corresponds  with  that  of  the  cavities  in  which 
they  are  contained.  The  encephalon  is  broad  and  ovoid,  the  spinal  cord 
is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct,  though 
closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large  ovoid  mass,  occu- 
pying the  entire  upper  part  of  the  cranial  cavity;  (2)  the  cerebellum,  the 
wedge-shaped  portion  placed  beneath  the  posterior  part  of  the  cerebrum 
and  lodged  within  the  cerebellar  fossae  of  the  cranium;  (3)  the  isthmus  of 
the  encephalon,  the  more  or  less  pyramidal-shaped  portion  connecting  the 
cerebrum  and  cerebellum  with  each  other  and  both  with  (4)  the  medulla 
oblongata.     (Fig.  227.) 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies  the 
spinal  canal  as  far  as  the  second  or  third  lumbar  vertebra.  The  central 
nerve  system  is  bilaterally  symmetric,  consisting  of  distinct  halves  united  in 
the  median  line.  'The  cerebrum  is  subdivided  by  a  deep  fissure,  running 
antero-posteriorly,  into  two  ovoid  masses  termed  cerebral  hemispheres;  the 
cerebellum  is  also  partially  subdivided  into  hemispheres;  the  isthmus  like- 
wise presents  in  the  median  line  a  partial  division  into  halves;  the  medulla 
oblongata  and  spinal  cord  are  subdivided  by  an  anterior  or  ventral  and  a 
posterior  or  dorsal  fissure  into  halves,  a  right  and  a  left. 

The  peripheral  organs  of  the  nerve  system  in  anatomic  and  phy- 
siologic relation  with  the  central  organs  are  the  encephalic  and  the  spinal 
nerves.  The  encephalic  nerves,  twelve  in  number  on  each  side  of  the 
median  line,  are  in  relation  with  the  base  of  the  encephalon,  and  because  of 
the  fact  that  they  pass  through  foramina  in  the  walls  of  the  cranium  they  are 
usually  termed  cranial  nerves. 

The  spinal  nerves,  thirty-one  in  number  on  each  side,  are  in  relation 
with  the  spinal  cord,  and  because  of  the  fact  that  they  pass  through  foramina 
in  the  walls  of  the  spinal  column  they  are  termed  spinal  nerves.  As  both 
cranial  and  spinal  nervTS  are  ultiniately  distributed  to  the  structures  of  the 
body — i.e.,  the  general  periphery — they  collectively  constitute  the  periph- 
eral organs  of  the  nerve  system. 

The  central  organs  of  the  nerve  system  are  supported  and  protected 
by  three  membranes  named,  in  their  order  from  without  inward,  the  dura 
mater,  the  arachnoid,  and  the  pia  mater. 

The  dttra  mater  is  a  tough  membrane  composed  of  fibrous  tissue.  It 
consists  of  two  layers,  the  outer  of  which  lines  the  cranial  cavity  and  forms 

491 


492 


TEXT-BOOK  (^F  PHYSIOLOGY. 


an  internal  periosteum;  the  inner  layer  is  closely  attached  to  the  outer 
except  at  certain  regions  where  it  separates  and  forms  supporting  structures, 
such  as  the  falx  cerebri,  falx  cerebelli,  tentorium  cerebelli,  etc.;  at  the  margin 
of  the  foramen  magnum  the  outer  layer  becomes  continuous  with  the 
periosteal  tissue,  while  the  inner  layer  invests  the  cord  down  to  its  ultimate 
termination.      (Fig.   228.) 

The  arachnoid  is  a  delicate  serous  membrane. 
The  external  surface  is  smooth  and  well  defined 
and  separated  from  the  dura  by  a  narrow  space, 
the  subdural  space.  The  inner  surface  sends  in- 
ward fine  connective-tissue  processes  which  inter- 
lace in  every  direction,  constituting  the  subarach- 
noid tissue.  This  tissue  is  abundant  in  the  cra- 
nium, much  less  so  in  the  spinal  canal.  The 
spaces  between  the  connective  tissue,  taken  collec- 
tively, constitute  the  general  subarachnoid  space. 
Around  the  spinal  cord  this  space  is  well  defined, 
and  at  the  base  of  the  encephalon  expands  to 
form  large  cavities  known  as  the  cisterna  magna, 
cisterna  pontis,  etc. 

The  pia  mater  is  a  delicate  membrane  com- 
posed of  areolar  tissue.  It  closely  invests  the  en- 
cephalon and  spinal  cord,  dipping  into  the  various 
fissures.  It  is  exceedingly  vascular  and  sends  small 
blood-vessels  for  some  distance  into  the  brain  and 
spinal  cord. 

The  Encephalo-spinal  Fluid. — The  general 
subarachnoid  space,  as  well  as  certain  cavities  within 
the  encephalon,  contain  a  clear  transparent  fluid, 
termed  the  encephalo-spinal  fluid.  This  fluid  has 
an  alkaline  reaction  and  a  specific  gravity  of  1.007 
or  1.008.  It  is  composed  of  water,  proteins  (pro- 
teoses and  serumglobulin) ,  and  pyrocatechin 
CgHJOH)2,  capable  of  reducing  copper  salts, 
though  not  exhibiting  any  other  of  the  properties 
of  sugar.  In  many  respects  this  fluid  resembles 
lymph.  The  subarachnoid  space  and  the  general 
encephalic  cavities,  termed  ventricles,  communicate 
with  one  another  by  an  opening  in  the  pia  mater 
(the  foramen  of  Magendie)  as  it  passes  over  the 
lower  part  of  the  fourth  ventricle. 

It  was  stated  in  Chapter  VIII  that  the  entire 
nerve  or  neuron  system  can  be  resolved  into  single 
morphologic  units,  the  neurons:  the  histologic  fea- 
tures and  the  physiologic  properties  of  the  neuron 
were  there  also  described;  the  anatomic  relation  of  the  neurons  constitut- 
ing the  peripheral  organs  of  the  nerve  system  to  the  neurons  constituting 
the  central  organs  of  the  nerve  system,  were  also  stated  and  illustrated  in 
part  diagrammatically,  page  49.     From  the  statements  made  regarding  the 


Fig.  227.— The  Central 
Organs  of  the  Nerve 
System,  f.  t.  o.  Frontal, 
temporal,  and  occipital 
lobes  of  the  cerebrum,  c. 
Cerebellum,  p.  Pons.  mo. 
Medulla  oblongata.  nis., 
ms.  The  upper  and  lower 
limits  of  the  spinal  cord. 
The  remaining  letters  in- 
dicate the  region  and  num- 
ber of  the  spinal  nerves  — - 
{Quain,  after  Boiirgery.) 


THE  ENCEPHALO-SPINAL  MEMBRANES. 


493 


functions  of  the  different  neurons  in  their  individual  and  collective  capacity 
the  functions  of  the  nerve  system  will  become  apparent. 

The  Functions  of  the  Nerve  System. — The  functions  of  the  nerve 
system  are  twofold:  (i)  It  unites  and  associates  the  organs  and  tissues  of 
the  body  in  such  a  manner  that  they  are  enabled  to  cooperate  for  the  accom- 
plishment of  a  definite  object.  (2)  It  serves  to  arouse  in  the  individual  a 
consciousness  of  the  existence  of  an  external 
world,  by  virtue  of  the  impressions  which  it 
makes  on  his  sense  organs,  and  consequently 
to  enable  him  to  adjust  himself  to  his  environ- 
ment. 

By  virtue  of  the  anatomic  and  physiologic 
association,  a  stimulus,  if  of  sufficient  inten- 
sity, applied  to  one  organ  or  tissue  will  call 
forth  activity  in  one  or  more  organs  near  or 
remote  from  the  part  stimulated.  This  coor- 
dination of  action  is  accomplished  mainly  by 
the  spinal  cord  and  the  medulla  oblongata. 
All  actions  which  take  place  in  response  to  a 
peripheral  stimulus  and  independently  of  voli- 
tion are  termed  retiex  actions.  The  reflex 
activities  connected  with  digestion,  the  circu- 
lation of  the  blood,  with  respiration,  excretion, 
etc.,  are  illustrations  of  the  coordinating  capa- 
bilities of  the  nerve-centers  located  in  these 
portions  of  the  central  nerve  system. 

Consciousness  of  the  existence  of  the  ex- 
ternal world  and  of  the  relation  existing  be- 
tween it  and  the  individual  is  associated  with 
the  physiologic  activities  of  the  encephalon, 
and  more  particularly  of  the  cerebral  hemi- 
spheres. This  portion  of  the  nerve  system  is 
the  chief,  though  perhaps  not  the  sole,  organ 
of  the  mind,  and  its  functions  are  for  the  most  part  mental. 

The  function  of  a  part  at  least  of  the  peripheral  nerve  system  is  to  afford 
a  means  of  communication  between  the  central  organ  of  the  nerve  system 
and  the  remaining  structures  of  the  body.  The  nerve-trunks  constituting 
this  part  may  be  divided  into  two  groups,  as  follows : 

1.  The  first  group  comprises  nerves  in  connection  with  the  special  sense- 

organs,  e.g.,  skin,  eye,  ear,  nose,  tongue,  as  well  as  nerves  in  connection 
with  the  general  or  organic  sense-organs,  e.g.,  mucous  membranes, 
viscera,  etc.,  which  are  connected  primarily  with  nerve-cells  in  the  spinal 
cord  and  medulla  oblongata,  and  secondarily  with  nerve  cells  in  local- 
ized areas  of  the  cerebral  cortex. 

2.  The   second   group   comprises  nerves   which  terminate   mainly  in   the 

muscle  apparatus  and  which  constitute  the  continuation  of  nerve  paths 
which  have  their  origin  in  nerve-cells  of  localized  areas  of  the  cerebral 
cortex. 
The  first  group  of  nerves,  the  afferent,  especially  those  connected  with 


Fig.  22S. — The  Membranes  of 
THE  Spinal  Cord.  i.  Dura 
mater.  2.  .Arachnoid.  3.  Poste- 
rior root  of  spinal  nerve.  4.  An- 
terior root  of  spinal  ner\-e.  5. 
Ligamentum  dentatum.  6.  Linea 
splendens. — {Morris,    after  Ellis.) 


494  TEXT-BOOK  OF  PHYSIOLOGY. 

the  special  sense-organs,  are  excited,  to  activity  by  impressions  made  on  their 
peripheral  terminations  by  agencies  in  the  external  world.  The  nerve 
impulses  thus  generated  are  transmitted  in  part  only  as  far  as  the  spinal 
cord  and  medulla  oblongata  while  the  remainder  ascend  to  nerve-cells  in 
localized  areas  of  the  cerebral  cortex  where  they  evoke  sensations.  These 
sensations  by  their  grouping  and  combinations  become  the  primary  elements 
of  intelligence.  The  afferent  nerves  thus  become  a  means  of  communication 
between  the  physical  and  the  mental  worlds. 

The  second  group  of  nerves,  the  efferent,  are  excited  to  activity  by  those 
molecular  disturbances  in  their  related  nerve-cells  which  accompany  voli- 
tional efforts.  The  nerve  impulses  thus  developed  and  discharged  from 
localized  areas  in  the  cerebral  cortex  are  transmitted  by  way  of  the  med- 
ulla and  spinal  cord  to  the  muscles  of  the  face,  trunk  and  extremities 
which  are  in  consequence  excited  to  activity.  The  muscle  movements  thus 
become  physical  expressions  of  mental  states,  and  if  directed  in  a  definite 
manner  to  the  overcoming  of  the  resistance  offered  by  the  external  world 
they  become  capable  of  modifying  it  in  accordance  with  the  mental  states. 
The  efferent  nerves  thus  become  a  means  of  communication  between  the 
mental  and  the  physical  worlds. 

The  central  nerve  system  is  thus  composed  of  a  number  of  separate 
though  closely  related  parts,  to  each  of  which  a  separate  function  has  been 
assigned.  In  the  study  of  the  structure  and  function  of  these  separate  parts 
it  will  be  found  convenient,  and  conducive  to  clearness,  to  consider  them  in 
the  order  of  their  complexity,  beginning  with  the  spinal  cord  and  ending  with 
the  cerebrum. 

THE  SPINAL  CORD. 

The  spinal  cord  is  the  narrow  elongated  portion  of  the  central  nerve 
system  contained  within  the  spinal  canal.  It  is  cyUndric  in  shape  though 
presenting  an  enlargement  in  both  the  lower  cervical  and  lower  lumbar 
regions  corresponding  to  the  origins  of  the  nerves  distributed  to  the  upper 
and  lower  extremities.  The  cord  varies  in  length  from  40  to  45  cm.,  measures 
12  mm.  in  diameter,  weighs  42  gms.,  and  extends  from  the  atlas  to  the  second 
lumbar  vertebra,  beyond  which  it  is  continued  as  a  narrow  thread,  the 
filum  terminale.  (Fig.  229.)  It  is  divided  by  the  anterior  and  posterior 
longitudinal  fissures  into  halves,  and  is  therefore  bilaterally  symmetric.  A 
transverse  section  of  the  cord  shows  that  it  is  composed  of  both  white  and 
gray  matter,  the  former  covering  the  surface,  the  latter  occupying  the  center. 

Structure  of  the  Gray  Matter.— The  gray  matter  is  arranged  in  the 
form  of  two  crescents,  united  in  the  median  line  by  a  transverse  band  or 
commissure  forming  a  figure  resembling  the  letter  H.  Though  varying 
in  shape  in  different  regions  of  the  cord,  the  gray  matter  in  all  situations 
presents  on  either  side  an  anterior  or  ventral  and  a  posterior  or  dorsal  horn. 
Between  the  two  horns  there  is  a  portion  termed  the  intermediate  gray  sub- 
stance. The  commissure  presents  in  its  center  a  narrow  canal  which  extends 
throughout  the  entire  length  of  the  cord.  This  canal  is  lined  by  cylindric 
epithelium  and  surrounded  by  gelatinous  material.     (Fig.  230.) 

The  anterior  horn  is  short  and  broad  and  entirely  surrounded  by  white 


THE  SPINAL  CORD. 


495 


matter.  The  posterior  horn  is  narrow  and  elongated  and  extends  quite 
up  to  the  surface  of  the  cord,  where  it  is  capped  by  gelatinous  matter,  the 
substantia  gelatinosa.  In  the  lower  cervical  and  thoracic  regions  a  portion 
of  the  intermediate  gray  substance  projects  outward  and  forms  the  so-called 
lateral  horn.  The  gray  matter  fundamentally  consists  of  a  framework  of 
fine  neuroglia  supporting  blood-vessels,  lymphatics,  medullated  and  non- 
medullated  nerves,  and  groups  of  nerve-cells. 


Superior  or  Cervical  Segment       Middle  or  Dorsal  Portion       Inferior  Portion  of  Cord 
of  Spinal  Cord.  of  Cord.  and  Cauda  Equina. 

Fig.  229. — Superior,  Middle,  and  Inferior  Portions  of  Spinal  Cord.  i.  Floor  of 
fourth  ventricle.  2.  Superior  cerebellar  peduncle.  3.  Middle  cerebellar  peduncle.  4.  Infe- 
rior cerebellar  peduncle.  5.  Enlargement  at  upper  extremit)'  of  postero-median  column.  6. 
Glosso-pharyngeal  nerve.  7.  Vagus.  8.  Spinal  accessory.  9,  9,  9,  9.  Ligamentum  denticula- 
tum.  10,  ID,  10,  10.  Posterior  roots  of  spinal  nerves.  11,  11,  11,  11.  Postero-lateral  fissure. 
12,  12,  12,  12.  Ganglia  of  posterior  roots.  13,  13.  Anterior  roots.  14.  Division  of  united  roots 
into  anterior  and  posterior  nerves.  15.  Terminal  e.xtremity  of  cord.  16,  16.  Filum  terminale. 
17,17.  Cauda  equina.  I,  VIII.  Cervical  nerves.  I,  XII.  Dorsal  nerves.  I,  V.  Lumbar  nerves. 
I,  V.  Sacral  nerves. — (Sappey.) 

The  Nerve-cells. — The  nerve  cells  of  the  cord  are  very  numerous  and 
they  present  a  variety  of  shapes  and  sizes  in  different  regions.  They  are 
usually  arranged  in  groups  which  extend  for  some  distance  up  and  down 
the  gray  matter,  forming  columns  more  or  less  continuous. 

In  the  anterior  horn  two  wxll-marked  groups  are  found,  one  situated  at 
the  anterior  and  inner  angle,  known  at  the  antero-median  group,  the  other 
situated  at  the  posterior  and  lateral  angle  and  known  as  the  postero-lateral 
group.     In  the  lower  cervical  and  upper  thoracic  regions,  in  the  region  of 


496 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  lateral  horn,  another  group  of  cells  is  found,  known  as  the  intermediate 
group.     In  the  central  portion  of  the  horn  there  is  also  a  central  group. 

The  cells  of  the  anterior  horns  are  of  large  size,  nucleated  and  multi- 
polar. They  are  the  modified  descendants  of  pear-shaped  cells,  the  neuro- 
blasts, which  migrated  from  the  medullary  tube  (see  page  95).  In  the  cousre 
of  their  migration  they  developed  dendrites  which  form  an  intricate  felt- 
work  throughout  the  anterior  horn.  One  of  the  processes,  the  axon,  ap- 
proached the  surface  of  the  cord,  penetrated  it, 
grew  outward,  became  covered  with  myelin  and 
neurilemma,  and  developed  into  an  anterior 
root-fiber.  These  nerve-cells,  with  their  den- 
drites, axons,  and  terminal  branches,  form 
efferent  neurons  of  the  first  order.  The  inti- 
mate histologic  and  physiologic  relationship 
existing  between  the  nerve-cell  and  the  axon  is 
revealed  by  the  degenerative  changes  which 
arise  in  the  latter  when  separated  from  the 
former.  The  cell  apparently  determines  the 
nutrition  of  the  axon  and  may  be  regarded  as 
trophic  in  function.  Some  of  the  cells  of  the 
anterior  horn  send  their  axons  into  the  im- 
mediately surrounding  white  matter  of  the  same 
side,  after  which  they  divide  into  two  branches, 
one  passing  up,  the  other  down,  the  cord,  to 
re-enter  the  gray  matter  at  different  levels. 
They  are  probably  associative  in  function. 
Other  cells  send  their  axons  into  that  portion 
of  the  white  matter  on  the  same  and  opposite 
sides  known  as  Gower's  antero-lateral  tract. 
(Fig.  231.) 

In  the  posterior  horn  nerve-cells  are  also 
present,  though  they  are  not  so  numerous  as 
in  the  anterior  horn.  At  the  base  of  the  horn 
and  on  its  inner  side  there  is  a  well-marked 
group  of  cells  which  extends  from  the  seventh 
or  eighth  cervical  nerves  downward  to  the  sec- 
ond or  third  lumbar  nerves,  being  most  promi- 
nent in  the  thoracic  region.  This  column  is 
known  as  Clarke's  vesicular  column.  From  the 
nerve-cells  constituting  this  column  axons  pass 
of  the  lumbar  obliquely  outward  into  the  portion  of  the  white 
,    r  „  ^^   ,^^?    "PP^'^  matter  known  as   the   direct   cerebellar  tract. 

part  of  the  conus  medullaris.      i.         ""-'-'-^     «.    w  •  i  i  • 

Posterior  roots.    2.  Anterior  roots.  Other  ncrve-cells  Send  their  axons  mto  the  white 
3.  Posterior    fissure.    4.  Anterior  matter    in    the  posterior  portion  of  the   cord 

fissure.     K.  Central  canal. — (Mor-   ^         ,.         ,i  ,.  j-         r  c,~,„ 

m'  "Anatomyr  after  Schwalbe.)  bordering  the  postcHor  median  hssure.     Some 

of  the  nerve-cells,  their  situation  and  the  dis- 
tribution of  their  axons  are  shown  in  Fig.  231. 

Classification  of  Nerve-cells. — The  cells  of  the  gray  matter  may  be 
divided  into  three  main  groups:  viz.,  intrinsic,  efferent,  and  afferent. 


yi^W€. 


Fig.  230. — Sections  through 
Different  Regions  of  the 
Spinal  Cord.  A.  At  the  level 
of  the  sixth  cervical  nerve.  B. 
At  the  mid-dorsal  region.  C 
At  the  center 
enlargement.     D 


THE  SPINAL  CORD. 


497 


The  intrinsic  cells  are  associative  in  function.  The  axons  to  which  these 
cells  give  origin  pass  more  or  less  horizontally  into  the  white  matter,  where 
they  divide  into  two  branches,  one  of  which  passes  upward,  the  other  down- 
ward. At  various  levels  they  re-enter  the  gray  matter  and  arborize  around 
other  intrinsic  cells. 

The  efferent  cells,  independently  of  their  trophic  influence,  are  also  motor 
in  function,  inasmuch  as  the  excitation  arising  in  them  is  transmitted  out- 
wardly through  their  axons  to  muscles,  blood-vessels,  glands  and  viscera, 
imparting  to  them  motion,  either  molar  or  molecular.  As  the  efferent 
fibers  in  the  ventral  roots  of  the  spinal  nerves  are  classified  (see  page  97) 
in  accordance  with  their  physiologic  action  into  motor,  vaso-motor,  secretor, 
viscero-motor  and  pilo-motor  nerves,  so  the  nerve-cells  of  which  the  nerves 


l/enXral 

Fig.  231. — Scheme  of  the  Structure  of  the  Cord. — {Howell  after  Lenhossek.)  On  the 
right  the  nerve  cells;  on  the  left  the  entering  nerve  fibers.  Right  side:  i,  Motor  cells,  anterior 
horn,  giving  rise  to  the  fibers  of  the  anterior  root;  2,  tract  cells  whose  axons  pass  into  the  white 
matter  of  the  anterior  and  lateral  columns;  3,  commissural  cells  whose  axons  pass  chiefly  through 
the  anterior  commissure  to  reach  the  anterior  columns  of  the  other  side;  4,  Golgi  cells  (second  type), 
whose  axons  do  not  leave  the  gray  matter;  5,  tract  cells  whose  a.xons  pass  into  the  white  matter 
of  the  posterior  column.  Left  side:  i,  Entering  fibers  of  the  posterior  root,  ending,  from  within 
outward,  as  follows:  Clarke's  column,  posterior  horn  of  opposite  side,  anterior  horn  same  side  (re- 
flex arc),  lateral  horn  of  same  side,  posterior  horn  of  same  side;  2,  collaterals  from  fibers  in  the 
anterior  and  lateral  columns;  3,  collaterals  of  descending  pyramidal  fibers  ending  around  motor 
cells  in  anterior  horn.  . 


are  integral  parts  may  be  classified  physiologically  as  motor,  vaso-motor, 
secretor,  viscero-motor  and  pilo-motor.  Collections  or  groups  of  such  cells 
are  termed  "centers." 

The  afferent  cells  are  largely  sentient  or  receptive  in  function,  inasmuch 
as  the  excitations  brought  to  the  spinal  cord  by  the  afferent  ner\'es  in  the 
dorsal  roots  from  the  general  periphery  are  received  by  them  and  transmitted 
by  and  through  their  axons  to  the  cortex  of  the  cerebrum,  where  they  are 
translated  into  conscious  sensations.  As  the  nerve-fibers  in  the  dorsal  roots 
of  the  spinal  nerves  are  classified,  in  accordance  with  the  sensations  to  which 
they  give  rise,  as  sensor,  thermal,  tactile,  etc.,  so  these  nerve-cells  may  be 
similarly  classified  according  as  they  transmit  their  excitations  to  those 


498 


TEXT-BOOK  OF  PHYSIOLOGY. 


specialized  areas  in  the  cerebral  cortex  in  which  these  different  sensations 
arise. 

Structure  of  the  White  Matter. — A  transverse  section  of  the  cord 
shows  that  the  white  matter  completely  covers  the  gray  matter  except  where 
the  posterior  horns  reach  the  surface.  Anteriorly  the  white  matter  of  each 
lateral  half  is  connected  by  a  narrow  strip  or  bridge  of  white  matter,  the 
anterior  commissure.  Microscopic  examination  shows  that  the  white  matter 
is  composed  of  vertically  disposed  medullated  nerve-libers  which  are  devoid 
of  a  neurilemma.  These  fibers  are  supported  partly  by  a  framework  of 
connective  tissue,  and  partly  by  neuroglia.  The  white  matter  of  each  side 
of  the  cord  is  anatomically  divided  into  an  anterior,  a  lateral,  and  a  posterior 
column  by  the  anterior  and  posterior  roots  of  the  spinal  nerves. 


Fig. 


52. — Transection  of  the  Cervical  Spinal  Cord  showing  Its  Chief  Subdivisions. 

{After  Mills.) 


Classification  of  the  Nerve-fibers. — From  a  study  of  the  embryologic 
development  of  the  white  matter  and  of  the  degenerative  changes  which  follow 
its  pathologic  and  experimental  destruction,  it  has  been  differentiated  into 
a  number  of  specialized  tracts  which  have  different  origins,  destinations, 
and  functions.  Some  of  the  more  important  tracts  are  shown  in  Fig.  232. 
They  may  be  divided,  however,  into  efferent,  afferent,  and  associative  libers. 

I.  The  anterior  column,  comprising  that  portion  between  the  anterior 
longitudinal  fissure  and  the  anterior  roots,  has  been  subdivided  into: 

{a)  The  direct  pyramidal  tract,  or  column  of  Tiirck.  This  tract  borders 
the  longitudinal  fissure  and  extends  from  the  upper  extremity  of  the  cord  as 
far  down  as  the  mid-thoracic  region.  From  above  downward  this  tract 
diminishes  in  size,  for  the  reason  that  its  libers  or  their  collaterals  cross  at 
successive  levels  to  the  opposite  side  of  the  cord  by  way  of  the  anterior  com- 
missure to  enter  the  gray  matter  of  the  anterior  horn.  These  fibers  are  the 
continuations  of  fibers  which  take  their  origin  in  cells  which  are  located  in 


THE  SPINAL  CORD.  499 

the  cortex  of  the  cerebral  hemisphere  of  the  same  side.  The  terminal 
filaments  of  these  fibers  or  axons  are  in  physiologic  relation  either  directly 
or  indirectly  through  intercalated  neuron  cells  with  the  dendrites  of  the 
cornual  cells.  When  divided  in  any  part  of  their  course,  these  fibers  undergo 
descending  degeneration. 

(6)  The  antero-lateral  ground  bundle  or  root  zone.  This  tract  lies  external 
to  the  pyramidal  tract,  surrounds  the  anterior  horn  of  the  gray  matter,  and 
extends  throughout  the  length  of  the  cord.  It  is  composed  of  short  com- 
missural or  associative  fibers  which  come  from  nerve-cells  in  the  gray  matter 
from  the  same  and  opposite  sides  of  the  cord.  After  entering  the  white 
matter  they  divide  into  two  branches,  pursue  opposite  directions,  then  re- 
enter the  gray  matter  at  higher  and  lower  levels  and  come  into  relation  with 
other  nerve-cells: 

2.  The  lateral  column,  comprising  that  portion  between  the  ventral 
and  dorsal  roots,  has  been  divided  into: 

(a)  The  antero-lateral  tract  of  Gowers.  This  tract  is  somewhat  crescentic 
in  shape  and  situated  on  the  lateral  aspect  of  the  cord  external  to  the  antero- 
lateral root  zone.  It  extends  throughout  the  entire  length  of  the  cord. 
When  divided  it  undergoes  ascending  degeneration,  which  would  indicate 
that  the  axons  originate  in  nerve-cells  in  the  gray  matter.  This  tract  is 
therefore  probably  afferent  in  function. 

The  majority  of  the  fibers  composing  this  tract  on  reaching  the  pons 
turn  backward,  pass  through  the  superior  medullary  velum  to  terminate  in 
the  dorsal  vermis  of  the  cerebellum. 

{h)  The  lateral  limiting  tract.  This  tract,  which  is  quite  narrow,  lies 
close  to  the  external  border  of  the  gray  matter.  It  is  composed  of  fibers 
which  do  not  degenerate  to  any  considerable  extent  after  transverse  section 
and  are  in  all  probability  associative  fibers  which  come  from  nerve  cells  in 
the  gray  matter  to  re-enter  at  lower  and  higher  levels.  It  is  also  believed  by 
some  investigators  that  the  anterior  portion  contains  efferent  and  the  pos- 
terior portion  afferent  fibers;  for  this  reason  it  is  frequently  termed  the 
mixed  lateral  tract. 

(c)  The  crossed  pyramidal  tract.  This  tract  occupies  the  posterior  por- 
tion of  the  lateral  column,  though  its  exact  position  varies  somewhat  in 
different  regions  of  the  cord.  In  the  cervical  and  thoracic  regions  it  is 
covered  by  a  layer  of  fibers.  In  the  lumbar  region,  however,  it  comes  to  the 
surface.  From  above  downward  this  tract  gradually  diminishes  in  size, 
for  the  reason  that  its  fibers  and  their  collaterals  enter  the  gray  matter  at 
successive  levels.  The  terminal  branches  of  these  fibers  are  in  close  physi- 
ologic relation  either  directly  or  indirectly  through  intercalated  neuron 
cells  with  the  dendrites  of  the  cornual  cells.  These  fibers  are  the  continua- 
tions of  fibers  which  take  their  origin  in  cells  which  are  located  in  the  cortex 
of  the  cerebral  hemispheres  of  the  opposite  side.  When  divided  in  any  part 
of  their  course,  they  undergo  descending  degeneration.  They  are  therefore 
efferent  neurons  and  of  the  second  order. 

{d)  The  direct  cerebellar  tract,  or  column  of  Flechsig.  This  tract  is 
situated  on  the  surface  of  the  lateral  column  external  to  the  crossed  pyramidal 
tract.  It  slightly  increases  in  size  from  below  upward.  It  is  composed  of 
fibers  the  cells  of  which  are  found  on  the  inner  side  and  base  of  the  posterior 


500  TEXT-BOOK  OF  PHYSIOLOGY. 

horn  (Clark's  vesicular  column).  From  this  origin  the  fibers  pass  obliquely 
outward  to  the  surface  and  then  directly  upward  to  terminate,  as  its  name 
implies,  in  the  cerebellum.  Decussation  of  these  fibers  takes  place  in  the 
superior  vermiform  lobe  of  the  cerebellum.  When  divided  this  tract  degener- 
ates upward.  It  is  therefore  in  all  probability  an  afferent  tract  and  of  the 
second  order. 

3.  The  posterior  column,  comprising  that  portion  between  the  dorsal 
roots  and  the  posterior  longitudinal  fissure,  has  been  subdivided  into : 

(a)  The  poster o-external  tract  of  Burdach.  This  tract  lies  just  within 
the  posterior  horns.  A  portion  of  this  tract  is  composed  of  ground  fibers 
which,  though  vertically  disposed,  have  but  a  short  course.  They  take  their 
origin  in  cells  in  the  gray  matter,  and  after  entering  this  tract  divide  into 
ascending  and  descending  branches,  which  with  their  collaterals  re-enter 
the  gray  matter  at  different  levels.  Another  portion  of  this  tract  is  made  up 
of  nerve-fibers  derived  from  the  dorsal  roots  of  the  spinal  nerves,  which 
cross  this  column  toward  the  median  line  in  an  oblique  or  horizontal  direc- 
tion. The  fibers  of  the  upper  portion  of  this  tract  terminate  around  the 
nucleus  cuneatus  at  the  medulla  oblongata.  When  divided,  these  fibers 
degenerate  for  but  a  short  distance.  The  ground  fibers  are  probably  as- 
sociative in  function. 

{b)  The  postero-internal  tract,  or  column  of  Goll.  This  tract  is  separated 
from  the  former  by  a  septum  of  connective  tissue  which  is  most  marked 
above  the  eleventh  thoracic  segment.  The  fibers  which  compose  this  tract 
are  long  and  derived  for  the  most  part  from  the  dorsal  roots  of  the  spinal 
nerves  of  the  same  side.  This  is  shown  by  the  fact  that  division  of  these 
roots  central  to  the  ganglion  is  followed  by  ascending  degeneration  of  the 
column  of  Goll  as  far  as  the  nucleus  gracilis  in  the  medulla  oblongata. 
Fibers  derived  from  cells  in  the  gray  matter  are  also  contained  in  this  column. 
This  tract  is  largely  afferent  in  function. 

(c)  Lissauer''s  tract.  This  tract  embraces  the  tip  of  the  posterior  horn 
and  is  composed  principally  of  fibers  from  the  dorsal  roots  of  the  spinal 
nerves.  After  entering  the  tract  the  fibers  di\dde  into  ascending  and  de- 
scending branches,  w^hich  finally  terminate  around  cells  in  the  posterior 
horn. 

In  addition  to  the  tracts  described  in  foregoing  paragraphs  a  number 
of  small  narrow  tracts  have  been  discovered  in  different  regions  of  the  spinal 
cord  the  functional  significance  of  which,  however,  has  not  been  determined. 
Of  these  may  be  mentioned: 

1.  The  antero-lateral  tract  of  Marchi  and  Lowenthal,  situated  at  the 
anterior  and  inner  angle  of  the  anterior  column,  which  degenerates  down- 
ward after  removal  of  one-half  of  the  cerebellum. 

2.  The  comma  tract,  a  narrow  bundle  of  fibers  situated  in  the  anterior 
portion  of  the  column  of  Burdach.  When  it  is  divided  it  degenerates  down- 
ward. 

3.  The  septo-marginal  tract,  an  oval-shaped  tract  situated  along  the 
margin  of  the  posterior  longitudinal  fissure. 

4.  The  cornu-commissitral  tract  found  along  the  border  of  the  anterior 
portion  of  the  posterior  column  as  far  forward  as  the  posterior  commissure. 
Both  of  these  tracts  are  best  developed  in  the  lumbosacral  region.     They 


THE  SPINAL  CORD.  501 

arise    from   nerve-cells   in    the    gray    matter.     They   undergo    descending 
degeneration  when  divided,  but  not  after  division  of  the  dorsal  roots. 

The  Relation  of  the  Spinal  Nerves  to  the  Spinal  Cord. — The  spinal 
nerves  present  near  the  spinal  cord  two  divisions  which  from  their  connection 
with  the  anterior  or  ventral  and  the  posterior  or  dorsal  surfaces  are  known 
as  the  ventral  and  dorsal  roots. 

The  ventral  roots  are  the  axons  of  various  groups  of  nerve-cells  situated 
in  the  anterior  horns  of  the  gray  matter.  From  their  origin  these  axons 
pass  almost  horizontally  forward  through  the  anterior  column  in  three 
distinct  bundles.  After  emerging  from  the  cord  they  curve  downward  and 
backward  to  join  the  dorsal  roots. 

The  dorsal  roots  are  the  centrally  directed  axons  of  nerve-cells  in  the 
spinal  ganglia.  After  entering  the  cord  they  divide  into  two  main  groups, 
a  lateral  and  a  mesial.  A  portion  of  the  lateral  group  enters  the  posterior 
horn  directly  through  the  caput  cornu;  the  other  portion  turns  upward  and 
runs  through  Lissauer's  tract  and  ultimately  enters  the  posterior  horn.  The 
mesial  group  passes  into  the  postero-extemal  column  (Burdach),  where 
the  fibers  divide  into  descending  and  ascending  branches.  The  former 
probably  constitute  the  comma  tract,  the  terminal  branches  of  which  sur- 
round cells  in  the  gray  matter;  the  latter  (ascending)  cross  the  column 
obliquely  and  enter  the  postero-internal  column  (GoU),  in  which  they  pass 
upward  to  terminate  around  the  cells  of  the  nucleus  gracilis  of  the  same  side. 
As  these  root  fibers  pass  up  and  down  the  cord,  collateral  branches  are 
given  off  which  enter  the  gray  matter  at  successive  levels  and  come  into 
physiologic  relation  with  the  cells  of  Clark's  vesicular  column  on  the  same  and 
opposite  sides  and  with  the  cells  of  the  anterior  horn. 

The  peripherally  directed  axons  of  the  nerve-cells  in  the  spinal  nerve 
ganglia  become  associated  with  the  axons  of  the  ventral  roots  and  together 
they  pass  as  a  spinal  nerve  to  peripheral  organs. 

The  ventral  root  axons  are  distributed  to  skeletal  muscles,  blood-vessels, 
glands  and  viscera.  The  dorsal  root  axons  are  distributed  to  skin,  mucous 
membranes,  and  muscles.  The  classification  of  the  nerve-fibers  in  the 
ventral  and  dorsal  roots  in  accordance  w^ith  the  functions  they  subserve 
will  be  found  on  pages  97,  98. 

Though  both  the  eiJerent  and  afferent  fibers  of  the  spinal  nerves  are 
directly  connected  with  nerve-cells  in  the  spinal  cord,  they  are  also  indirectly 
connected  by  efferent  and  afferent  nerve-tracts  with  the  cerebral  cortex. 

Experimentally,  it  has  been  determined  that  the  anterior  or  ventral 
roots  contain  all  the  e_fferent  fibers,  the  posterior  or  dorsal  roots  all  the  a^erent 
fibers.     The  proofs  in  support  of  this  view  are  as  follows: 

Stimulation  oj  the  ventral  root  fibers  produces: 

1.  Tetanic  contraction  of  skeletal  muscles. 

2.  Discharge  of  secretions  from  glands. 

3.  Variations  in  the  degree  of  the  contraction,  the  tonus,  of  the  muscle 

walls  of  the  peripheral  arteries  either  in  the  way  of  augmentation 
or  inhibition. 

4.  Variations  in  the  degree  of  the  contraction,  the  tonus,  of  the  muscle 

walls  of  certain  viscera  either  in  the  way  of  augmentation  or  in- 
hibition. 


502 


TEXT-BOOK  OF  PHYSIOLOGY. 


Division  of  the  ventral  root  fibers  is  followed  by: 

1.  Relaxation  of  skeletal  muscles  and  loss  of  movement. 

2.  Cessation  in  the  discharge  of  secretions  from  glands. 

3.  Temporary  dilatation  and  loss  of  the  tonus  of  blood-vessels. 

4.  Temporary   impairment   of    the    normal    activities   of    the  visceral 

muscles  from  loss  of  central  nerve  control;  the  degree  of  impair- 
ment depending  on  the  nature  of  the  viscus  involved. 

Peripheral  stimulation  of  the  dorsal  root  fibers  produces : 

1.  Reflex  excitation  of  spinal  centers,  in  consequence  of  which  there  is  an 

increased  activity  of  skeletal  muscles,  blood-vessels,  glands,  and 
visceral  walls. 

2.  Reflex  inhibition  of  spinal  nerve  centers,  in  consequence  of  which 

there  may  be  a  decrease  in  the  activities  of  skeletal  muscles,  blood- 
vessels, glands,  and  viscera. 

3.  Sensations  of  touch,  temperature,  pressure,  and  pain. 

4.  Sensations  of  the  duration  and  direction  of  muscle  movements,  of  the 

resistance  offered  and  of  the  position  of  the  body  or  of  its  individual 
parts  (muscle  sensations). 

Division  oj  the  dorsal  root  fibers  is  followed  by: 

1.  Loss  of  the  power  of  exciting  or  inhibiting  reflexly  the  activities  of 

spinal  nerve  centers  and  in  consequence  a  loss  of  the  power  of 
exciting  or  inhibiting  the  activities  of  peripheral  organs. 

2.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

The  ventral  roots  are  therefore  efferent  in  function,  transmitting  nerve 
impulses  from  the  spinal  cord  to  the  peripheral  organs  which  excite  them  to 
activity. 

The  dorsal  roots  are  afferent  in  function,  transmitting  nerve  impulses 
from  the  general  periphery  to  {a)  the  spinal  cord  where  they  excite  its  con- 
tained nerve-centers  to  activity  or  to  a  more  or  less  complete  cessation  of 
activity  (inhibition),  and  {b)  to  the  cerebrum  where  they  excite  its  centers 
to  activity  with  the  development  of  sensations. 

Segmentation  of  the  Spinal  Cord. — For  the  elucidation  of  many 
problems  connected  with  the  physiologic  actions  of  the  spinal  cord,  as  well  as 
of  the  symptoms  which  follow  its  pathologic  impairment,  it  will  be  found 
helpful  to  consider  the  cord  as  consisting  physiologically  of  a  series  of  segments 
placed  one  above  the  other,  the  number  of  segments  corresponding  to  the 
number  of  spinal  nerves.  Each  spinal  segment  would  therefore  comprise 
that  portion  of  the  cord  to  which  is  attached  a  pair  of  spinal  nerves.  The 
nerve-cells  in  each  segment  are  in  histologic  and  physiologic  relation  with 
definite  areas  of  the  body,  embracing  muscles,  blood-vessels,  glands, 
skin,  etc. 

If  the  exact  distribution  of  the  nerves  of  any  segment  were  known, 
its  function  could  be  readily  stated.  By  virtue  of  this  segmentation  it 
becomes  possible  for  each  segment  to  act  independently  of  or  in  cooperation 
with  other  segments  near  or  remote,  with  which  they  are  associated  by  the 
intrinsic  or  associative  cells  and  their  axons;  and  by  the  same  cooperative 
action  the  spinal  cord  itself  is  enabled  to  act  as  a  unit. 


THE  SPINAL  CORD.  503 

THE  FUNCTIONS  OF  THE  SPINAL  CORD. 

Anatomic  investigation  has  demonstrated  that  the  spinal  cord  .is  com- 
posed of  a  series  of  segments  which  are  associated  through  their  related 
spinal  nerves  with  the  organs  and  tissues  of  definite  areas  of  the  body. 
Physiologic  investigation  has  also  demonstrated  that  the  segments  by  reason 
of  the  presence  of  nerA'e-cells  and  nerve  fibers  may  be  regarded  as  composed 
of: 

1.  Nerve  centers,  each  of  which  has  certain  special  functions,  and 

2.  Conduction  paths  by  which  these  centers  are  brought  into  relation  not 

only  with  one  another,  but  with  the  cerebrum  and  its  subordinate  or 
underlying  parts,  e.g.,  the  medulla  oblongata,  pons  varolii  and 
cerebellum. 

A.     THE  SPINAL  CORD  SEGMENTS  AS  LOCAL  NERVE  CENTERS. 

The  efferent  cells  of  the  spinal  segments  are  the  immediate  sources  of 
the  ner^'e  energy  that  excites  activity  in  skeletal  muscles,  glands,  vascular, 
and  to  some  extent  visceral  muscles. 

The  discharge  of  their  energy  may  be  caused : 

1.  By  variations  in  the  composition  of  the  blood  or  lymph  by  which  they 

are  surrounded  or  as  the  outcome  of  a  reaction  between  the  chemic 
constituents  of  the  lymph  on  the  one  hand  and  the  chemic  constituents 
of  the  nerve-cell  on  the  other  hand.  The  excitation  of  the  cell  thus 
occasioned  is  termed  automatic  or  autochthonic  excitation. 

2.  By  the  arrival  of  nerve  impulses,  coming  through  afferent  nerves  from 

the  general  periphery,  skin,  mucous  membrane,  etc. 

3.  By  the  arrival  of  nerve  impulses  descending  the  spinal  cord  from  cells  in 

the  cortex  of  the  cerebrum  or  subordinate  regions.     The  excitation  in 
the  former  instances  is  said  to  be  reflex  or  peripheral  in  origin;  in  the 
latter  instance  direct  or  cerebral  in  origin.     In  the  direct  or  cerebral 
excitations  the  skeletal  muscle  movements  are  due  to  volitional,  the 
gland  discharges  and  vascular  and  visceral  muscle  movements  to  emo- 
tional phases  of  cerebral  activity. 
Automatic  Activity. — By  this  expression  is  meant  a  discharge  of  energy 
from  the  spinal  ner\' e-cells  occasioned  by  (a)  a  change  in  the  chemic  composi- 
tion of  the  blood  or  lymph  by  which  they  are  surrounded  or  probably  a 
reaction  between  the  constituents  of  the  lymph  and  the  constituents  of  the 
nerve-cell  or  (b)  the  development  within  the  cell  of  a  stimulus,  the  so- 
called  "inner  stimulus,"  the  outcome  of  metabolic  activity. 

As  no  effect  arises  without  a  sufficient  cause  the  term  automatic  has  been 
objected  to  and  the  term  autochthonic  has  been  suggested,  as  more  nearly 
expressing  the  facts  stated.  A  center  so  acting  could  not  be  regarded  as 
primarily  a  center  for  reflex  activity,  however  much  it  might  be  influenced 
secondarily  by  afferent  impulses.  If  the  cell  excitation  is  continuous  though 
variable  from  time  to  time,  it  is  said  to  possess  tonus  and  the  organ  or  tissue 
thus  excited  is  also  said  to  possess  tonus  or  to  be  in  a  state  of  tonic  activity. 
If  the  cell  discharge  is  intermittent  in  character  it  imparts  to  certain  muscles, 
e.g.,  the  respiratory  muscles,  a  rhythmic  activity.  It  must,  however,  be  kept 
in  mind  that  the  tonus  of  nerve  centers  as  well  as  of  peripheral  organs  can  also 
be  developed  and  maintained  by  the  inflow  of  nerve  impulses  transmitted 


504  TEXT-BOOK  OF  PHYSIOLOGY. 

from  the  periphery.  The  reason  for  the  belief  that  the  cord  and  its  u^per 
prolongation,  the  medulla  oblongata,  are  endowed  with  autochthonic  activity 
is  based  on  the  fact  that  certain  peripheral  organs  are  in  a  state  of  contin- 
uous activity  and  apparently  uninfluenced  to  any  marked  extent  except  tem- 
porarily by  nerve  impulses  transmitted  to  the  cord  through  afferent  nerves. 
As  illustrations  of  such  continuous  activity  may  be  mentioned:  (a)  the 
contractionof  the  abductor  muscle  of  the  larynx  (the  posterior  crico-arytenoid) 
whereby  the  vocal  membranes  are  separated  and  the  glottis  kept  open  under 
all  circumstances  except  during  the  emission  of  a  vocal  sound;  (b)  the  con- 
traction of  the  dilatator  muscle  of  the  iris;  (c)  the  contraction  of  the  anal 
and  vesic  sphincters;  (d)  the  periodic  contractionof  the  respiratory  muscles 
(see  page  417);  (e)  the  acceleration  of  the  heart-beat  (page  313). 

Though  automatic  activity  of  the  spinal  cord  is  yet  upheld  by  some 
physiologists,  the  fact  must  be  recognized  that  with  increasing  knowledge 
of  reflex  activities  many  phenomena  previously  regarded  as  automatic  have 
been  found  to  be  dependent  on  peripheral  stimulation  and  therefore  reflex 
in  origin.  Whether  this  will  eventually  be  found  true  for  all  instances  of 
so-called  automatic  or  autochthonic  activity  remains  to  be  seen.  Among 
the  phenomena  removed  from  the  sphere  of  automatic  to  the  sphere  of 
reflex  activity  may  be  mentioned  muscle  tonus,  vascular  tonus  and,  trophic 
tonus. 

Trophic  Tonus. — The  normal  metabolism  of  muscle,  gland,  and 
connective  tissue  which  underlies  the  assimilation  of  food,  the  production 
and  storage  of  energy-holding  compounds,  and  the  production  of  new 
compounds,  is  dependent,  in  the  higher  animals  at  least,  on  the  connection 
of  these  tissues  with  the  central  ners^e  system;  for  if  the  efferent  nerves  be 
divided,  not  only  will  they  themselves  undergo  degeneration  in  their  per- 
ipheral portions,  but  the  muscles,  glands,  and  connective  tissues  to  which 
they  are  distributed  will  also  undergo  similar  changes.  This  is  to  be  attrib- 
uted not  merely  to  inactivity,  but  rather  to  a  loss  of  nerve  influence.  It 
would  appear  from  facts  of  this  character  that  the  normal  metabolism  is 
dependent  for  its  continuance  on  nerve  influences.  There  is  no  evidence, 
however,  as  to  the  existence  of  special  trophic  nerves,  separate  from 
those  which  impart  to  glands  and  muscles  their  customary  activities.  The 
trophic  centers  and  the  motor  centers  are  identical,  though  the  two 
modes  of  their  activity  are  separate  and  distinct.  The  activity  of  the  so- 
called  trophic  centers  which  was  at  one  time  believed  to  be  automatic  is 
now  regarded  as  due  to  reflex  influences. 

Vascular  Tonus. — The  arteriole  muscles  throughout  the  vascular 
apparatus  are  also  constantly  in  a  state  of  slight  but  continuous  contraction 
which  assists  in  the  maintenance  of  an  average  arterial  pressure  and  is  due 
to  the  continuous  discharge  of  nen^e  energy  from  the  general  or  dominating 
vasomotor  (constrictor)  center  in  the  medulla  oblongata.  The  automaticity 
of  this  center  has  also  largely  been  discredited;  but  whether  it  is  automatic 
or  not  it  is  capable  of  being  influenced  in  its  activity  not  only  by  variations 
in  the  composition  of  the  blood  but  by  nerve  impulses  reflected  to  it  from  all 
regions  of  the  body  (see  page  372.) 

Muscle  Tonus. — All  the  skeletal  muscles  of  the  body  are  at  all  times^in 
a  state  of  slight  but  continuous  contraction,  termed  tonus,  by  virtue  of  which 


THE  SPINAL  CORD. 


505 


their  efiiciency  as  quickly  responsive  organs  is  increased.  That  such  a 
slight  contraction  is  present  even  in  a  state  of  rest  is  shown  by  the  fact  that 
if  a  muscle  be  divided  in  the  living  animal  the  two  portions  will  contract 
and  separate  to  a  certain  distance.  The  condition  of  the  muscle  was 
formerly  attributed  to  an  automatic  and  continuous  discharge  of  energy  from 
the  nerve-cells.  Brondgeest,  however,  showed  that  this  tonus  is  entirely 
reflex  in  origin  and  immediately  disappears  on  division  of  the  posterior 
roots  of  the  spinal  nerv^es,  which  would  not  be  the  case  if  the  cells  in  the  cord 
were  acting  automatically.  The  afferent  nerves  in  this  reflex  arise  in  the 
muscle  or  its  tendons,  and  the  stimulus  is  the  slight  degree  of  extension  to 
which  the  muscle  is  subjected  in  virtue  of  its  attachments  and  the  ever-varying 
position  of  the  limbs  and  trunk,  (see  page  54.) 

The  tonic  contraction  of  the  visceral  muscles — e.g.,  the  pyloric,  the 
vesical,  the  anal  sphincters — though  regarded  as  automatic  by  some,  is 


sp.c. 


Fig.  233. — D1AGR.A.M:  Showing  the  Structures  Involved  in  the  Production  of  Reflex 
Actions,  G.  Bachman.  r.s.  Receptive  surface;  af.n.  afferent  nerve;  ex.  emissive  or  motor  cells  in 
the  anterior  horn  of  the  gray  matter  of  the  spinal  cord,  sp.c;  ef.n.  efferent  nerves  distributed  to 
responsive  organs,  e.  g.,  directly  to  skeletal  muscles,  sk.m.,  and  indirectly  through  the  interme- 
diation of  sympathetic  ganglia,  sym.g.,  to  blood-vessels,  b.v.,  and  to  glands,  g.  The  nerves 
distributed  to  viscera  are  not  represented. 


probably  reflex  in  origin,  dependent  on  the  arrival  of  afferent  impulses  from 
the  periphery.  It  is  probable  that  future  investigation  will  disclose  the 
existence  and  pathway  of  these  afferent  fibers. 

Reflex  Activity. — It  has  already  been  stated  that  the  nerve-cells  in  the 
spinal  cord  are  capable  of  receiving  and  transforming  afferent  nerv'e  impulses, 
the  result  of  peripheral  stimulation,  into  efferent  nerve  impulses,  which  are 
transmitted  outward  to  skeletal  muscles,  exciting  contraction;  to  glands, 
provoking  secretion;  to  blood-vessels,  changing  their  caliber;  and  to  organs, 
inhibit  ing  or  augmenting  their  activity.  All  such  actions  taking  place  through 
the  spinal  cord  and  medulla  oblongata  independently  of  sensation  or  volition 
are  termed  reflex  actions.  The  mechanism  involved  in  every  reflex  action 
consists  of  at  least  the  following  structures  (Fig.  233) : 


5o6 


TEXT-BOOK  OF  PHYSIOLOGY. 


A  receptive  surface;  e.g.,  skin,  mucous  membrane,  sense  organ,  etc. 
An  afferent  fiber  and  cell. 
An  emissive  cell,  from  which  arises — 
An  efferent  nerve,  distributed  to — 
A  responsive  organ,  as  muscle,  gland,  blood-vessel,  etc. 
In  this  connection  the  reflex  contractions  of  skeletal  muscles  only  will  be 
considered. 

If  a  stimulus  of  sufficient  intensity  be  applied  to  the  receptive  surface, 
there  will  be  developed  in  the  terminals  of  the  afferent  nerve  a  series  of 
nerve  impulses  which  will  be  transmitted  by  the  afferent  nerve  to,  and  re- 
ceived by,  the  dendrites  of  the  emissive  cell  in  the  anterior  horn  of  the  gray 
matter.     With  the  reception  of  these  impulses  there  will  be  a  disturbance  in 

the  equilibrium  of  the  molecules  of  the  cells, 
a  liberation  of  energy,  and  a  transmission  of 
nerve  impulses  outward  through  the  efferent 
nerve  to  the  muscle. 

A  reflex  mechanism  or  arc  of  this  simplicity 
would  subserve  but  a  simple  movement.  The 
majority  of  the  reflexes,  however,  are  ex- 
tremely complex  and  involve  the  cooperation 
and  coordination  of  a  number  of  centers  at 
different  levels  of  the  spinal  cord  and  medulla, 
on  the  same  and  opposite  sides,  and  of  mus- 
cles situated  at  distances  more  or  less  remote 
from  one  another.  The  transference  of  nerve 
impulses  coming  from  a  localized  area  of  a 
receptive  surface,  to  emissive  cells  situated  at 
different  levels  is  accomplished  by  the  inter- 
mediation cf  a  third  neuron  situated  in  the 
gray  matter,  which  is  in  connection  on  the  one 
hand  with  the  central  terminals  of  the  afferent 
nerve  and,  on  the  other  hand  through  colla- 
teral branches  with  the  dendrites  of  the  efferent 
neurons  situated  at  different  levels.  (Fig.  234.) 
A  histologic  and  physiologic  mechanism  of 
this  character  readily  explains  how  a  localized 
stimulation  can  give  rise  to  reflex  actions  extremely  complex  in  character. 
The  reflex  contractions  of  skeletal  muscles  are  best  studied  after  division 
of  the  central  nerve  system  at  the  upper  limit  of  the  spinal  cord.  After 
this  procedure  the  spinal  centers  can  act  independently  of,  and  uninfluenced 
by  either  sensation  or  volitional  efforts  on  the  part  of  the  animal.  Though 
it  is  possible  to  provoke  reflex  contractions  under  such  circumstances  in 
warm-blooded  animals,  they  are,  as  a  rule,  incomplete  and  of  short  duration, 
owing  to  disturbances  of  the  circulation  and  respiration  and  the  consequent 
loss  of  tissue  irritability.  In  frogs  and  in  cold-blooded  animals  generally, 
the  spinal  cord  retains  its  irritability  for  a  long  period  of  time  after  removal 
of  the  brain,  and  therefore  is  well  adapted  to  the  study  of  reflex  actions. 
The  separation  of  the  spinal  cord  from  the  brain  is  readily  effected  by 
destroying  the  medulla  oblongata.     This  can  be  done  by  inserting  a  pin 


Fig.  234. — Diagram  Showing 
THE  Relation  of  the  Third 
Neuron  a,  to  the  Afferent 
Neuron  b,  and  to  the  Efferent 
Neurons  c,  c,  c. — {After  Kdlliker.) 


THE  SPINAL  CORD.  507 

through  the  skin  and  the  occipito-atlantal  membrane  covering  the  space  be- 
tween the  occipital  bone  and  the  atlas,  until  it  strikes  the  bodies  of  the 
vertebrae  below.  If  the  pin  is  properly  directed  it  passes  through  the  medulla. 
Care  should  be  taken  to  avoid  injury  to  the  blood-vessels  on  either  side. 
The  brain  itself  should  then  be  destroyed,  so  as  to  remove  all  conscious- 
ness, by  inserting  the  pin  into  the  brain  cavity  through  the  foramen  mag- 
num, and  giving  it   a  few  rotatory  movements. 

A  frog  so  prepared,  and  placed  on  the  table  and  allowed  to  remain  at 
rest  for  a  few  moments  until  the  shock  of  the  operation  passes  away,  will 
draw  the  limbs  close  to  the  body  and  assume  a  position  not  unlike  that  of  a 
normal  frog.  If  then  the  posterior  limbs  be  extended,  they  will  immediately 
be  drawn  close  to  the  side  of  the  trunk  in  the  usual  flexed  position.  If  the 
toes  are  pinched  with  forceps,  the  foot  will  execute  a  series  of  movements  as 
if  the  frog  were  trying  to  free  itself  from  the  source  of  irritation. 

If  the  frog  be  suspended,  the  limbs,  through  the  force  of  gravity,  will  be 
gradually  extended  and  hang  down  freely.  In  this,  as  in  the  sitting  position, 
the  animal  will  remain  perfectly  quiet  and  will  not  execute  spontaneous 
movements.  Any  stimulus  applied  to  the  skin,  however,  provided  it  is  of 
sufficient  intensity,  will  be  followed  by  a  more  or  less  pronounced  move- 
ment. Mechanic,  chemic,  or  electric  stimuli  applied  to  any  part  of  the 
skin  will  call  forth  the  characteristic  reflex  movements.  Chemic  stimuli 
such  as  weak  solutions  of  sulphuric  or  acetic  acid  placed  on  the  toes  will  be 
followed  by  feeble  flexion  of  the  corresponding  leg,  to  be  succeeded  in  a 
short  time  by  extension.  Stronger  solutions  will  produce  more  extensive 
and  vigorous  movements,  the  foot  at  the  same  time  being  rubbed  against  the 
thigh,  apparently  for  the  purpose  of  freeing  it  from  the  irritant.  Similar 
phenomena  follow  the  application  of  the  acid  to  the  fingers  or  the  trunk. 
As  a  rule,  the  extent  and  complexity  of  the  movements  is,  within  limits, 
proportional  to  the  strength  of  the  stimulus.  By  limiting  the  sphere  of 
action  of  the  stimulus  to  definite  but  different  areas  of  the  skin  a  great  variety 
of  movements,  more  or  less  complex  and  coordinated  and  apparently  pur- 
posive and  defensive  in  character,  can  be  produced.  The  coordinated  and 
purposive  character  of  the  movements  exhibited  by  a  brainless  frog  led 
Pfliiger  to  the  assumption  that  the  spinal  cord  in  this  as  well  as  in  other  cold- 
blooded animals  is  possessed  of  sensorial  functions,  and  endowed  with 
rudimentary  consciousness.  This  view,  however,  is  not  generally  accepted, 
the  movement  being  attributed  to  specialized  mechanisms  in  the  cord, 
partially  inherited,  which  permit  of  one  and  the  same  movement  with 
mechanic  regularity  and  precision,  so  long  as  the  conditions  of  the  experi- 
ment remain  the  same. 

In  warm-blooded  animals  similar  results  may  be  obtained  for  a  short 
time  after  division  of  the  cord,  especially  if  artificial  respiration  is  maintained 
and  the  circulation  of  the  blood  continued.  The  cord  will  then  retain  its 
irritability  for  some  time.  If  the  conditions  of  experimentation  were  favor- 
able, it  is  highly  probable  that  the  human  spinal  cord  would  execute  similar 
movements.  Thus  it  was  observed  by  Robin  in  a  man  who  had  been  decap- 
itated that  reflex  muscle  contractions  could  be  elicited  by  stimulating  the  skin 
after  the  lapse  of  an  hour  after  execution.  "While  the  right  arm  was  lying 
extended  by  the  side,  with  the  hand  about  25  centimeters  distant  from  the 


5o8  TEXT-BOOK  OF  PHYSIOLOGY. 

upper  part  of  the  thigh,  I  scratched  with  the  point  of  a  scalpel  the  skin  of 
the  chest  at  the  areola  of  the  nipple,  for  a  space  of  loor  ii  centimeters  in 
extent,  without  making  any  pressure  on  the  subjacent  muscles.  We  im- 
mediately saw  a  rapid  and  successive  contraction  of  the  great  pectoral 
muscle,  the  biceps,  probably  the  brachialis  anticus,  and  lastly  the  muscles 
covering  the  internal  condyle.  The  result  was  a  movement  by  which  the 
whole  arm  was  made  to  approach  the  trunk;  with  rotation  inward  and  half- 
flexion  of  the  forearm  upon  the  arm;  a  true  defensive  movement,  which 
brought  the  hand  toward  the  chest  as  far  as  the  pit  of  the  stomach.  Neither 
the  thumb,  which  was  partially  bent  toward  the  palm  of  the  hand,  nor  the 
fingers,  which  were  half  bent  over  the  thumb,  presented  any  movements. 
The  arm  being  replaced  in  its  former  position,  we  saw  it  again  execute  a 
similar  movement  on  scratching  the  skin,  in  the  same  manner  as  before,  a 
little  below  the  clavicle.  This  experiment  succeeded  four  times,  but  each 
time  the  movement  was  less  extensive;  and  at  last  scratching  the  skin  over 
the  chest  produced  only  contractions  in  the  great  pectoral  muscle  which 
hardly  stirred  the  limb"  (Dalton). 

Laws  of  Reflex  Action  (Pfluger). 

1.  Law  oj  Unilaterality. — If  a  feeble  irritation  be  applied  to  one  or  more 

sensory  nerves,  movement  takes  place  usually  on  one  side  only,  and 
that  the  same  side  as  the  irritation. 

2.  Law  oj  Symmetry. — If  the  irritation  becomes  suflficiently  intense,  motor 

reaction  is  manifested,  in  addition,  in  corresponding  muscles  of  the 
opposite  side  of  the  body. 

3.  Law  oj  Intensity. — Reflex  movements  are  usually  more  intense  on  the  side 

of  irritation;  at  times  the  movements  of  the  opposite  side  equal  them  in 
intensity;  but  they  are  usually  less  pronounced. 

4.  Law  oj  Radiation. — If  the  excitation  still  continues  to  increase,  it  is  pro- 

pagated upward,  and  motor  reaction  takes  place  through  centrifugal 
nerves  coming  from  segments  of  the  cord  higher  up. 

5.  Law  oj  Generalization. — When  the  irritation  becomes  very  intense,  it  is 

propagated  to  the  medulla  oblongata;  motor  reaction  then  becomes 
general,  and  it  is  propagated  up  and  down  the  cord,  so  that  all  the  mus- 
cles of  the  body  are  thrown  into  action,  the  medulla  oblongata  acting 
as  a  focus  whence  radiate  all  reflex  impulses. 

Special  Reflex  Movements. — Among  the  reflexes  connected  with  the 
more  superficial  portions  of  the  body  there  are  some  which  are  so  frequently 
either  increased  or  diminished  in  pathologic  conditions  of  the  spinal  cord 
that  their  study  affords  valuable  indications  as  to  the  seat  and  character  of 
the  lesions.     They  may  be  divided  into: 

1.  The  skin  or  superficial  reflexes. 

2.  The  tendon  or  deep  reflexes. 

3.  The  organ  reflexes. 

The  skin  reflexes,  characterized  by  contraction  of  underlying  muscles, 
are  induced  by  stimulation  of  the  skin — e.g.,  pricking,  pinching,  scratching, 
etc.     The  following  are  the  principal  skin  reflexes: 
I.  Plantar  reflex  consisting  of  contraction  of  the  muscles  of  the  foot,  induced 


THE  SPINAL  CORD.  509 

by  stimulation  of  the  sole  of  the  foot;  it  involves  the  integrity  of  the 
reflex  arc  through  the  second  and  third  sacral  nerves. 

2.  Gluteal  reflex,  consisting  of  contraction  of  the  glutei  muscles  when  the  skin 

over  the  buttock  is  stimulated;  it  takes  place  through  the  segments 
giving  origin  to  the  fourth  and  fifth  lumbar  nerves. 

3.  Cremasteric  reflex,  consisting  of  a  contraction  of  the  cremaster  muscle 

and  a  retraction  of  the  testicle  toward  the  abdominal  ring  when  the  skin 
on  the  inner  side  of  the  thigh  is  stimulated;  it  depends  upon  the  integ- 
rity of  the  segments  giving  origin  to  the  first  and  second  lumbar 
nerves. 

4.  Abdominal  reflex,  consisting  of  a  contraction  of  the  abdominal  muscles 

when  the  skin  upon  the  side  of  the  abdomen  is  gently  scratched;  its 
production  requires  the  integrity  of  the  spinal  segments  from  the 
eight  to  the  twelfth  dorsal  nerves. 

5.  Epigastric  reflex,  consisting  of  a  slight  muscular  contraction  in  the  neigh- 

borhood of  the  epigastrium  when  the  skin  between  the  fourth  and 
sixth  ribs  is  stimulated;  it  requires  the  integrity  of  the  cord  between 
the  fourth  and  seventh  dorsal  nerves. 

6.  Scapular  reflex  consisting  of  a  contraction  of  the  scapular  muscles  when 

the  skin  between  the  scapulae  is  stimulated;  it  depends  upon  the  in- 
tegrity of  the  cord  between  the  fifth  cervical  and  third  dorsal  nerves. 
The  skin  or  superficial  reflexes,  though  variable,  are  generally  present  in 
health.     They  are  increased  or  exaggerated  when  the  gray  matter  of  the 
cord  is  abnormally  excited,  as  in  tetanus,  strychnin-poisoning,  and  disease 
of  the  lateral  columns. 

The  so-called  ^'tendon  reflexes,''''  are  characterized  by  the  contraction  of  a 
muscle  and  are  elicited  by  a  sharp  tap  on  its  tendon.  They  are  also  of  much 
value  in  the  diagnosis  of  lesions  of  the  cord.  The  fundamental  condition 
for  the  production  of  the  tendon  reflex  is  a  certain  degree  of  tonus  of  the 
muscle,  which  is  a  true  reflex,  maintained  by  afferent  nerve  impulses 
developed  in  the  muscle  itself  in  consequence  of  its  extension  and  hence 
compression  of  the  end-organs  of  the  afferent  nerves,  the  muscle  spindles. 
When  the  muscle  is  passively  extended,  as  it  is  when  the  reflex  is  to  be 
elicited,  there  is  an  exaltation  of  the  tonus  and  an  increase  in  the  irri- 
tability. To  this  condition  of  the  muscle  due  to  passive  tension,  the  term 
myotatic  irritability  has  been  given.  If  the  muscle  extension  be  now  sud- 
denly increased,  as  it  is  when  the  tendon  is  sharply  tapped,  the  increased 
compression  of  the  muscle  spindles  will  develop  additional  afferent  impulses 
which  after  transmission  to  the  spinal  cord  will  give  rise  to  contraction  of  the 
corresponding  muscle. 

The  following  are  the  principal  forms  of  the  tendon  reflexes : 
I.  The  Patellar  tendon  reflex  or  hiee-jerk.  This  phenomenon  is  characterized 
by  a  contraction  of  the  extensor  muscles  of  the  thigh  imparting  to  the  leg  a 
forward  movement  when  the  ligamentum  patellae  is  struck  between  the 
patella  and  tibia.  This  reflex  is  best  observed  when  the  legs  are  freely 
hanging  over  the  edge  of  a  table.  The  patella  reflex  is  generally  present 
in  health,  being  absent  in  only  2  per  cent. ;  it  is  greatly  exaggerated  in 
lateral  sclerosis,  in  descending  degeneration  of  the  cord;  it  is  absent  in 
locomotor  ataxia  and  in  atrophic  lesions  of  the  anterior  gray  cornua. 


5IO  TEXT-BOOK  OF  PHYSIOLOGY. 

2.  The  tendo  achillis  reflex  or  ankle- jerk.     This  is  characterized  by  a  con- 

traction of  the  gastrocnemius  muscle  and  a  flexion  of  the  foot.  To 
elicit  the  contraction  the  leg  should  be  extended  and  the  dorsum  of  the 
foot  be  pressed  toward  the  leg  so  as  to  give  to  the  gastrocnemius 
a  slight  degree  of  extension.  If  the  tendon  be  now  sharply  struck  a  quick 
extension  of  the  foot  is  produced. 

3.  Ankle  clonus. — This  consists  of  a  series  of  rhythmic  reflex  contractions 

of  the  gastrocnemius  muscle,  varying  in  frequency  from  six  to  ten  per 
second.  To  elicit  this  reflex,  pressure  is  made  upon  the  sole  of  the  foot 
so  as  to  flex  the  foot  at  the  ankle  suddenly  and  energetically,  thus  putting 
the  tendo  Achillis  and  the  gastrocnemius  muscle  upon  the  stretch. 
The  rhythmic  movements  thus  produced  continue  so  long  as  the  ten- 
sion within  limits  is  maintained.  Ankle  clonus  is  never  present  in 
health,  but  is  very  marked  in  lateral  sclerosis  of  the  cord. 

The  toe  reflex,  peroneal  reflex,  and  wrist  re/?gx  are  also  present  in  sclerosis 
of  the  lateral  columns  and  in  the  late  rigidity  of  hemiplegia. 

The  organ  reflexes,  e.g.,  the  activities  of  the  genito-urinary  organs,  the 
stomach,  intestines,  gall-bladder,  etc.,  which  are  induced  by  peripheral 
stimulation  have  been  considered  in  connection  with  the  physiologic  action 
of  these  organs.  The  genito-urinary  center  is  located  in  the  lumbar  region 
of  the  spinal  cord.  In  diseased  conditions  of  this  region  the  genito-urinary 
reflexes  are  sometimes  increased,  at  other  times  decreased  or  even  abolished. 

Reflex  Irritability. — The  general  irritability  or  c[uickness  of  response  of 
the  mechanism  involved  in  reflex  action  can  be  approximately  determined 
by  observation  of  the  length  of  time  that  elapses  between  the  application  of 
a  minimal  stimulus  and  the  appearance  of  the  muscle  response.  The  method 
of  Tiirck  is  sufficiently  accurate  for  general  purposes.  This  consists  in 
suspending  a  frog,,  after  removal  of  the  brain,  and  immersing  the  foot  in 
a  0.2  per  cent,  solution  of  sulphuric  acid.  The  time  is  determined  by  means 
of  a  metronome  beating  one  hundred  times  a  minute.  Stimulation  of  the 
skin  can  also  be  effected  by  the  induced  electric  current,  as  suggested  by 
Gaskell.  A  single  shock  is,  however,  ineffective.  The  currents  must 
follow  each  other  with  a  rapidity  sufficient  to  give  rise  to  a  summation  of 
effects  in  the  nerve-centers  which  will  then  be  followed  by  a  muscle  response. 
It  is  highly  probably  that  the  chemic  stimulation  gives  rise  to  a  similar  sum- 
mation of  effects. 

The  period  of  time  thus  obtained  is  distributed  over  the  entire  mechan- 
ism. The  true  reflex  time,  however — i.e.,  the  time  occupied  in  the  passage 
of  the  nerv'e  impulses  through  the  spinal  mechanism — is  shorter  and  is  obtained 
by  subtracting  from  the  whole  period  the  time  occupied  by  the  passage  of  the 
impulses  through  the  afferent  and  efferent  nerves  as  well  as  the  latent  period 
of  muscle  contraction.  This  corrected  period,  the  true  reflex  time,  has  been 
found  to  be  twelve  times  longer  than  the  time  occupied  by  the  passage  of  the 
nerve  impulse  through  the  nerves,  including  the  latent  period  of  the  muscle. 

The  reflex  irritability  is  increased  by : 
I.  Separation  of  the  Brain  from  the  Cord. — This  is  at  once  followed  by  an  in- 
crease in  reflex  irritability,  and  is  taken  as  evidence  that  the  brain 
normally  exerts  an  inhibitor  influence  over  the  reflex  centers  of  the  cord. 


THE  SPINAL  CORD. 


511 


olf.L 


The  same  increase  is  observed  upon  hemisection  of  the  cord,  though 
the  increase  is  limited  to  the  same  side. 

The  Toxic  Action  of  Drugs. — Many  drugs  increase  the  irritabiUty  of  the 
spinal  cord,  though  the  most  efficient  is  strychnin.  This  drug,  even  in 
small  doses,  increases  the  irritability  to  such  an  extent  that  a  minimal  stim- 
ulus is  sufficient  to  call  forth  spasmodic  contractions  of  all  the  skeletal 
muscles.  Under  its  influence  the  usual  coordinated  reflexes  disappear 
and  are  succeeded  by  incoordinated  reflexes.  The  explanation  of  this 
fact  is  believ^ed  to  be  a  diminution  in 
the  resistance  offered  by  the  cord  to 
the  passage  of  the  afferent  impulses 
rather  than  to  a  direct  stimulation  of 
the  efferent  cells.  So  much  is  this 
resistance  decreased  that  the  nervx 
impulses  instead  of  being  confined 
to  their  accustomed  paths,  are  ra- 
diated in  all  directions.  Absolute 
repose  of  the  animal  and  the  ex- 
clusion of  all  external  stimuli  greatly 
diminish  the  tendency  to  the  occur- 
rence of  spasms. 

Degeneration  oj  the  Pyramidal  Tracts. 
— In  primary  lateral  sclerosis,  a 
pathologic  condition  characterized 
primarily  by  a  degeneration  of  the 
terminal  filaments  of  the  pyramidal 
tract  fibers,  the  reflex  activity  of  the 
cord  becomes  exalted.  As  the  dis- 
ease progresses  the  irritability  in- 
creases to  such  an  extent  that  vio- 
lent spasmodic  contractions  of  the 
arms  and  legs  arise  when  the  skin 
or  tendons  are  mechanically  stimu- 
lated. The  explanation  offered  is 
practically  the  same  as  in  division 
of  the  cord:  viz.,  withdrawal  of  the 
inhibitor  and  controlling  influence 
of  the  brain. 
The  reflex  excitability  may  be  decreased  by : 

Stimulation  oJ  Certain  Regions  oj  the  Brain. — It  was  discovered  by  Setche- 
now  that  when  the  frog  brain  is  divided  just  anterior  to  the  optic  lobes 
(Fig.  235)  and  the  reflex  time  subsequently  determined  according  to  the 
method  of  Tiirck,  the  time  can  be  considerably  lengthened  by  stimula- 
tion of  the  optic  lobes.  This  is  readily  accomplished  by  placing  small 
crystals  of  sodium  chlorid  on  the  optic  lobes.  It  was  concluded  from 
this  fact  that  these  lobes  contain  centers  which  exert  an  inhibitor  in- 
fluence over  centers  in  the  spinal  cord  through  descending  nerve-fibers. 
This  conclusion  is  strengthened  by  the  fact  that  division  of  the  brain 
just  behind    the    optic    lobes  causes    a  temporary  inhibition    of    the 


•-■.op.l. 


mcd.  ob. 


Yyq,.  235. — Diagram  of  the  Brain  of 
THE  Frog.  ol].  n.  olfactory  nerves;  o//./. 
olfactory  lobes;  c.  It.  cerebral  hemispheres; 
op.  thl.  optic  thalamus;  op.  I.  optic  lobes; 
c.  cerebellum;  vied.  cb.  medulla  oblon- 
gata; IV.  V.  fourth  ventricle. 


512  TEXT-BOOK  OF  PHYSIOLOGY. 

reflexes  in  consequence  of  a  mechanical  irritation  of  these  fibers.  It  is 
quite  probable  that  the  volitional  inhibition  of  certain  reflexes  is  accom- 
plished through  the  intermediation  of  this  center  localized  by  Sctchenow. 

2.  Stimulation  oj  Sensor  Nerves. — If   during   the  application  of  a  stimulus 

sufficient  to  call  forth  a  characteristic  reaction  in  a  definite  period  of  time, 
a  sensor  nerve  in  a  distant  region  of  the  body  be  sim.ultaneously  stimu- 
lated, it  will  be  found  that  the  reflex  time  will  be  lengthened  or  the  reac- 
tion completely  inhibited. 

3.  Lesions  oj  spinal  cord;  e.g.,  atrophy  of  the  multipolar  cells  of  the  anterior 
^^     horns  of  the  gray  matter;  degeneration  of  the  terminals  of  the  dorsal 

root  fibers. 

4.  The  toxic  action  of  various  drugs — e.g.,  chloroform,  chloral — which  are 

believed  to  exert  a  depressing  action  on  the  nerve-cells  themselves. 

B.     THE  SPINAL  CORD  SEGMENTS  AS  CONDUCTORS. 

The  white  matter  of  the  spinal  cord  consists  of  nerve-fibers  the  special 
function  of  which  is 

1.  To  conduct  nerve  impulses  from  one  segment  of  the  cord  to  another. 

2.  To  conduct  nerve  impulses  coming  to  the  cord  through  afferent  nerves, 

directly  or  indirectly  to  various  areas  of  the  encephalon. 

3.  To  conduct  nerve  impulses  from  the  encephalon  to  the  spinal  cord 

segments. 

Intersegmental  or  Associative  Conduction. — The  spinal  cord  con- 
sists of  a  series  of  physiologic  segments  each  of  which  has  specific  functions 
and  is  associated  through  its  related  spinal  nerve  with  a  definite  segment 
of  the  body.  For  the  harmonious  cooperation  and  coordination  of  all  the 
spinal  segments  it  is  essential  that  they  should  be  united  by  commissural 
or  associative  fibers.  This  is,  in  fact,  accomplished  by  the  axons  of  the  intrin- 
sic cells  of  the  gray  matter,  which  constitute  such  a  large  part  of  the  antero- 
lateral and  posterior  root  zones.  In  consequence  of  this  association, 
the  cord  becomes  capable  of  complex  coordinated  and  purposive  reflex 
actions. 

Spino-encephalic  or  Sensor  Conduction. — The  nerve  impulses  that 
arise  in  consequence  of  impressions  made  on  the  terminals  of  the  nerves 
in  the  cutaneous  and  mucous  surfaces,  in  the  viscera  and  in  the  muscles,  are 
transmitted  through  the  dorsal  roots  of  the  spinal  nerves  to  the  cord.  On 
reaching  the  cord  they  are  received  by  nerve-cells,  the  axons  of  which  pass  up- 
ward to  and  through  the  medulla,  the  posterior  part  of  the  pons,  the  poste- 
rior part  of  the  crura  cerebri,  and  for  the  most  part  to  the  ventral  portion  of  the 
thalamus  opticus,  forming  what  is  known  as  the  spino-thalamic  system. 
On  reaching  the  thalamus  they  are  received  by  nerve  cells,  the  axons  of 
which  pass  by  way  of  the  internal  capsule  to  the  cells  of  the  cortex  of  the 
cerebrum  forming  what  is  known  as  the  thalamo-cortical  system.  It  is 
probable  however  that  some  fibers  from  the  cord  and  medulla  pass  direct 
to  the  cortex.  When  thus  transmitted  through  the  cord  to  the  cerebral 
hemispheres  directly  or  indirectly,  they  are  received  by  specialized  nerve- 
cells  in  the  cortex  and  translated  into  conscious  sensations.  The  sensations 
thus  arising  may  be  divided  into  special  and  general  sensations.     Of  the 


THE  SPINAL  CORD.  513 

former  may  be  mentioned  the  sensations  of  pain,  touch,  pressure,  tem- 
perature, passive  position  and  movements  of  parts  due  to  the  activity  of 
skeletal  muscles;  of  the  latter  may  be  mentioned  hunger,  thirst,  fatigue, 
well-being,  etc. 

Though  all  the  impulses  that  give  rise  -to  these  varied  sensations  are 
contained  within  the  fibers  of  the  afferent  peripheral  nerves,  they  are  on 
reaching  the  cord  distributed  by  the  intraspinal  mechanisms  to  different 
tracts  of  nerve  fibers,  each  of  which  transmits  to  localized  areas  of  the  cerebral 
cortex,  the  somesthetic  areas,  a  special  group  of  impulses  which  give  rise  to 
sensations  of  various  kinds,  but  especially  to  sensations  of  pain,  temperature 
(heat  and  cold),  touch,  passive  position,  and  movement  of  parts,  due  to  the 
action  of  skeletal  muscles. 

The  pathways  through  the  spinal  cord  that  conduct  these  afferent  im- 
pulses to  the  brain  are  ill-defined  and  imperfectly  known,  and  only  for  a  few 
sensations  can  it  be  said  that  their  pathways  have  been  determined.  The 
reason  for  this  obscurity  lies  partly  in  the  difficulties  of  experimentation, 
partly  in  the  difficulties  of  interpretation.  Clinical  observations  are  for 
special  reasons  more  or  less  untrustworthy. 

As  the  outcome  of  many  investigations  it  may  be  said  that  a  transverse 
section  of  one  lateral  half  of  the  cord  in  the  monkey,  or  a  lesion  involv- 
ing the  one  lateral  half  in  man,  as  a  rule  abolishes  many  if  not  all  forms  of 
cutaneous  sensibility  on  the  opposite  side  below  the  injury.  This  would  seem 
to  prove  that  the  nerve  impulses  cross  the  median  line  of  the  cord  immedi- 
ately or  very  shortly  after  entering  and  then  ascend  the  corresponding  half 
of  the  cord  on  their  way  to  the  thalamus.  At  the  same  time,  muscle  sen- 
sibility is  abolished  on  the  same  below  the  injury.  This  would  seem  to 
prove  that  the  fibers  of  the  posterior  roots  that  enter  and  cross  the 
column  of  Burdach  and  ascend  in  the  column  of  Goll  to  terminate  around  the 
cells  of  the  gracile  and  cuneate  nuclei  are  derived  mainly  from  the  muscles. 
It  is,  however,  believed  by  some  investigators  that  those  fibers  which  sub- 
serve the  sense  of  touch  do  not  decussate  at  once,  but  ascend  in  the  column 
of  Goll  as  far  as  the  medulla  oblongata,  where  they,  in  common  with  the 
fibers  coming  from  the  muscles,  arborize  around  the  nerve-cells  in  the  gracile 
and  cuneate  nuclei.  The  afferent  path  originating  in  the  spinal  cord  in- 
creases in  "size  at  successive  levels  as  it  passes  upward,  to  and  through  the 
medulla  and  successive  structures,  to  the  thalamus.  The  fibers  that  com- 
pose the  afferent  path  originating  in  the  cells  of  the  gracile  and  cuneate 
nuclei,  cross  over  the  median  plane  and  after  decussating  with  the  fibers 
coming  from  the  opposite  side,  join  the  afferent  path  from  the  spinal  cord. 
These  fibers  are  known  as  the  internal  arcuate  fibers  and  assist  in  the  forma- 
tion of  the  lemniscus  or  fillet.  (Fig.  236.)  The  sensor  pathway  decussates 
in  part  at  different  levels  of  the  spinal  cord  and  in  part  at  the  level  of  the  gra- 
cile and  cuneate  nuclei.  The  former  is  often  termed  the  lower,  the  latter 
the  upper  sensor  decussation. 

The  afferent  pathway  on  passing  toward  the  thalamus  receives  addi- 
tional fibers  at  the  level  of  the  medulla  and  pons  from  the  cells  with  which 
the  terminations  of  the  afferent  cranial  nerves,  the  trigeminal,  glosso- 
pharyngeal, and  vagus,  are  associated. 

The  pathways  for  the  impulses  that  give  rise  to  the  different  sensations 
2,i 


514 


TEXT-BOOK  OF  PHYSIOLOGY. 


have  been  variously  located  by  different  observers,  e.g.,  in  the  gray  matter, 
in  the  limiting  layer,  and  in  the  antero-lateral  tract  of  Gowers;  the  pathway 
for  the  impulses  that  give  rise  to  temperature  sensations  has  been  located  in 


Fig.  236. — Diagram  of  the  Sensor  Pathways  in  the  Spinal  Cord   enlarged  above  by 
Fibers  of  the  Sensor  Cranial  Nerves  and  Nerves  of  Specul  Sense. 

the  gray  matter;  the  pathway  for  tactile  impressions  has  been  located  in  the 
posterior  columns,  though  this  is  not  beyond  dispute.  The  pathway  for 
pain  sensations  has  been  located  in  Gowers'  tract. 


THE  SPINAL  CORD. 


515 


The  current  \dews  regarding  the  physiologic  activities  of  the  afferent 
portion  of  the  peripheral  nerve  S3^stem  and  its  relation  to  the  production  of 
different  forms  of  sensibility  have  been  enlarged  by  the  results  of  the  investi- 
gations that  have  been  made  by  Head.  Thus,  he  has  shown  that  the  afferent 
nerves  consist  of  three  systems,  each  of  which  when  excited  to  activity  evokes 
in  consciousness  a  different  and  distinct  group  of  sensations  as  follows  : 

1.  One  group  of  nerves  which  when  stimulated  evoke  sensations  through 

which  is  gained  the  power  of  cutaneous  localization,  of  the  discrimina- 
tion of  two  points  of  a  compass,  of  the  finer  grades  of  temperature,  and 
of  light  touch.  To  this  form  of  cutaneous  sensibility  the  term  epicritic 
has  been  applied. 

2.  A  second  group  of  nerves  which  when  vigorously  stimulated,  as  by  the 

prick  of  a  pin  or  by  the  application  of  a  hot  or  cold  object,  evoke  sensa- 
tions of  pain  or  heat  and  cold.  To  this  form  of  cutaneous  sensibility 
the  term  protopathic  has  been  applied.  This  form  of  sensibility  is 
unaccompanied  by  a  definite  appreciation  of  the  locality  stimulated  for 
the  reason  that  the  stimulus  causes  a  widespread  or  radiating  sensation 
which  at  times  is  referred  to  parts  far  removed  from  the  part  stimulated. 

3.  A  third  group  of  nerves  which  when  stimulated  evoke  sensations  of  pres- 

sure, of  the  passive  position  and  the  movements  of  parts  of  the  body,  and 
sensations  of  pain  as  well,  if  the  stimulus  (pressure)  be  severe,  or  if  the 
underlying  structures  are  injured,  e.g.,  the  rupture  of  a  joint.     The 
nerves  subserving  this  form  of  sensibility  are  contained  in  the  trunks 
of  the  motor  (muscle)  nerves  and  are  distributed  to  muscles,  tendons 
and  joints.     To  this  form  of  sensibility  the  term  deep  has  been  applied. 
As  previously  stated,  the  pathways  in  the  spinal  cord  for  the  transmission 
of  afferent  nerve  impulses  are  imperfectly  known,  and  for  their  determina- 
tion in  human  beings  reliance  must  be  placed  on  observations  of  the  results 
of  disease  of  the  cord,  supplemented  by  experiments  made  on  the  cord  in 
mammals  such  as  monkeys  and  apeS. 

From  the  classification  of  the  various  forms  of  sensibility  by  Head,  the 
following  table  showing  the  effects  or  symptoms  following  a  transverse  lesion 
of  one-half  of  the  cord  in  man  has  been  constructed  by  Turner  and  Stewart. 

Scheme   showing   the   Brown-Sequard    "Symptom-Complex," 
BASED  ON  Head's  Observations. 
Side  of  lesion.  Side  opposite  lesion. 

Motor  paralysis.  No  paralysis. 

Retention  of  tactile,  light  pressure.       Tactile  and  light  pressure  sensibili- 
painful,  and  thermal  sensibilities.  ties  may  or  may  not  be  impaired. 

Painful    and    thermal    sensibilities 
abolished. 
Painful  pressure  retained.  Painful  pressure  abolished. 

Impairment  or  abolition  of  tactile       Retention  of  sense  of  position  and 
discrimination,  and  sense  of  posi-  of  tactile  discrimination, 

tion  of  limbs. 
Retention    of    cutaneous    localiza-       Cutaneous  localization  depends  up- 
tion.  on  the  state  of  tactile  sensibility. 


5'i6  TEXT-BOOK  OF  PHYSIOLOGY. 

From  a  study  of  this  table  it  is  apparent:  (i)  that  some  forms  of  sensor 
impulses  (those  of  pain  and  temperature  sensibility)  cross  soon  after  their 
entrance  and  pass  up  the  opposite  side  of  the  cord;  (2)  that  other  forms  of 
sensor  impulses  (those  of  the  sense  of  passive  position  and  of  movement  and 
tactile  discrimination,  Head)  do  not  cross,  but  pass  up  on  the  same  side  as 
the  entering  posterior  nerve  roots;  (3)  that  tactile  sensibility  may  or  may  not 
be  abolished  on  the  side  opposite  the  lesion;  and  (4)  that  the  sense  of  cuta- 
neous localization  may  be  dissociated  from  the  sense  of  passive  position,  and 
remain  intact  when  the  latter  is  absent  (Head). 

Encephalo-spinal  or  Motor  Conduction. — At  birth  the  child  is  cap- 
able of  performing  all  the  functions  of  organic  life,  such  as  sucking,  swallow- 
ing, breathing,  etc.  It  is,  however,  deficient  in  psychic  activity  and  in 
volitional  control  of  its  muscles.  Its  movements  are  therefore  largely,  if  not 
entirely,  reflex  in  character. 

Embryologic  and  histologic  examination  of  the  spinal  cord  and  medulla 
show  that  so  far  as  their  mechanisms  for  independent  physiologic  activities 
are  concerned  both  are  fully  developed.  Similar  investigations  of  the  cere- 
bral hemispheres  and  of  the  nerve- fibers  which  bring  their  nerve-cells  into 
relation  with  the  spinal  segments  show  that  the  cells  of  the  cortex  are  not  only 
immature,  but  that  their  descending  axons  are  incompletely  invested  with 
myelin.  With  the  growth  of  the  child,  psychic  life  unfolds  and  volitional 
control  of  muscles  is  acquired.  Coincidently  the  cells  of  the  cerebral  cortex 
grow  and  develop  and  the  fibers  become  covered  with  myelin. 

The  nerv^e-fibers  which  have  their  origin  in  the  cells  of  the  cerebral  cortex, 
and  which  terminate  in  tufts  around  the  cells  in  the  anterior  horns  of  the 
gray  matter  of  the  spinal  segments,  are  to  be  regarded  as  long  commissural 
tracts  uniting  and  associating  these  two  portions  of  the  central  nerve  system. 

Experimental  investigations  and  observations  of  pathologic  lesions 
accord  with  the  view  that  physiologically  these  fibers  are  efferent  pathways 
for  the  transmission  of  motor  or  volitio'nal  impulses  from  the  cortex  to  the 
spinal  segments.  The  nerve-cells  in  which  the  motor  impulses  originate 
are  located  for  the  most  part,  as  will  be  fully  stated  later,  in  the  central 
portion  of  the  cortex  of  the  cerebral  hemispheres  in  the  neighborhood  of  the 
central  or  Rolandic  fissure.  The  axons  of  these  cells  from  each  hemisphere 
descend  through  the  corona  radiata  to  and  through  the  internal  capsule, 
along  the  inferior  surface  of  the  crura  cerebri,  behind  the  pons  to  the  medulla, 
of  which  they  constitute  the  anterior  pyramids.  (Fig.  237.)  At  this  point 
the  pyramidal  tract^  of  each  side  divides  into  two  portions,  viz. : 

1.  A  large  portion,  containing  from  80  to  90  per  cent,  of  the  fibers,  which 

decussates  at  the  lower  border  of  the  medulla  and  passes  downward  in 
the  posterior  part  of  the  lateral  column  of  the  opposite  side,  constituting 
the  crossed  pyramidal  tract;  as  it  descends  it  gradually  diminishes  in  size 
as  its  fibers  or  their  collaterals  enter  the  gray  matter  of  each  successive 
segment. 

2.  A  small  portion,  containing  from  20  to  10  per  cent,  of  the  fibers,  which 

'  From  the  fact  that  the  region  included  between  the  origin  of  these  fibers  and  the  internal 
capsule  presents  somewhat  the  form  of  a  pyramid  with  four  sides,  Charcot  designated  it  the 
pyramidal  region  and  the  fibers  composing  it  the  pyramidal  tract.  The  base  of  the  pyramid  in- 
cludes the  convolutions  of  the  cortex  around  the  Rolandic  fissure.  The  summit  of  the  pyramid 
is  truncated  and  covers  the  pyramidal  region  of  the  internal  capsule. 


THE  SPINAL  CORD. 


517 


does  not  decussate  at  the  medulla  but  passes  downward  on  the  inner 

side  of  the  anterior  column  of  the  same  side,  constituting  the  direct 


Fig.  237. — DiAGR.\M  of  the  Pyr.\midal  Tr.act  or  Motor  Path.  III.  Common  oculo-motor 
nerve.  IV.  Pathetic  nerve.  V.  Motor  di\-ision  of  the  trigeminal  nerve.  VI.  The  abducens 
nerve.  MI.  Facial  nerve.  IX.  and  X.  Motor  divisions  of  the  glos50-phar\^ngeal  and  pneumogas- 
tric  nerves.     XI.  Spinal  accessory  nerve.     XII.     Hypoglossal  nerve. — {Van  Gehuchten.) 

pyramidal  tract  or  column  Turck.     This  tract  can  be  traced  down,  as 
a  rule,  only  as  far  as  the  mid-dorsal  region.     As  it  descends  it  becomes 


5i8  TEXT-BOOK  OF  PHYSIOLOGY. 

smaller  as  its  fibers  cross  the  anterior  commissure  to  enter  the  gray 
matter  of  the  opposite  side.  Thus  all  the  fibers  of  the  pyramidal  tract 
from  each  cerebral  hemisphere  eventually  are  brought  into  relation  with 
the  cells  of  the  gray  matter  of  the  opposite  side  of  the  cord. 
That  the  pyramidal  tracts  are  the  conductors  of  volitional  impulses 
throughout  the  length  of  the  cord  to  its  various  segments  has  been  made 
evident  by  the  results  of  section,  electric  stimulation,  and  disease.  Division 
of  the  anterior  and  lateral  columns  of  one  side  of  the  cord  in  any  part  of  its 
extent  is  invariably  followed  by  a  loss  of  motion  or  paralysis  of  the  muscles 
below  the  section,  while  electric  stimulation  of  the  peripheral  end  of  the 
isolated  crossed  pyramidal  tract  .is  followed  by  marked  characteristic  move- 
ments of  the  muscles.  Similar  results  follow  division  of  the  pyramidal  tract 
in  any  part  of  its  course  from  the  cerebral  cortex  downward.  Electric 
stimulation  of  the  cortical  cells  which  give  origin  to  the  pyramidal  tract  is 
also  followed  by  contraction  of  the  muscles  of  the  opposite  side,  while  their 
destruction  is  attended  by  paralysis  of  the  same  muscles.  As  the  nutrition  of 
the  fibers  is  governed  by  the  cells,  it  follows  that  when  the  axon  is  separated 
from  its  cell-body  it  degenerates.  It  has  been  found  that  a  lesion  of  the 
pyramidal  tract  in  any  part  of  its  course  is  followed  by  descending  degenera- 
tion, which  is  taken  in  evidence  that  it  conducts  ners^e  impulses  from  above 
downward.  Thus  experimental  investigation  and  pathologic  observ^ation  are 
in  accord  in  the  view  that  physiologically  these  nerve-fibers  are  the  pathways 
for  the  transmission  of  motor  or  volitional  impulses  from  the  encephalon 
to  the  spinal  cord. 


CHAPTER  XX. 

THE  ANATOMIC  RELATIONS  OF  THE  MEDULLA  OBLONGATA; 

THE  ISTHMUS  OF  THE  ENCEPHALON ;  THE  CORPORA 

QUADRIGEMINA ;  THE  BASAL  GANGLIA. 

THE  MEDULLA  OBLONGATA. 

The  medulla  oblongata  is  that  portion  of  the  central  nerve  system  im- 
mediately superior  to  and  continuous  with  the  spinal  cord.  It  has  the  shape 
of  a  truncated  cone,  the  base  of  which  is  directed  upward,  the  truncated 
apex  downward.  It  is  38  mm.  in  length,  18  mm.  in  breadth,  and  12  mm. 
in  thickness.  By  the  continuation  upward  of  the  anterior  and  posterior 
median  fissures,  the  medulla  is  di^dded  into  symmetric  halves  (Figs.  238  and 
239).  Like  the  cord,  of  which  it  is  a  continuation,  it  is  composed  of  white 
matter  externally  and  gray  matter  internally. 

Structure  of  the  Gray  Matter. — The  gray  matter  of  the  medulla  is 
continuous  with  that  of  the  cord,  though  owing  to  the  shifting  of  position  of 
the  different  tracts  of  the  white  matter  it  is  arranged  with  much  less  regular- 
ity. The  appearance  which  the  gray  matter  presents  on  transverse  section 
varies  also  at  different  levels. 

At  the  level  of  the  first  cer\dcal  nervT  the  posterior  horns  are  narrow, 
elongated,  and  directed  outward.  The  lateral  horns  are  well  developed  and 
present  a  collection  of  cells  near  their  bases  which  can  be  traced  upward  and 
downward  for  some  distance.  At  the  level  of  the  decussation  of  the  py- 
ramidal tracts  the  head  of  the  anterior  horn  becomes  detached  from  the  rest  of 
the  gray  matter  and  is  pushed  backward  toward  the  posterior  horn;  the  bases 
of  the  anterior  horns  become  spread  out  to  form  a  layer  of  gray  matter  near  the 
dorsal  aspect  of  the  medulla.  Transverse  sections  of  the  medulla  at  all 
levels  show  a  more  or  less  extensive  network  of  nerve-fibers  known  as  the 
reticular  formation.  In  its  meshes  are  found  collections  of  nerve-cells  of 
varying  size.  Toward  the  dorsal  aspect  of  the  medulla  special  groups 
of  cells  are  found  from  which  axons  arise  to  become  the  fibers  of  various 
efferent  cranial  nerves,  e.g.,  hypoglossal,  efferent  fibers  of  the  vagus,  and 
glossopharyngeal . 

Structure  of  the  White  Matter. — The  white  matter  is  composed  of 
nerve  fibers  supported  by  connective  tissue  and  neuroglia.  It  is  subdivided 
on  either  side  by  grooves  into  three  main  columns:  viz.,  an  anterior  column 
or  pyramid,  a  lateral  column,  and  a  posterior  column. 

The  anterior  column  or  pyramid  is  composed  partly  of  fibers  continuous 
with  those  of  the  anterior  column  of  the  spinal  cord  (the  direct  pyramidal 
tract),  and  partly  of  fibers  continuous  with  those  of  the  lateral  column  of  the 
cord  of  the  opposite  side  (the  crossed  pyramidal  tract) ,  which  decussate  at 
the  anterior  portion  of  the  medulla.  The  united  fibers  can  be  traced  up- 
ward to  the  pons,  where  they  disappear  from  view. 

519 


520 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  lateral  column  is  composed  of  fibers  continuous  with  those  of  the  lateral 
column  of  the  cord.  As  the  fibers  pass  upward,  however,  they  diverge  in  several 
directions.     The  fibers  of  the  crossed  pyramidal  tract  cross  the  median  line, 


Fig.  238. — Anterior  or  Ventral 
View  of  the  Medulla  Oblongata 
AND  Isthmus,  i.  Infundibulum.  2. 
Tuber  cinereum.  3.  Corpora  albi- 
cantia.  4.  Cerebral  peduncle.  5. 
Tuber  annulare.  6.  Origin  of  the 
middle  peduncle  of  the  cerebellum. 
7.  Anterior  pyramids  of  the  medulla 
oblongata.  (S.  Decussation  of  the  an- 
terior pyramids,  g.  Olivary  bodies. 
10.  Restiform  bodies.  11.  Arciform 
fibers.  12.  Upper  extremity  of  the 
spinal  cord.  13.  Ligamentum  dentic- 
ulatum.  14,  14.  Dura  mater  of 
the  cord.  15.  Optic  tracts.  16. 
Chiasm  of  the  optic  nerves.  17. 
Motor  oculi  communis.  18.  Patheti- 
cus.  ig.  Fifth  nerve.  20.  Motor 
oculi  externus.  21.  Facial  nerve.  22. 
Auditory  nerve.  23.  Nerve  of  Wris- 
berg.  24.  Glosso-pharyngeal  nerve 
25.  Pneumogastric.  26,  26.  Spinal 
accessory.  27.  Sublingual  nerve.  28, 
29,  30.  Cervical  nerves. — (Sappey.) 


Fig.  239. — Posterior  or  Dorsal  \tew 
of  the  Medulla  Oblongata,  Isthmus, 
AND  Basal  Ganglia,  i.  Corpora  quad- 
rigemina.  2.  Corpus  quadrigeminum  an- 
terius  (pregeminum).  3.  Corpus  quadri- 
geminum posterius  (post-geminum).  4. 
Tract  of  fibers  (brachium)  passing  to  the 
corpus  geniculatum  externum.  5.  Tract  of 
fibers  (brachium)  passing  to  6,  the  corpus 
geniculatum  internum.  7.  Posterior  com- 
missure. 8.  Pineal  gland.  9.  Superior  cere- 
bellar peduncle.  10,  11,  12.  The  valve  of 
Vieussens.  13.  The  pathetic  ner\-e.  14. 
Lateral  groove  of  the  isthmus.  15.  Triangu- 
lar bundle  of  the  isthmus.  16.  Superior 
cerebellar  j)eduncle.  17.  Middle  cerebellar 
peduncle.  18.  Inferior  cerebellar  peduncle, 
ig.  Antero-inferior  wall  of  the  fourth  ventri- 
cle. 20.  Acoustic  nerve.  21.  Spinal  cord. 
22.  The  postero-median  column.  23.  The 
posterior  pyramids. — (Sappey.) 


as  previously  stated,  to  enter  into  the  formation  of  the  anterior  column; 
the  fibers  of  the  direct  cerebellar  tract  gradually  curve  backward,  and  in  so 
doing  unite  with  other  fibers  to  form  the  restiform  body,  after  which  they 
enter  the  cerebellum  by  way  of  the  inferior  peduncle.     Situated  between  the 


ISTHMUS  OF  THE  ENCEPHALOX.  521 

anterior  pyramid  and  the  restiform  body  is  a  small  oval  mass,  the  olivary 
body,  composed  of  both  white  and  gray  matter. 

The  posterior  column  is  composed  largely  of  fibers  continuous  w^ith  those 
of  the  posterior  column  of  the  cord.  The  subdivision  of  this  column  into  a 
postero-external  (Burdach)  and  a  postero-internal  (Goll)  is  more  marked 
in  the  medulla  than  in  the  cord.  The  former  is  here  known  as  the  funiculus 
cuneatus,  the  latter  as  the  funiculus  gracilis.  These  two  strands  of  fibers 
are  apparently  continued  into  the  restiform  body.  Owing  to  the  divergence 
of  the  restiform  bodies  a  V-shaped  space  is  formed,  the  floor  of  which  is 
covered  with  epithelium  resting  on  the  ependyma.  At  the  upper  extremity 
of  the  funiculus  cuneatus  and  funiculus  graciUs,  two  collections  of  gray 
matter  are  found,  known  respectively  as  the  nucleus  cuneatus  and  nucleus 
gracilis.  Around  the  cells  of  these  nuclei  many  of  the  fibers  of  the  posterior 
column  end  in  brush-like  expansions,    (see  Fig.  236). 

The  Fillet  or  Lemniscus. — From  the  ventral  surface  of  the  cuneate  and 
gracile  nuclei  axons  emerge  which  pass  forward  and  upward  through  the 
gray  matter  and  decussate  with  corresponding  fibers  coming  from  the  op- 
posite nuclei.  They  then  assume  a  position  just  posterior  to  the  pyramids 
and  between  the  olivary  bodies.  These  fibers  thus  form  a  new  tract,  termed 
the  fillet  or  lemniscus.  As  this  tract  ascends  toward  the  cerebrum  it  receives 
additional  axons  from  the  sensor  end-nuclei  of  all  the  afferent  cranial 
nerves  of  the  opposite  side  with  the  exception  of  the  auditory.  From  the 
end-nuclei  of  the  auditory  nerve,  new  axons  ascend  as  a  distinct  tract  situated 
near  the  lateral  aspect  of  the  pons.  From  their  position,  these  two  separate 
tracts  have  been  termed  the  mesial  and  lateral  fillets  respectively. 

THE  ISTHMUS  OF  THE  ENCEFHALCN. 

The  isthmus  of  the  encephalon  comprises  that  portion  of  the  central 
nerve  system  connecting  the  cerebrum  above,  the  cerebellum  behind,  and  the 
medulla  below.  Its  ventral  surface  presents  below  an  enlargement,  convex 
from  side  to  side,  the  pons  Varolii.  On  each  side  the  fibers  of  which  the 
pons  consists  converge  to  form  a  compact  bundle,  the  middle  peduncle, 
which  enters  the  corresponding  half  of  the  cerebellum.  Above  the  pons, 
this  surface  presents  two  large  columns  of  white  matter  which,  diverge  some- 
what from  below  upw^ard,  enter  the  base  of  the  cerebrum  and  are  known  as 
crura  cerebri.  Embracing  the  crura  above  are  two  large  bands  of  white 
matter,  the  optic  tracts  (Fig.  238). 

The  dorsal  surface  presents  below  two  diverging  columns  of  white  matter, 
the  inferior  peduncles;  above,  two  converging  columns,  the  superior  peduncles 
of  the  cerebellum  (Fig.  239).  At  the  extreme  upper  part  of  this  surface 
there  are  four  small  grayish  eminences,  the  corpora  quadrigemina.  From 
the  disposition  of  the  white  matter  on  the  dorsal  surface  of  the  isthmus  and 
medulla,  there  is  formed  a  lozenge-shaped  space,  ih.^  fourth  ventricle.  The 
space  is  an  expansion  of  the  central  cavity  of  the  cord,  the  result  of  the 
changed  relations  of  the  white  and  gray  matter  in  this  region  of  the  central 
nerve  system.  Above,  this  ventricle  communicates  by  a  narrow  canal,  the 
aqueduct  of  Sylvius,  with  the  third  ventricle.  The  floor  of  the  fourth 
ventricle  is  covered  with  a  layer  of  epithelium  resting  on  the  ependyma  con- 


522 


TEXT-BOOK  OF  PHYSIOLOGY. 


tinuous  with  that  lining  the  central  canal  of  the  cord.     Beneath  this  is  a 
layer  of  gray  matter. 

The  pons  varolii  comprises  in  a  general  way  that  portion  of  the  central 
nerve  system  situated  between  the  medulla  oblongata  and  the  crura  cerebri. 
The  ventral  surface  is  convex  from  side  to  side;  the  lateral  surface,  owing  to 
the  convergence  of  the  fibers  of  which  it  is  composed,  is  contracted  to  form  the 
middle  peduncle  of  the  cerebellum;  the  posterior  surface  is  flat  and  forms 
the  upper  half  of  the  floor  of  the  fourth  ventricle.  The  pons  consists  of 
white  fibers  and  gray  matter  supported  by  connective  tissue  and  neuroglia. 
Transverse  sections  of  the  pons  show  that  it  is  divided  into  an  anterior 
or  ventral,  and  a  posterior  or  dorsal  portion,  the  latter  being  usually  termed 
the  tegmentum. 

The  ventral  portion  consists  for  the  most  part  of  white  fibers,  arranged 
longitudinally  and  transversely  (Fig.  240).  The  longitudinal  fibers  are 
largely  continuations  of  the  pyramidal  tracts,  or  the  fibers  composing  in  part 
the  anterior  pyramid  of  the  medulla.     In  the  lower  part  of  the  pons  these 

fibers  are  compactly  arranged,  but  at  higher 
levels  they  are  separated  into  a  number  of 
bundles  by  the  interlacing  of  the  transverse 
fibers.  The  transverse  fibers  are  divided  into 
a  superficial  and  a  deep  set.  Among  these 
fibers  are  groups  of  nerve-cells  which  collec- 
tively are  known  as  the  nucleus  pontis.  Some 
of  the  transverse  fibers,  especially  the  super- 
ficial ones,  are  commissural  in  character — i.e., 
they  connect  corresponding  parts  of  the  gray 
matter  of  the  lateral  halves  of  the  cerebellum; 
others  coming  from  the  gray  matter  of  the 
cerebellum  cross  the  median  line  and  terminate 
around  the  cells  of  the  nucleus  pontis;  others 
again  are  connected  with  the  gray  cells  of  the 
same  side.  Through  the  intermediation  of 
the  nucleus  pontis  and  certain  of  the  longitudinal  fibers  of  the  pons,  the 
cerebellum  is  brought  into  relation  with  the  cerebrum. 

The  dorsal  or  tegmental  portion  consists  of:  (i)  The  fillet;  (2)  the  formatio 
reticularis;  (3)  the  medial  longitudinal  bundle;  (4)  groups  of  efferent  and 
afferent  nerve-cells. 

The.  fillet  or  lemniscus  in  this  region  is  divided  into  a  mesial  and  a  lateral 
portion.  The  fibers  of  the  mesial  portion  are  partly  the  axons  of  the  nerve- 
cells  of  the  gracile  and  cuneate  nuclei  of  the  opposite  side  of  the  medulla,  and 
partly  of  the  axons  of  the  sensor  nerve-cells  of  the  afferent  cranial  nerves  with 
the  exception  of  the  auditory.  The  fibers  of  the  lateral  portion  are  mainly 
the  axons  of  the  cells  in  the  floor  of  the  fourth  ventricle  around  which  the 
auditory  nerve-fibers  end.  They  are  therefore  a  continuation  of  the  acoustic 
tract. 

The  formatio  reticularis  is  a  continuation  of  that  of  the  medulla. 
The  medial  longitudi?ial  bundle  is  a  band  of  nerve-fibers,  triangular  in 
shape,  placed  on  either  side  of  the  median  line  just  beneath  the  floor  of  the 
fourth  ventricle  and  the  aqueduct  of  Sylvius.     It  consists  of  both  afferent 


Fig.  240. — Transkction  of  the 
Pons  through  its  Middle  Por- 
tion, Showing  the  Relation 
of  the  Nerve  Tracts  of  Which 
it  is  Composed.  D.  l.f.  Dorsal 
longitudinal  fasciculus,  i.e.  and 
c.  Locus  ceruleus.  L.f.  Lateral 
fillet. 


ISTHMUS  OF  THE  ENCEPHALON. 


523 


(ascending)  and  efferent  (descending)  fibers.  The  afferent  fibers  are  the 
axons  of  sensor  end-nuclei  located  in  the  upper  segments  of  the  spinal  cord 
as  well  as  of  axons  of  sensor  end-nuclei  of  cranial  nerves.  As  they  pass 
upward  some  of  the  fibers,  as  well  as  collateral  branches,  arborize  around 
the  nuclei  of  origin  of  the  various  motor  cranial  nerves  of  the  same  and 
opposite  sides.  The  ascending  fibers  thus  associate  anatomically  sensor 
end-nuclei  of  both  spinal  and  cranial  nerves  with  the  nuclei  of  the  motor 
cranial  nerves.  The  efferent  fibers  are  the  axons  of  nerve-cells  located  in 
the  corpora  quadrigemina  and  in  a  special  nucleus  in  the  floor  of  the  third 
ventricle.  From  this  origin  the  fibers  soon  cross  the  median  line,  pursue  a 
downward  course  and  come  into  close  relation  with  the  ascending  fibers. 
In  their  descent  fibers  pass  successively  to  the  nuclei  of  origin  of  the  motor 
cranial  nerves  and  to  nuclei  in  the  upper  segments  of  the  spinal  cord.  The 
efiferent  fibers  thus  associate  anatomically  the  nuclei  from  which  they  arise 
with  the  nuclei  just  alluded  to. 

The  superior  olive  is  a  cylindric  mass  of  gray  matter  situated  in  the  pons 
in  the  anterior  part  of  the  formatio  reticularis.  It  consists  of  nerve-cells 
the  axons  of  which  pass  dorso-later- 
ally,  decussate  in  the  median  line, 
and  form  the  lateral  fillet  of  the  oppo- 
site side.  Some  few  axons  go  to  the 
lateral  fillet  of  the  same  side. 

The  groups  of  efferent  nerve-cells 
lying  just  beneath  the  floor  of  the 
fourth  ventricle  give  origin  to  axons 
composing  the  motor  portion  of  the 
fifth,  the  sixth,  the  seventh  cranial 
nerves.  The  groups  of  afferent  cells 
are  the  sensor  end-nuclei  of  the  fifth 
and  eighth  cranial  nerves  from  which 
new  axons  pass  as  a  part  of  the 
mesial  and  lateral  fillets  toward  the 
cerebrum. 

The  crura  cerebri  comprise  that 
portion  of  the  central  nerve  system 
situated  between  the  pons  below  and 
the  cerebrum  above.  They  are  com- 
posed of  strands  of  nerve-fibers  which  are  divided,  as  shown  on  cross-section, 
into  a  ventral  and  a  dorsal  portion  by  a  crescentic  shaped  layer  of  gray  matter, 
the  substantia  nigra  (Fig.  241).  Of  the  fibers  which  compose  the  ventral 
portion  of  each  crus,  the  crusta  or  pes,  the  larger  part  is  continuous  below, 
through  the  longitudinal  fibers  of  the  pons,  with  the  pyramid  of  the  medulla 
and  the  pyramidal  tract;  above  they  assist  in  the  formation  of  the  internal 
capsule.  On  the  inner  and  on  the  outer  side  of  each  crusta  there  is  a  bundle 
of  fibers  derived  from  the  frontal,  and  from  the  temporal  and  occipital  por- 
tions of  the  cerebrum  respectively.  These  fibers  are  connected  directly 
with  the  nuclei  pontis  and  indirectly  with  the  cerebellum  of  the  same  and 
opposite  sides.  The  fibers  which  compose  the  dorsal  portion,  the  tegmentum, 
are  continuous  with  those  which  pass  upward  from  the  medulla  and  pons 


Fig.  241. — Scheme  of  Transverse  Sec- 
tion OF  THE  Cerebral  Peduncles.  CQ 
Corpora  quadrigemina.  Aq.  Aqueduct 
p. Lb.  Posterior  longitudinal  bundle.  F 
Fillet  or  lemniscus.  RN.  Red  nucleus.  SN 
Substantia  nigra.  III.  Third  nerve.  P} 
Pyramidal  tracts.  Fc.  Fronto-cerebellar; 
and  TOC,  temporo-occipital  fibers  of  the 
crusta.  CC.  Caudato-cerebellar  fibers  in 
upper  part  of  crusta. — {After  Wernicke  and 
Gowers.) 


524  TEXT-BOOK  OP^  PHYSIOLOGY. 

e.g.,  the  fillet,  both  mesial  and  lateral,  the  formatio  reticularis,  the  medial 
longitudinal  bundle,  and,  in  addition,  the  fibers  of  the  superior  peduncles  of 
the  cerebellum.  Above,  the  fibers  terminate  largely  in  collections  of  gray 
matter  at  the  base  of  the  cerebrum. 

The  aqueduct  of  Sylvius  is  a  short  narrow  canal  which  connects  the  caxaty 
of  the  fourth  with  the  cavity  of  the  third  ventricle.  It  is  lined  by  the  epen- 
dyma  and  surrounded  by  a  layer  of  gray  matter  continuous  with  that  forming 
the  floor  of  the  fourth  ventricle.  In  that  portion  of  the  gray  matter  lying 
beneath  or  ventral  to  the  aqueduct  there  are  groups  of  nerve-cells  which 
give  origin  to  axons  which  unite  to  form  the  third  and  fourth  cranial  nerves. 

THE   CORPORA   QUADRIGEMINA. 

The  corpora  quadrigemina  are  four  small  grayish  eminences  situated 
beneath  the  posterior  border  of  the  corpus  callosum  and  behind  the  third 
ventricle.  They  rest  upon  the  lamina  quadrigemina,  which  forms  the 
roof  of  the  aqueduct  of  Sylvius.  The  superior  pair  are  the  larger  and  are 
known  as  the  superior  quadrigeminal  bodies,  the  superior  collicuU  or  the  pre- 
gemina;  the  inferior  pair  are  the  smaller  and  are  known  as  the  inferior  quad- 
rigeminal bodies,  the  inferior  collicuU,  or  the  post-gemina. 

External  and  somewhat  inferior  to  the  corpora  quadrigemina  are  two 
small  collections  of  gray  matter  the  more  external  of  which  has  been  termed 
the  external  geniculate  body  or  the  pregeniculum,  the  more  internal  of  which 
has  been  termed  the  internal  geniculate  body  or  the  post-geniculum. 

Though  these  bodies  are  closely  associated  anatomically,  they  dift'er  in 
origin,  in  their  relations,  and  in  their  functions. 

On  either  side  the  fibers  composing  the  optic  tract  pass  to  and  through 
the  geniculate  bodies  in  which  some  of  the  fibers  terminate,  while  others 
pass  onward  to  the  superior  and  inferior  quadrigeminal  bodies  and  there 
terminate.  The  bands  of  white  matter  associating  the  superior  or  external 
and  the  inferior  or  internal  geniculate  bodies,  with  the  corresponding  quad- 
rigeminal bodies  are  known  as  the  superior  and  inferior  brachia  respectively. 
The  internal  geniculate  body  gives  origin  to  and  receives  fibers  from  the 
mesial  portion  of  the  optic  tract  which  is  in  reality  not  a  portion  of  the  optic 
tract  proper,  but  a  commisural  band  (Gudden)  which  associates  the  body 
from  which  it  arises  w4th  that  of  the  opposite  side.  The  point  of  decussa- 
tion is  in  the  posterior  part  of  the  optic  chiasm. 

The  external  geniculate  body  is  a  terminal  station  for  a  portion  of  the 
fine  visual  fibers  coming  from  the  retina.  From  the  cells  of  this  body 
new  axons  arise  which  course  forward  and  upward,  enter  the  internal 
capsule  and  pass  by  way  of  the  optic  radiation  to  the  cortex  of  the  occipital 
region  of  the  cerebrum. 

The  corpora  c[uadrigemina  show  on  microscopic  examination  that  they 
are  composed  of  nerve-cells  and  nerve-fibers,  both  of  which  are  so  intricately 
arranged  that  it  is  difficult  to  trace  their  relation  one  to  another  and  to  ad- 
joining structures.  Some  of  the  cells  of  the  superior  quadrigeminal  body  give 
origin  to  axons  v/hich  pass  downward  and  forward  and  terminate  in  brush- 
like expansions  around  the  nuclei  of  origin  of  the  oculo-motor,  trochlear,  and 
abducent  nuclei;  other  cells  are  surrounded  by  the  terminal  branches  of  some 


BASAL  GANGLIA.  525 

of  the  libers  of  the  optic  tract,  though  it  is  not  probable  that  they  are  true  visual 
fibers.  Still  other  cells  receive  the  terminal  branches  of  axons  the  cells  of 
origin  of  which  are  located  in  the  occipital  cortex  of  the  cerebrum  and  which 
reach  the  superior  quadrigeminal  body  by  way  of  the  optic  radiation  and 
internal  capsule. 

The  cells  of  the  post-geminum  give  origin  to  axons  which  pass  upward, 
forward,  and  outward,  enter  the  internal  capsule,  and  pass  by  way  of  the 
auditory  tract  to  the  cortex  of  the  temporo-sphenoidal  region  of  the  cerebrum. 
Many  of  the  fibers  of  the  lateral  fillet,  a  portion  of  the  auditory  tract,  termi- 
nate in  brush-like  expansions  around  these  same  cells.  There  is  thus  es- 
tablished a  connected  pathway  between  the  cochlea  and  the  temporo-sphe- 
noidal cortex.  The  cells  of  the  temporal  cortex,  however,  send  axons  in  the 
reverse  direction  by  way  of  the  auditory  tract  to  the  cells  of  the  post-geminum. 
There  is  thus  established  a  double  communication  between  the  occipital 
and  temporal  region  of  the  cerebral  cortex,  and  the  pre-geminal  and  post- 
geminal  bodies  respectively. 

THE  BASAL  GANGLIA;  THE  CORPORA  STRIATA  AND  OPTIC 

THALAMI. 

The  basal  ganglia  are  collections  of  ganglionic  matter,  situated  at  the 
base  ot  the  cerebrum  along  the  course  of  the  ;ierve-fibers  that  pass  to  and 
from  its  cortical  expansion.  Among  these  ganglia  the  more  important  are 
the  corpora  striata  and  the  optic  thalami.  They  are  made  visible  upon 
removal  of  the  cerebrum.  The  general  relations  of  these  ganglia  are  shown 
in  Fig.  242. 

The  corpus  striatum,  the  more  anterior  of  the  two,  is  an  ovoid  col- 
lection of  gray  and  white  matter  and  receives  its  name  from  the  fact  that  it 
presents  on  cross-section  a  striated  appearance.  The  larger  portion  of  this 
body  is  embedded  in  the  cerebral  white  matter,  while  the  smaller  portion 
projects  into  the  anterior  part  of  the  lateral  ventricle.  .  A  dissection  of  this 
nucleus  shows  that  it  is  subdivided  by  a  band  of  white  matter  into  two 
smaller  nuclei,  viz,  the  caudate  and  the  lenticular  nuclei. 

1.  The  caudate  nucleus  is  a   pyriform  body  which  corresponds  with    the 

intra-ventricular  portion  of  the  corpus  striatum.  It  consists  of  a  head, 
an  arching  body  and  a  tail.  The  head,  which  is  thick  and  large,  projects 
into  the  anterior  cornu  of  the  ventricle;  the  body  arches  across  the  ven- 
tricle from  before  backward  and  from  within  outward,  while  the  tail  is 
directed  downward  and  forward  to  become  associated  with  the  collection 
of  gray  matter  situated  beneath  the  lenticular  nucleus  and  known  as  the 
amygdaline  nucleus.  Anteriorly  the  caudate  nucleus,  is  united  with  the 
lenticular  nucleus  by  a  narrow  bridge  of  gray  matter,  partially  subdivided 
by  small  bands  or  strands  of  nerve  fibers  passing  through  it. 

2.  The  lenticular  nucleus  is  an  irregularly  triangular  pyramidal-shaped  body 

and  corresponds  with  the  extra-ventricular  portion  of  the  corpus  striatum, 
the  portion  embedded  in  the  cerebral  white  matter.  The  apical  extremity 
of  the  nucleus  is  directed  toward  the  median  line  while  its  convex  base 
is  directed  toward  and  runs  almost  parallel  with  the  gray  matter  of  the 


526  TEXT-BOOK  OF  PHYSIOLOGY. 

Island  of  Rcil.  The  general  appearance  and  relation  of  these  nuclei, 
are  shown  in  Figs.  243  and  244.  A  horizontal  section  of  the  lenticular 
nucleus  shows  that  it  is  divided  by  two  lamina  of  white  matter  into  three 
portions.  The  two  inner,  from  their  pale  yellow  color,  form  the  globus 
pallidus,  the  outer,  somewhat  darker  in  color,  is  termed  the  putamen. 
External  to  the  lenticular  nucleus  is  a  thin  stratum  of  gray  matter, 
arranged  more  or  less  vertically,  and  placed  between  the  outer  surface 
of  the  lenticular  nucleus  and  the  cortex  of  the  Island  of  Reil,  and 
known  as  the  daustriim. 


Fig.  242. — Corpora  Striata,  Optic  Thalami,  Corpora  Quadrigemina,  Cerebellum  and 
Associated  Structures,  i,  Corpora  quadrigemina;  2,  valve  of  Vieussens;  3,  pre-peduncle ; 
4,  upper  part  of  medi-peduncle;  5,  upper  part  of  crus;  6,  lateral  fillet;  7,  band  of  Reil;  8,  post- 
brachium;  g,  frenulum;  10,  gray  matter  of  valve  of  Vieussens;  11,  medi-commissure ;  12,  pre-com- 
missure;  13,  14,  center  of  cerebellum;  15,  post-commissure;  16,  peduncles  of  the  pineal  gland;  17, 
pineal  gland;  18,  19,  posterior  and  anterior  tubercles  of  the  thalamus;  20,  tenia semicircularis;  21, 
vessels  of  the  corpus  striatum;  22,  fornicolumn;  23,  corpus  striatum;  24,  septum  lucidum. — 
(Sappey.) 


The  Optic  Thalamus  is  an  oblong  mass  of  gray  matter  situated  between 
the  sensor,  afferent  pathway  and  the  cortex  of  the  cerebrum.  The  anterior 
and  posterior  extremities  of  each  thalamus  present  enlargements  known 
respectively  as  the  anterior  tubercle  and  the  posterior  tubercle  or  ptdvinar. 
The  mesial  surface  of  the  thalamus  forms  the  lateral  wall  of  the  third  ventri- 
cle and  is  covered  by  epithelium  resting  on  a  thin  layer  of  ependyma. 

A  transection  of  the  thalamus  shows  that  it  is  not  only  covered  externally 
but  penetrated  by  white  matter,  which  subdivides  its  contained  gray  cells 


BASAL  GANGLIA. 


527 


ANrouAt  J 


into  four  more  or  less  distinct  masses  termed  nuclei,  viz.,  an  anterior,  a 
lateral,  occupying  the  external  part  of  the  thalamus,  a  ventral,  close  to  the 
entire  ventral  surface,  and  a  posterior,  situated  beneath  the  pulvinar.  Be- 
neath and  somewhat  internal  to  each  optic  thalamus  there  is  a  region,  the 
subthalamic,  consisting  of  an  intricate  network  of  nerve-fibers  and  several 
nuclei  of  gray  matter,  e.g.,  the  red  or  tegmental  nucleus,  the  subthalamic 
nucleus,  or  Luys'  body,  and  the  substantia  nigra. 

Though  the  thalamus  has  extensive  connections  with  many  portions  of 
the  central  nerve  system,  the  most  important  are  with  the  cortex,  the  teg- 
mentum, and  the  optic  tracts. 

From  the  cells-  of  these  various  nuclei  axons  emerge  which  pass  into  the 
internal  capsule,  and  through  the  corona  radiata  to  various  portions  of  the 
cortex.  Those  which  come  from  the 
pulvinar  and  pass  to  the  occipital  lobe 
constitute  a  part  of  the  optic  radiation; 
those  from  the  lateral  and  ventral  nuclei 
ultimately  reach  the  parietal  lobe;  those 
from  the  anterior  nucleus  pass  to  the 
hippocampal  and  uncinate  convolutions. 
In  a  similar  manner  various  portions  of 
the  cortex  are  brought  into  relation  with 
the  thalamus,  axons  from  the  cortical 
cells  passing  downward  to  terminate  in 
tufts  around  the  thalamic  nuclei. 

The  tegmentum  is  intimately  related 
to  the  thalamus,  though  the  exact  dis- 
tribution of  various  strands  of  fibers  is 
a  subject  of  much  discussion.  Most 
of  the  fibers  of  the  mesial  fillet  end  in 
tufts  around  the  cells  of  the  ventral  and 
lateral  nuclei;  other  fibers  pass  directly 
to  the  cortex. 

The  optic  tract  sends  fibers  directly 
into  the  pulvinar,  the  external  geniculate 
body,  and  the  superior  corpus  quadri- 
geminum,  around  the  cells  of  which  they 
terminate  in  brush-like  expansions. 

The  Internal  Capsule. — The  lenti- 
cular nucleus  is  enclosed  on  all  sides 
by  ascending  and  descending  nerve-fibers.  From  the  manner  in  which 
they  surround  and  enclose  the  nucleus  they  have  collectively  been  called  the 
lenticular  capsule.  If  a  horizontal  section  of  the  cerebrum  be  made  at  a 
certain  level  so  as  to  cut  across  the  capsule  and  the  enclosed  nucleus  an 
appearance  similar  to  that  shown  in  Fig.  244  will  be  presented.  That  portion 
of  the  capsule  that  lies  between  the  caudate  nucleus  and  the  optic  thalamus 
internally  and  the  lenticular  nucleus  externally  is  known  as  the  internal 
portion  of  the  lenticular  capsule  or  in  its  abbreviated  form  as  the  internal 
capsule,  while  that  portion  between  the  external  convex  border  of  the  len- 
ticular nucleus  and  the  claustrum  is  known  as  the  external  portion  of  the 


^jpB^^' 


(^'^  // 


AMYGDALA 


Fig.  243. — Two  Views  of  a  Model 
OF  THE  Striatum.  A,  Lateral  aspect;  B, 
mesial  aspect.     {Spitzka.) 


52^ 


TEXT-BOOK  OF  PHYSIOLOGY. 


lenticular  capsule  or  in  its  abbreviated  form  as  the  external  capsule.  At  a 
given  level  the  internal  capsule  may  be  said  to  consist  of  two  segments  or 
limbs,  an  anterior,  situated  between  the  caudate  nucleus  and  the  anterior 
extremity  of  the  lenticular  nucleus,  and  a  posterior,   situated  between  the 


Fig.  244.- 


-HORIZONTAL  SeCTIOX  THROUGH  THE  CEREBRUM  SHOWIXG  THE  NaTURAl'^ReLATIONS 

OF  THE  Various  Structures. 


optic  thalamus  and  the  posterior  extrernity  of  the  lenticular  nucleus.  The 
two  segments  unite  at  an  obtuse  angle,  termed  the  knee,  which  is  directed 
toward  the  median  line.  The  appearance  which  is  presented  at  different 
levels  varies  however  considerablv. 


MEDULLA  AND  BASAL  GANGLIA.  529 

SUMMARY  OF  THE  STRUCTURE   OF  THE  MEDULLA,  ISTHMUS,  AND 

BASAL  GANGLIA. 

Structure  of  the  Central  Gray  Matter. — Though  the  general  arrange- 
ment of  the  central  gray  matter  has  been  incidentally  alluded  to  in  the  fore- 
going presentation  of  the  anatomic  features  of  the  medulla  and  isthmus,  it 
will  be  convenient  to  summarize  its  arrangement  and  structure  at  this  point. 

The  gray  matter  of  the  cord,  of  the  dorsal  aspect  of  the  medulla  and  pons, 
of  the  region  surrounding  the  aqueduct  of  Sylvius,  and  of  the  lining  of  the 
third  ventricle,  constitute  practically  a  continuous  system,  though  presenting 
modifications  in  various  parts  of  its  extent.  In  the  transition  region  of  the 
spinal  cord  and  medulla  the  gray  matter  of  the  former  becomes  much  changed 
in  shape  owing  to  the  shifting  of  position  of  the  various  tracts  of  white  matter, 
until  in  the  medulla  and  pons  it  is  spread  out  in  the  form  of  a  thin  layer  near 
their  dorsal  surfaces,  where,  together  with  the  ependyma,  it  forms  the  floor 
of  the  fourth  ventricle. 

In  the  region  of  the  acjueduct  of  Sylvius  the  gray  matter  again  converges 
and  ultimately  surrounds  the  canal,  to  again  expand  at  its  anterior  extremity 
to  form  the  lining  of  the  third  ventricle. 

The  Nerve-cells. — The  nerve-cells  in  these  different  regions  do  not 
differ  morphologically  from  those  in  the  gray  matter  of  the  spinal  cord.  The 
corpus,  or  body  of  the  cell,  presents  a  number  of  dendrites  as  well  as  the 
sharply  defined  axon.  As  a  rule,  the  cells  are  arranged  in  groups,  or  clusters, 
or  nests,  partially  surrounded  and  enclosed  by  supporting  tissue,  and  situ- 
ated beneath  the  floor  of  the  fourth  ventricle  and  the  floor  of  the  aqueduct  of 
Sylvius.  From  some  of  the  cell  groups  axons  pass  ventrally  through  the 
white  matter  to  merge  on  the  ventral  and  lateral  surfaces  of  the  medulla, 
pons,  and  crura,  where  they  are  known  as  efferent  or  motor  cranial  ner\'es. 
From  other  groups  of  cells,  axons  cross  the  median  line,  and  after  joining 
the  mesial  fillet  ascend  toward  the  cerebrum.  Around  these  latter  cells  the 
terminal  filaments  of  the  afferent  or  sensor  cranial  nerves  arborize.  The 
collection  of  cells  found  in  the  central  gray  matter  may  be  divided  into  two 
groups — efferent  and  afferent. 

The  efferent  cells  are  motor  in  function,  inasmuch  as  the  excitation 
arising  in  them  is  transmitted  outward  through  their  related  axons  to,  and 
exciting  movement  in,  skeletal  muscles,  glands,  viscera  or  blood-vessels. 

The  afferent  cells  are  largely  sentient  or  receptive  in  function,  inasmuch 
as  the  excitations  brought  to  them  by  the  afferent  cranial  nerves  from  skin 
and  mucous  membranes  and  from  sense-organs,  such  as  the  tongue  and  ear, 
are  received  by  them  and  transmitted  through  their  ascending  axons  to  the 
cortex  of  the  cerebrum,  where  they  are  translated  into  conscious  sensations. 

Structure  of  the  White  Matter. — The  white  matter  is  composed  of 
medullated  nerve-fibers,  and  though  arranged  in  a  very  complex  manner 
may  be  divided  into  longitudinal  and  transverse  fibers. 

The  longitudinal  fibers  which  compose  the  main  portion  of  the  isthmus 
may  be  subdivided  into  (i)  a  ventral  or  pedal  portion  and  (2)  a  dorsal  or 
tegmental  portion.  The  fibers  constituting  the  ventral  or  pedal  portion  may 
for  convenience  be  said  to  extend  from  the  cerebral  cortex  through  the  crus 
cerebri  to  the  pons,  medulla,  and  spinal  cord.     They  may  be  divided  into 

34 


530 


TEXT-BOOK  OF  PHYSIOLOGY. 


three  distinct  tracts:  e.g.,  the  pyramidal  tract,  the  fronto-cerebellar  tract, 
and  the  occipito-temporo-cerebellar  tract  (Fig.  245). 

The  pyramidal  or  motor  tract  descends  from  the  cortex  of  the  cerebrum 
mainly  from  the  gyru§  anterior  to  the  central  fissure,  passes  through  the 
posterior  one-third  of  the  anterior  segment  and  the  anterior  two-thirds  of  the 
posterior  segment  of  the  internal  capsule,  the  middle  two-fifths  of  the  crusta, 
behind  the  transverse  fibers  of  the  pons,  to  become  the  anterior  pyramid  of 
the  medulla,  beyond  which  it  divides  into  the  direct  and  crossed  pyramidal 
tracts  of  the  cord.  In  its  course  some  of  the  fibers  and  their  collaterals 
arborize  around  efferent  cells  from  the  anterior  extremity  of  the  aqueduct  of 
Sylvius  to  the  termination  of  the  spinal  cord. 


Fig.  245. — Schema  of  the  Projection  Fibers  of  the  Cerebrum  and  of  the  Peduncles  of 
THE  Cerebellum;  Lateral  View  of  the  Internal  Capsule.  A,  Tract  from  the  frontal  gyri 
to  the  pons  nuclei,  and  so  to  the  cerebellum  (frontal  cerebro-cortico-pontal  tract);  B,  the  motor 
(pyramidal)  tract;  C,  the  sensory  (body  sense)  tract;  D,  the  visual  tract;  E,  the  auditory  tract;  i^, 
the  fibers  of  the  superior  peduncle  of  the  cerebellum;  G,  fibers  of  the  middle  peduncle  uniting  with 
A  in  the  pons;  H,  fibers  of  the  inferior  peduncle  of  the  cerebellum;  /,  fibers  between  the  auditory 
nucleus  and  the  inferior  quadrigeminal  body;  K,  motor  (pyramidal)  decussation  in  the  bulb;  Vl, 
fourth  ventricle.     The  numerals  refer  to  the  cranial  nerves.— (Modified  from  Stan.) 

The  fronto-cerebellar  tract  descends  from  the  cortex  of  the  frontal  gyri 
of  the  anterior  lobe,  passes  through  the  anterior  portion  of  the  anterior 
segment  of  the  internal  capsule,  the  inner  fifth  of  the  crusta  to  the  pons, 
where  its  fibers  terminate  or  arborize  around  the  nucleus  pontis  of  the  same 
and  opposite  sides. 

The  occipito-temporo-cerebellar  tract  descends  from  the  occipital  and 
temporal  lobes,  passes  to  the  inner  side  of  the  lenticular  nucleus,  and  con- 
tinues downward  on  the  outer  side  of  the  crusta,  occupying  about  one-fifth  of 
its  bulk,  to  the  pons,  where  its  fibers  also  arborize  around  the  nucleus  pontis 
of  the  same  and  opposite  sides.  By  means  of  fibers  in  the  middle  peduncle 
these   descending   fibers   are   brought   into   relation   with   the   cerebellum. 


FUNCTIONS  OF  THE  MEDULLA  OBLONGATA.  531 

In  this  tract,  not  shown  in  the  figure,  are  to  be  found  the  afferent  fibers 
constituting  the  visual  tract,  D  and  the  auditory  tract  E. 

The  fibers  constituting  the  dorsal  or  tegmental  portion  of  the  longitudinal 
system  may  be  said  for  convenience  to  extend  from  the  posterior  portion  of 
the  medulla  and  pons  to  the  optic  thalamus  and  cerebrum.  They  may  be 
subdivided  into  several  tracts,  the  more  important  of  which  are  the  fillet  and 
the  dorsal  longitudinal  bundle. 

The  fillet  or  lemniscus,  Qonshiingoi  fibers  having  their  origin  partly  from 
the  cells  of  the  cuneate  and  gracile  nuclei  and  partly  from  the  cells  of  the 
sensor  end-nuclei  of  the  spinal  and  various  sensor  cranial  nerves,  occupies 
a  region  in  the  ventral  and  mesial  portion  of  the  tegmentum  throughout  its 
entire  extent.  Superiorly  this  mesial  fillet  terminates  for  the  most  part 
around  nerve  cells  in  the  nuclei  of  the  thalamus.  From  these  nuclei  new 
fibers  arise  which  pass  for  the  most  part  to  the  cortex  of  the  post-central 
and  parietal  gyri.  The  fibers  coming  from  the  sensor  end-nucleus  of  the 
auditory  nerve  (the  lateral  fillet)  lie  on  the  lateral  aspect  of  the  pons  and 
crus.  Superiorly  they  terminate  in  the  post-geminum  (the  inferior  quadri- 
geminal  body)  and  in  the  internal  geniculate  body.  From  these  nuclei  the 
fibers  composing  the  auditory  tract  pass  to  the  super-temporal  convolution. 

The  dorsal  longitudinal  bundle,  an  upward  extension  of  the  fibers  com- 
posing a  portion  of  the  ground  bundle  of  the  spinal  cord,  is  located  on 
either  side  of  the  median  line  just  beneath  the  floor  of  the  fourth  ventricle  and 
the  aqueduct  of  Syhdus.  As  it  passes  upward  collateral  branches  are  given 
off,  some  of  which  arborize  around  the  cell  nuclei  of  the  third,  fourth,  and 
sixth  cranial  nerves  of  the  same  side,  while  others  cross  the  median  line  and 
arborize  around  the  corresponding  cell  nuclei  of  the  opposite  side.  Supe- 
riorly some  of  the  fibers  become  related  to  cells  in  the  thalamus  and  sub- 
thalamic region.  This  bundle  of  fibers  appears  to  be  mainly  commissural  in 
character. 

The  transverse  fibers  of  the  isthmus  are  found  in  the  pons.  The  fibers 
of  the  ventral  as  well  as  those  of  the  more  dorsal  regions  have  their  origin 
in  nerve-cells  in  the  cortex  of  the  cerebellum.  From  their  origin  they  pass 
through  the  cerebellar  white  matter,  and  through  the  middle  peduncle  as 
far  as  the  median  line,  where  they  decussate  with  fibers  coming  from  the  op- 
posite side.  Beyond  this  point  they  pass  to  the  cerebellar  cortex.  From 
their  anatomic  relations  it  is  probable  that  these  transverse  fibers  are  com- 
missural in  character,  bringing  into  relation  opposite  but  corresponding 
regions  of  the  cerebellar  cortex.  In  addition  to  the  commissural  fibers  other 
transverse  fibers  associate  the  cerebellar  cortex  with  the  gray  matter  in  the 
pons  on  both  the  same  and  opposite  sides.  In  this  way  the  cerebellum  is 
brought  into  relation  with  longitudinal  fibers  coming  from  and  going  to  the 
cerebrum. 

FUNCTIONS   OF  THE   MEDULLA   OBLONGATA,   ISTHMUS,  CORPORA 
QUADRIGEMINA,  AND  BASAL  GANGLIA. 

Microscopic  examination  of  the  white  and  gray  matter  of  these  various 
parts  of  the  central  nerve  system  shows  that  they  are  composed  of  nerve- 
cells  and  nerve-fibers  which  morphologically  do  not  differ  in  essential  respects 
from  those  found  in  the  spinal  cord,  though  their  arrangement  is  far  more 


532  TEXT-BOOK  OF  PHYSIOLOGY. 

complicated  and  involved.  The  functions  of  these  closely  related  structures 
are  in  consequence  equally  complex  and  involved  and  but  imperfectly  known. 
In  a  general  way  it  may  be  said  that  by  virtue  of  the  presence  of  nerve- 
cells  and  definite  tracts  of  nerve-fibers  these  structures  collectively  may  be 
regarded  as  consisting: 

1.  Of  centers  for  reflex  actions;  and — 

2.  Of  conducting  paths  by  which  the  various  parts  are  brought  into  relation 

one  with  another  and  with  the  spinal  cord,  the  cerebellum,  and  the 
cerebrum. 

The  Medulla  Oblongata  and  Pons. — The  gray  matter  situated  in  these 
structures — i.e.,  just  beneath  the  floor  of  the  fourth  ventricle — contains 
nerve-cells  arranged  in  more  or  less  well-defined  groups  which  may  be  divided 
into  efferent  and  afferent. 

The  efferent  cells  are  the  immediate  sources  of  nerve  impulses  which  are 
transmitted  through  efferent  axons  to  various  peripheral  organs — skeletal 
muscles,  glands,  viscera,  and  blood-vessels.  Their  activity  may  be  excited 
by  the  same  influences  as  excite  the  'efferent  cells  of  the  spinal  cord:  e.g., 
variations  in  the  composition  of  the  blood  or  lymph;  the  arrival  of  nerve 
impulses  coming  through  afferent  pathways  in  the  spinal  cord  and  through 
afferent  cranial  nerves;  the  arrival  of  nerve  impulses  coming  through  efferent 
pathways  from  the  cerebrum.  The  peripheral  activity  resulting  from  their 
excitation  may  therefore  be  automatic  or  autochthonic,  peripheral  (reflex) 
or  cerebral  (volitional)  in  origin. 

The  afferent  cells  are  sentient  or  receptive  in  function,  inasmuch  as  they 
receive  nerve  impulses  coming  through  lower  afferent  pathways  and  transmit 
them  through  their  related  axons  to  the  cortex  of  the  cerebrum,  where  they 
evoke  sensations. 

The  efferent  cells  give  origin  to  nerve-fibers  which  pass  ventrally  and  be- 
come the  efferent  or  motor  cranial  nerves. 

The  afferent  cells  give  origin  to  fibers  which  pass  to  the  cerebral  cortex. 
Around  both  groups  of  cells,  the  afferent  or  sensor  cranial  nerves  terminate 
in  tuft-like  expansions.  (In  a  subsequent  section  the  origin,  course,  dis- 
tribution, and  functions  of  the  various  cranial  nerves  will  be  considered). 
But  as  the  function  of  the  nerve  is  only  to  transmit  energy  from  the  cell  of 
which  it  constitutes  a  part,  the  function  ascribed  to  the  nerve  may  without 
impropriety  be  transferred  to  the  cell  itself. 

Since  it  is  by  means  of  nerve-cells  and  their  associated  fibers  that  many 
important  functions  of  organic  life  are  initiated  and  maintained,  it  would 
naturally  be  expected  from  its  extensive  nerve  connections  that  this  region 
of  the  nerve  system  plays  an  extensive  role  in  this  respect.  As  the  accomplish- 
ment of  these  functions  requires  the  cooperation  and  coordination  of  a  number 
of  separate  but  related  structures,  it  is  evident  that  there  must  exist  in  the 
medulla  and  pons  a  number  of  coordinating  mechanisms  consisting  of  nerve- 
cells  and  nerve-fibers  which  are  associated  in  various  ways  for  the  accomplish- 
ment of  definite  functions.  To  such  a  coordinating  mechanism  the  term 
"center"  has  been  given:  e.g.,  respiratory,  cardiac,  deglutitory,  etc.  ^ 

'  By  the  term  center  as  here  employed  is  meant  a  collection  of  nerve-cells  and  nerve-fibers 
occupving  an  area  of  greater  or  less  extent,  though  its  exact  anatomic  limits  may  not  be  accurately 
defined.  That  an  area  may  merit  the  term  center,  it  is  necessary  that  its  stimulation  should 
increase,  its  destruction  should  abolish  or  impair,  functional  activit}'. 


FUNCTIONS  OF  THE  CRURA  CEREBRI.  533 

The  Medulla  Oblongata  and  Pons  as  Centers  for  Reflex  Activities,— 

Experimentation  has  shown  that  the  medulla  and  pons  contain  a  number 
of  such  centers,  the  more  important  of  which  are  as  follows: 

1.  Cardiac  centers,  which  exert   (i)  an  accelerator  action  over  the  heart's 

pulsations  through  nerve-fibers  emerging  from  the  spinal  cord  in  the 
roots  of  the  first  and  second  dorsal  nerves  and  reaching  the  heart  through 
the  sympathetic  nerve;  (2)  an  inhibitor  or  retarding  action  on  the  rate 
of  the  heart-beat  through  efferent  fibers  in  the  trunk  of  the  pneumogastric 
or  vagus  nerve.     (See  pages  313,  314-) 

2.  A  vaso-motor  center,  which  regulates  the  caliber  of  the  blood-vessels 

throughout  the  body  in  accordance  with  the  needs  of  the  organs  and 
tissues  for  blood,  through  nerve-fibers  passing  by  way  of  the  spinal 
nerves  to  the  walls  of  the  blood-vessels.     (See  page  372.) 

3.  A  respiratory  center,  which  coordinates  the  muscles  concerned  in  the 
•  production  of  the  respiratory  movements.     (See  page  416.) 

4.  A  mastication  center,  which  excites  to  activity  and  coordinates  the  muscles 

of  mastication.     (See  page  142.) 

5.  A  deglutition  center,  which  excites  and  coordinates  the  muscles  concerned 

in  the  transference  of  the  food  from  the  mouth  to  the  stomach.  (See 
page  162.) 

6.  An  articulation  center,  which  coordinates  the  muscles  necessary  to  the 

production  of  articulate  speech. 

7.  A  diabetic  center  stimulation  of  which  gives  rise  to  glycosuria. 

In  addition,  the  gray  matter  contains  centers  which  influence  the  secretion 
of  saliva,  provoke  vomiting,  coordinate  the  muscles  of  the  face  concerned  in 
expression,  and  control  the  secretion  of  the  perspiration. 

As  Conducting  Pathways. — The  anterior  pyramids  of  the  medulla  and 
their  continuations  through  the  more  ventral  portions  of  the  pons,  being 
portions  of  the  general  pyramidal  tract,  serv^e  to  conduct  volitional  efferent 
nerve  impulses  from  higher  portions  of  the  brain  to  the  spinal  cord.  Di\d- 
sion  of  these  pathways  is  at  once  followed  by  a  loss  of  volitional  control  of  the 
muscles  below  the  section. 

The  dorsal  or  tegmental  portion,  containing  the  fillet,  serves  to  transmit 
afferent  ner\'e  impulses  from  the  spinal  cord  to  higher  portions  of  the  brain. 
Transverse  division  of  one-half  of  the  dorsal  portion  of  the  pons  is  followed 
by  complete  anesthesia  of  the  opposite  half  of  the  body  without  any  im- 
pairment of  motion. 

The  restiform  bodies  constitute  a  pathway  between  the  spinal  cord  and 
the  cerebellum.  The  transverse  fibers  of  the  pons  associate  opposite  but 
corresponding  portions  of  the  cerebellar  hemispheres. 

The  Crura  Cerebri. — The  crura  cerebri  consists  ventrally  of  fibers 
which  are  largely  derived  from  the  pyramidal  tracts  and  are  continuous 
with  the  longitudinal  fibers  of  the  ventral  portion  of  the  pons  and  medulla; 
and  dorsally  of  fibers  continuous  with  those  coming  through  the  lower  por- 
tions of  the  tegmentum.  Hence  they  are  conductors  of  motor  impulses  in 
the  former  and  of  sensor  impulses  in  the  latter  region.  It  is  not  definitely 
known  as  to  whether  reflex  actions  take  place  through  the  gray  matter,  the 
locus  niger,  or  not. 

The  gray  matter  beneath  the  aqueduct  of  Sylvius  contains  nerve-cell 


534  TEXT-BOOK  OF  PHYSIOLOGY. 

groups  which  are  centers  for  reflex  actions  in  connection  with  ocular  move- 
ments: e.g.,  closure  of  the  lids,  contraction  of  the  sphincter  pupillae,  con- 
vergence of  the  eyes,  etc. 

The  Corpora  Quadrigemina. — From  the  anatomic  relation  of  the 
superior  quadrigeminal  body  (the  pre-geminum)  to  the  optic  tract,  the 
inference  can  be  drawn  that  it  is  in  some  way  essential  to  the  performance  of 
various  reflex  ocular  movements  and  perhaps  to  the  variations  in  size  of  the 
pupil.  Experimental  investigations  and  pathologic  changes  support  the 
inference. 

Irritation  of  the  pre-geminum  in  monkeys  on  one  side  is  followed  by 
diminution  of  the  pupils  first  on  the  opposite  side  and  then  almost  immedi- 
atelv  on  the  same  side.  The  eyes  at  the  same  time  are  also  widely  opened  and 
the  eyeballs  turned  upward  and  to  the  opposite  side.  If  the  irritation  be 
continued,  motor  reactions  are  exhibited  in  various  parts  of  the  body. 
Destruction  of  the  pre-geminum  in  both  monkeys  and  rabbits  is  followed  by 
blindness,  dilatation  and  immobility  of  the  pupils,  with  marked  disturbance 
of  eciuilibrium  and  locomotion  (Ferrier). 

From  the  anatomic  relation  of  the  inferior  quadrigeminal  body  (the 
post-geminum)  to  the  lateral  fillet,  the  basal  tract  for  hearing,  the  inference 
may  be  drawn  that  it  is  in  some  way  connected  with  the  auditory  process. 

Stimulation  of  the  post-geminum  gives  rise  to  cries  and  various  forms 
of  vocalization.  Pathologic  states  of  this  body  are  also  attended  by  impair- 
ment of  hearing  and  disorders  of  the  equilibrium. 

From  the  foregoing  facts  it  is  probable  that  the  corpora  quadrigemina  are 
associated  with  station  and  locomotion.  Ferrier  assumes  that  in  these 
bodies  "sensory  impressions,  retinal  and  others,  are  coordinated  with  adap- 
tive motor  reactions  such  as  are  involved  in  equilibration  and  locomotion." 

The  Corpora  Striata. — The  relation  of  these  bodies  to  the  pyramidal 
motor  tract  would  indicate  that  they  are  in  some  way  connected  with  motor 
activities.  Their  function,  however,  is  obscure.  WTiile  stimulation  of  one 
corpus  produces  convulsion  of  the  muscles  of  the  opposite  side  of  the  body, 
and  destruction  gives  rise  to  paralysis  of  the  corresponding  muscles,  it  is 
difiicult,  owing  to  the  intimate  association  of  the  white  and  the  gray  matter, 
to  state  to  which  the  phenomena  are  to  be  attributed.  The  evidence  at 
hand  points  to  the  conclusion  that  if  a  lesion  is  limited  to  the  gray  matter  the 
paralysis  which  might  result  would  be  but  temporary  and  of  short  duration. 
The  pathologic  evidence  is  of  a  similar  character.  Gowers  is  of  the  opinion, 
that  if  the  lesion  is  small  and  at  a  sufl&cient  distance  from  the  white  fibers  of 
the  capsule,  there  may  even  be  no  initial  hemiplegia;  neither  motor  nor 
sensory  paralysis  ^^ill  arise  if  the  lesion  is  confined  to  the  gray  matter. 

It  is  stated  by  some  experimenters  that  localized  injuries,  both  experi- 
mental and  pathologic,  are  followed  by  a  persistent  rise  of  temperature, 
varying  from  i°  to  2.6°  C. 

The  Optic  Thalami. — From  the  anatomic  relation  of  the  optic  thalami 
to  the  general  and  special  sense  nerve-tracts,  on  the  one  hand,  and  to  the 
cerebral  cortex,  on  the  other  hand,  it  is  assumed  that  they  are  connected 
with  the  production  of  sensations  both  general  and  special,  and  act  as 
intermediaries  between  the  peripheral  sense-organs  and  the  cortex. 

The  results  of  experimental  stimulation  and  destruction  of  the  thalami 


FUNCTIONS  OF  THE  INTERNAL  a\PSULE. 


535 


are  extremely  contradictory  and  fail  to  throw  much  light  on  their  functions. 
Ferrier  states  that  destruction  of  the  posterior  part  of  one  thalamus  pro- 
duced bhndness  in  the  opposite  eye  and  impairment  of  the  sense  of  touch 
and  pain  in  the  opposite  side  of  the  body.  In  a  patient  under  the  care  of 
Hughlings  Jackson  there  was  blindness  in  the  right  half  of  each  eye,  loss  of 
hearing  in  the  left  ear,  impairment  of  taste  on  the  left  side  of  the  tongue, 
and  a  diminution  of  the  sense  of  touch  on  the  left  side  of  the  body.  Post- 
mortem examination  showed  a  patch  of  softening  in  the  posterior  part  of 
the  right  thalamus,  the  remainder  of  the  organ  being  normal. 

It  is  probable  that  in  the  thalamus  visual,  tactile,  and  labyrinthine  im- 
pressions are  received,  coordinated,  and  reflected  outward,  with  the  result 
of  producing  various  adaptive  motor  reactions  connected  with  station  and 


Fig.  246. — Horizontal  Section  of  the  Internal  Cu-slle  Showing  the  Position  and 
Relation  of  the  Motor  Tilacts  for  the  Eye,  Head,  Tongue,  Mouth,  Shoulder  (Shi.), 
Elbow  (Elb.),  Digits  of  Hand  (Dig.),  Abdomen  (Abd.),  Hip,  Knee  (Kn.),  Digits  of  Foot 
(Dig.).     S.  Sensor  tract.     O.  T.  Optic  tract.     A.  T.  Auditory  tract. 

equilibrium.  Tlie  thalamus  is  believed  by  some  investigators  to  act  also  as 
an  intermediary  between  emotional  states  and  their  expression  in  the  muscles 
of  the  face,  this  power  being  lost  in  certain  pathologic  conditions.  The 
power  of  regulating  the  temperature  of  the  body  has  been  also  assigned  to 
the  thalamus,  as  destruction  of  its  anterior  extremity  is  usually  followed  bv 
a  rise  in  temperature. 

The  Internal  Capsule. — The  internal  capsule  has  been  shown  by  the 
results  both  of  experiment  and  of  pathologic  processes  to  be,  first,  a  pathway 
for  the  transmission  of  ner\'e  impulses  from  the  cerebral  cortex  to  the  pons, 
medulla,  and  spinal  cord,  which  give  rise  to  contraction  of  the  muscles  of  the 
opposite  side  of -the  body;  and,  second,  a  pathway  for  the  transmission  of 
ner^-e  impulses  coming  from  skin,  mucous  membrane,  muscles,  and  special 


536  TEXT-BOOK  OF  PHYSIOLOGY. 

sense-organs  lo  the  cortex,  where  they  give  rise  to  sensations  general  and 
special.  It  is  therefore  the  common  motor  and  sensor  pathway.  For  the 
reason  that  it  transmits  both  motor  and  sensor  impulses,  and  for  the  further 
reason  that  it  is  frequently  the  seat  of  pathologic  lesions  which  are  followed 
by  either  a  loss  of  motion  or  sensation  or  both,  the  internal  capsule  is  one 
of  the  most  interesting  parts  of  the  central  nerve  system.  As  shown  in  Fig. 
246,  it  consists  of  two  segments  or  limbs  united  at  an  obtuse  angle,  the  knee 
or  elbow,  which  is  directed  toward  the  median  line.  The  motor  tract  is 
confined  to  the  posterior  one-third  of  the  anterior  segment  and  the  anterior 
two-thirds  of  the  posterior  segment.  The  sensor  tract  is  confined  to  the 
posterior  one-third  of  the  posterior  segment,  the  extreme  end  of  which  also 
contains  the  optic  and  auditory  tracts. 

The  region  of  the  anterior  segment  in  front  of  the  motor  tract  contains 
the  fibers  of  the  fronto-cerebellar  tract,  the  function  of  which  is  unknown. 

The  motor  region  contains  fibers  which  descend  from  the  cerebral  cortex 
to  nerve-centers  situated  in  the  gray  matter  beneath  the  aqueduct  of  Sylvius, 
in  the  gray  matter  beneath  the  floor  of  the  fourth  ventricle,  and  in  the  anterior 
horns  of  the  gray  matter  of  the  spinal  cord,  and  which  in  turn  are  connected 
by  the  cranial  and  spinal  nerves  with  the  muscles  of  the  eye,  head,  face, 
trunk,  and  limbs.  The  positions  occupied  by  these  different  tracts  are 
shown  in  Fig.  246. 

The  relation  of  the  internal  capsule  to  the  caudate  nucleus  and  the  optic 
thalamus  internally,  and  to  the  lenticular  nucleus  externally,  is  also  shown 
in  a  vertical  section  of  the  cerebrum  made  in  front  of  the  gray  commissure 
(Fig.  237).  From  the  fact  that  the  internal  capsule  contains  efferent  or 
motor  tracts,  and  afferent  or  sensor  tracts,  it  is  evident  that  a  destructive 
lesion  of  the  motor  tract  would  be  followed  by  a  loss  of  motion;  and  of  the 
sensor  tract,  by  a  loss  of  sensation  on  the  opposite  side  of' the  body. 


CHAPTER  XXI. 
THE  CEREBRUM. 

The  cerebrum  is  the  largest  portion  of  the  encephalon,  constituting 
about  85  per  cent,  of  its  total  weight.  In  shape  it  is  ovate,  convex  on  its 
outer  surface,  narrow  in  front  and  broad  behind.  It  is  divided  by  a  deep 
longitudinal  cleft  or  fissure  into  halves,  known  as  the  cerebral  hemispheres. 
The  hemispheres  are  completely  separated  anteriorly  and  posteriorly  by  this 
fissure,  but  in  their  middle  portions  are  united  by  a  broad  white  band  of 
nen-e  fibers,  the  corpus  callosum.  Each  hemisphere  or  hemi-cerebrum  is 
convex  on  its  outer  aspect,  and  corresponds  in  a  general  way  with  each  side 
of  the  cavity  of  the  skull;  the  inner  or  mesial  surface  is  flat  and  forms  the 
lateral  boundary  of  the  longitudinal  fissure. 

The  surface  of  each  hemi-cerebrum  presents  a  series  of  alternate  indenta- 
tions and  elevations,  known  respectively  as  fissures  or  sulci,  and  convolutions 
or  gyri.  A  knowledge  of  the  situation  and  extent  of  the  principal  fissures 
and  convolutions,  as  well  as  of  their  relation  one  to  another,  is  essential  to  a 
clear  understanding  of  many  physiologic  processes,  clinical  phenomena, 
and  surgical  procedures.  The  general  arrangement  of  the  primary  fissures 
and  convolutions  is  represented  in  Figs.  247  and  248. 

Fissures. 

1.  The  Sylvian  fissure,  one  of  the  most  important  of  the  primary  fissures,  is 

found  on  the  side  of  the  cerebrum.  It  begins  at  the  base  and  extends 
upward,  outward,  and  backward  to  a  point  corresponding  to  the  emi- 
nence of  the  parietal  bone,  where  it  usually  terminates  in  a  more  or  less 
vertically  directed  branch,  the  epi-sylvian  branch.  Anteriorly  a  short 
branch  is  given  off  which  passes  upward  and  forward  into  the  frontal 
lobe  and  known  as  the  pre-syhaan;  a  horizontal  branch  is  known  as  the 
sub-sylvian.  The  Sylvian  fissure  is  the  first  to  appear  in  the  develop- 
ment of  the  fetal  brain,  becoming  visible  at  the  third  month.  In  the 
adult  it  is  deep  and  well  marked  and  divides  the  hemi-cerebrum  into  a 
frontal  and  a  temporo-sphenoidal  lobe. 

2.  The  Rolandic  or  central  fissure,  equally  important,  is  found  on  the  superior 

and  lateral  aspects  of  the  cerebrum.  It  runs  from  a  point  on  the  con- 
vexity of  the  hemisphere  near  the  median  line  transversely  outward  and 
downward  toward  the  fissure  of  Sylvius,  but  as  a  rule  does  not  pass  into 
it.  It  divides  the  frontal  from  the  parietal  lobe.  The  inclination  of 
the  central  fissure  is  such  as  to  form  with  the  longitudinal  fissure  an  angle 
of  about  70  degrees. 

3.  The  intra-parieial fissure  arises  a  short  distance  behind  the  central  fissure. 

It  then  runs  upward,  backward,  and  downward  to  terminate  near  the 
posterior  extremity  of  the  parietal  lobe.  It  divides  the  parietal  lobe 
into  a  superior  and  an  inferior  portion. 

537 


538 


TEXT-BOOK  OF  PHYSIOLOGY. 


4.  The  parieto-occipital  fissure,  situated  on  the  mesial  surface  of  the  hemi- 
spheres, divides  the  latter  into  a  parietal  and  an  occipital  lobe.  It  begins 
as  a  deep  notch  on  the  surface  of  the  hemisphere,  and  is  then  continued 
downward  and  forward  until  it  enters  the  calcarine  fissure.     (Fig.  248.) 


SVfilRCfnTfiAt   f. 


Fig.  247. — Fissures  and  Gyri  on  the  Lateral  Surface  of  the  Left  Hemi-cerebrum. — 
F.  Fissure.     G.  Gyrus.     R.  Ramus. 


Fig.  248. — Fissures  and  Gyri  of  the  Mesial  Surface  of  the  Left  Hemi-cerebrum 

{Spitzka^ 

5.  The  calcarine  fissure  begins  on  the  posterior  extremity  of  the  mesial  surface 
of  the  occipital  lobe.  From  this  point  it  passes  downward  and  forward 
to  unite  with  the  parieto-occipital  fissure. 


THE  CEREBRUM. 


539 


6.  The  para-central  fissure  begins  at  the  supero-mesial  border  of  the  hemis- 

phere. It  then  passes  downward  and  forward  for  a  variable  distance 
and  then  turns  upward  enveloping  a  lobule  known  as  the  para- 
central lobule. 

7.  The  super-callosal  fissure  extends  from  a  point  just  anterior  to  the  para- 

central lobule  downward  and  forward  below  the  rostrum  of  the  corpus 
callosum. 

Secondary  fissures  of  more  or  less  importance  are  present  in  the  different 
lobes,  subdividing  the  surface  into  convolutions:  e.g.,  in  the  frontal  lobe  are 
found  the  pre-central,  the  super-frontal,  medi-frmital  and  snb-frantal  fissures; 
in  the  temporal  lobe  the  super-temporal  and  medi-temporal  fissures. 

Convolutions.^ — The  convolutions  or  gyri  are  the  portions  of  the  cere- 
bral surface  comprised  between  the  fissures.  The  arrangement  of  the  sur- 
face is  such  that  only  the  more  superficial  portions  are  visible.  The  depth 
of  the  convolution,  the  portion  bordering  the  fissure,  is  concealed  from  view. 
Each  lobe  presents  a  series  of  such  convolutions  which  differ  considerably 
in  their  relative  physiologic  importance. 

The  Frontal  Lobe. — The  frontal  lobe  presents  on  its  convex  surface 
four  convolutions:,  viz.,  the  anterior  or  pre-central  convolution,  and  the 
super-,  medi-,  and  sub-frontal  convolutions. 

1.  The  anterior  or  pre-central  convolution  or  gyrus  is  situated  just  in  front  of 

the  Rolandic  or  central  fissure,  with  which  it  corresponds  in  direction. 
It  is  continuous  above  with  the  super-frontal  and  below  with  the  sub- 
frontal  convolution. 

2.  The  super-frontal  convolution  or  gyrus  is  bounded  internally  by  the  longi- 

tudinal fissure  and  externally  by  the  super-frontal  fissure.  From  the 
upper  end  of  the  pre-central  convolution,  with  which  it  is  continuous, 
it  runs  forward  and  downward  to  the  anterior  extremity  of  the  frontal 
lobe,  where  it  turns  backward  and  rests  on  the  orbital  plate  of  the 
frontal  bone. 

3.  The  medi-frontal  convolution  or  gyrus  is  situated  on  the  side  of  the  lobe, 

between  the  super-frontal  fissure  above  and  the  medi-frontal  fissure 
below.     Its  general  direction  is  downward  and  forward. 

4.  The  sub-frontal  convolution  or  gyrus  winds  around  the  pre-sylvian  branch 

of  the  fissure  of  Sylvius  in  the  anterior  and  inferior  portion  of  the  frontal 

lobe.     It  is  continuous  posteriorly  with  the  lower  end  of  the  pre-central 

convolution. 

The   Parietal  Lobe. — The  parietal   lobe  presents  three  well-marked 

convolutions:  viz.,  the  posterior  or  post-central  convolution,  and  the  super- 

and  sub-parietal.     The  latter  is  again  subdivided  into  the  marginal  and  the 

angular  convolution. 

1.  The  posterior  or  post-central  convolution  or  gyrus  is  situated  just  behind  the 

Rolandic  or  central  fissure,  with  which  it  corresponds  in  direction. 
Above,  it  is  continuous  with  the  super-parietal  convolution;  below,  with 
the  marginal  and  the  pre-central  convolutions. 

2.  The  super-parietal  convolution  or  gyrus  is  bounded  internally  by  the  longi- 

tudinal fissure  and  externally  by  the  intra-parietal  fissure.  From  the 
upper  end  of  the  post-central  convolution,  with  which  it  is  connected, 
it  runs  downward  and  backward  as  far  as  the  parieto-occipital  fissure. 


540  TEXT-BOOK  OF  PHYSIOLOGY. 

3.  The  sub-parietal  convolution  or  gyrus  is  connected  anteriorly  with  the 
post-central    convolution.     Passing    backward,    it    winds    around    the 
superior  extremity  of  the  fissure  of  Sylvius,  in  which  situation  it  is  known 
as  the  supra-marginal  convolution.     Beyond  this  point  it  divides  into 
two  portions,  one  of  which  runs  forward  into  the  temporal  lobe  above 
the  super-temporal  fissure,  while  the  other  runs  downward  and  back- 
ward, following  the  intra-parietal  fissure  to  its  termination.     At  this 
point  it  makes  a  sharp  bend  and  runs  forward  into  the  temporal  lobe 
just  beneath  the  super-temporal  fissure.     In  the  neighborhood  of  the 
bend  it  is  generally  known  as  the  angular  convolution  or  gyrus. 
The  Temporo-sphenoidal  Lobe.— The  temporo-sphenoidal  lobe  presents 
on  its  external  surface  three  well-marked  convolutions:  viz.,  the  super-,  the 
medi-,  and  the  sub-temporal,  separated  by  the  super-  and  medi-temporal 
fissures.     These  three  convolutions  are  in  a  general  way  parallel  with  each 
other,  and  pursue  a  direction  from  before  backward  and  upward.     Ante- 
riorly, they  are  fused  together,  but  posteriorly  their  connections  are  some- 
what different.     The  supertemporal  is  continuous  behind  and  above  with 
the  supra-marginal  convolution,  and  behind  and  below  with  the  angular 
convolution  or  gyrus.     The  medi-temporal  blends  with  the  preceding  and 
with  the  middle  occipital.     The  sub-temporal  is  continuous  with  the  in- 
ferior occipital. 

The  Occipital  Lobe. — The  occipital  lobe  is  triangular  in  shape  and 
forms  the  posterior  apex  of  the  hemisphere.  Its  base  on  the  external 
surface  is  formed  by  an  imaginary  line  drawn  from  the  parieto-occipital 
fissure  to  the  pre-occipital  notch  on  the  inferior  and  lateral  border.  The 
external  surface  presents  three  convolutions — the  superior,  middle,  and  in- 
ferior occipital. 

The  inner  or  mesial  surface  of  the  hemisphere,  formed  in  part  by  the 
frontal,  the  parietal,  the  occipital,  and  the  temporal  lobes,  presents  several 
convolutions  of  much  physiologic  interest,  viz. : 

1.  The  callosal  convolution,  or  gyrus,  situated  between  the  super-callosal 

fissure  and  the  corpus  callosum.  From  its  origin  anteriorly  at  the  base 
of  the  brain  this  convolution  passes  backward,  gradually  increasing  in 
width  as  it  approaches  the  posterior  extremity  of  the  corpus  callosum. 
At  this  point  it  again  narrows  and  descends  between  the  calcarine  and 
hippocampal   fissures   to   blend   with    the   hippocampal    convolution. 

2.  The  hippocampal  gyrus,  formed  by  the  union  of  the  posterior  extremity 

of  the  callosal  convolution  and  the  sub-calcarine  convolution  is  situated 
just  below  the  dentate  or  hippo-campal  fissure.  Anteriorly  it  becomes 
enlarged,  and  just  behind  the  apex  of  the  temporal  lobe  turns  backward 
and  inward  to  form  a  hook-shaped  eminence,  the  uncinate  gyrus  or 
uncus. 

The  limbic  lobe  is  the  name  given  to  an  area  of  the  brain  which 
includes,  among  other  structures,  the  callosal  convolution,  the  gyrus 
hippocampus,  and  the  uncus.  As  forming  a  part  of  this  general 
lobe  may  be  mentioned  the  dentate  fascia,  the  striae  and  peduncle  of 
the  corpus  callosum,  the  septum  lucidum,  the  fornix,  and  the  in- 
fra-callosal  gyrus. 

3.  The  sub-collateral  convolution  or  gyrus  is  bounded  by  the  collateral  fissure 


THE  CEREBRUM. 


541 


M^ 


above,  and  its  inferior  border  extends  from  the  occipital  lobe  to  the 
anterior  pole  of  the  temporal  lobe. 

4.  The  quadrate  lobule,  or  precuneus,  a  square-shaped  convolution,  is  situated 

between  the  posterior  termination  of  the  para-central  fissure  and  the 
parieto-occipital  fissure.  It  blends  with  the  callosal  convolution,  on  the 
one  hand,  and  with  the  parietal  lobule  on  the  other. 

5.  The  cuneus,  a  triangular  or  w^edge-shaped  convolution  or  lobule,  is  situated 

on  the  mesial  surface  of  the  occipital  lobe 
between  the  parieto-occipital  and  calca- 
rine  fissures. 
The    Insula   or   Island   of  Reil. — This 
anatomic    structure   consists  of  a  triangular- 
shaped  cluster  of  six  small  convolutions  situa- 
ted  at  the  bifurcation  of  the  Sylvian  fissure 
and  concealed  from  view  by  the  convolutions 
bordering  it,  spoken  of  collectively  as  the  oper- 
culum.    These    convolutions    are    connected 
with  the  frontal,  the  parietal,  and  the  tem- 
poral lobes. 

Structure  of  the  Gray  Matter  of  the 
Cortex. — The  gray  matter,  the  cortex  "of  the 
cerebrum,  varies  from  two  to  four  millime- 
ters in  thickness.  When  examined  with  a  lens 
of  low  power,  it  presents  a  laminated  appear- 
ance, due  to  differences  in  color  and  arrange- 
ment of  its  constituent  elements.  With 
higher  magnification  the  cortex  is  seen  to  con- 
sist of  neuroglia  cells,  nerve-cells  with  special- 
ized dendrites  and  axons,  medullated  and  non- 
medullated  nerve-fibers,  blood-vessels,  con- 
nective tissue,  etc.,  all  of  which  are  arranged 
and  interblended  in  a  most  intricate  man- 
ner. Notwithstanding  the  complexity  of  its 
structure,  modern  histologic  methods  have 
enabled  Cajal  to  divide  it  into  four  fairly  dis- 
tinct layers  or  zones,  from  without  inward,  as 
follows  (Fig.  249) : 

I .  The  Molecular  Layer. — The  most  superficial 
portion  of  this  layer  consists  mainly  of 
neuroglia  or  glia  cells,  the  processes  of 
which  interlace  in  all  directions,  forming 
a  distinct  sheath  just  beneath  the  pia. 
The  deeper  portions  of  this  layer  con- 
tain a  specialized  type  of  ner\'e-cells  (Cajal  cells),  of  which  there 
are  several  varieties.  These  cells  give  oft'  nerve-fibers  which  purr 
sue  a  horizontal  direction  for  a  variable  distance,  but  in  their 
course  give  off  collateral  branches  which  ascend  to  the  outer  surface 
of  the  layer.  Among  these  structures  are  to  be  found,  also,  den- 
dritic processes  of  cells  situated  in  the  subjacent  layer.     The  terminal 


Fig.  249. — Section  of  the 
Cerebr.\l  Cortex  (Motor  Area) 
OF  Child,  Stained  by  Golgi's 
Silver  Method.  A.  Layer  of 
neuroglia  cells.  B.  Layer  of  small 
pyramidal  ganglion  cells.  C 
Laver  of  large  pyramidal  cells.  D. 
Laver  of  irregular  smaller  cells. — 
(Pier  sol.) 


542  TEXT-BOOK  OF  PHYSIOLOGY. 

filaments  of  meduUated  nerve-fibers  coming  from  nerve-cells  in  lower 
regions  of  the  encephalo-spinal  axis  are  also  present,  but  for  the  most 
part  they  pursue  a  tangential  direction. 

2.  The  Layer  of  Small  Pyramidal  Cells. — This  layer   consists    mainly    of 

nerve-cells,  the  majority  of  which  are  pyramidal  in  shape  and  of  small 
size.  Other  cells,  however,  are  present,  which  present  a  variety  of 
shapes,  for  which  reason  the  layer  was  at  one  time  termed  the  ambiguous 
layer.  The  apical  process  of  the  pyramidal  cells  is  broad  at  the  base, 
but  narrows  rapidly  as  it  passes  upward.  It  frequently  divides  into  sev- 
eral branches,  each  of  which  develops  club-shaped  processes  or  gem- 
mules,  which  give  to  it  a  feathery  appearance.  Dendrites  are  also  given 
off  from  the  sides  and  base  of  the  cell-body.  From  the  base  a  single 
axon  descends  which  ultimately  becomes  the  axis-cylinder  of  a  mod- 
ulated nerve. 

3.  The  Layer  of  Large  Pyramidal  Cells. — The  nerve-cells  of  this  layer,  as  the 

name  implies,  are  also  pyramidal  in  shape,  but  of  large  size.  Each 
cell  presents  the  same  features  as  the  cells  of  the  preceding  layer,  with 
the  exception  that  the  apical  process  is  larger,  better  developed,  and 
branches  more  freely.  All  the  dendrites  are  extensively  provided  with 
gemmules.  The  axon  is  well  developed,  sharply  defined,  and  smooth. 
After  giving  off  collateral  branches,  the  axon  descends  into  the  cere- 
brum and  becomes  a  medullated  nerve-fiber. 

4.  The  Layer  of  Polymorphous  Cells. — In  this  layer  the  nerve-cells  present  a 

variety  of  forms:  e.g.,  spindle,  polygonal,  pyramidal,  etc.     The  spindle 
form  is  the  most  common.     From  either  end  of  the  spindle  a  large 
dendrite  emerges,  soon  branches,  and  becomes  gemmulated.     The  axon 
is  well  defined  and  it  soon  descends  into  the  white  matter. 
The  Number  of  Cortical  Cells. — Attempts  have  been  made  by  various 
histologists  to  estimate  the  total  number  of  functional  nerve-cells  in  the  cere- 
bral cortex  of  man.     Though  the  estimates  are  widely  different,  the  lowest 
presents   numbers   which   are   beyond   comprehension.     Thus,   Meynert's 
estimate  is  612  millions;  Donaldson's  estimate  for  the  entire  brain  is  12000 
millions;  and  Thompson's  9283  millions. 

Structure  of  the  White  Matter. — The  white  matter  of  the  cerebrum 
consists  of  medullated  nerve-fibers  which,  though  intricately  arranged,  may 
be  divided  into  three  systems:  viz.,  the  commissural,  the  association,  and  the 
projection. 

1.  The  commissural  system.     The  fibers  which  compose  this  system  unite 

corresponding  areas  of  the  cortex  of  each  hemisphere.  The  fibers 
from  the  frontal,  parietal,  and  occipital  lobes  cross  in  the  median  line 
and  form  a  band  of  transversely  arranged  fibers,  the  corpus  callosum. 
The  fibers  which  unite  the  corresponding  areas  of  the  temporo-sphe- 
noidal  lobes  cross  in  the  anterior  commissure.  All  the  commissural  fib- 
ers are  the  axons  of  nerve-cells  in  the  cortex,  the  terminals  of  which  are  to 
be  found  in  the  cortex  of  the  opposite  side. 

2.  The  association  system.     The  fibers  which  compose  this  system  unite 

neighboring  as  well  as  distant  parts  of  the  same  hemisphere,  and  may 
therefore  be  divided  into  long  and  short  fibers.  They  associate  the  in- 
excitable  or  association  areas  with  the  excitable  or  projection  areas. 


THE  CEREBRUM.  543 

3.   The  projection  system.     The  fibers  composing  this  system  unite  certain 
areas  of  the  cortex  of  the  cerebrum  with  the  basal  gangha,  the  pon^, 
medulla  oblongata,  and  spinal  cord.     They  may  be  divided  into:  (i) 
afferent  fibers  which  have  their  origin  in  the  lower  nerve-centers  at  dif- 
ferent levels  and  thence  pass  to  the  cortex;  and  (2)  efferent  fibers  which 
have  their  origin  in  the  cortex  and  thence  pass  to  the  lower  nerve-centers, 
terminating  at  different  levels.     The  former  are  also  lermed  the  cortico- 
afferent  or  corticopetal;  the  latter,  corticoefferent  or  corticofugal. 
The  afferent  fibers,  the  so-called  sensor  tract,  which  transmit  nerve  im- 
pulses coming  from  the  general  periphery  and  the  sense-organs,  pass  through 
the  tegmentum  as  the  mesial  and  lateral  fillets,  and  thence  to  the  cortex 
directly  by  way  of  the  internal  capsule,  or  indirectly  through  the  intermedia- 
tion of  the  thalamic  and  subthalamic  nuclei.     See  Fig.  245,  page  530.     The 
distribution  of  these  fibers  to  the  various  areas  of  the  cortex  will  be  stated  in 
following  paragraphs. 

The  efferent  fibers  of  the  so-called  motor  tract  w^hich  transmit  motor  or 
volitional  nerv'c  impulses  from  the  cortex  to  the  pons,  medulla,  and  spinal 
cord,  emerge  from  the  layer  of  pyramidal  cells  of  the  gray  matter  of  the  an- 
terior or  the  pre-central  convolution,  the  paracentral  lobule,  and  immediately 
adjacent  areas.  From  this  origin  the  axons  descend  through  the  white 
matter  of  the  corona  radiata,  converging  toward  the  internal  capsule,  into 
and  through  which  they  pass,  occupying  the  anterior  two-thirds  of  the  poste- 
rior limb  or  segment.  Beyond  the  capsule  they  continue  to  descend,  occupy- 
ing the  middle  three-fifths  of  the  pes  or  crusta  of  the  crus  cerebri,  the  ventral 
portion  of  the  pons,  and  eventually  the  anterior  pyramid  of  the  medulla 
oblongata.     At  this  point  the  tract  divides  into  two  portions,  viz.: 

1.  A  large  portion,  containing  from  ninety-one  to  ninety-seven  per  cent,  of 

the  fibers,  which  decussates  at  the  lower  border  of  the  medulla  and 
passes  down  the  lateral  column  of  the  cord,  constituting  the  crossed 
pyramidal  tract.  , 

2.  A  small  portion,  containing  from  three  to  nine  per  cent,  of  the  fibers, 

which  does  not  decussate  at  the  medulla,  but  passes  down  the  inner  side 

of  the  anterior  column  of  the  same  side,  constituting  the  direct  pyramidal 

tract  or  column  of  TUrck. 

After  passing  through  the  internal  capsule,  and  as  it  descends  through 

the  crus,  pons,  and  medulla,  the  cortico-efferent  tract  gives  off  a  number  of 

fibers  which  cross  the  median  line  and  arborize  around  the  nerve-cells  of 

the  gray  matter  beneath  the  aqueduct  of  SyKaus  (the  nuclei  of  origin  of  the 

third  and  fourth  cranial  nerves),  and  around  the  nerve-cells  in  the  gray 

matter  beneath  the  floor  of  the  fourth  ventricle  (the  nuclei  of  origin  of  the 

remainder  of  the  motor  cranial  nerves).     The  remaining  fibers  go  to  form 

the  crossed  and  direct  pyramidal  tracts  and  arborize  around  the  cells  in  the 

anterior  horn  of  the  gray  matter  of  the  opposite  side  of  the  cord  at  successive 

levels.     By  this  means  the  cortex  is  brought  into  anatomic  and  physiologic 

relation  with  the  general  musculature  of  the  body  through  the  various  cranial 

and  spinal  motor  nerves.     (See  Fig.  237,  page  517.) 

The  fronto-cerebellar  and  the  occipito-temporo-cerebeUar  tracts  are  also 
efferent  tracts  and  parts  of  the  projection  system.     The  fronto-cerebellar, 


544  TEXT-BOOK  OF  PHYSIOLOGY. 

originating  in  the  nerve-cells  of  the  cortex  of  the  frontal  lobe,  passes  down  to 
and  through  the  internal  capsule,  occupying  the  anterior  one-third  of  the 
anterior  segment.  It  then  descends  along  the  inner  side  of  the  crus  cerebri 
to  the  ])ons,  where  its  fibers  arborize  around  the  cells  of  the  nucleus  pontis. 
Through  the  intermediation  of  these  cells  this  tract  is  brought  into  relation 
with  the  cerebellum  of  the  same  but  chiefly  of  the  opposite  side.  The 
occipito-temporal  tract,  originating  in  the  cells  of  the  cortex  of  both  the 
occipital  and  temporal  lobes,  passes  downward  and  inward  toward  the 
lenticular  nucleus,  beneath  which  it  passes  to  enter  the  outer  one-fifth  of  the 
crusta.  It  then  enters  the  pons,  and  through  the  nucleus  pontis  also  comes 
into  relation  with  the  cerebellum  of  both  sides.     (See  Fig.  245,  page  530.) 

THE  FUNCTIONS  OF  THE  CEREBRUM. 

The  functions  of  the  cerebrum  comprehend,  in  man  at  least,  all  that 
pertains  to  sensation,  cognition,  feeling,  and  volition.     All  subjective  experi 
ences,  which  in  their  totality  constitute  mind,  are  dependent  on  and  asso- 
ciated with  the  anatomic  integrity  and  the  physiologic  activity  of  the  cere- 
brum and  its  related  sense-organs,  the  eye,  ear,  nose,  tongue,  etc. 

From  an  examination  of  the  anatomic  development  of  the  brain  in  difTerent 
classes  of  animals,  in  different  men  and  races  of  men,  and  from  a  study  of  the 
pathologic  lesions  and  the  results  of  experimental  lesions  of  the  brain,  evi- 
dence has  been  obtained  which  reveals  in  a  striking  manner  the  intimate 
connection  of  the  cerebrum  and  all  phases  of  mental  activity. 

1.  Comparative  anatomic  investigations  show  that  there  is  a  general  connec- 

tion between  the  size  of  the  brain,  its  texture,  the  depth  and  number  of 
convolutions,  and  the  exhibition  of  mental  power.  Throughout  the 
entire  animal  series  an  increase  in  intelligence  goes  hand  in  hand  with 
an  increase  in  the  development  of  the  brain.  In  man  there  is  an  enor- 
mous increase  in  size  over  that  of  the  highest  animals,  the  anthropoid 
apes.  The  most  cultivated  races  of  men  have  the  greatest  cranial 
capacity,  that  of  the  educated  European  or  American  being  approxi- 
mately 92.1  cubic  inches  (1835  c.c);  while  that  of  the  Australian  is  but 

81.7  cubic  inches  (1628  c.c).  Men  distinguished  for  great  mental 
power  usually  have  large  and  well-developed  brains;  e.g.,  that  of  Cuvier 
weighed  64.4  ounces  (1830  grams);  that  of  Abercrombie,  63  ounces 
(1786  grams).  A  large  intelligence,  however,  is  not  incompatible  w^ith 
a  much  smaller  brain  weight;  thus,  the  brain  of  Helmholtz  weighed  but 

50.8  ounces  (1440  grams);  that  of  Leidy,  49.9  ounces  (141 5  grams); 
that  of  Liebig,  47.7  ounces  (1352  grams).  The  average  brain 
weight  of  96  distinguished  men  has  been  found  to  be  51.9  ounces 
(1473  grams)  (Spitzka). 

2.  Pathologic  lesions  and  mechanic  injuries  which  disorganize  the  cerebrum 

are  at  once  followed  by  a  disturbance  or  an  entire  suspension  of  mental 
activity.  Concussion  of  the  brain  or  sudden  compression  from  a  hem- 
orrhage destroys  consciousness.  Physical  and  chemic  alterations  of 
the  gray  matter  of  the  cerebrum  have  been  shown  to  coexist  with  insan- 
ity, loss  of  memory,  loss  of  articulate  speech,  etc.  Congenital  defects  of 
organization  are  accompanied  by  a  deficiency  in  mental  capacity  and 
the  higher  instincts.     Under  such  circumstances  no  great  advance  in 


THE  CEREBRUM.  545 

brain  development  is  possible  and  the  intelligence  remains  at  a  low 
level.  In  congenital  idiocy  the  brain  is  small,  imperfectly  developed, 
and  wanting  in  proper  chemic  composition. 

Experimental  lesions  of  the  brain  in  lower  animals  are  attended  by  results 
similar  to  those  observed  in  disease  or  after  injury  in  man.  Removal 
of  the  cerebrum  in  the  pigeon  completely  abolishes  intelligence  and 
destroys  the  capability  of  performing  volitional  movements.  The 
pigeon  remains  in  a  state  of  profound  stupor,  though  retaining  the  cap- 
ability of  executing  reflex  or  instinctive  movements.  It  can  temporarily 
be  aroused  by  loud  noises,  light  placed  before  the  eyes,  pinching  of  the 
toes,  etc.,  but  it  soon  relapses  into  a  condition  of  quietude.  Coincident 
with  the  destruction  of  the  cerebrum  there  occurs  a  loss  of  memory, 
reason,  and  judgment,  and  the  animal  fails  to  associate  the  impressions 
with  any  preA"ious  train  of  ideas.  The  higher  the  animal  in  the  scale  of 
development,  the  more  striking  is  the  loss  of  mentality  after  removal 
of  the  cerebrum. 

Experimental  interference  with  the  blood-supply  to  the  cerebrum  is  followed 
by  a  diminished  or  complete  cessation  of  its  activities.  There  is  per- 
haps no  organ  of  the  body  that  is  so  directly  dependent  upon  its  blood- 
supply  for  the  continuance  of  its  activities  as  the  cerebrum.  The 
supply  of  blood  is  furnished  by  four  large  blood-vessels:  viz.,  the  two 
carotid  and  the  two  vertebral  arteries.  These  vessels,  after  entering 
the  cavity  of  the  skull,  give  off  branches  which  unite  to  form  the  "circle 
of  Willis."  From  this  circle,  large  branches  are  given  off  which  enter 
the  cerebrum  and  distribute  blood  to  all  its  parts.  After  passing  through 
the  capillaries  the  blood,  greatly  altered  in  chemic  composition,  is 
returned  by  large  veins.  The  large  volume  of  blood  that  is  present 
in  the  brain  and  the  marked  changes  in  composition  that  it  undergoes 
while  passing  through  the  brain  indicate  a  very  active  and  complex  met- 
abolism in  this  organ.  By  means  of  the  anatomic  arrangement  of  the 
blood-vessels  at  the  base  of  the  brain,  the  blood-supply  is  equalized. 
It  also  explains  why,  when  one,  or  even  two,  of  the  four  large  vessels 
are  occluded  by  pathologic  deposits  or  surgical  procedures,  brain 
activity  continues,  though  perhaps  diminished  in  degree.  Occlusion 
of  all  four  vessels,  however,  is  at  once  followed  by  a  complete  aboHtion 
of  all  forms  of  cerebral  activity.  An  experiment  performed  by  Brown- 
Sequard  illustrates  the  dependence  of  cerebral  activity  on  the  blood- 
supply.  A  dog  was  beheaded  at  the  junction  of  the  neck  and  chest. 
After  a  period  of  ten  minutes  all  evidences  of  life  had  entirely  ceased. 
Four  tubes  connected  with  a  reservoir  of  warm  defibrinated  blood 
were  then  connected  with  the  four  arteries  of  the  head.  By  means  of  a 
pumping  apparatus  imitating  the  action  of  the  heart  the  blood  was 
driven  into  and  through  the  brain.  After  a  few  minutes  cerebral  activ- 
ity returned,  as  shown  by  contractions  of  the  muscles  of  the  face  and 
eyes.  The  character  of  the  contractions  were  such  as  to  convey  the 
idea  that  they  were  directed  by  the  will.  These  vital  manifestations 
continued  for  a  period  of  fifteen  minutes,  when  on  the  cessation  of  the 
artificial  circulation  they  disappeared,  and  the  head  exhibited  once 
more  the  usual  phenomena  observed  in  dying:  viz.,  contraction  and 
35 


546  TEXT-BOOK  OF  PHYSIOLOGY. 

then  dilatation  of  the  pupils  and  convulsive  movements  of  the  muscles 

of  the  face. 

Localization  of  Functions  in  the  Cerebrum. — By  the  term  localiza- 
tion of  functions  is  meant  the  assignment  of  definite  physiologic  functions  to 
definite  anatomic  areas  of  the  cerebral  cortex.  From  experiments  made  on 
the  brains  of  animals,  by  the  observation  and  association  of  clinical  symp- 
toms with  pathologic  lesions  of  the  central  nerve  system,  and  from  obser- 
vation of  the  developmental  stages  of  the  embryonic  brain,  it  has  been  es- 
tablished in  recent  years: 

1.  That  the  general  and  special  sense-organs  of  the  body  are  associated 

through  afferent  nerve-tracts  with  definite  though  perhaps  not  sharply 
delimited  areas  of  the  cerebral  cortex;  and — 

2.  That  certain  areas  of  the  cortex  are  associated  through  efferent  nerve- 

tracts  with  special  groups  of  skeletal  or  voluntary  muscles. 

Experimental  excitation  of  a  cortical  area  associated  with  a  sense-organ  is 
undoubtedly  attended  by  the  production  of  a  sensation  at  least  similar  to 
that  produced  by  peripheral  excitation  of  the  sense-organ  itself;  destruction 
of  the  area  is  followed  by  an  abolition  of  all  the  sensations  associated  with  the 
sense-organ.     For  these  reasons  such  areas  are  termed  sensor. 

Experimental  excitation  of  a  cortical  area  associated  with  a  group  of  skele- 
tal muscles  is  attended  by  their  contraction;  destruction  of  the  area  is  followed 
by  their  relaxation  or  paralysis.  For  these  reasons  such  areas  are  termed 
motor. 

Since  the  sense-organs  are  remote  from  the  brain  and  the  impressions 
made  upon  them  by  the  objective  world  can  be  utilized  by  the  mind  only 
after  they  have  been  reproduced  in  the  cortical  areas,  it  may  be  said  that 
each  sense-organ  has  its  special  area  in  the  cortex  by  which  it  is  represented, 
or,  in  other  words,  each  sense-organ  has  a  cortical  area  of  representation. 

Since  the  muscles  are  remote  from  the  brain  and  since  they  contract  in 
response  to  the  discharge  of  nerve  impulses  from  the  cells  of  the  cortical  motor 
areas,  it  may  be  said  that  the  activities  of  the  motor  areas  are  represented 
by  the  contractions  of  the  muscles;  in  other  words,  that  the* cortical  motor 
areas  have  areas  of  representation  in  the  general  skeletal  musculature.  It 
is  usually  stated,  however,  in  the  reverse  way:  viz.,  that  the  muscle  move- 
ments have  areas  of  representation  in  the  cortex. 

The  cortex  of  the  cerebrum  may  therefore  be  compared  to  a  mosaic 
made  up,  partially  at  least,  of  sensor  and  motor  areas  which  respectively 
represent  sense-organs  and  motor  organs,  and  which  bear  a  definite  anatomic 
and  physiologic  relation  one  to  the  other.  Their  cooperation  is  essential  to 
the  normal  performance  of  many  forms  of  cerebral  activity. 

A  knowledge  of  the  situation  of  these  areas,  the  order  of  their  develop- 
ment, the  effects  that  arise  from  their  stimulation  or  follow  their  destruction, 
are  matters  of  the  highest  importance  in  the  study  of  cerebral  activity  and 
indispensable  to  the  physician  in  the  localization  of  lesions  which  manifest 
themselves  in  perversions  or  abolition  of  sensations  and  in  convulsive  seizures 
or  paralyses. 

The  Sensor  Areas. — The  sensor  areas  which  should  theoretically  be 
present  in  the  cortex  are  primarily  those  which  receive  and  translate  into  con- 
scious sensations  nerve  impulses,  developed  by  changes  going  on  in  the  body 


THE  CEREBRUM.  547 

itself;  and  secondarily  those  which  receive  and  translate  into  conscious  sensa- 
tions the  ner\^e  impulses  developed  in  the  special  sense-organs  by  the  impact  of 
the  external  or  objective  world.  In  the  former  areas,  are  received  the  nerve 
impulses  that  come  from  the  mucous  membranes,  muscles,  joints,  viscera,  etc., 
and  give  rise  to  muscle,  and  visceral  sensations.  In  the  latter  areas  are 
received  the  nerv'e  impulses  that  come  from  the  sense-organs  and  give  rise 
to  cutaneous,  e.g.,  tactile,  thermal,  painful,  gustatory,  olfactory,  auditory, 
and  visual  sensations.  A  number  of  such  sense  areas  may  be  predicated: 
e.g.,  areas  of  cutaneous  and  muscle  sensibility,  of  gustatory,  olfactory,  auditory, 
and  visual  sensibility. 

The  Motor  Areas. — The  motor  areas  which  should  theoretically  be 
present  in  the  cortex  are  those  which  in  consequence  of  the  discharge  of  nerve 
impulses  excite  contraction  of  special  groups  of  muscles  and  which,  from 
their  coordinate  and  purposive  character,  are  conventionally  termed  voli- 
tional. Five  such  general  motor  areas  may  be  predicated:  e.g.,  one  for  the 
muscles  of  the  head  and  eyes,  one  for  the  muscles  of  the  face  and  associated 
organs,  and  others  for  the  muscles  of  the  arm,  leg,  and  trunk.  They  are 
usually  designated  as  head  and  eye,  face,  arm,  leg,  and  trunk  motor  areas. 

The  existence  and  anatomic  location  of  these  areas  in  the  cortex  of 
animals  have  been  determined  by  the  employment  of  two  methods  of  ex- 
perimentation: viz.,  stimulation  and  destruction  or  extirpation;  the  first 
by  means  of  the  rapidly  repeated  induced  electric  currents,  the  second  by 
the  electric  cautery  and  the  knife.  If  the  stimulation  or  excitation  of  any 
given  area  is  followed  by  contraction  and  its  destruction  by  paralysis  of 
muscles,  it  is  assumed  that  the  area  is  motor  in  function — is  a  center  of 
motion.  If  the  stimulation  of  a  given  area  is  attended  by  phenomena 
which  indicate  that  the  animal  is  experiencing  sensation,  and  its  destruction 
by  a  loss  of  this  capability  or  the  loss  of  a  special  sense,  it  is  assumed  that  the 
area  is  sensor  in  function —  is  an  area  of  special  sense.  The  animals  gener- 
ally employed  for  experiments  of  this  character  are  dogs  and  monkeys,  though 
other  animals  have  frequently  been  employed  by  different  investigators. 
Of  all  animals,  the  monkey  is  the  most  frequently  selected,  as  the  configura- 
tion of  the  brain  in  its  general  outlines  more  closely  resembles  that  of  man 
than  does  the  brain  of  any  other  animal.  The  results  therefore  which  are 
obtained,  there  is  every  reason  to  believe,  are  the  results,  in  their  general 
outlines,  that  would  follow  stimulation  of  the  human  brain  if  this  were 
possible  under  the  same  conditions.  Indeed,  the  clinical  symptoms  which 
arise  .during  the  development  of  pathologic  processes,  and  the  phenomena 
which  occur  during  surgical  procedures  for  the  removal  of  growths  and 
pathologic  cortical  areas,  justify  the  conclusion  that  the  chart  of  the  sensor 
and  motor  areas  of  the  monkey  brain  may  be  transferred  to  the  human  brain 
without  introducing  any  serious  errors. 

The  Sensor  Areas  of  the  Monkey  Brain. — From  experiments  made  on 
the  brains  of  monkeys,  P^rrier,  Schafer,  Horsley,  and  many  others  have 
mapped  out,  though  not  with  a  high  degree  of  definiteness  and  certainty, 
the  sensor  areas,  stimulation  of  which  gives  rise  to  sensation,  destruction  to 
loss  of  sensation.  A  diagrammatic  representation  of  these  areas  is  shown 
in  Fig.  250  and  Fig.  251. 

The  tactile  area  or  area  of  tactile  perception  has  not  been  accurately  or 


548 


TEXT-BOOK  OF  PHYSIOLOGY. 


definitely  located.  Ferrier  assigned  it  to  the  hippocampal  region.  Schafer 
and  Horsely  assigned  it  to  the  limbic  lobe,  and  especially  to  that  portion 
known  as  the  gyrus  fornicatus,  or  callosal  gyrus,  as  destruction  of  this  convolu- 
tion was  followed  by  hemianesthesia  of  the  opposite  side  of  the  body  which  was 
more  or  less  marked  and  persistent.  These  observers  conclude  that  the  limbic 
lobe  "is  largely  if  not  exclusively  concerned  in  the  appreciation  of  sensation, 
painful  and  tactile."  Other  experimenters  question  this  conclusion  and 
locate  the  area  near  to,  if  not  within,  the  Rolandic  area.  The  difference  of 
opinion  regarding  the  location  and  probable  limitation  of  the  area  of  tactile 
sensibility  renders  necessary  additional  and  more  conclusive  exyjerimcnts. 
The  olfactory  and  gustatory  areas  or  areas  of  olfactory  and  gustatory 


Fig.  250. 


-DlAGR,\M   OF   THE    MOTOR   AND    SeNSOR   ArEAS    ON   THE    LATERAL    StJRFACE     OF   THE 

Monkey  Brain. — {After  Horsley  and  Schafer.) 


perception  have  been  located  in  the  uncinate  gyrus  or  uncus  and  the  adjacent 
region,  though  their  exact  limits  have  not  been  determined  by  the  experi- 
ments thus  far  performed. 

The  auditory  area  or  area  of  auditory  perception  was  located  by  Ferrier 
in  the  upper  two-thirds  of  the  superior  temporo-sphenoidal  convolution. 
Bilateral  cauterization  of  this  region  gave  rise  to  complete  deafness,  which 
endured  to  the  time  of  the  animal's  death,  more  than  a  year  later.  Unilateral 
destruction  of  this  region  gave  rise  to  deafness  in  the  opposite  ear  only.  The 
results  of  experiments  made  subsequently  by  other  observers  would  indi- 
cate that  the  auditory  area  is  somewhat  more  extended  than  that  designated 
by  Ferrier,  as  apparently  animals  regained  their  hearing,  to  some  extent 
at  least,  after  complete  recovery  from  the  operation.  The  limit  or  extent  of 
the  area  is,  however,  uncertain. 

The  visual  area  or  area  of  visual  perception  has  been  located  in-the  occipital 
lobe,  though  in  this,  as  in  the  previous  instances,  its  exact  limits  have  not 
been  positively  determined.  Experimenters  also  are  not  in  accord  as  to  the 
relative  functions  of  its  different  parts.  Ferrier  located  this  area  in  the 
occipital  lobe  and  that  adjacent  portion  of  the  parietal  lobe  on  the  outer 


THE  CEREBRUM. 


549 


surface  known  as  the  angular  gyrus.  He  found  that  extirpation  of  the  an- 
gular gyrus  alone  was  followed  by  a  temporary  blindness  of  the  opposite  eye, 
which  was,  however,  not  hemianopsic  in  character.^  He  also  found  that 
destruction  of  the  occipital  lobe  together  with  the  angular  gyrus  gave  rise 
to  a  more  or  less  enduring  hemianopsia,  in  addition  to  the  transient 
blindness  of  the  opposite  eye.  From  these  and  similar  facts  he  concluded 
that  the  angular  gyrus  is  the  area  of  representation  for  the  macular  or  central 
region  of  the  retina,  and  the  occipital  lobes  for  the  corresponding  halves 
of  the  peripheral  portions  of  the  retina. 

It  was,  how^ever,  found  by  Munk,  Schafer,  and  others  that  the  angular 
gyrus  was  not  concerned  in  any  way  with  vision;  that  extirpation  of  the 


Fig.  2>i.- 


-DlAGR-AM  OF  THE  ZMOTOR  AND  SeNSOR  ArEAS  ON  THE  MESIAL  SURFACE  OF  THE  MON- 

KEY  Br-AIN. — {After  Horsley  and  Schafer.) 


occipital  lobe  alone,  especially  if  the  line  of  division  be  carried  a  Httle  further 
forward  on  the  mesial  and  inferior  surfaces,  was  followed  by  homonymous 
hemianopsia.  Additional  experiments  lead  to  the  conclusion  that  the 
area  for  macular  vision  is  near  the  anterior  extremity  of  the  calcarine  fissure, 

^  In  a  consideration  of  this  subject  certain  facts  connected  with  visual  perception,  both  in 
physiologic  ^nd  pathologic  conditions,  must  be  kept  in  mind.  Thus,  visual  sensation  may  arise 
from  stimulation  of  either  the  central  portion,  the  macula,  or  the  peripheral  portion  of  the  retina 
or  both.  In  the  first  instance  the  vision  is  termed  central  or  macular;  in  the  second  instance, 
peripheral  or  retinal.  Macular  vision  is  clear,  sharp,  and  distinct;  retinal  vision  progressively 
indistinct  from  the  center  toward  the  periphery.  Division  of  one  optic  tract  is  followed,  in  con- 
sequence of  the  partial  decussation  of  the  optic  nerve-fibers  at  the  chiasma,  by  a  loss  of  function 
in  the  outer  two-thirds  of  the  retina  of  the  same  side,  both  in  the  central  (macular)  as  well  as  in 
its  peripheral  portions,  and  the  inner  one-third  of  the  retina  of  the  opposite  side.  To  this  condition 
the  term  hemianopsia  has  been  applied.  As  a  result  of  this  want  of  functional  activity  of  these  ret- 
inal portions  on  the  side  of  the  lesion,  rays  of  hght  emanating  from  objects  situated  in  the  opposite 
side  of  the  field  of  vision  \\\\\  not  be  perceived  when  both  eyes  are  directed  to  the  fixation  point. 
To  this  "  blindness  "  in  the  opposite  half  of  the  field  of  vision  the  name  hemianopsia  is  given.  In 
the  lesion  under  consideration  (division  of  one  optic  tract)  the  hemianopsia  is  bilateral,  and  as 
it  aft"ects  the  corresponding  portions  associated  in  normal  \ision  it  is  of  the  homonymous  variety 
Division  of  the  right  optic  tract  is  followed  by  left  lateral  homonymous  hemianopsia,  inchcative  of  the 
fact  that  objects  in  the  field  of  vision  to  the  left  of  the  binocular  fixation  point  are  invisible. 


550  TEXT-BOOK  OF  PHYSIOLOGY. 

while  the  area  for  peripheral  \dsion  is  in  the  posterior  portion  of  the  mesial 
surface  and  for  a  variable  distance  on  the  outer  surface.  Moreover,  there 
is  reason  to  believe  that  the  area  for  macular  vision  is  in  relation  with  homony- 
mous halves  of  the  two  maculae  luteae.  The  supposed  error,  the  assignment 
of  macular  vision  to  the  angular  gyrus,  has  been  attributed  to  destruction  of 
the  fibers  of  the  optic  radiation,  which  in  their  course  to  the  occipital  lobe 
pass  close  to  this  gyrus. 

The  Motor  Areas  of  the  Monkey  Brain. — From  experiments  made  on 
the  brains  of  monkeys  Ferrier  mapped  out  a  number  of  areas  stimulation  of 
which  give  rise  to  muscle  contractions  on  the  opposite  side  of  the  body  which 
are  so  purposive  and  coordinate  in  character  that  they  may  be  regarded  as 
identical  with  those  produced  voHtionally.  Destruction  of  these  areas  is_ 
followed  by  paralysis.  Collectively  these  areas  are  known  as  the  motor  area 
or  motor  zone;  and  as  it  is  ranged  along  the  Rolandic  fissure,  it  is  sometimes 
termed  the  Rolandic  area. 

The  experiments  of  Horsley  and  Schafer  added  additional  facts  and 
enabled  them  to  construct  a  new  diagrammatic  representation  of  the  motor 
area  and  more  accurately  define  the  special  areas  upon  the  lateral  and  mesial 
aspects  of  the  brain  of  the  monkey.  The  boundaries  of  the  general  and 
special  areas,  as  determined  by  these  observers,  will  be  readily  apparent 
from  an  examination  of  Fig.  249.  Their  experiments  have  enabled  them 
also  to  subdivide  the  general  into  special  areas  as  follows: 

1.  The  head  area  or  area  for  visual  direction  into  areas  excitation  of  which 

causes  "opening  of  the  eyes,  dilatation  of  the  pupils  and  turning  the 
head  to  the  opposite  side  with  conjugate  deviation  of  the  eyes  to  that 
side." 

2.  The  leg  area  may  be  subdivided  into  {a)  an  area  both  on  the  lateral  and 

mesial  surfaces  which  presides  over  the  movements  of  the  hip  and 
thigh;  (h)  an  area  in  the  posterior  part  which  presides  over  the  move- 
ments of  the  legs  and  toes;  (c)  an  area  in  the  paracentral  lobule  for  the 
movements  of  the  hallux  or  great  toe. 

3.  The  trunk  area,  situated  largely  on  the  mesial  surface,  may  be  subdivided 

into  an  anterior  and  a  posterior  area,  which  respectively  preside  over 
the  movements  of  the  spinal  column  as  arching  and  rotation,  and  the 
movements  of  the  pelvis  and  tail. 

4.  The  arm  area  may  be  subdivided  as  follows:  (a)  an  area  superiorly 

which  controls  the  movements  of  the  shoulder;  {h)  an  area  posteriorly 
and  below  this,  which  controls  the  movements  of  the  elbow;  (c)  an 
area  anteriorly  and  below  the  preceding,  governing  the  movements  of 
the  wrist  and  fingers;  {d)  an  area  posteriorly  and  below  governing  the 
movements  of  the  thumb. 

5.  The /ace  area  may  be  divided  into  an  upper  part,  comprising  about  one- 

third,  and  a  lower  part,  comprising  the  remaining  two-thirds.  In  the 
upper  part  are  areas  governing  the  movements  of  the  opposite  angle 
of  the  mouth  and  of  the  lower  face.  In  the  lower  part  anteriorly  there 
is  an  area  governing  the  movements  of  the  vocal  membranes  or  bands 
(the  laryngeal  area);  posteriorly  areas  governing  the  opening  and 
closing  of  the  mouth,  the  protrusion  and  retraction  of  the  tongue. 


THE  CEREBRUM.  551 

Electric  stimulation  of  the  sensor  areas  is  attended  by  certain  motor 
reactions  which  vary  in  accordance  with  the  area  stimulated.  Thus,  when 
the  electrodes  are  applied  to  different  portions  of  the  occipital  lobe  the  eye- 
balls are  conjugately  turned  upward,  downward,  or  laterally  and  to  the 
opposite  side;  when  placed  on  the  upper  portion  of  the  superior  temporal 
convolution,  the  ear  is  pricked  up  or  retracted,  the  head  is  turned  to  the' 
opposite  side  and  the  pupils  are  dilated;  when  placed  on  the  hippocampal 
convolution,  there  is  movement  of  torsion  of  the  nostril  and  lips  of  the  same 
side. 

Ferrier  assumed  that  these  movements  were  the  result  of  the  origination 
of  subjective  sensations  and  not  an  evidence  that  the  area  in  question  is  a 
motor  area,  in  the  sense  that  this  term  is  applied  to  the  areas  of  the  Rolandic 
region,  especially  as  their  destruction  is  not  followed  by  paralysis  of  any  of 
the  corresponding  muscles.  This  interpretation  is  supported  by  the  ex- 
periments of  Schafer,  which  showed  that  the  contraction  of  the  eye-muscles 
which  followed  stimulation  of  the  occipital  lobe  took  place  between  0.2  and 
0.3  second  later  than  when  the  frontal  lobe  was  stimulated;  and  that  as  the 
motor  reaction  takes  place  after  extirpation  of  the  frontal  region,  the  route  of 
the  efferent  impulse  cannot  be  to  and  through  the  frontal  lobe,  but  probably 
through  some  lower  center.  The  same  facts  hold  true  for  the  reactions  of 
the  ear-muscles  following  stimulation  of  the  temporal  lobe. 

The  view  that  the  cortex  of  the  cerebrum  can  be  di\dded  into  separate  and 
independent  though  physiologically  related  motor  and  sensor  areas  has 
been  questioned  in  recent  years,  and  a  somew^hat  different  interpretation 
given  to  the  facts.  It  is  believed  by  many  physiologists  and  neurologists 
that  the  so-called  motor  and  sensor  areas  are  so  closely  related  that  it  is 
almost  impossible  to  distinguish  one  from  the  other  either  anatomically  or 
physiologically.  Thus  the  Rolandic  region  is  believed  to  be  both  motor  and 
sensor  in  function,  the  former,  however,  being  more  predominant  in  the  per- 
central,  the  latter  in  the  post-central,  convolution.  As  these  two  functions 
are  so  intimately  blended  and  their  anatomic  substrata  so  difl&cult  of  separa- 
tion, it  is  thought  the  term  sensori-motor  should  be  employed  as  more  descrip- 
tive and  more  in  accordance  with  the  facts  to  the  entire  Rolandic 
region. 

This  \'iew  has  been  strengthened  by  the  results  of  the  embryologic 
investigation  of  Flechsig,  which  show  that  different  nerve-tracts  become 
meduUated  or  receive  their  myelin  investment  at  successively  later  periods  and 
that  the  tracts  which  first  become  myelinated  and  are  hence  first  functionally 
active,  belong  to  the  afferent  system.  Among  the  first  to  undergo  myelini- 
zation  are  three  tracts  numbered  by  Flechsig  i,  2  and  3,  which  arise  largely 
from  the  median  nucleus  of  the  thalamus  and  the  medial  lemniscus  and  pass 
to  the  anterior  and  posterior  convolutions,  to  the  para-central  lobule  and 
foot  of  the  superior  frontal  convolution,  and  to  the  foot  of  the  third  frontal 
convolution  respectively.  It  is  these  fibers  which  convey  nerve  impulses  to 
the  cortex  and  furnish  information  regarding  changes  taking  place  in  the 
body  itself  and  thus  lead  to  the  performance  of  muscle  movements.  This 
area  is  therefore  primarily  a  sensor  area,  an  area  for  body-feelings,  cutaneous, 
tactile,  muscle,  and  visceral,  and  secondarily  a  motor  area.  The  afferent 
fibers  to  this  region  become  myelinated  during  the  ninth  month  of  intra- 


552 


TEXT-BOOK  OF  PHYSIOLOGY. 


uterine  life,  the  efferent  fibers  from  it  become  myelinated  during  the  third 
month  of  extra-uterine  life. 

By  the  same  method  of  reasoning  the  gustatory,  olfactory,  auditory,  and 
visual  sense  areas  are  to  be  regarded  assensori-motor  in  character,  for  embryo- 
logic  investigations  show  that  subsequently  to  the  myelinization  of  the 
afferent  tracts  connecting  the  sense-organs  with  the  cortex,  efferent  nerve- 
tracts  arise  from  or  near  to  the  same  centers  and  undergo  myelinization.  In 
other  words,  these  areas  are  primarily  sensor  and  secondarily  motor,  and 
therefore  should  be  termed  sensori-motor.  In  Flechsig's  own  terminology 
each  corticopetal  or  afferent  tract  is  accompanied  by  a  corticofugal  or 
eft'erent  tract. 


Fig.  2^2.- 


CONCRLTE    CONCEPT 

-The  Areas  and  Centers  of  the  Later.al  Aspect  of  the  HuaL-^N  Hemicerebrum.- 

(C.  K.  Mills.) 


In  this  view  sensations,  or  the  mental  processes  the  outcome  of  sensations, 
are  the  immediate  cause  of  the  movements  of  the  muscles  connected  with  both 
the  sense-organs  and  skeletal  structures.  Though  this  interpretation — viz., 
the  coincidence  of  sensor  and  motor  areas — appears  more  in  accordance 
with  the  facts  than  the  earlier  view,  it  must  be  admitted  that  there  are  many 
facts  both  of  a  physiologic  and  pathologic  character  which  it  is  difficult  to 
harmonize  \\dth  it. 

The  Motor  Area  of  the  Chimpanzee  Brain. — In  a  series  of  experi- 
ments made  by  Sherrington  and  Griinbaum  on  the  brain  of  the  chimpanzee 
it  was  discovered  that  the  so-called  motor  area  was  not  so  widely  distributed 
as  in  the  monkeys  generally,  but  was  confined  almost  exclusively  to  the 
convolution  just  in  front  of  the  fissure  of  Rolando,  as  it  was  impossible  to 
obtain  any  movement  on  direct  stimulation  of  the  convolution  just  behind  it. 
All  points  on  the  surface  of  the  pre-central  convolution,  including  the  portion 
forming  the  wall  of  the  Rolandic  fissure  itself,  were  found  to  be  excitable 
and  productive  of  movement  when  stimulated.     The  sequence  of  representa- 


THE  CEREBRUM. 


553 


tion  from  below  upward  is  similar  to  that  obser\'ed  in  the  monkey.  One 
peculiarity,  however,  was  the  location  of  the  area  for  conjugate  deviation  of 
the  eyeballs  to  the  opposite  side.  This  is  situated  far  forward  in  the  middle 
and  inferior  frontal  convolutions,  and  separated  from  the  areas  in  the  pre- 
ccntral  convolution  by  a  region  apparently  inexcitable.  These  facts  are  of 
great  interest  and  value  in  the  assignment  of  the  motor  areas  in  the  cortex 
of  the  human  brain,  as  in  its  development  and  configuration  the  chimpanzee 
brain  more  closely  resembles  the  human  brain  than  does  the  monkey's. 
The  Localization  of  Sensor  and  Motor  Areas  in  the  Human  Brain. — 
The  obser\'ation  of  clinical  symptoms  and  their  interpretation  by  post-mortem 
findings,  the  phenomena  observed  during  surgical  procedures,  and  the  results 


Fig. 


253- 


-The  Areas  and  Centers  of  the  Mesial  Aspect  of  the  Hum.an  Hemicerebrum.^ 
(C.  K.  Mills.) 


of  embryologic  investigations,  point  to  the  conclusion  that  corresponding 
areas  both  for  sensations  and  movements  exist  in  the  cerebral  cortex  of  the 
human  brain,  though  it  is  probable  that  their  locations  do  not  in  all  respects 
coincide  with  those  characteristic  of  the  monkey  or  even  the  ape  brain.  In 
the  following  diagrams  (Figs.  252  and  253),  the  sensor  and  motor  areas  are 
at  least  provisionally  located,  in  accordance  with  recent  observations.  They 
are  represented  as  limited  or  bounded  by  a  serrated  line  to  indicate,  as 
suggested  by  Mills,  that  they  are  not  sharply  delimited,  but  that  they  inter- 
fuse or  interdigitate  with  surrounding  regions. 

The  Sensor  Areas. — The  sensor  areas  occupy  regions  corresponding  in 
a  general  way  with  those  of  the  monkey  brain. 

The  cutaneous  and  muscle  sense  areas  have  been  assigned  to  the  post- 
central, a  portion  of  the  super-  and  sub-parietal  convolutions  on  the  lateral 
aspect,  and  to  portions  of  the  frontal  convolution  and  of  the  callosal  convolu- 
tion on  the  mesial  aspect.  It  is  also  probable  that  the  tactile  (cutaneous) 
area  may  be  assigned,  though  in  less  degree,  to  the  pre-central  convolution, 
the  general  motor  area.     This  is  in  accordance  with  the  embryologic  investi- 


554  TEXT-BOOK  OF  PHYSIOLOGY. 

gations  of  Flechsig,  who  concludes  that  the  entire  Rolandic  region  is  to  be 
regarded  as  sensor  as  well  as  motor  in  function,  and  names  it  the  area  of 
body  feelings,  or  the  somesthetic  area. 

The  clinic  and  post-mortem  evidence  as  to  the  extent  of  the  area  of  tactile 
sensibility  and  its  coincidence  with  the  motor  area  is  somewhat  contradictory, 
and  in  some  respects  apparently  in  opposition  to  the  view  of  Flechsig.  Thus, 
Dr.  C.  K.  Mills,  whose  skill  in  interpreting  the  phenomena  of  disease  is  well 
known,  states  in  this  connection  in  his  work  on  nervous  diseases  that  "  innu- 
merable cases  have  been  reported  of  lesions  of  the  motor  cortex  without  the 
slightest  impairment  of  sensibility."  In  several  cases  of  excision  of  the 
human  cortex  in  the  Rolandic  region  by  surgical  operations  careful  studies  of 
the  patients  failed  to  show  any  impairment  of  sensation.  Other  competent 
observers,  however,  have  reported  a  number  of  cases  in  which  anesthesia 
more  or  less  pronounced  and  persistent  has  accompanied  lesions  of  the  motor 
area.     The  explanation  of  these  contradictory  observations  is  not  apparent. 

The  olfactory  area  has  been  assigned  to  the  uncinate  convolution,  the 
anterior  part  of  the  callosal  convolution,  and  the  posterior  part  of  the  base  of 
the  frontal  lobe.  Lesions  in  this  region  are  frequently  accompanied  by 
subjective  olfactory  sensations. 

The  gustatory  area  has  been  assigned  to  the  collateral  convolution. 

The  auditory  area  has  been  assigned  to  the  posterior  portion  of  the  super 
temporal  convolution  and  to  the  retro-insular  convolutions,  the  island  of 
Reil.  Unilateral  destruction  of  this  region  is  followed  by  only  a  partial 
loss  of  hearing  in  the  opposite  ear  (owing  to  the  partial  decussation  of  the 
cochlear  nerve),  which,  however,  may  be  recovered  from  after  a  time,  owing 
probably  to  a  compensatory  activity  of  the  insular  convolutions.  Bilateral 
disease  of  this  region  is  followed  by  complete  deafness.  Within  this  area 
there  is  a  smaller  region,  disease  of  which  is  accompanied  by  word-deafness 
only,  the  patient  being  unable  to  distinguish  the  tone  intervals  between 
words  and  syllables  and  therefore  hearing  only  confused  noises.  Object- 
hearing  has  also  a  separate  area  of  representation. 

The  visual  area  has  been  assigned  to  a  triangular  shaped  area  on  the 
mesial  surface  of  the  occipital  lobe,  which  includes  the  gray  matter  above 
and  below  the  calcarine  fissure  (the  cuneus  and  upper  part  of  the  lingual 
lobe),  and  to  the  gray  matter  of  the  first  occipital  convolution  on  the  lateral 
aspect  of  the  occipital  lobe.  Focal  lesions  of  this  area  on  one  side  are  followed 
by  lateral  homonymous  hemianopsia,  which,  however,  does  not  involve,  as  a 
rule,  the  fovea  or  macula.  It  is,  therefore,  the  area  of  homonymous  half- 
retinal  representation.  The  location  of  the  area  for  macular  or  central  vision 
is  uncertain.  Henschen  locates  it  in  the  anterior  part  of  the  area  near  the 
extremity  of  the  calcarine  fissure,  and  asserts  that  in  each  area  both  maculae 
are  represented.  From  experiments  made  on  monkeys  Schafer  locates  it  in 
the  same  region.  Beyond  the  limits  of  this  visual  area  and  on  the  lateral 
aspect  of  the  parietal  lobe  there  is  a  region  (the  supra-marginal  convolution 
and  angular  gyrus)  in  which  impressions  of  words  and  letters  seen  have 
their  representation.  Destruction  of  this  area  by  diseases  is  followed  by 
word-  and  perhaps  letter-blindness,  the  patient  being  unable  to  recognize 
words  and  letters  seen  because  of  failure  to  revive  the  memory  images  of 
words  and  letters.     The  areas  for  visual  sensations  and  optic  memory 


THE  CEREBRUM. 


555 


pictures  are  therefore  separate,  a  fact  which  has  led  to  a  division  of  the 
visual  area  into  a  lower  and  a  higher  area. 

It  was  stated  in  a  previous  paragraph  that  electric  stimulation  of  the 
sensor  areas  of  the  monkey  brain  is  attended  by  certain  motor  reactions 
which  vary  with  the  area  stimulated.  Corresponding  areas  are  believed  to 
be  present  in  the  human  brain  and  that  their  stimulation  would  be  followed 
by  similar  motor  reactions.  Their  location  is  shown  in  Figs.  252  and  253, 
and  named  visual,  auditory,  olfactory,  and  gustatory  motor. 

The  stereognostic  area  or  area  of  stereo  gnostic  perception,  by  which  objects 
are  recognized  through  their  form  independent  of  vision  and  by  the  sense  of 
touch  alone,  has  been  located  in  the  super-parietal  convolution  and  the 
precuneus  (Mills).     The  existence  of  such  an  area  is  rendered  probable  by 


Fig.  254.— Scheme  of  the  Motor  Area  of  the  Hum.\n  Brain  and  its  Subdivisions. — {After 

Mills.) 

the  fact  that  cases  have  been  recorded  in  which  there  was  a  loss  of  this 
power  (astereognosis)  unaccompanied  by  either  sensor  or  motor  distur- 
bances. Post-mortem  investigations  showed  that  in  these  cases  there  was  a 
destruction  of  the  superior  parietal  convolution. 

Equilibratory,  intonation,  and  orientation  areas  have  been  provisionally 
located  in  the  spheno-temporal  lobe. 

The  Motor  Areas. — The  general  motor  area  (Fig.  252)  is  represented  as 
occupying  the  pre-central  convolution,  the  base  of  the  super-frontal  con- 
volution, both  on  its  lateral  and  mesial  aspects,  and  the  paracentral  lobule. 
The  exclusion  of  the  post-central  convolution  from  the  motor  area  is  in  ac- 
cordance with  the  embryologic  researches  of  Flechsig,  which  indicate  that 
the  efferent  fibers  which  compose  the  pyramidal  tract  come  from  the  region 
anterior  to  the  central  fissure,  and  with  the  experiments  of  Sherrington  and 
Griinbaum  on  the  brain  of  the  chimpanzee,  which  demonstrate  that  the 
post-central  convolution  is  absolutely  inexcitable  to  electric  stimulation. 


556  TEXT-BOOK  OF  PHYSIOLOGY. 

It  is  quite  probable  that  with  the  growth  of  the  brain  in  size  and  complexity, 
the  motor  area  has  come  to  occupy  a  position  somewhat  farther  forward  in 
the  human  brain  than  in  the  monkey  brain. 

This  general  area  is  also  capable  of  subdivision  into  areas  of  variable  size, 
in  which  the  movements  of  the  face  and  associated  structures,  the  head  and 
eyes,  the  arm,  trunk,  and  leg,  are  represented.  (Fig.  254.)  The  sequence 
of  their  representation  from  below  upward  is  similar  to  that  observed  in  the 
monkey  and  chimpanzee.  In  each  of  these  five  main  areas  there  are  yet 
smaller  areas  in  which  the  movements  of  localized  regions  of  the  body  are 
in  part  represented  and  which  are  indicated  in  diagram  (Fig.  254)  by  corre- 
sponding words.  The  words  in  the  areas  marked,  eyes  and  head,  face, 
arm,  trunk,  and  leg,  indicate  the  location  of  nerve-cells  which  through  the 
discharge  of  nerve  impulses  excite  to  contraction  the  muscles  which  im- 
part to  the  regions  indicated  by  these  words  their  characteristic  movements. 
A  localized  irritative  lesion  of  any  one  of  these  areas  gives  rise  to  convulsive 
movements  of  the  muscles  of  the  opposite  side  of  the  body,  similar  in  char- 
acter to  those  resulting  from  electric  simulation  of  the  corresponding  areas 
of  the  monkey  and  ape  brains.  Destruction  of  these  areas  from  the  growth 
of  tumors,  softening,  etc.,  is  followed  by  paralysis  of  the  muscles.  Electric 
stimulation  of  these  areas  of  the  human  brain  for  the  purpose  of  localizing 
obscure  irritative  lesions  prior  to  surgical  procedures  on  the  brain  gives 
rise  to  the  same  convulsive  movements. 

Language. — The  succession  of  motor  acts  by  which  ideas  are  expressed, 
is  known  as  language,  which  may  be  divided  into  (i)  articulate  or  spoken, 
and  (2)  written. 

The  expression  of  ideas  both  by  words  and  signs  depends  primarily 
on  the  power  of  reviving  the  images  or  memories  of  words  and  letters  heard 
and  seen;  and  secondarily  of  the  power  or  reviving  the  images  or  memories 
of  the  muscle  movements  which  were  previously  employed  in  an  effort  to 
imitate  or  reproduce  the  words  (speech)  or  the  verbal  signs  (writing). 

Clinico-pathologic  investigations  have  shown  that  words  or  letters  heard 
and  seen  have  areas  of  representation  in  the  cortex,  in  the  general  auditory 
area,  in  the  supra-marginal  convolution  and  angular  gyrus  respectively  (Fig. 
252).  Destruction  of  these  areas  is  followed  by  word-deafness  and  word- 
blindness.  The  same  methods  of  investigation  have  shown  that  the  muscle 
movements  employed  to  reproduce  the  words  and  the  verbal  signs  also 
have  areas  of  representation  in  the  cortex;  the  former  in  the  sub-frontal 
convolution  (Fig.  252),  and  probably  in  the  adjacent  region,  the  island  of 
Reil,  on  the  left  side  in  the  great  majority  of  people;  the  latter  in  front  of 
the  arm  region  of  the  general  motor  area.  Destruction  of  these  areas  is 
followed  in  the  first  instance  by  a  loss  of  the  power  of  executing  the  move- 
ments of  the  muscles  employed  in  speech,  and  in  the  second  instance,  of 
those  employed  in  writing. 

These  different  areas  are  connected  with  one  another  by  association 
fibers,  and,  taken  collectively,  constitute  the  language  zone.  Their  situ- 
ation and  relations  are  shown  in  Fig.  255.  In  this  figure  the  dotted  lines 
coming  from  the  ear  (a)  and  the  eye  (v)  represent  the  auditory  and  visual 
tracts  through  which  nerve  impulses  pass  to  the  auditory  (A)  and  the  visual 
centers  (V)  respectively.     Similar  fines  coming  from  the  muscles  involved 


THE  CEREBRUM. 


557 


in  speech  and  writing  might  also  be  represented  to  indicate  the  paths  of  the 
nerve  impulses  to  the  motor  speech  (M)  and  the  motor  writing  centers  (E). 
The  continuous  lines  on  the  surface  of  the  cortex  represent  nerve-fibers 
which  associate  the  auditory  and  visual  centers  with  the  speech  and  writing 
centers  and  with  higher  psychic  centers  (O  O)  as  well.  The  dotted  lines 
coming  from  the  speech  and  writing  centers  represent  the  tracts  through 
which  nerv-e  impulses  pass  to  the  muscle  of  the  larynx,  tongue,  mouth,  and 
lips,  and  to  the  muscles  of  the  hand.  The  anatomic  and  physiologic 
association  of  the  various  areas  is 
essential  to  the  registration  of  the 
impressions  made  on  the  ear  and  eye 
and  for  the  expression  of  the  ideas 
evolved  from  them  by  words  (speech) 
and  signs  (writing).  Their  collective 
action  is  essential  to  the  acquisition 
of  language.  Destruction  of  any 
part  of  this  cerebral  mechanism  is  at- 
tended by  an  impairment  of  or  a  total 
loss  in  either  the  power  of  obtaining 
auditory  images  of  words  heard  and 
visual  images  of  words  seen,  or  the 
power  of  expressing  ideas  by  speech 
and  writing.  To  this  pathologic  con- 
dition the  term  aphasia  has  been 
given. 

Aphasia. — It  was  discovered  by 
Bouillaud  that  a  destructive  lesion  of 
the  third  frontal  convolution  on  the  left 
side  was  accompanied  by  a  partial  or 
complete  loss  of  the  faculty  of  articu- 
late speech,  the  power  to  express  ideas 
with  words.  To  this  condition  the 
term  aphasia  was  given.  Though  of 
limited  application  etymologically,  the 
word  is  now  employed  in  a  wider 
sense  to  signify  "partial  or  complete 
loss  of  the  power  of  expression  or  com- 
prehension of  the  conventional  signs 
of  language,"  words  either  spoken  or 
written,  due  to  lesicns  of  different  por- 
tions of  the  cortex,  and  especially  on 
the  left  side. 

Aphasias  are  of  many  degrees  and  kinds,  though  they  may  be  included 
in  the  two  general  di\asions,  motor  and  sensor. 

Motor  aphasia  may  be  either  ataxic  or  agraphic.  In  ataxic  aphasia  the 
patient  is  unable  to  express  or  communicate  his  thoughts  by  spoken  words, 
owing  to  an  inability  to  execute  those  movements  of  the  mouth,  tongue,  etc., 
necessary  for  speech  without  there  being  any  paralysis  of  these  muscles. 
the  lesion  is  usually  in  the  third  frontal  convolution  and  most  frequently 


Fig.  255. — Diagram  Showing  the  Re- 
latiox  of  the  centers  of  language  and 
THEIR  Princip.al  ASSOCIATIONS.  A.  Audi- 
tory center.  V.  Visual  center.  M.  Motor 
speech  center.  E.  Motor  writing  center. 
O    O.  Intellectual  center. — {After  Grasset.) 


558  TEXT-BOOK  OF  PHYSIOLOGY. 

associated  with  right  hemiplegia.  In  agraphic  aphasia  the  patient  is  unable 
to  communicate  his  ideas  by  writing  through  an  inability  to  execute  the 
necessary  movements,  though  retaining  his  mental  processes.  In  this  form  of 
aphasia  the  lesion  is  in  the  writing  area.  These  two  forms  of  motor  aphasia 
are  not  infrequently  associated. 

Sensor  aphasia  or  amnesia  may  be  either  visual  or  auditory.  In  visual 
aphasia  or  amnesia  the  patient  is  unable  to  recognize  a  letter  or  word,  printed 
or  written  (though  capable  of  seeing  other  objects),  a  condition  known  as 
letter-  or  word-blindness.  It  is  usually  associated  with  lesions  in  the  neighbor- 
hood of  the  supra-marginal  convolution.  In  auditory  aphasia  or  amnesia 
the  patient  cannot  understand  articulate  or  vocal  speech,  though  capable  of 
hearing  and  understanding  other  sounds,  through  an  inability  to  distinguish 
the  associations  of  words  and  letters — a  condition  known  as  word-deafness. 
It  is  associated  with  lesions  of  the  auditory  area. 

Paraphasia  is  an  inability  to  recall  the  proper  words  to  associate  with 
ideas  and  necessary  to  their  expression. 

Concept  aphasia  is  the  inability  to  recall  the  names  of  objects.  It  is 
associated  with  lesions  of  the  cortex  of  the  mid-temporal  or  third  temporal 
convolution  (Mills).     This  area  is  known  as  the  concept  or  naming  area. 

Bilateral  Representation. — Though  highly  specialized  movements, 
such  as  those  performed  by  the  arms  and  hands,  legs  and  feet,  have  their 
areas  of  representation  on  one  side  of  the  cerebrum  only,  and  that  opposite 
to  the  side  of  the  movement,  less  highly  specialized  movements,  such  as  the 
masticatory,  phonatory,  respiratory  and  various  trunk  movements,  which 
require  for  their  performance  the  cooperation  of  muscles  on  both  sides  of 
the  body,  have  their  areas  of  representation  on  both  sides  of  the  cerebrum ; 
the  area  of  either  side  exciting  to  action  the  muscles  on  both  sides  of  the 
body.  In  the  case  of  specialized  movements  the  representation  is  unilateral; 
in  the  case  of  the  more  general  movements  the  representation  is  bilateral. 

Association  Centers. — The  sensor  and  motor  areas  to  which  specific 
functions  have  been  assigned  do  not  constitute  more  than  one-third  of  the 
total  cerebral  cortex.  There  yet  remain  large  regions  to  which  it  has  been 
impossible  to  assign  specific  functions  based  on  physiologic  experiments. 
Three  or  four  such  regions  separated  by  the  sensor  and  motor  centers  are  to 
be  recognized  on  the  lateral  and  mesial  aspects  of  the  hemisphere.  In  Fig. 
256  the  location,  extent,  and  names  of  these  regions  are  represented.  The 
fibers  which  are  found  in  these  regions  belong  almost  exclusively  to  the 
association  system,  and  become  medullated  at  a  later  period  than  do  the 
fibers  of  the  projection  system;  moreover,  from  the  method  of  their  medulliza- 
tion  it  would  appear  that  many  of  these  fibers  grow  out  directly  from  the 
sensor  centers  into  these  regions  and  become  related  to  the  nerve-cells  of 
their  convolutions,  while  others  grow  out  from  adjacent  as  well  as  distant 
convolutions.  From  histologic  and  pathologic  evidence  these  regions  were 
termed  by  Flechsig  association  centers  or  areas,  implying  the  idea  that 
through  the  intervention  of  their  cell  mechanisms  the  sense  areas  are  in- 
directly associated  anatomically  and  physiologically,  and  together  constitute 
a  mechanism  by  which  sensations  are  associated  and  elaborated  into  concrete 
forms  of  knowledge  or  related  to  definite  forms  of  movement. 

It  has  been  assumed  by  Flechsig  that  the  frontal  association  center,  from 


THE  CEREBRUM. 


559 


its  connections  with  the  sensor  and  motor  areas  of  the  Rolandic  region,  the 
olfactory,  and  perhaps  other  regions,  is  engaged  in  associating  and  registering 
body  sensations  and  vohtional  acts,  and  that  the  knowledge  thus  gained  has 
reference  largely  to  the  personality  of  the  individual ;  that  the  paricto-occipital 
association  area,  from  its  relation  to  the  visual,  auditory,  and  tactile  sense 
areas,  is  engaged  in  associating  and  registering  visual,  auditory,  and  tactile 

Motor  and  tactile  area.  , 


Parietal  association  area 


Frontal 

issociation 

area. 


Island  of  Reil. 


Auditory  area. 


Occipito-temporal 
association  area. 


Motor  and  tactile  area. 


Parietal  association  area       y^ 
(Precuneus).  '^ 


Visual  area        •.;..)-•    ''  ^'>     '    '•\ 
(cuneus).  ^'•"■'  ■ 


Frontal 

association 

area. 


Occipito-temporal 
association  area 


Olfactory  lobe. 
Olfactory  tract. 


Olfactory  area. 


Fig. 


Gyrus  hippocampus. 

256. — Diagrams  to  show  the  Position  and  the  Relation  of  the  Association  ani> 
Projection  Areas.    The  Projection  Areas  are  Dotted. — {After  Flechsig.) 


sensations,  and  that  the  knowledge  thus  gained  has  reference  mainly  to  the 
external  world.  These  assumptions  in  a  general  way  are  supported  by  the 
phenomena  of  disease.  In  certain  lesions  of  the  frontal  lobe  the  symptoms 
indicate  a  loss  or  change  of  ideas  regarding  personality  rather  than  of  the 
objective  world,  while  the  reverse  is  true  in  disease  of  the  parieto-occipital. 
lobe. 


56o  TEXT-BOOK  OF  PHYSIOLOGY. 

The  Intra-cranial  Circulation. — The  circulation  within  the  cranium 
presents  certain  peculiarities  which  distinguish  it  from  that  in  other  parts  of 
the  body.  These  peculiarities  reside  in  part  in  the  anatomic  arrangement  of 
the  blood-vessels,  in  the  probable  absence  of  vaso-motor  nerves  to  the  blood- 
vessels, and  in  greater  part  in  the  fact  that  the  brain  and  its  blood-vessels 
are  contained  in  a  case  with  rigid,  unyielding,  and  closed  walls. 

The  Blood-supply. — As  stated  in  a  previous  paragraph  the  arteries 
supplying  the  brain  with  blood  are  four  in  number,  viz. :  the  two  internal 
carotids  and  the  two  vertebrals.  These  four  arteries  anastomose  very  freely 
at  the  base  of  the  brain,  the  anastomosis  constituting  the  circle  of  Willis. 
From  this  circle  there  arise  the  anterior,  middle  and  posterior  cerebral  arteries 
which  are  distributed  to  the  cortex  and  the  underlying  white  matter.  The 
basal  ganglia,  the  capsule  and  adjacent  white  matter  are  suppHed  by  a  num- 
ber of  branches  which  arise  from  the  circle  of  Willis  or  from  the  three  cerebral 
arteries  immediately  after  their  origin.  From  the  distribution  of  these  two 
sets  of  vessels  they  have  been  named  the  cortical  and  the  central  ganglionic 
respectively. 

The  venous  blood  is  returned  by  a  system  of  vessels  which  present  charac- 
teristics of  physiologic  interest.  These  vessels  consist  of  large  sinuses  formed 
by  folds  of  the  dura  mater  or,  as  at  the  base  of  the  cranium,  by  the  dura  mater 
and  the  bone.  These  sinuses,  from  the  very  nature  of  the  tissues  which 
enter  into  their  formation,  have  rigid  walls  and  will  therefore  withstand  any 
pressure  to  which  they  may  be  subjected  under  physiologic  conditions.  The 
same  obtains  at  their  points  of  exit  from  the  cranium  where  a  free  outflow 
is  in  consequence  always  assured. 

The  various  sinuses  have  opening  into  them,  the  veins  which  return  the 
blood  from  the  cortex  and  subjacent  white  matter,  and  from  the  inner  struc- 
tures of  the  brain.  Neither  sinuses  nor  veins  have  valves  and  most  of  the 
veins  which  empty  into  the  superior  longitudinal  sinus  have  their  mouths 
directed  forward,  hence  the  blood  discharged  from  these  veins  must  flow 
against  the  current  in  the  sinus.  The  venous  blood  leaves  the  cranium 
mainly  by  way  of  the  internal  jugular  veins  which  are  direct  continuations  of 
the  lateral  sinuses. 

The  Intra-cranial  Lymph  Spaces.^ — In  order  to  understand  the  phe- 
nomena attending  the  circulation  of  blood  through  the  cranium  it  is  necessary 
to  take  into  consideration  an  important  fact,  viz.:  that  the  brain  and  spinal 
cord  are  surrounded  on  all  sides  by  a  relatively  large  and  continuous  lymph 
space.  This  space  which  is  found  between  the  arachnoid  and  the  pia  mater 
is  filled  with  a  liquid,  the  so-called  cerebrospinal  fluid,  which  being  interposed 
between  the  brain  and  the  skull  on  the  one  hand  and  the  spinal  cord  and  the 
vertebrae  on  the  other  hand,  acts  as  a  water  cushion  protecting  these  delicate 
organs  from  the  injury  which  might  result  from  sudden  jars.  The  ventricles 
of  the  brain  are  also  filled  with  cerebrospinal  fluid  which  is  in  communication 
with  that  in  the  subarachnoid  space  through  the  foramen  of  Magendie  and  the 
foramina  of  Key  and  Retzius.  The  cerebrospinal  fluid  may  also  penetrate 
into  the  perineural  lymph  spaces  surrounding  the  cranial  and  spinal  nerves. 
The  quantity  of  the  cerebrospinal  fluid  is  relatively  small,  amounting  to 
from  60  to  80  c.c. 


THE  CEREBRUM.  561 

The  Mechanism  of  the  Intra-cranial  Circulation. — As  previously 
stated,  by  virtue  of  the  physical  relations  existing  between  the  blood,  the 
brain,  the  cerebrospinal  fluid  and  rigid  walls  of  the  cranium,  the  flow  of  the 
blood  through  the  brain  and  cranial  cavity,  is  attended  by  certain  phenomena 
which  are  peculiar  to  this  region  and  present  in  no  other  situation. 

Taking  as  a  point  of  departure  the  condition  of  the  arteries  during  the 
cardiac  diastole,  the  relations  of  these  structures  are  somewhat  as  follows:  the 
cerebrospinal  fluid  occupies  all  the  available  lymph  space,  but  under  a  pres- 
sure approximately  equal  to  that  in  the  large  veins  and  hence  not  materially 
above  that  of  the  atmosphere;  the  pressure  in  the  arteries,  capillaries  and 
veins  presents  the  usual  values  in  these  different  regions  of  the  vascular 
apparatus;  the  brain  presents  a  volume  which  may  be  termed  diastolic. 

With  the  occurrence  of  the  succeeding  cardiac  systole,  the  cerebral 
vessels,  receiving  an  additional  volume  of  blood,  expand  and  occasion  a 
corresponding  increase  in  the  volume  of  the  brain,  which  is  accomplished  by 
a  partial  displacement  of  the  cerebrospinal  fluid  into  extra-cranial  lymph 
spaces.  Because  of  the  fact  that  the  displacement  of  the  cerebrospinal 
fluid  is  insufficient  to  permit  of  the  complete  expansion  of  the  brain,  there  is 
developed  in  the  intra-cranial  lymph  spaces  a  counter-pressure  (the  so-called 
intra-cranial  pressure)  which  would  keep  pace  with  and  finally  equalize  the 
rising  pressure  in  the  arteries.  In  consequence  of  this,  the  brain  tissue,  it  is 
believed,  would  be  subjected  to  a  pressure  sufficiently  great  to  interfere  with 
its  activities,  even  to  the  point  of  unconsciousness.  If  this  is  not  to  occur 
the  maximum  expansion  of  the  arteries,  and  hence  the  brain,  must  be  checked 
and  controlled.  This  is  accomplished  in  the  following  way:  As  the  brain 
approaches  that  degree  of  expansion  permitted  by  the  displacement  of  the 
cerebrospinal  fluid,  it  begins  to  exert  a  compression  of  the  pial  veins.  This 
compression  by  narrowing  the  lumen  of  the  veins  diminishes  their  capacity 
and  hence  increases  the  pressure  of  their  contained  blood  until  it  is  equiva- 
lent to  the  pressure  exerted  by  the  brain  against  the  veins.  At  this  moment 
the  pressures  in  the  arterioles,  capillaries  and  veins  approximate  each  other 
in  value. 

From  these  factors  it  will  be  seen  that  the  circulation  through  the  brain 
approximates  a  circulation  through  a  system  of  rigid  tubes.  The  result  is  an 
increase  in  the  velocity  of  the  outflow  and  a  diminution  of  the  blood-pressure. 
As  an  additional  result  the  pulse  wave  of  the  arterial  system  is  transmitted 
to  the  blood  of  the  large  veins  and  sinuses  which  therefore  exhibit  normally 
pulsations  synchronous  with  those  of  the  arteries.  The  rise  of  the  pressure 
in  the  cerebral  veins  is  regarded  therefore  as  the  factor  which,  by  limiting 
brain  expansion,  checks  the  rise  of  the  intra-cranial  pressure  beyond  physio- 
logic limits.  With  the  diastole  of  the  heart  and  the  recoil  of  the  arteries, 
the  former  relation  of  the  blood,  brain,  cerebrospinal  fluid  and  cranial  walls 
is  regained.  Because  of  this  change  of  relation  with  each  heart-beat, 
the  brain  pulsates  synchronously  with  the  arteries. 

The  brain  dift'ers  from  other  organs,  also,  in  that  normally  its  volume  is 
more  influenced  in  a  positive  direction  by  the  expiratory  rise  of  venous  pres- 
sure than  by  the  inspiratory  rise  of  general  arterial  pressure.  Thus  the  rise 
of  pressure  in  the  thoracic  veins  which  occurs  with  each  expiratory  act, 
causes  a  damming  back  of  the  venous  blood  in  the  sinuses  and  pial  veins, 
36 


562  TEXT-BOOK  OF  PHYSIOLOGY. 

resulting  in  a  further  increase  in  the  volume  of  the  brain  and  in  the  intra- 
cranial pressure.     The  reverse  takes  place  in  inspiration. 

It  has  been  ascertained  experimentally  that  the  intra-cranial  pressure 
may  vary  considerably  and  consciousness  still  be  preserved.  Hill  found  it 
to  be  40  to  50  mm.  of  Hg.  in  the  convulsions  of  strychnin  poisoning  and  a 
little  less  than  zero  in  a  patient  standing  erect. 

The  Regulation  of  the  Volume  of  Blood  Entering  the  Brain. — It  is 
generally  believed  that  the  cerebral  vessels  are  not  provided  with  vaso-motor 
nerves.  Every  attempt  to  prove  their  existence  either  by  physiologic  or 
histologic  methods  has  thus  far  failed  of  convincing  proof.  In  the  absence 
of  vaso-motor  nerves,  the  regulation  of  the  circulation  in  the  brain  must  neces- 
sarily be  dependent  on  changes  affecting  the  arterial  and  venous  pressures  in 
other  regions  of  the  body. 

The  most  effective  factor  in  increasing  or  decreasing  the  blood-supply 
to  the  brain  resides  in  the  power  of  the  vaso-motor  center  to  cause  a  contrac- 
tion or  dilatation  of  the  cutaneous  and  splanchnic  vessels.  Thus  if  the 
vaso-motor  center  declines  in  its  tonus  from  any  cause  whatever,  there  is  a 
relaxation  of  the  blood-vessels  in  one  or  both  of  these  regions,  an  increase  in 
the  volume  of  the  blood  flowing  into  them,  and  in  consequence,  a  decrease 
in  the  volume  of  the  blood  flowing  through  the  brain.  If  on  the  contrary  the 
vaso-motor  center  is  increased  in  its  tonus,  the  reverse  conditions  prevail  in 
the  cutaneous  and  splanchnic  vessels  and  the  quantity  of  blood  flowing  into 
the  brain  is  increased.  Thus  in  an  indirect  way  the  vaso-motor  center,  by 
bringing  about  a  rise  or  a  fall  in  the  general  arterial  pressure,  regulates  the 
blood-supply  to  the  brain,  and  controls  its  amount  in  accordance  with  its 
needs. 

Brain  Activity .^ — Brain  activity  is  characterized  by  an  active  conscious- 
ness, the  development  of  sensations,  ideas,  feelings,  and  the  exercise  of 
volitional  power  (which  manifests  in  muscle  movement)  and  is  the  result  of  a 
physiologic  condition  of  the  body  at  large.  For  the  manifestation  of  brain 
activity  it  is  essential  that  the  irritability  of  the  brain  cells  and  more  especially 
of  those  composing  in  large  measure  the  cerebral  cortex  be  maintained  at  a 
normal  physiologic  level,  so  that  they  may  respond  in  the  manner  peculiar 
to  them  to  the  action  of  nerve  impulses  transmitted  through  afferent  nerves 
from  all  regions  of  the  body.  Here  as  elsewhere  throughout  the  body,  the 
irritability  depends  on,  and  is  maintained  by,  the  presence  of  blood  flowing 
into  and  out  of  the  brain  in  varying  quantity  from  moment  to  moment,  with 
a  given  velocity  and  under  a  definite  pressure.  So  long  as  these  conditions 
are  maintained  in  the  strictly  physiologic  condition,  so  long  will  the  brain 
respond  to  stimuli  by  the  development  of  sensations.  The  avenues  through 
which  nerve  impulses  pass  to  the  cortical  cells  are  those  beginning  in  the 
special  and  general  sense  organs  of  the  body  in  contact  with  the  external 
world,  viz.:  the  eyes,  ears,  nose,  tongue,  and  skin.  The  maintenance  of 
these  structures  in  a  strictly  physiologic  condition  is  also  one  of  the  essential 
conditions  for  brain  activity. 

Judging  from  the  changes  in  the  character  and  composition  of  the  blood 
which  occur  during  its  passage  through  the  brain  capillaries,  there  is  coin- 
cidently  with  brain  activity  an  active  metabolism,  which  eventuates,  at  the 
end  of  a  variable  number  of  hours,  in  the  decline  of  the  irritabflity,  a  reduc- 


THE  CEREBRUM.  563 

tion  of  functional  activity,  and  the  establishment  of  the  condition  of  fatigue. 
The  irritability  of  the  sense  organs,  especially  of  the  eyes  and  ears,  in  all 
probability  declines  in  a  similar  manner.  These  structures  pass  into  the 
condition  of  fatigue  and  become  less  responsive  to  external  stimuli.  The 
results  of  all  these  conditions  is  a  less  active  stimulation  of  the  brain  cells, 
which  in  connection  with  other  factors  predisposes  to — 

Brain  Repose  or  Sleep. — Brain  repose  or  sleep  is  characterized  by  a 
greater  or  less  degree  of  unconsciousness,  the  non-development  of  sensations, 
ideas,  feelings  and  volitional  acts,  and  is  the  result  of  a  diminution  in  the 
physiologic  activities  of  the  body  at  large  and  more  especially  of  the  brain, 
sense  organs,  and  spinal  cord.  Coincident  with  the  cessation  of  brain  activity 
and  the  onset  of  sleep,  there  is  a  diminution  in  the  rate  and  force  of  the  heart- 
beat, and  in  the  frequency  and  depth  of  the  respiratory  movements,  and  a 
relaxation  of  the  skeletal  muscles,  especially  those  employed  in  voluntary 
movements. 

The  sense  organs  are  in  part  protected  from  the  action  of  external  stimuli. 
The  eyeball  is  so  turned  that  its  anterior  pole  is  directed  far  upward  under 
the  eyelid,  while  the  pupil  is  markedly  diminished  in  size,  and  in  consequence 
the  entrance  of  light  largely  prevented.  The  ear  is  protected  against  the 
reception  of  sounds  of  ordinary  pitch  by  an  increased  tension  of  the  tympanic 
membrane.  The  nose  and  mouth  are  less  responsive  to  various  stimuli 
because  of  the  dryness  of  their  mucous  membranes  from  diminished  secre- 
tion. The  skin  appears  to  be  less  sensitive  to  mechanic  pressure  and  other 
forms  of  stimulation. 

In  addition  to  the  foregoing  phenomena,  experimental  investigations 
have  shown  that  there  is  a  shunting  of  a  portion  of  the  blood  stream  from 
the  brain  to  other  regions  of  the  body,  especially  to  the  skin  and  perhaps  to 
the  abdominal  viscera  as  well,  whereby  it  becomes  incapable  of  functionating 
physiologically.  The  fact  that  the  brain  receives  a  lessened  quantity  of 
blood  during  sleep  has  been  shown  by  trephining  the  skull  and  inserting  in 
the  orifice  a  glass  plate  through  which  the  circulatory  conditions  of  the  brain 
can  be  observed.  In  the  waking  condition  the  blood-vessels  on  the  surface 
of  the  brain  are  prominent,  and  turgid  with  blood  and  the  whole  organ 
completely  fills  the  cranial  ca\'ity,  indicating  that  the  blood-vessels  in  the 
interior  of  the  brain  are  in  a  similar  condition.  With  the  onset  of  sleep  the 
larger  blood-vessels  begin  to  diminish  in  size,  the  smaller  vessels  disappear 
from  view,  the  brain  tissues  become  pale  and  the  volume  of  the  brain  shrinks. 
During  the  continuance  of  deep  sleep,  this  anemic  condition  persists.  As 
the  period  of  sleep  approaches  its  termination,  the  smaller  blood-vessels 
again  fill  with  blood,  the  surface  of  the  brain  flushes,  and  in  a  very  short 
time  the  former  circulatory  conditions  return,  the  volume  of  the  brain 
increases  and  the  waking  state  is  reestablished. 

The  fact  that  the  skin  receives  an  increased  volume  of  blood  during  sleep, 
has  been  shown  by  inserting  an  arm  or  leg  in  a  plethysmograph  by  which 
means  a  record  of  any  change  in  volume  can  be  obtained.  Howell  thus 
succeeded  in  obtaining  graphic  records  in  the  variations  of  the  volume  of  the 
arm  during  sleep.  These  records  disclosed  the  fact  that  with  the  onset  of 
sleep  the  volume  of  the  arm  gradually  increased  in  size  until  it  attained  a 
maximum  which  was  from  one  to  two  hours  after  the  beginning  of  sleep. 


564  TEXT-BOOK  OF  PHYSIOLOGY. 

After  this  period  the  volume  remains  practically  the  same  for  several  hours, 
diminishing  as  the  intensity  of  sleep  diminishes  and  the  waking  ^state  is 
approached.  Just  previous  to  the  return  of  consciousness  there  is  a  rapid 
diminution  in  the  volume  of  the  arm.  If  it  be  accepted  that  the  enlargement 
of  the  cutaneous  vessels  is  followed  by  a  diminution  in  size  of  the  cerebral 
vessels,  it  follows  that  the  former  condition  stands  to  the  latter  in  the  relation 
of  cause  and  effect,  whereby  a  portion  of  the  blood  is  diverted  from  the  brain 
to  the  skin.  It  also  naturally  follows  that  the  withdrawal  of  the  blood  from 
the  brain  to  the  skin  and  possibly  other  regions  as  well,  is  the  fundamental 
condition  for  brain  repose. 

The  Intensity  of  Sleep. — Observations  of  individuals  during  sleep 
show  that  the  intensity  or  the  depth  of  sleep  varies  from  hour  to  hour. 
Attempts  have  been  made  to  estimate  the  intensity  by  measuring  the 
loudness  of  a  sound  caused  in  seyeral  ways  that  is  necessary  to  awaken  the 
sleeper.  Accepting  this  criterion  it  may  be  stated  from  the  results  of  m.any 
experiments,  that  sleep  increases  in  intensity  or  depth  and  reaches  its  max- 
imum between  the  first  and  second  hours,  after  which  it  rapidly  decreases  un- 
til the  end  of  the  third  hour,  when  consciousness  is  so  nearly  restored,  that 
but  a  very  slight  stimulus  is  required  to  awaken  the  sleeper.  It  is  during 
the  latter  period  when  the  brain  is  reviving  that  dreams  arise,  the  elements 
of  which  are  formed  of  previous  sensations. 

The  Causes  of  Sleep. — Different  theories  have  been  proposed  to  account 
for  the  causes  of  sleep,  none  of  which  have  been  wholly  satisfactory.  From 
all  the  facts  which  have  been  presented  it  would  appear  that  one  cause  is  a 
decline  in  the  irritability  of  the  nerve-cells  of  the  brain  and  associated  sense 
organs,  and  the  development  of  fatigue  conditions,  the  result  of  prolonged 
activity. 

A  second  cause  is  the  withdrawal  of  a  large  portion  of  the  blood  from  the 
brain,  on  the  presence  of  which,  here  as  elsewhere,  normal  activity  depends. 
As  to  whether  the  diminished  activity  of  the  brain  is  the  cause  of,  or  the  result 
of  the  withdrawal  of  the  blood  there  has  been  much  difference  of  opinion. 
Howell  has  offered  a  plausible  explanation  for  the  withdrawal  of  the  blood 
from  the  brain  to  the  cutaneous  vessels,  based  on  the  activity  of  the  vaso- 
motor center.  He  assumes  that  for  a  variable  number  of  hours,  correspond- 
ing to  the  usual  waking  state,  this  center  possesses  a  certain  average  tonus, 
due  in  all  probability  to  reflex  influences,  by  virtue  of  which  it  maintains  a 
certain  average  contraction  of  the  cutaneous  vessels.  But  at  the  end  of 
this  period  it  too  becomes,  fatigued,  declines  in  irritability,  becomes  less 
responsive  to  reflex  influences,  and  hence  loses  its  control  over  the  vessels. 
As  a  result  they  dilate  and  thus  reduce  the  amount  of  blood  flowing  to  the  brain 
to  a  level  insufficient  to  maintain  its  activity,  after  which  sleep  supervenes. 
During  sleep  the  irritability  and  tonus  of  the  center  are  restored,  when  its 
control  of  the  blood-vessels  is  regained.  Unless  the  brain  in  its  functional 
activities  differs  from  all  other  organs  of  the  body,  it  may  be  inferred  that 
cessation  of  activity  or  repose  is  the  result  partly  of  fatigue  and  partly  of  a 
diminution  of  the  blood-supply. 


CHAPTER  XXII. 
THE  CEREBELLUM. 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital  bone, 
beneath  the  posterior  lobes  of  the  cerebrum,  from  which  it  is  separated  by  the 
tentorium  cerebelli,  a  semilunar  fold  of  the  dura  mater.  It  is  partially 
divided  into  hemispheres  by  a  longitudinal  fissure,  more  apparent  on  the 
inferior  surface,  though  united  by  a  central  lobe,  the  vermiform  process. 
Each  hemisphere  is  connected  with  the  cerebrum,  the  pons,  medulla,  and 
spinal  cord  by  three  bundles  of  nerve-fibers  known  respectively  SiS  the  superior, 
middle,  and  inferior  peduncles.  The  surface  of  the  cerebellum  presents  a 
series  of  lobes  and  fissures  of  which  the  former  have  received  more  or  less 
fanciful  names.  A  section  of  the  cerebellum  shows  that  it  is  composed 
of  gray  matter  externally  and  white  matter  internally.  The  general 
appearance  presented  on  section  is  shown  in  Fig.  257. 

Structure  of  the  Gray  Matter. — The  gray  matter  consists  mainly  of 
nerve-cells  of  varying  size  and  shape,  which  are  arranged  in  two  layers: 
viz.,  an  outer  or  molecular  and  an  inner  or  granular. 

The  molecular  layer  consists  of  stellate  and  multipolar  cells  of  small 
size,  from  which  dendrites  and  axons  pass  horizontally  and  vertically.  The 
granular  layer  consists,  as  its  name  implies,  of  granular-shaped  cells  and 
large  stellate  cells.  These  cells  are  characterized  by  the  possession  of  den- 
drites and  axons,  the  course  and  relation  of  which  have  not  been  clearly 
determined. 

The  inner  border  of  the  molecular  layer  presents  a  series  of  large  cells 
originally  described  by  Purkinjc  and  known  by  his  name.  From  the  outer 
end  of  the  cell-body  one  or  more  dendrities  emerge  which  soon  divide  and 
subdivide  into  a  number  of  branches  which  pass  toward  the  cerebellar  sur- 
face. The  general  arrangement  of  these  dendrities  gives  to  the  entire  cell  a 
tree-like  appearance  (Fig.  258).  From  the  inner  end  of  the  cell  an  axon 
emerges  which  passes  centrally  into  the  w^hite  matter. 

Structure  of  the  White  Matter. — The  white  matter  consists  of  nerve- 
fibers  which  are  arranged  in  association  and  projection  systems. 

The  Association  System. — The  fibers  which  compose  this  system  are  of 
variable  lengths  and  unite  adjacent  as  well  as  distant  regions  of  the  cer- 
bellar  cortex.  They  doubtless  associate  them  both  anatomically  and  physio- 
logically. 

The  Projection  System. — The  fibers  composing  this  system  connect  the 
cerebellar  cortex  with  certain  structures  in  the  cerebrum,  pons,  medulla, 
and  spinal  cord.     They  may  be  divided  into  efferent  and  afferent  systems. 

The  efferent  fibers  have  their  origin  in  the  cells  of  Purkinje  and  the 
dentate  nucleus.  Some  of  these  fibers  emerge  from  the  cerebellum  in  the 
superior  peduncles  through  which  they  pass  toward  and  beneath  the  corpora 

565 


566 


TEXT-BOOK  OF  PHYSIOLOGY. 


quadrigemina  to  terminate  around  the  cells  of  the  red  nucleus.  As  they 
approach  this  nucleus  some  of  the  fibers  cross  the  median  line  and  decussate 
with  those  coming  from  the  opposite  side,  while  others  pursue  a  straight 
direction,  terminating  on  the  same  side.  Through  the  intervention  of  fibers 
which  arise  in  the  red  nucleus  and  ascend  to  the  cerebral  cortex,  the  hemi- 
sphere is  thus  connected  with  both  sides  of  the  cerebellum,  though  chiefly 
with  the  opposite  side. 

Efferent  fibers  also  leave  the  cerebellum  by  the  middle  peduncle  and  pass 
directly  to  the  nucleus  pontis,  around  the  cells  of  which  their  terminals 
arborize.     Efferent  fibers  also  descend  the  inferior  peduncles  and  constitute 


Fig.    257. — ^\'IEW   OF   CEREBELLUil  IX  SECTION,  AND 

OF  Fourth  Ventricle,  with  the  Neighboring  Parts. 
— {From  Sappey.)  i.  Median  groove  fourth  ventricle, 
ending  below  in  the  calamus  scriptorius,  with  the  longi- 
tudinal eminences  formed  by  the  fascicuU  teretes, 
one  on  each  side.  2.  The  same  groove,  at  the  place 
where  the  white  streaks  of  the  auditory  nerve  emerge 
from  it  to  cross  the  floor  of  the  ventricle.  3.  Inferior 
peduncle  of  the  cerebellum,  formed  by  the  restiform 
body.  4.  Posterior  pyramid;  above  this  is  the  calamus 
scriptorius.  5,  5.  Superior  peduncle  of  cerebellum,  or 
processus  e  cerebello  ad  testes.  6,  6.  Fillet  to  the  side 
of  the  crura  cerebri.  7,7.  Lateral  grooves  of  the  crura 
cerebri.  8.  Corpora  quadrigemina. — {After  Hirschfeld 
and  Leveille.) 


Fig.  258. — Section  of  Cerebellar 
Cortex.  A.  Outei"  or  molecular 
layer.  B.  Inner  or  granular  layer. 
C."  White  matter,  a.  Cell  of  Purk- 
inje.  h.  Small  cells  of  inner  layer. 
c.  Dendrites  of  these  cells,  d.  A 
similar  cell  lying  in  the  white  matter. 
— {Stirling.) 


the  tract  known  as  the  Lowenthal  and  Marchi  tract,  situated  in  the  antero- 
lateral region  of  the  spinal  cord  in  its  upper  part. 

The  afferent  fibers  come  from  a  variety  of  sources.  Those  found  in  the 
superior  peduncles  come  from  the  red  nucleus;  those  in  the  middle  peduncles 
from  the  nucleus  pontis  of  the  opposite  side,  having  crossed  or  decussated  at 
the  raphe  near  the  anterior  surface  of  the  pons;  those  contained  in  the  in- 
ferior peduncles  are  the  most  abundant  and  important,  and  are  represented 
by  (i)  the  direct  cerebellar  tract,  which  terminates  in  the  superior  vermis 
after  decussation;  (2)  the  anterior  and  posterior  arcuate  fibers,  the  former 
coming  from  the  gracile  and  cuneate  nuclei  of  the  opposite  side,  the  latter 


THE  CEREBELLUM.  567 

from  the  same  side,  which  also  pass  to  the  superior  vermis;  (3)  the  acoustico- 
cerebellar  tract,  composed  of  fibers  which  are  the  axons  of  the  sensory 
end-nuclei  (Deiters)  of  the  vestibular  portion  of  the  auditory  ner\^e.  It  is 
probable  that  all  these  fibers  decussate  prior  to  their  final  termination. 

The  cerebellum  through  this  system  of  efferent  and  afferent  fibers  is 
brought  into  relation  with  many  different  regions  of  the  cerebrum,  pons, 
medulla,  and  spinal  cord.  Each  half  of  the  cerebellum  is  connected  with  the 
foregoing  structures  of  the  same  side,  and  of  the  opposite  side. 

THE  FUNCTIONS  OF  THE  CEREBELLUM. 

From  the  observations  of  the  results  of  experimental  lesions,  from  the 
analysis  of  clinico-pathologic  facts,  and  from  the  comparative  anatomic 
development  in  different  animals,  the  deduction  has  been  drawn  that  the 
cerebellum  coordinates  and  harmonizes  the  action  of  those  muscles  the 
activities  of  which  are  necessary  to  the  maintenance  of  body  equilibrium 
both  during  station  and  progression. 

By  equilihrium  of  the  body  is  understood  a  condition  which  may  be  main- 
tained for  a  variable  length  of  time  without  displacement,  and  is  possible  only 
so  long  as  a  vertical  line  passing  through  the  center  of  gravity  falls  within  the 
base  of  support.  The  support  offered  by  the  earth  to  the  feet  neutralizes  and 
counteracts  the  force  of  gravity.  In  standing,  when  the  body  is  in  the  erect  or 
military  position,  the  arms  by  the  side,  the  center  of  gravity  lies  between  the 
sacrum  and  the  last  lumbar  vertebra,  and  the  line  of  gravity  falls  between  the 
feet  and  within  the  base  of  support.  The  entire  skeleton  for  the  time  being 
is  rendered  fixed  and  rigid  at  all  its  joints  by  the  combined  action  of  the 
muscles  connected  with  it.  That  this  position  may  be  maintained  all  the 
different  groups  of  antagonistic  but  cooperative  muscles  must  be  accurately 
coordinated  in  their  actions.  Any  failure  in  this  respect  is  at  once  attended 
by  a  disturbance  of  the  equilibrium  and  displacement. 

In  progression,  walking,  running,  dancing,  etc.,  the  body  is  translated 
from  point  to  point  by  the  alternate  action  of  the  legs.  Whether  the  direction 
of  the  translation  be  rectilinear  or  curvilinear,  as  the  legs  change  their  position 
from  moment  to  moment,  the  center  of  gravity  also  changes,  and  at  once  the 
equilibrium  is  menaced.  If  it  is  to  be  maintained  and  displacement  prevented 
there  must  be  a  prompt  readjustment  in  the  relation  of  all  parts  of  the 
body  so  that  the  line  of  gravity  falls  again  within  the  base  of  support.  The 
more  complicated  the  moments  of  progression,  or  the  narrower  the  base  of 
support,  the  greater  is  the  danger  to  the  equilibrium,  and  hence  the  necessity 
for  rapid  and  compensatory  changes  in  coordinated  muscle  activity.  All 
movements  of  this  character,  in  man  at  least,  are  primarily  volitional  and 
require  for  their  performance  the  constant  exercise  of  the  attention.  With 
frequent  repetition  they  gradually  come  to  be  performed  independently  of 
consciousness  and  fall  into  the  category  of  secondary  or  acquired  reflexes. 

Though  coordinating  power  is  exhibited  by  the  spinal  cord,  medulla, 
and  basal  ganglia,  it  is  only  in  the  cerebellum  that  this  power  attains  its 
highest  development  and  differentiation.  To  it  is  assigned  the  power  of 
selecting  and  grouping  muscles,  not  in  any  restricted  part,  but  in  all  parts  of 
the  body,  and  coordinating  their  actions  in  such  a  manner  as  to  preserve  the 
equilibrium. 


568 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  Results  of  Experimental  Lesions. — If  the  cerebellum  in  it?  totality 
coordinates  and  harmonizes  the  action  of  the  muscles  on  the  opposite  sides  of 
the  body,  any  derangement  of  its  structure  or  its  connections  with  the  cord, 
medulla,  pons^  or  basal  ganglia  should  at  once  be  followed  by  incoordination 
of  muscles  and  a  want  of  harmony  in  their  action.  Experimental  lesions  of 
the  cerebellum  are  attended  by  such  results.  The  phenomena  observed  are 
many  and  complex.  They  differ  in  extent  and  character  in  different  animals 
and  in  accordance  with  the  extent  and  location  of  the  lesion,  though  the  note 
of  incoordination  runs  through  them  all. 


Fig.  259. — Attitude  Assumed  After  Destruction  of  the  Left  Half  of  the  Cerebellum. — 

{Moral  and  Doyon,  after  Thomas.) 

Removal  of  one  lateral  half  of  the  cerebellum  in  the  dog  is  followed  by  an 
inability  to  maintain  the  equilibrium  necessary  to  the  erect  position.  On 
attempting  to  stand,  the  animal  at  once  falls  toward  the  side  of  the  lesion,  the 
muscles  of  which  at  the  same  time  contract  and  give  to  the  body  a  distinctly 
curved  condition  (Fig.  259).  The  anterior  limbs  are  extended  to  the  opposite 
side.     On  making  efforts  to  regain  the  standing  position,  the  animal  may 

roll  over  around  the  long  axis  of  its 
body.  Conjugate  deviation  of  the 
eyes  is  frequently  observed  as  well  as 
nystagmus. 

After  a  few  days  the  symptoms  par- 
tially subside  and  the  animal  acquires 
the  power  of  sitting  on  the  abdomen 
when  the  anterior  limbs  are  widely 
extended  (Fig.  260).  As  the  days  go 
by  the  improvement  continues,  and  the 
animal  recovers  the  power  of  walking, 
though  each  step  is  attended  with 
tremor  and  oscillations  of  the  body. 
Any  change  in  the  center  of  gravity 
such  as  results  when  one  leg  is  lifted  may  result  in  a  fall  toward  the  side  of 
the  lesion,  owing  to  an  inability  to  promptly  bring  about  the  necessary 
compensatory  muscle  actions.  With  time  the  animal  continues  to  improve 
in  its  power  of  adjustment,  though  it  never  completely  recovers  it.  Move- 
ments of  progression  are  apt  to  be  characterized  by  stiffness  and  accom- 
panied by  tremor  suggestive  of  volitional  efforts. 

Total  removal  of  the  cerebellum  is  followed  by  a  different  train  of  symp- 
toms. The  extensor  muscles  apparently  preponderate  in  their  action,  for 
the  limbs  are  extended  and  abducted,  the  head  and  neck  are  retracted,  and 


Fig.  260. — Attitude  in  Repose  after 
the  Complete  Removal  of  the  Cere- 
bellum but  during  the  Period  of  Res- 
toration OF  Function. — {Morat  and  Doyon, 
after  Thomas.) 


THE  CEREBELLUM. 


569 


opisthotonos  is  established.  In  time  these  effects  also  partially  subside, 
though  all  attempts  at  walking  are  permanently  accompanied  by  tremor  and 
oscillations.  The  characteristic  effect  which  follows  section  of  the  peduncles 
is  again  incoordination,  manifesting  itself  in  deviation  of  the  head,  eyes,  in- 
ability to  walk,  tremor  on  exertion,  etc.  The  effects  vary,  however,  according 
to  the  peduncle  divided.  Section  of  the  middle  peduncle  gives  rise  to  the 
most  pronounced  effects.  The  head  and  the  anterior  part  of  the  body  are 
at  once  drawn  toward  the  pelvis  on  the  side  of  the  section.  A  voluntary 
effort  on  the  part  of  the  animal  causes  it  to  lose  all  control  of  its  muscles  and 
the  body  is  rotated  around  its  longitudinal  axis  from  40  to  60  times  a 
minute  before  it  comes  to  rest.  According  as  the  lesion  is  made  from  be- 
hind or  before,  the  rotation  is  from  or  to  the  side  of  the  section.  In  time 
these  symptoms  subside,  though  the  animal  never  completely  recovers. 

The  partial  recovery  of  the  power  of  coordination,  observed  after  removal 
of  a  portion  or  the  whole  of  the  cerebellum,  indicates  that  the  centers  in  the 
cord,  medulla,  pons,  and  cerebrum 


endowed  with  corresponding 
though  less  developed  power,  de- 
velop compensatory  activity  and 
acquire  to  some  extent  the  capa- 
bilities of  the  cerebellum  itself 
(Fig.  261). 

Clinico- pathologic  facts  partly 
corroborate  the  results  of  physio- 
logic investigations.  In  various 
forms  of  uncomplicated  cerebellar  of  the  Vermis^ 
disease,  vertigo,  tremor  on  making 
voluntary  efforts,  difficulty  in  maintaining  the  erect  position,  unsteadiness 
in  walking,  opisthotonos,  pleurothotonos,  are  among  the  symptoms  generally 
observed. 

Comparative  anatomic  investigations  reveal  a  remarkable  correspondence 
between  the  development  of  the  cerebellum  and  the  complexity  of  the  move- 
ments exhibited  by  animals.  In  those  animals  whose  movements  are  com- 
plex and  require  for  their  performance  the  cooperation  of  many  groups  of 
muscles  the  cerebellum  attains  a  much  greater  development  in  reference  to 
the  rest  of  the  brain  than  in  animals  whose  movements  are  relatively  simple 
in  charftcter.  This  relative  increase  in  the  development  of  the  cerebellum 
is  found  in  many  animals,  as  the  kangaroo,  the  shark,  the  swallow,  and  the 
predaceous  birds  generally. 

The  Coordinating  Mechanism. — Though  it  is  not  known  how  the 
cerebellum  selects  and  coordinates  groups  of  muscles  for  the  performance 
of  any  complex  movement,  it  is  known  that  its  acti\dty  is  largely  reflex  in 
origin  and  excited  by  impulses  which  come  to  it  from  peripheral  organs.  In 
this  as  in  other  forms  of  reflex  activity  the  mechanism  involves  (i)  afferent 
nerves,  e.g.,  cutaneous,  muscle,  optic,  and  vestibular,  and  their  related  end- 
organs,  tactile  corpuscles,  muscle  spindles,  retina,  and  semicircular  canals, 
all  indirectly  connected  with  (2)  the  cerebellar  centers;  (3)  efferent  nerves 
indirectly  connected  with  (4)  the  general  musculature  of  the  body.  Both 
station  and  progression  are  directly  dependent  on  the  development  and 


Fig.    261. 


Progression    after    Destruction 
{Moral  and  Doyon,  after  Thomas.) 


S70  TEXT-BOOK  OF  PHYSIOLOGY. 

transmission  of  afferent  impulses  from  the  previously  mentioned  peripheral 
sense-organs  to  the  cerebellum.  Tactile,  muscle,  visual,  and  labyrinthine 
impressions  and  sensations  not  only  cooperate  in  the  development  and  or- 
ganization of  the  motor  adjustments  necessary  to  the  maintenance  of  the 
equilibrium  and  locomotive  coordination,  but  even  after  their  organization 
they  are  necessary  to  the  excitation  of  cerebellar  activity.  The  manner  in 
which  they  lead  to  the  development  of  this  capability  on  the  part  of  the  cere- 
bellum is  conjectural.  Their  ever-present  influence  is  shown  by  the  effects 
which  follow  their  removal,  as  the  following  facts  indicate. 

The  prevention  of  the  development  of  tactile  impulses  by  freezing  or 
anesthetizing  the  soles  of  the  feet,  and  the  blocking  of  normally  developed 
impulses  through  destruction  of  afferent  pathways  in  diseases  of  the  spinal 
cord  lead  at  once  to  make  impairment  in  the  coordinating  power.  The 
removal  of  the  skin  from  the  hind  legs  of  the  frog,  previously  deprived  of  its 
cerebrum,  destroys  its  coordinating  power,  which  it  would  otherwise  possess 
in  a  high  degree. 

The  blocking,  in  consequence  of  destructive  lesions  of  the  spinal  cord, 
of  the  impulses,  which  come  from  the  muscles,  tendons,  etc.,  and  which  inform 
us  of  the  activity  and  the  degree  of  activity  of  our  muscles,  the  location  of 
limbs,  the  amount  of  effort  necessary  to  produce  a  given  movement,  etc., 
also  gives  rise  to  much  incoordination.  'A  blocking  of  both  tactile  and 
muscle  impulses  frequently  exists  in  degeneration  or  sclerosis  of  the  posterior 
columns  of  the  spinal  cord.  The  coordinating  power  is  so  much  impaired 
in  this  disease  that  the  patient  is  unable  to  maintain,  without  strained  effort, 
the  erect  position  and  especially  if  the  directive  power  of  the  eyes  be  removed 
by  closure  of  the  lids.  Walking  becomes  extremely  difficult;  the  gait  is 
irregular  and  jerky,  and  equilibrium  is  maintained  only  by  keeping  the 
eyes  fixed  on  the  ground  in  front  and  by  artificially  increasing  the  basis  of 
support  by  the  use  of  canes. 

An  interference  with  the  development  of  the  customary  visual  impres- 
sions which  in  a  measure  maintain  the  sense  of  relation  of  the  individual 
to  surrounding  objects  also  gives  rise  to  equilibratory  disturbances.  A 
rapid  change  in  the  relation  of  the  individual  to  surrounding  objects  or  the 
reverse;  a  change  in  the  direction  of  one  optic  axis  from  the  use  of  a  prism 
or  from  paralysis  of  an  eye  muscle;  the  destruction  of  an  eye; — these  and 
similar  conditions  frequently  give  rise  to  such  marked  disturbances  of  the 
equilibratory  power  that  displacement  is  difficult  to  prevent. 

An  interference  with  the  development  of  the  so-called  labyrinthine  im- 
pressions by  destruction  of  the  semicircular  canals  gives  rise  to  the  most 
remarkable  disturbances  in  this  respect.  Section  of  one  horizontal  canaP 
in  the  pigeon  is  followed  by  oscillations  of  the  head  in  a  horizontal  plane 
around  a  vertical  axis.  Bilateral  section  so  increases  these  oscillations  that 
the  pigeon  is  unable  to  maintain  equilibrium  and  forced  to  fall  and  turn  con- 
tinuously around  the  vertical  axis.  Bilateral  section  of  the  posterior  vertical 
canals  gives  rise  to  oscillations  around  a  horizontal  axis  which  frequently  be- 
come so  exaggerated  as  to  eventuate  in  the  turning  of  backward  somersaults, 

'  The  physiologic  anatomy  of  the  semicircular  canals  is  described  in  the  chapter    devoted 
to  the  ear,  to  which  the  reader  is  referred. 


THE  CEREBELLUM.  571 

head  over  heels.  Similar  phenomena  follow  di\dsion  of  the  superior  vertical 
canals. 

Bilateral  destruction  of  both  sets  of  canals  is  attended  by  extraordinary 
disturbances  in  the  equilibrium.  From  the  moment  of  the  operation  the 
animal,  the  pigeon,  loses  all  control  of  its  motor  mechanisms.  It  can  neither 
maintain  a  fixed  attitude  nor  execute  orderly  movements  of  progression;  its 
activity,  continuous  and  uncontrollable,  is  characterized  by  spinning  around 
a  vertical  axis,  turning  somersaults,  dashing  itself  against  surrounding  objects 
until  life  is  endangered.  If  the  animal  be  protected  from  injury,  these  dis- 
turbances gradually  subside,  and  in  the  course  of  a  few  months  the  equilibra- 
tory  power  is  so  far  regained  that  standing  and  walking  at  least  become 
possible.  In  this  condition,  however,  the  coordinating  power  is  directly  de- 
pendent on  \asual  impulses,  for  with  the  closure  of  the  eyes  all  the  previous 
motor  disturbances  at  once  recur.  These  and  similar  facts  indicate  that  the 
semicircular  canals  are  the  peripheral  sense-organs  from  which  come  the 
nerve  impulses  most  essential  to  the  excitation  of  the  cerebellar  coordina- 
tive  centers  in  their  control  of  equilibrium  and  of  progression. 

The  cerebellum  may  therefore  be  regarded  as  the  essential,  most  highly 
differentiated  portion  of  the  coordinating  mechanism  concerned  in  the  main- 
tenance of  equilibrium,  during  both  station  and  progression.  The  manner 
in  which  the  cerebellum  accomplishes  this  result  is  unknown,  though  it  is 
certain,  from  the  foregoing  facts,  that  its  special  mode  of  actixity  is  dependent 
on  the  excitatory  action  of  nerve  impulses  transmitted  from  a  variety  of 
peripheral  sense-organs. 


CHAPTER  XXIII. 
THE  CRANIAL  NERVES. 

The  nerve-trunks  which  serv^e  as  channels  of  communication  between  the 
encephalon  and  the  structures  of  the  head,  the  face,  and  in  part  the  organs 
of  the  thorax  and  abdomen,  pass  through  foramina  in  the  wails  of  the  cranium, 
and  for  this  reason  are  termed  cranial  nerves. 

According  to  the  classification  now  generally  adopted,  there  are  twelve 
cranial  nerves  on  either  side  of  the  median  line,  which,  enumerated  from 
before  backward,  are  as  follows  (Fig.  262) : 

First  or  Olfactory.  Seventh  or  Facial. 

Second  or  Optic.  Eiphth  or  Acoustic. 

Third  or  Oculo-motor.  Ninth  or  Glosso-pharyngeal. 

Fourth  or  Trochlear.  Tenth  or  Pneumogastric  or  Vagus. 

Fifth  or  Trigeminal.  Eleventh  or  Spinal  Accessory. 

Sixth  or  Abducent.  Twelfth  or  Hypoglossal. 

The  cranial  nerves  may  be  classified  physiologically  in  accordance  with  their 
functional  manifestations  into  three  groups,  viz. : 

1.  Nerves  of  Special  Sense:  e.g.,  Olfactory,  Optic,  Acoustic,  Gustatory  (Glosso-pharyngeal) 

2.  Nerves  of  General  Sensibility:  e.g.,  Large  root  of  the  Trigeminal,  Glosso-pharyngeal,  and 
Pneumogastric. 

3.  Nerves  of  Motion:  e.g.,  Oculo-motor,  Trochlear,  the  small  root  of  the  Trigeminal,  Abducent, 
Facial,  Spinal  Accessory,  and  Hypoglossal. 

Though  this  classification  in  the  main  holds  true,  it  must  be  borne  in  mind 
that  modern  investigations  have  demonstrated  that  the  glosso-pharyngeal 
and  pneumogastric  nerves  contain  even  at  their  junction  with  the  medulla 
oblongata  a  number  of  efferent  or  motor  fibers,  and  to  this  extent  are  mixed 
nerves. 

The  Origins  of  the  Cranial  Nerves. — In  accordance  with  modern 
views  as  to  the  origins  of  nerv^es  in  general,  it  may  be  stated  that — 

The  nerves  of  special  sense  have  their  origin  respectively  in  the  neuro- 
epithehal  cells  in  the  mucous  membrane  of  the  olfactory  region  of  the  nose, 
in  the  ganglion  cells  of  the  retina,  in  the  cells  of  the  spiral  ganglion  of  the 
cochlea  and  the  gangHon  of  Scarpa,  and  in  the  cells  of  the  petrous  and  jugular 
ganglia.  From  the  cells  of  these  ganglia  dendrites  pass  peripherally  to 
become  associated  with  specialized  end-organs,  while  axons  pass  centrally 
in  well-defined  bundles  to  become  related  by  means  of  their  end-tufts  with 
primary  basal  ganglia. 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia  on  their 
trunks,  and  in  this  respect  resemble  the  spinal  nerves.  From  the  ganglion 
cell  there  emerges  a  short  axon  process  which  soon  divides  into  a  central  and  a 
peripheral  branch.  The  former  passes  toward  and  into  the  gray  matter 
located  beneath  the  floor  of  the  fourth  ventricle,  where  its  end-tufts  arborize 
about  nerve-cells.  The  latter  (the  peripheral  branch)  passes  toward  the 
general  periphery  to  be  distributed  to  skin  and  mucous  membranes. 

572 


THE  CRANIAL  NERVES. 


573 


The  nerves  of  motion  have  their  origin  in  the  nerve-cells  in  the  gray  matter 
beneath  the  aqueduct  of  Sylvius  and  beneath  the  floor  of  the  fourth  ventricle. 
The  axons  emerging  from  these  cells  course  peripherally  to  be  distributed 
to  skeletal  muscles.  In  some  of  the  motor  nerves,  and  in  some  sensor 
nerves  as  well,  there  are  to  be  found  efferent  fibers  of  smaller  size  which 
have  a  similar  origin  and  which  become  related  through  the  intervention 
of  sympathetic  ganglia  (peripheral  neurons)  with  visceral  muscles  and 
glands.     These  nerves  have  been  termed  autonomic  nerves. 

The  Cortical  Connections  of  the  Cranial  Nerves. — Each  of  these  three 
groups  of  cranial  nerves  has  special  connections  with  the  cerebral  cortex. 

The  nerves  of  special  sense  for  the 
most  part  terminate  in  primary  basal 
ganglia,  around  the  cells  of  which  their 
central  end-tufts  arborize.  From  these 
cells  axons  arise  which  pass  upward  and 
directly  or  indirectly  come  into  physio- 
logic relation  with  sensor  nerve-cells  in 
the  cerebral  cortex. 

The  nerves  of  general  sensibility  ter- 
minate in  the  gray  matter  beneath  the 
floor  of  the  fourth  ventricle,  around  the 
nerve-cells  of  which  their  end-tufts  arbor- 
ize. These  groups  of  nerve-cells  are 
known  as  sensor  end-nuclei.  Though 
once  regarded  as  the  centers  of  origin  of 
the  sensor  nerves,  they  are  now  regarded 
as  the  centers  of  origin  of  axons  which 
pass  upward  to  the  cortex  of  the  cere- 
brum, where  they  also  come  into  physio- 
logic relation  with  sensor  nerve-cells. 

The  axons  in  both  of  these  classes  of 
nerves  thus  originate  in  the  cells  of  the 
central  nerv^e  system  and  continue  up- 
ward to  the  cerebrum,  the  primary  affer- 
ent path. 

The  motor  nerves  which  have  their 
origin  in  the  cells  of  the  gray  matter 
beneath  the  aqueduct  of  Sylvius  and  beneath  the  floor  of  the  fourth  ventricle 
are  in  physiologic  relation  with  nerve-cells  in  the  motor  region  of  the 
cortex  through  descending  axons  contained  in  the  pyramidal  tract,  the  end- 
tufts  of  which  arborize  around  the  nerve-cells.  The  efferent  path  be- 
ginning in  the  cerebral  cortex  is  thus  continued  by  the  motor  nerves  to  the 
general  periphery. 

The  three  groups  of  ners^s,  those  of  special  sense,  of  general  sensibility, 
and  the  motor  nerves,  are  neurons  of  the  first  order;  the  nerve-cells  and 
fibers  which  constitute  the  cerebral  connections  are  neurons  of  the  second 
order.  It  is  probable  that  the  sensor  cells  in  the  cerebral  cortex  are  neurons 
of  a  third  order. 


Fig.  262. — Superficial  Origin  of 
THE  Cranial  Nerves  from  the  Base 
OF  THE  Encephalon.  I.  Olfactory.  2. 
Optic.  3.  Motor  oculi.  4.  Trochlear. 
5.  Trigeminal.  6.  Abducent  7.  lacial. 
7'.  Ner%-e  of  Wrisberg.  8.  Acoustic.  9. 
Glosso-pharyngeal.  10.  Pneumogastric. 
II.  Spinal  accessory.  12.  Hypoglossal. — 
{Moral  and  Doyon.) 


574 


TEXT-BOOK  OF  PHYSIOLOGY 


FIRST  NERVE.     THE  OLFACTORY. 

The  first  cranial  nerv^e,  the  olfactory,  is  situated  in  the  upper  third  of  the 
nasal  fossa,  in  the  regio  olfacloria.  It  consists  of  from  20  to  30  branches,  the 
fibers  of  which  are  non-medullated. 

Origin. — The  olfactory  nerve  is  composed  of  centrally  coursing  axons 
which  have  their  origin  in  the  central  ends  of  bipolar,  rod-shaped,  or  spindle- 
shaped  nerve-cells  interspersed  among  the  epithelial  cells  covering  the  mucous 
membrane  in  the  regio  olfactoria;  the  peripheral  ends  of  these  cells  give  off 
a  number  of  dendrites  which  are  spread  out  to  form  a  delicate  feltwork  over 
the  surface  of  the  mucous  membrane.     From  their  origin  the  axons  gradually 

converge  to  form  bundles  which 
ascend  to  the  cribriform  plate  of 
the  ethmoid  bone,  through  the 
foramina  of  which  th^y  pass  to  be- 
come related  by  their  end-tufts  with 
structures  in  the  gray  matter  of 
the  olfactory  bulb  (Fig.  263), 

Cortical  Connections. — The 
olfactory  bulb  and  olfactory  tract, 
formerly  called  the  olfactory  ner\^e, 
are  portions  of  the  cerebrum  (the 
olfactory  lobe)  which  arise  em- 
bryologically  by  a  protrusion  of 
the  walls  of  the  cerebral  cavity. 
The  bulb  is  oval-shaped  and  con- 
sists of  both  gray  and  white  matter. 
It  rests  on  the  cribriform^plate  of 
the  ethmoid  bone  and  is  embraced 
by  the  olfactory  nerves.  As  seen 
on  sagittal  section,  there  is  just  be- 
neath the  surface  a  layer  of  large 
pyramidal  and  spindle-shaped 
cells  (termed  also  mitral  cells), 
each  provided  with  an  apical  and 
two  lateral  dendrites.  The  apical  dendrite  passes  toward  the  surface  and 
ends  in  a  brush-  or  basket-like  expansion  which  interlaces  with  the  end- 
tufts  of  the  olfactory  nerves,  forming  what  are  known  as  the  olfactory  glom- 
erules.     The  lateral  dendrites  end  free. 

The  axons  of  the  pyramidal  cells  pass  toward  the  center  of  the  bulb  and 
bend  at  right  angles,  after  which  they  pursue  a  horizontal  direction  toward 
and  into  the  olfactory  tract.  This  tract  is  about  five  centimeters  in  length, 
prismatic  in  shape  on  cross-section  and  divisible  into  a  ventral  and  a  dorsal 
portion.  It  emerges  from  the  posterior  extremity  of  the  bulb,  passes  back- 
ward to  the  posterior  part  of  the  anterior  lobe,  where  it  di\'ides  into  three 
roots:  viz.,  a  lateral  or  external,  a  mesial  or  internal,  a  middle  or  dorsal. 
The  fibers  of  the  lateral  and  mesial  roots  are  derived  almost  exclusively 
from  the  ventral  portion  of  the  tract,  the  fibers  of  which  come  from  the 
mitral  cells  in  the  bulb.     The  lateral  root-fibers  pass  outward  into  the  fossa  of 


Fig.  263 . — The  Relation  of  the  Olfactory 
Nerves  to  the  Olfactory  Tract,  i.  Ol- 
factory nerve-cell.  2.  Axon  process.  3.  Epi- 
thelial cells.  4.  Glomerulus.  5.  Mitral  cells. 
6.  Centrally  coursing  axons  of  the  olfactory 
tract. — {Morat  and  Doyon.) 


THE  CRANIAL  NERVES. 


575 


Sylvius  and  come  into  relation  with  nerve-cells  in  the  inferior  extremity  of 
the  gyrus  hippocampus  and  the  gyrus  uncinatus.  The  mesial  fibers  pass 
inward  and  come  into  relation  with  nerve-cells  in  the  pre-callosal  part  at 
least  of  the  gyrus  fornicatus.  The  fibers  thus  far  considered  are  undoubt- 
edly true  olfactory  fibers,  pursuing  a  centripetal  direction,  carrying  ner^^e 
impulses  from  the  olfactory  cells  to  the  cerebrum  (Fig.  264). 

Histologic  and  embryologic  methods  of  research  have  shown  that  some 
of  the  fibers  in  the  olfactory  tract  are  centrifugal  in  function.  They  originate 
in  the  olfactory  cortical  areas,  pass  toward  the  periphery  as  far  as  the  an- 
terior commissure,  where  they 
cross    to  become   the   dorsal  /^ 

root,  enter  the  olfactory  tract,  -  '—  ^ 
and  finally  terminate  in  the 
bulb.  This  tract  serves  to 
connect  the  cortex  with  the 
bulb  of  the  opposite  side,  and 
carries  impulses  from  the  cor- 
tex to  the  bulb.  The  two  op- 
posite cerebral  olfactory  areas 
are  also  united  by  commissural 
fibers  which  decussate  at  the 
anterior  commissure. 

Function. — The  function 
of  the  olfactory  system  in  its 
entirety  is  the  transmission  of 
nerv^e  impulses  from  its  origin 
in  the  olfactory  region  of  the 
nose  to  the  cerebral  cortex, 
where  they  evoke  sensations 
of  odor.  The  stimulus  to  its 
excitation  is  the  impact  and 
chemic  action  of  gaseous  or 
volatile  organic  matter  on  the 
dendrites  of  the  olfactory  cells. 
The  sensitiveness  of  the  olfac- 
tory end-organ  to  the  action 
of  many  substances  is  remark- 
able, responding,  for  example,  to  the  t2t1u"o¥ 
the  -2T6io"{ro"  o^  ^  gram  of  mercaptan. 

Division  or  destruction  of  the  olfactory  path  at  any  point  is  followed 
by  an  aboHtion  of  the  sense  of  smell  on  the  corresponding  side.  Destructive 
lesions  of  the  hippocampal  and  uncinate  gyri  are  followed  by  similar  results. 

SECOND  NERVE.     THE  OPTIC. 

The  second  cranial  nerve,  the  optic,  consists  of  centrally  coursing  axons 
of  neurons  which  connect  the  essential  part  of  the  organ  of  vision,  the  retina, 
with  sensory  end-nuclei  or  ganglia  situated  at  the  base  of  the  cerebrum. 

Origin. — The  axons  which  constitute  the  optic  nerve  have  their  origin 
in   the  ganglion  cells  in   the  anterior  part  of  the  retina.     Through  their 


Fig.  264. — Olfactory  Lobe  of  the  Humax  Braix. 
—Bii.  Olfacton-  bulb.  T.  Tract.  Tr.o.  Trigone.  R. 
Rostrum  of  corpus  callosum.  p.  Peduncle  of  corpus 
callosum,  passing  into  C.  s.,  gyrus  subcallosus  (diagonal 
tract,  Broca).  Br.  Broca's  area.  F.p.  Fissura  prima. 
F.s.  Fissura  serotina.  C.a.  Position  of  anterior  com- 
missure. L.t.  Lamina  terminalis.  Ch.  Optic  chiasma. 
T.o.  Optic  tract,  p.  olf.  Posterior  olfactory  lobule  (or 
anterior  perforated  space),  m.r.  Mesial  root.  l.r. 
Lateral  root  of  tract. — (His.) — After  Quain.) 

of  a  gram  of  oil  of  roses  and  to 


576 


TEXT-BOOK  OF  PHYSIOLOGY. 


dendrites  these  cells  are  brought  into  relation  posteriorly  with  successive 
layers  of  cells  which  collectively  constitute  the  retina.  Though  the  retina  is 
said  to  consist  of  ten  or  eleven  layers,  it  may  be  reduced  practically  to  three, 
viz.^  (Fig.  265) : 

1.  The  layer  of  visual  cells. 

2.  The  layer  of  bipolar  cells. 

3.  The  layer  of  ganglionic  cells. 

The  visual  cells  present  peripherally  modified  dendrites,  known  as  the 
rods  and  cones;  centrally  they  give  off  an  axon  which  after  a  short  course 
terminates  in  an  end-tuft.  The  bipolar  cells  also  possess  dendrites  and  an 
axon;  the  former  interlace  with  the  end-tufts  of  the  visual  cell  axon,  the  latter 
with  the  dendrites  of  the  ganglion  cell.  The  retina  may 
be  regarded  therefore  as  the  peripheral  end-organ  in 
which  the  optic  nerve  originates.  From  their  origin  the 
axons  turn  backward,  at  the  same  time  converging  to 
form  a  distinct  bundle  which  passes  through  the  cho- 
rioid  coat  and  sclera.  After  emerging  from  the  eyeball 
the  nerve-bundle  (the  optic  nerve)  passes  backward  as 
far  as  the  sella  turcica,  traversing  in  its  course  the  orbit 
cavity  and  the  optic  foramen.  At  the  sella  turcica  there 
is  a  union  and  partial  decussation  in  man  and  other 
mammals  of  the  two  nerves,  forming  the  optic  chiasm.^ 

Decussation  of  the  Optic  Nerves. — The  extent  to 
which  the  fibers  from  each  eye  decussate  at  the  chiasm 
is  a  subject  of  dispute,  but  the  results  of  various  methods 
of  research  would  seem  to  indicate  that  the  fibers  from 
the  nasal  third  of  the  retina  of  the  left  eye  cross  in  the 
chiasm,  to  unite  with  the  fibers  from  the  temporal  two- 
thirds  of  the  retina  of  the  right  eye.  In  a  similar  man- 
ner the  fibers  from  the  nasal  third  of  the  retina  of  the 
right  eye  cross  in  the  chiasm,  and  unite  with  the  fibers 
from  the  temporal  two-thirds  of  the  retina  of  the  left 
eye  (Fig.  266).  Posterior  to  the  chiasm  the  crossed  and 
uncrossed  fibers  form  the  so-called  optic  tracts,  which  after 
winding  around  the  crura  cerebri  enter  the  optic  basal  ganglia.  Tran- 
section of  the  optic  nerv'e  shows  that  it  is  composed  of  an  enormous  number 
of  non-medullated  nerve-fibers,  estimated  by  Salzer  at  from  450,000  to 
800,000,  enclosed  in  a  sheath  of  the  dura  mater. 

The  visual  fibers  comprising  tlie  optic  nerve  may  be  physiologically 
divided  into  two  clases,  (a)  those  coming  from  the  peripheral  portion  of  the 
retina,  and  (b)  those  coming  from  that  central  area  known  as  the  macula 
lutea.  The  retinal  fibers  are  by  far  the  more  abundant,  and  make  up  the 
major  portion  of  the  nerve;  the  macular  fibers  are  less  abundant.     An  ex- 

'  Though  the  foregoing  is  the  usual  method  of  stating  the  origin  and  course  of  the  optic  nerve, 
nevertheless  morphologically  the  true  optic  nerve  lies  wholly  within  the  retina  and  is  composed 
of  the  visual  cells  there  found.  The  remainder  of  the  visual  system  from  and  including  the 
ganghon  cells  of  the  retina  to  the  optic  basal  ganglia,  is  the  optic  tract,  there  being  no  anatomic 
or  physiologic  distinction  between  the  optic  nerve  so  called  and  the  optic  trvct.  Both  are  out- 
growths from  the  brain  and  hence  possess  properties  which  differentiate  them  from  other  cranial 
nerves. 


Fig.  265. — Reti- 
nal Cells,  s',  z'. 
Visual  cells  with 
their  peripheral  ter- 
minations, s.  Rods. 
2.  Cones,  h.  B  i- 
polar  cells,  g.  Gan- 
glion cells  froin  which 
arise  the  axons  of  the 
optic  nerve. 


THE  CRANIAL  NERVES. 


577 


VISUAL  mw 

M     * 


VISUAL  IIEW 


amination  of  a  cross-section  of  the  optic  nerve  shows  the  presence  of  a  wedge- 
shaped  tract  occupying  the  center  of  the  nerve  which  is  regarded  as 
composed  of  the  macular  fibers.  At  the  chiasm  this  bundle  of  fibers  un- 
dergoes a  partial  decussation  similar  to  that  of  the  fibers  coming  from  the 
more  peripheral  portions  of  the  retina.  In  the  left  optic  tract,  therefore,  fibers 
from  at  least  four  different  regions  are  to  be  found:  viz.,  the  two-thirds  of 
the  temporal  side  of  the  left  retina,  the  temporal  half  of  the  left  macula, 
the  nasal  third  of  the  right  retina,  and  the  nasal  half  of  the  right  macula. 
Corresponding  fibers  are  to  be  found  in  the  right  optic  tract.  As  the  optic 
tract  passes  around  the  crus  cerebri 
it  divides  into  a  lateral  or  outer,  and 
a  mesial  or  inner  bundle,  which  then 
terminate  in  the  optic  basal  ganglia. 
The  fibers  of  the  lateral  bundle  are 
traceable  into  the  lateral  or  external 
geniculate  body  (the  pre-geniculum), 
the  pulvinar  of  the  optic  thalamus, 
and  the  anterior  quadrigeminal  body 
(the  pre-geminum).  With  the  excep- 
tion of  the  fibers  passing  to  the  ante- 
rior quadrigeminal  body,  these  are 
in  all  probabiHty  the  true  visual  fibers. 
The  fibers  of  the  mesial  bundle  are 
traceable  into  the  internal  geniculate 
body  (the  post-geniculum)  and  the 
posterior  quadrigeminal  body  (the 
post-geminum).  These  fibers  are 
not  a  part  of  the  optic  nerve  proper, 
but  commissural  fibers  associating 
the  internal  geniculate  bodies  of  the 
two  sides. 

Cortical  Connections. — After 
entering  the  pulvinar  and  the  lateral 
or  external  geniculate  body  the  visual 
fibers  terminate  in  end-tufts  which 
arborize  around  nerve-cells.  From 
these  cells  new  axons  arise  which 
ascend  through  the  posterior  part  of  the  internal  capsule,  at  the  same  time 
curving  backward  to  form  the  optic  radiation  of  Gratiolet,  and  terminate 
finally  around  nerve-cells  in  the  gray  matter  of  the  cuneus  and  in  the  gray 
matter  bordering  the  calcarine  fissure,  both  situated  on  the  mesial  aspect 
of  the  occipital  lobe. 

Centrifugal  Fibers  of  the  Optic  Nerve. — All  the  fibers  previously  alluded 
to  have  been  afferent  or  centripetal  in  direction;  but  the  optic  nerve  also 
contains  efferent  or  centrifugal  fibers  which  come  from  nerve-cells  in  the 
basal  ganglia  and  ramify  around  special  cells,  the  amacrine  cells,  in  the 
retina.  Their  function  is  unknown.  It  has  been  suggested  that  they 
regulate  the  vascular  supply  to  the  retina.  Centrifugally  coursing  fibers  also 
connect  the  visual  areas  of  the  cortex  with  the  superior  quadrigeminal  body. 
37 


Fig.  266. — DiAGR.\M  Illustrating  Left 
Homonymous  Lateral  Hemianopsia  from  a 
Lesion  of  the  Right  Optic  Tract  or  the 
Right  Cuneus.  The  Sh.^ded  Lines  in  the 
Visual  Fields  Indicate  the  Darkened  Area. 


"578  TEXT-BOOK  OF  PHYSIOLOGY. 

Function. — The  function  of  the  visual  apparatus  in  its  entirety  is  the 
transmission  of  nerve  impulses  from  the  retina  to  the  cerebral  cortex  where 
they  evoke  the  sensations  of  light  and  its  different  qualities — colors.  The 
specific  physiologic  stimulus  to  the  retinal  visual  cells  is  the  im.pact  of  the 
undulations  of  the  ether.  In  general  it  may  be  said  that,  at  least  for  the 
same  color,  the  intensity  of  the  objective  undulation  or  vibration  determines 
the  intensity  of  the  sensation. 

Pupillary  Fibers. — The  optic  nerve  also  contains  nerve-fibers  some- 
what larger  in  caliber  than  the  usual  visual  fibers,  which  are  sup- 
posed to  form  the  afferent  path  for  those  nerve  impulses  which  excite  re- 
flexly  a  contraction  of  the  sphincter  pupillcB  muscle,  thus  varying  the  size 
of  the  pupil.  These  fibers,  termed  pupillary  fibers,  come  from  all  portions 
of  the  retina  but  most  abundantly  from  the  posterior  pole  in  and  around 
the  macula.  The  existence  of  these  fibers  is  confirmed  by  pathologic  find- 
ings. In  a  manner  similar  to  that  of  the  visual  fibers  they,  too,  undergo  a 
decussation  in  the  optic  chiasm,  so  that  in  the  optic  tract  there  are  pupil- 
lary fibers  which  come  from  the  temporal  side  of  the  eye  of  the  corresponding 
side,  and  fibers  which  come  from  the  nasal  side  of  the  eye  of  the  opposite 
side  (Fig.  270).  The  central  termination  of  these  fibers  is  not  positively 
known. 

Hemiopia  and  Hemianopsia. — Division  of  the  optic  nerve  between  the 
eyeball  and  the  optic  chiasm  is  followed  by  complete  iDlindness  in  the  eye  of 
the  corresponding  side.  Owing  to  the  partial  decussation  of  the  fibers  in 
the  chiasm,  division  of  an  optic  tract  is  followed  by  a  loss  of  sight  in  the  outer 
two-thirds  of  the  eye  of  the  same  side  and  in  the  inner  third  of  the  eye  of  the 
opposite  side.  To  this  loss  of  visual  power  in  the  retina  the  term  hemiopia 
is  given.  In  consequence  of  this  loss  of  visual  power  in  the  retina  there  is 
a  corresponding  obscuration  or  total  obliteration  of  nearly  one-half  of  the 
visual  field, ^  to  which  the  term  hemianopsia  is  given.  If,  for  example,  the 
right  optic  tract  is  divided  there  will  be  hemiopia  in  the  outer  two-thirds  of 
the  right  eye  and  the  inner  third  of  the  left  eye,  with  left  lateral  hemianopsia, 
and  as  the  portions  of  the  retina  which  are  alTected  are  associated  in  vision 
the  loss  of  the  visual  fields  is  spoken  of  as  homonymous  hemianopsia  (Fig. 
266).  A  destructive  lesion  of  the  cerebral  visual  area,  the  cuneus  and 
the  adjacent  gray  matter  on  the  right  side,  is  also  followed  by  left  lateral 
hemianopsia.^ 

The  existence  of  a  homonymous  hemianopsia  becomes  evident  when 

^  The  visual  field  comprises  that  portion  of  the  external  world  from  which,  with  the  eye 
stationary,  rays  of  light  pass  to  the  retina  and  is  the  area  included  between  the  extremes  of 
the  visual  lines  entering  the  pupil.  The  center  of  the  visual  field  is  the  area  the  rays  of  light  from 
which  are  focaUzed  on  the  fovea  centralis.  The  visual  field  is  somewhat  irregular  in  outUne  by 
reason  of  the  position  of  the  eyeball  in  the  orbit  cavity,  and  the  consequent  interference  with 
the  entrance  of  light  by  the  bridge  of  the  nose,  the  cheek  bones,  and  the  eye-brows.  The  hori- 
zontal diameter  of  the  visual  field  for  the  right  eye  is  about  150°,  of  which  go°  pertain  to  the 
temporal  and  60°  to  the  nasal  portion.  The  vertical  diameter  is  about  115°,  of  which  45°  pertain 
to  the  superior  and  70°  to  the  inferior  portion.  By  reason  of  the  position  of  the  eyes  in  the  orbit 
cavity  the  two  visual  fields,  viz.,  that  of  the  right  and  of  the  left  eye,  overlap  to  a  variable  extent  in 
their  nasal  divisions. 

^  It  should  be  borne  in  mind  that  in  both  instances  the  retina  itself  is  unaffected.  The  impact 
of  light  generates,  as  usual,  nerve  impulses  which  proceed  as  far  backward  as  the  point  of  division 
or  destruction.  In  consequence  those  portions  of  the  cerebral  cortex  stimulation  of  which  evokes 
the  sensation  of  light  remain  unaffected  and  the  individual  does  not  become  aware  through 
sensation,  of  the  presence  of  a  luminous  body  in  the  left  side  of  the  visual  field. 


THE  CRANIAL  NERVES. 


579 


the  individual  is  directed  to  focus  the  vision  on  an  object  placed  directly  in 
front  and  vi^ith  its  center  in  the  median  plane  of  the  body,  when  if  the  lesion 
be  on  the  right  side,  the  left  half  of  the  object  will  be  invisible.  The  reason 
for  this  will  be  apparent  on  reference  to  Fig.  267.  All  the  light  rays  emanating 
from  the  left  half  of  the  object  fall  on  the  retina  on  the  side  of  the  injury, 
and  hence  there  will  be  no  sensation.  If,  however,  the  object  be  moved  to 
the  right  without  change  in  the  position  of  the  head,  the  entire  object  will  be 
visible,  as  all  the  rays  fall  on  the  normal  side.  If,  on  the  contrary,  the 
object  be  moved  to  the  left,  it  will  be  invisible  for  the  opposite  reason. 

Hemianopsia  may  be  the  result  of 
either  destruction  of  the  optic  tract  or 
of  the  cortical  visual  area.  The  seat 
of  lesion  in  any  given  case  is  indicated 
by  a  peculiarity  of  the  iris  reflex 
pointed  out  by  Wernicke,  which  will 
be  referred  to  in  connection  with  the 
consideration  of  the  oculo-motor 
nerve. 

THIRD  NERVE.  THE  OCULO- 
MOTOR. 

The  third  cranial  nerve,  the  oculo- 
motor, consists  of  some  15,000  per- 
ipherally coursing  nerve-fibers  which 
serve  to  bring  the  nerve-cells  from 
which  they  arise  into  relation  with  a 
large  portion  of  the  general  muscula- 
ture of  the  eye. 

Origin. — The  axons  composing 
the  third  nerve  arise  from  a  series  of 
seven  or  eight  groups  of  nerve-cells, 
located  in  the  gray  matter  beneath  the  floor  of  the  aqueduct  of  Sylvius. 
From  each  of  these  groups  or  nuclei,  bundles  of  axons  emerge,  which  after 
a  short  course  unite  to  form  the  common  trunk.  The  large  majority  of 
the  fibers  in  the  nerve  come  directly  from  the  nuclei  of  the  same  side;  the 
remainder  come  from  a  group  of  cells  on  the  opposite  side  of  the  median 
line.     There  is  thus  a  partial  decussation  of  its  fibers  (Fig.  268). 

The  different  groups  of  cells,  the  nuclei  of  origin,  are  arranged  in  a  serial 
manner.  The  anatomic  arrangement  of  these  nuclei  would  indicate  that 
each  nucleus  is  related  to  an  individual  member  of  the  eye-group  of  muscles. 
Clinical  observation  and  the  investigation  of  the  results  of  pathologic  processes 
have  not  only  shown  that  this  is  the  case,  but  also  succeeded  in  locating  the 
position  of  the  nucleus  for  any  given  muscle.  Though  there  is  some  difference 
of  opinion  in  regard  to  the  exact  location  of  one  or  two  of  the  nuclei,  the 
tabulation  subjoined  is  approximately  correct. 

Enumerating  them  from  before  backward,  the  nuclei  occur  in  the  follow- 
ing order: 

1.  The  sphincter  pupillae. 

2.  The  tensor  chorioideae  (the  accommodation  nucleus). 


Fig.  267. — Diagram  to  Show  the  Exist- 
ence OF  Hemianopsia.  The  lesion  is  sup- 
posed to  be  in  the  right  optic  tract. 


58o 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  convergence  nucleus,  a  common  nucleus  for  the  conjoint  action  of 

the  two  internal  recti  muscles. 
The  superior  rectus. 
The  inferior  rectus. 
The  levator  palpebrae. 
The  inferior  oblique. 

Cortical  Connections. — The  ocuol-motor  nuclei  are  in  histologic  and 
physiologic  relation  with  the  motor  area  of  the  cerebrum.     Nerve-cells  in  the 

cortex  give  off  axons  which,  entering  the 
pyramidal  tract,  descend  through  the  inter- 
nal capsule,  and  the  crus  cerebri,  from  which 
they  cross  to  the  opposite  side.  The  end- 
tufts  arborize  around  the  nuclei  of  the 
oculo-motor  nerve  with  the  exception  of  the 
nucleus  for  the  iris  sphincter. 

Distribution. — After  their  origin  the 
axons  converge  to  form  a  common  trunk, 
which  emerges  from  the  base  of  the  enceph- 
alon,  on  the  inner  side  of  the  crus  cerebri, 
in  front  of  the  pons  Varolii.  The  nerve  then 
passes  forward  through  the  sphenoid  fissure 
into  the  orbit  cavity,  where  it  divides  into 
a  superior  and  an  inferior  branch.  The 
former  is  distributed  to  the  superior  rectus 
and  the  levator  palpebrcB  muscles;  the  latter 
is  distributed  to  the  internal  and  inferior 
recti  and  inferior  oblique  muscle  (Fig.  269). 
From  the  inferior  branch  a  short  bundle 
of  fibers  passes  to  the  ciliary  or  ophthalmic 
ganglion,  where  they  terminate,  arborizing 
around  the  ganglion  cells.  These  fibers  are 
smaller  in  size  than  those  constituting  the 
bulk  of  the  nerve  and  belong  to  the  system 
known  as  the  autonomic.  These  cells  give 
origin  to  new  axons,  the  ciliary  nerves ,  which 
enter  the  eyeball,  pass  forward  between 
the  sclera  and  chorioid  coat,  and  terminate 
in  the  ciliary  muscle  and  the  sphincter  of 
the  pupil.  The  ciliary  nerves  are  not  por- 
tions of  the  third  nerve  proper,  but  periph- 
eral sympathetic  neurons.  As  the  ciliary 
ganglion  receives  filaments  from  the  caver- 
nous plexus  of  the  sympathetic  and  fila- 
ments which  become  a  part  of  the  trigeminal  nerve,  it  is  probable  that  the 
ciliary  nerves  contain  not  only  motor,  but  vaso-motor  and  sensor  fibers 
as  well. 

Properties. — Stimulation  of  the  nerve  near  its  exit  from  the  enceph- 
alon  is  followed  by  contraction  of  the  muscles  to  which  it  is  distributed 
with  the  following  results,  viz. : 


Fig.  268. — Diagrammatic  View  of 
THE  Situation  and  Relation  of 
THE  Nuclei  of  Origin  of  the 
Oculo-motor  and  Patheticus 
(Trochlearis)  Nerves.  The  oculo- 
motor nuclei  consist  of  an  anterior 
nucleus,  the  Edinger-Westphal  nucleus 
(a  and  b),  and  a  posterior  nucleus; 
the  posterior  nucleus  has  a  dorsal,  a 
ventral,  and  a  mesial  portion;  the  de- 
cussation of  fibers  from  the  dorsal 
portion  of  the  posterior  nucleus  is  also 
shown.  The  decussation  of  the  fibers 
of  the  fourth  nerve  is  also  represented. 
' — (Edinger.) 


THE  CRANIAL  NERVES. 


581 


1.  Diminution  in  the  size  of  the  pupil. 

2.  Accommodation  of  the  eye  for  near  vision. 

3.  Elevation  of  the  upper  eyehd. 

4.  Internal  deviation  and  rotation  upward  and  inward  of  the  anterior  pole 

of  the  eye,  combined  with  a  small  amount  of  torsion  toward  the  mesial 
line,  due  to  preponderating  action  of  the  internal  rectus  and  inferior 
oblique  muscles. 
Division  of  the  nerve  either  experimentally  or  as  a  result  of  compression 

from  a  pathologic  cause  is  followed  by  a  relaxation  of  the  muscles,  with  the 

following  effects,  viz.: 

1.  Dilatation  of  the  pupil,  the  iris 

responding  neither  to  light  nor 
to  efforts  of  accommodation. 

2.  Loss     of     the     accommodative 


the    upper    eyelid 


Fig.  269. — Intra-orbital  Portiox  of  the 
Third  Nerve,  i.  Optic  nerve.  2.  Third 
nerve.  3.  Superior  branch.  4.  Injerior  branch. 
5.  Abducens.  6.  Trifacial.  7.  Ophthalmic 
branch  divided.  8.  Nasal  branch.  9.  Ciliary 
ganglion.  10.  Motor  branch  to  this  ganglion 
from  the  inferior  branch  of  the  third  nerve.  11. 
Sensory  fibers.  12.  Sympathetic  fibers.  13. 
Ciliary  nerves. — (Sappey.) 


power. 

3.  Falling     of 

(ptosis). 

4.  External  deviation  and  rotation 

downward  and  outward  of  the 
anterior  pole  of  the  eyeball 
combined  with  a  small  amount 
of  torsion  toward  the  mesial 
line  due  to  the  unopposed 
action  of  external  rectus  and 
the  superior  oblique  muscles. 

5.  Double  vision  or  diplopia.     The 

image  of  the  eye  of  the  para- 
lyzed side  is  projected  to  the 
opposite  side  of  the  true  image  and  to  the  upper  part  of  the  visual  field. 
Owing  to  the  slight  mesial  torsion  the  false  image  is  inclined  away 
from  the  true  image. 

6.  Immobility  and  slight  protrusion  of  the  eyeball. 

Function. — The  function  of  the  third  nerv'e  is  to  transmit  nerve  im- 
pulses from  the  nuclei  of  origin  to  all  the  muscles  of  the  eye  except  the  ex- 
ternal rectus  and  superior  oblique  and  excite  them  to  activity.  The  majority 
of  the  ocular  movements,  the  power  of  accommodation,  the  variations  in 
the  size  of  the  pupil  in  accordance  with  variations  in  the  intensity  of  the  light, 
the  power  of  convergence  of  the  visual  axes,  are  all  excited  by  the  trans- 
mission of  nerve  impulses  by  the  constituent  fibers  of  the  nerve  from  their 
related  nuclei.  This  is  made  evident  by  the  effects  which  follow  stimula- 
tion and  division  of  the  nerve  or  lesions  of  the  nuclei  themselves. 

The  central  nuclei  can  be  excited  to  activity  (i)  by  nerve  impulses  de- 
scending the  motor  tract,  from  the  cerebral  cortex,  (2)  by  ner\^e  impulses 
coming  through  various  afferent  nerves.  This  holds  true  more  especially 
for  the  sphincter  pupillae  nucleus. 

The  Iris  Reflex  or  the  Pupillary  Reflex.^ — These  are  terms  applied 
to  the  variations  in  the  size  of  the  pupil  that  follow  variations  in  the  inten- 
sity of  the  light.  In  the  absence  of  light  the  pupil  widely  dilates,  due 
largely  to  the  relaxation  of  the  sphincter  pupillcs  muscle  and  partly  to  a  con- 


582 


TEXT-BOOK  OF  PHYSIOLOGY. 


traction  of  the  radiating  fibers  of  the  iris,  which  collectively  constitute  the 
dilatator  pupilla  muscle.  With  the  entrance  of  light  into  the  eye,  the  pupil 
diminishes  in  size,  in  consequence  of  the  contraction  of  the  sphincter  ptpilla 
caused  by  a  stimulation  of  the  peripheral  ends  of  the  pupillary  fibers  of  the 
retina,  the  degree  of  contraction  depending  within  limits  on  the  intensity 
of  the  light. 

The  action  of  the  sphincter  pupilte  muscle  is  therefore  a  reflex  action 
and  involves  the  usual  mechanism,  viz.:     A  receptive  surface,  the  retina; 

afferent  nerves,  the  pupil- 
lary fibers  in  the  optic 
nerve;  an  emissive  center, 
the  sphincter  nucleus  of 
the  motor  oculi  center;  effer- 
ent nerves,  including  fibers 
in  the  trunk  of  the  motor 
oculi  and  in  the  ciliary 
nerves;  and  a  responsive 
organ,  the  muscle.  (See 
Fig.  270).  That  this  is  the 
mechanism  involved  in  this 
reflex,  is  shown  by  the  fact 
that  when  any  portion  of 
it  is  destroyed,  the  reflex 
contractions  of  the  sphinc- 
ter are  impaired  or  abol- 
ished. 

As  stated  in  a  preceding 
paragraph  the  central  ter- 
mination of  the  afferent 
pupillary  fibers  concerned 
in  this  reflex  is  not  posi- 
tively known.  No  one  has 
as  yet  succeeded  in  tracing 
these  fibers  directly  to  the 
sphincter  nucleus.  Experi- 
mental and  pathologic  data 
apparently  disprove  the 
probability  of  their  ter- 
minating 'in  the  superior 
corpora  quadrigemina.  It 
has  been  shown,  however, 
that  as  the  optic  tract  ap- 
proaches its  termination 
the  visual  and  the  pupillary  fibers  separate  and  it  has  been  assumed  that 
the  latter  come  into  anatomic  relation  with  some  intercalated  system  which 
in  turn  is  connected  with  the  sphincter  nucleus.  As  to  the  situation,  origin 
and  course  of  this  system  nothing  positively  is  known.  There  is  some  evi- 
dence for  the  view  that  these  two  systems  are  associated  by  commissural 
fibers. 


e 


Pupillaryrib 
in  Optic  Tract 

Hirevt,. 

Crossed, 


"^"t.Corp.qaad. 
rostffa/iff/ionic     fibers. 
Sup.  Ccnical  Ganglion. 
Pn^a/i^lionlc  fibers 


■•1-T}ioracicNeri>e 


Transection  of  Spinal  Cord. 


Fig.  270. — Diagram  Designed  TO  Show  THE  Mechanism 
OF  THE  Iris  Reflex.  The  central  termination  of  the 
pupillary  fibers  is  hypothetical. 


THE  CRANIAL  NERVES.  583 

The  contraction  of  the  sphincter  and  a  diminution  in  the  size  of  the  pupil 
may  be  direct,  as  when  the  Hght  which  enters  one  eye  causes  a  reflex  contrac- 
tion of  the  sphincter  of  one  and  the  same  side;  or  it  may  be  indirect  or 
consensual,  as  when  the  light,  which  enters  one  eye  only,  causes  a  contraction 
of  the  sphincter  not  only  in  the  eye  of  the  same,  but  in  the  eye  of  the  opposite 
side  also.  It  is,  however,  highly  probable  that  all  reflex  contractions  of 
the  sphincter  muscles  are  consensual,  that  is,  bilateral  reflex  actions  because 
of  the  decussation  of  the  pupillary  fibers  at  the  chiasm.  Contraction  of  both 
pupils  also  occurs  as  an  associated  movement  in  the  convergence  of  the  eyes 
during  accommodation. 

The  dilatation  of  the  pupil  is,  however,  not  due  exclusively  to  the 
relaxation  of  the  sphincter  pupillae  muscle,  but  partly  to  the  contraction  of 
the  dilatator  pupillae  muscle,  which  is  kept  normally  in  a  state  of  tonic  con- 
traction by  impulses  emanating  from  a  nerve-center  in  the  medulla 
oblongata. 

The  axons  which  arise  in  this  center  pass  down  the  cord,  emerge  through 
the  first  thoracic  nerve,  and  then  ascend  to  the  superior  cervical  ganglion 
(see  Fig.  270),  in  which  their  terminal  branches  arborize  around  its  nerve- 
cells.  From  these  cells  new  axons  of  the  sympathetic  system  arise  which  pass 
successively  to  the  ophthalmic  division  of  the  fifth  nerve,  the  nasal  nerve, 
the  long  ciliary  nerve  and  the  iris. 

Experimental  research  renders  it  highly  probable  that  the  dilatator 
center  is  in  a  state  of  continuous  activity  and  the  dilatator  muscle  in  a  state 
of  tonic  contraction.  Whatever  the  normal  stimulus  may  be,  the  center 
is  increased  in  activity  by  dyspneic  blood,  by  severe  muscle  exercise,  by 
emotional  excitement,  and  by  stimulation  of  various  sensor  nerves.  That 
the  efferent  pathway  just  alluded  to  transmits  the  impulses  to  the  iris  is  shown 
by  the  fact  that  division  in  any  part  of  the  course  is  followed  by  narrowing, 
stimulation  by  active  dilatation  of  the  pupil. 

The  variations  in  size  of  the  pupil,  though  largely  a  reflex  act  under  the 
control  of  the  oculo-motor  nerv^e,  are  nevertheless  partly  due  to  the  active 
cooperation  of  the  dilatator  nerves  and  their  related  muscle.  The  size  of  the 
pupil  necessary  from  moment  to  moment  for  the  admission  of  just  that 
amount  of  light  essential  to  the  formation  and  perception  of  a  distinct  image 
is  the  result  of  two  nicely  adjusted  and  delicately  balanced  forces. 

Wernicke's  Hemianopic  Pupillary  Reaction. — It  was  stated  on 
page  578  that  a  modification  of  the  pupillary  reaction  is  observed  in  some 
cases  of  hemianopsia,  which  indicates  approximately  the  seat  of  the  lesion. 
This  reaction,  or  inaction  as  it  is  sometimes  called,  is  present  when  the  lesion 
is  along  the  course  of  the  optic  tract  between  the  chiasma  and  the  anterior 
quadrigeminal  body.  In  a  case  of  left  lateral  hemianopsia,  the  lesion  being 
in  the  right  optic  tract,  the  method  of  testing  for  the  reaction  is  as  follows: 
The  eye  of  the  left  side  is  first  carefully  shielded  from  the  light.  A  fine  ray 
of  light  is  then  projected  into  the  right  eye  in  such  a  manner  that  it  falls  en- 
tirely on  the  non-sensitive  (the  temporal)  side  of  the  retina.  There  will  be  an 
absence  of  the  usual  pupillary  response,  or  rather  the  pupil  remains  inactive; 
but  if  the  light  is  gradually  directed  toward  the  sensitive  (the  nasal)  side  of 
the  retina,  there  will  come  a  moment,  as  the  central  line  is  crossed  and  the 
light  falls  on  the  sensitive  side,  when  the  usual  pupillary  response  manifests 


584 


TEXT-BOOK  OF  PHYSIOLOGY. 


itself,  viz.:  a  contraction  of  the  sphincter  pupillae  and  a  diminution  in  the 
size  of  the  pupil.  The  explanation  of  these  facts  will  become  apparent 
from  an  examination  of  Fig.  272  in  which  the  course  of  the  pupillary  fibers 
is  shown  and  especially  if  it  be  accepted  that  these  fibers  at  their  central 
terminations  decussate  or  are  in  relation  either  directly  or  indirectly  with  the 
sphincter  centers. 

The  eye  of  the  right  side  is  then  in  turn  shielded  from  the  light  and  the 
same  method  of  examination  is  carried  out.  In  this  case,  however,  the  light 
is  projected  first  on  the  nasal,  which  is  the  non-sensitive  side  of  the  retina; 
there  will  again  be  no  response  in  the  pupil.  But  if  the  light  is  gradually 
directed  toward  the  sensitive  (the  temporal)  side,  there  will  come  a  moment, 
as  the  central  line  is  crossed  and  the  light  falls  on  the  sensitive  portion  of 
the  retina,  when  the  usual  pupillary  response  manifests  itself.  The  course 
of  the  pupillary  fibers  in  this  instance  will  also  become  apparent  from  an 

examination  of  Fig.  270.  It  is  evident, 
however,  that  in  either  case  a  bilateral 
pupillary  reaction  will  follow  stimulation 
of  the  sensitive  side  of  either  eye  because 
of  the  central  decussation  of  the  pupil- 
lary fibers. 

FOURTH  NERVE.  THE  TROCHLEAR. 

The  fourth  cranial  nerve,  the  troch- 
lear, consists  of  peripherally  coursing 
axons  which  serve  to  bring  the  cells  from 
which  they  arise  into  relation  with  the 
superior  oblique  muscle. 

Origin. — The  axons  of  this  nerve 
arise  from  a  group  of  cells  located  be- 
neath the  aqueduct  of  Sylvius  just  pos- 
terior to  the  last  nucleus  of  the  third 
nerve.  After  emerging  from  the  nucleus 
the  nerve-fibers  pass  downward  for  a 
short  distance,  then  curve  dorsally 
around  the  aqueduct  of  Sylvius,  and 
enter  the  valve  of  Vieussens,  where  they 
completely  decussate  with  the  nerve- 
fibers  of  the  opposite  side. 
Cortical  Connections. — The  nucleus  of  the  trochlear  nerve  is  in  his- 
tologic and  physiologic  connection  with  the  motor  area  of  the  cerebral  cor- 
tex. Nerve-cells  in  this  region  give  off  axons  which  enter  the  pyramidal 
tract  and  descend  through  the  internal  capsule  and  the  crus  cerebri,  after 
which  they  cross  to  the  opposite  side.  Their  end-tufts  arborize  around  the 
cells  of  the  nuclei  already  described. 

Distribution. — After  its  decussation  the  nerve-trunk  emerges  just  be- 
low the  posterior  quadrigeminal  body,  crosses  the  superior  cerebellar  pe- 
duncle, and  winds  around  the  crus  cerebri  to  the  anterior  border  of  the  pons 
Varolii.  It  then  enters  the  orbit  cavity  through  the  sphenoid  fissure  and 
finally  terminates  in  the  superior  oblique  muscle.     In  its  course  the  nerve 


F,iG.  271.  —  Distribution  of  the 
Patheticus.  I.  Olfactory  nerve.  II. 
Optic  nerves.  III.  Motor  oculi  commu- 
nis. IV.  Trochlear,  by  the  side  of  V  the 
ophthalmic  branch  of  the  fifth,  and  passing 
to  the  superior  oblique  muscle.  VI.  Motor 
oculi  externus.  i.  Ganglion  of  Gasser. 
2,  3,  4,  5,  6,  7,  8,  9,  10.  Ophthalmic  divi- 
sion of  the  fifth  nerve,  with  its  branches. — 
{Hirschfeld.) 


THE  CRANIAL  NERVES. 


58s 


receives  filaments  from  the  cavernous  plexus  of  the  sympathetic  and  the 
ophthalmic  division  of  the  trigeminal  (Fig.  271). 

Properties. — Stimulation  of  the  nerve-trunk  is  followed  by  spasmodic 
contraction  of  the  superior  oblique  muscle,  the  anterior  pole  of  the  eyeball 
being  turned  downward  and  outward,  combined  with  slight  torsion  away 
from  the  middle  line. 

Division  of  the  nerve  is  followed  by  a  relaxation  or  paralysis  of  the 
muscle.  In  consequence  of  the  now  unopposed  action  of  the  inferior 
oblique  muscle,  the  anterior  pole  of  the  eyeball  is  turned  upward  and  in- 
ward with  slight  torsion  toward  the  middle  line.  The  diplopia  consequent 
upon  this  paralysis  is  homonymous,  the  images  appearing  one  above  the 
other.  The  image  of  the  paralyzed  eye  is  below  that  of  the  normal  eye  and 
its  upper  end  inclined  toward  that  of  the  normal  eye. 

Function. — The  function  of  the  trochlear  nerve  is  to  transmit  nerve 
mpulses  to  the  superior  oblique  muscle  and  to  excite  it  to  contraction. 

FIFTH  NERVE.     THE  TRIGEMINAL. 


The  fifth  cranial  nerve,  the  trigeminal,  consists  of  both  afferent  and 
efferent  axons  which  for  the  most  part  are  separate  and  distinct.  The 
afferent  axons  constitute 
by  far  the  major  portion, 
the  efferent  fibers  the 
minor  portion,  of  the 
nerve. 

Origin  of  the  Affer- 
ent Axons. — ^The  afferent 
axons  have  their  origin 
in  the  monaxonic  cells  in 
the  ganglion  of  Gasser, 
which  rests  on  the  apex  of 
the  petrous  portion  of  the 
temporal  bone.  The  cells 
of  this  ganglion  give  origin 
to  a  short  process  which 
soon  divides  into  two 
branches,  one  of  which 
passes  centrally,  the  other 
peripherally  (Fig.  272). 
The  centrally  directed 
branches  collectively  form 
the  so-called  large  or  sen- 
sor root;  the  peripherally  directed  branches  collectively  constitute  the  three 
main  divisions  of  the  nerve:  viz.,  the  ophthalmic,  the  superior  maxillary, 
and  the  inferior  maxillary.  Branches  of  the  carotid  plexus  of  the  sym- 
pathetic enter  the  nerve  in  the  neighborhood  of  the  ganglion  of  Gasser  and 
accompany  some  of  its  branches  to  their  terminations. 

Distribution. — i.  The  Central  Branches. — The  axons  of  the  large  root 
pass  backward  into  the  pons  Varolii  on  its  lateral  aspect.     After  entering 


3  4 

Fig.  272. — Scheme  of  Origin  and  Constitution  of  the 
Trigeminal  Nerve,  i.  Centrally  coursing  fibers.  2,  3, 
4.  Peripherally  coursing  fibers  of  the  cells  of  the  ganglion  of 
Gasser.  R,  N.  Nuclei  of  origin  of  the  efferent  fibers.  6. 
Motor  root.     Central  terininations  of  the  large  root. 


586 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  pons  each  axon  divides  into  two  branches,  one  of  which  passes  upward 
a  short  distance,  the  other  passes  downward,  descending  as  far  as  the 
second  cervical  segment.  Both  branches  give  off  a  number  of  collaterals, 
some  of  which  terminate  in  fine  end-tufts  around  nerve-cells  in  the  sub- 
stantia gelatinosa. 

2.   The  Peripheral  Branches. — The  peripheral  axons  emerge  from  the 

peripheral  end  of  the  ganglion  of  Gasser  in  three  distinct  and  separate 

branches,  each  of  which  is  distributed  to  a  different  region  of  the  face 

and  head. 

I.  The  ophthalmic  branch  passes  forward  and  subdivides  into  three  large 

branches,    the   frontal,  the  lachrymal,  and  the  nasal.     The  ultimate 

termination   of  the  branches  of 
these  nerves  is  as  follows:  viz., 
the  conjunctiva  and  skin  of  the 
upper  eyelid,  the  cornea,  the  skin 
of  the  forehead  and  the  nose,  the 
lachrymal    gland    and   caruncle, 
and    the    mucous   membrane   of 
the  nose  (Fig.  273). 
The     superior     maxillary     branch 
passes  forward  through  the  fora- 
men     rotundum,      crosses      the 
spheno-maxillary     fossa,     enters 
the  infra-orbital  canal,  and  emer- 
ges at  the  infra-orbital  foramen. 
In  its  course  it  gives  off  a  num- 
ber of  branches  which  are  dis- 
tributed  as   follows:  viz.,  to  the 
integument    and    conjunctiva    of 
the   lower   lid,    the  nose,  cheek, 
and    upper    lip,   the   palate,    the 
teeth  of  the  upper  jaw,  and  the 
alveolar  processes  (Fig.  274). 
3.  The  inferior  maxillary  branch  passes  through  the  foramen  ovale,  after 
which  it  subdivides  into  three  branches — the  auriculo-temporal,  the 
lingual,  and  the  inferior  dental.     The  ultimate  branches  are  distributed 
as  follows:  viz.,  the  external  auditory  meatus,  the  side  of  the  head,  the 
mucous  membrane  of  the  mouth,  the  anterior  portion  of  the  tongue,  the 
arches  of  the  palate,  the  teeth  and  alveolar  process  of  the  lower  jaw  and 
the  integument  of  the  lower  part  of  the  face  (Fig.  275). 
The  alferent  axons  thus  serve  to  bring  into  relation  the  skin,  mucous 
membranes  of  the  head  and  face,  and  other  sentient  structures,  with  certain 
sensor  end-nuclei  in  the  pons,  medulla  oblongata,  and  adjoining  structures. 
Cortical  Connections. — The  afferent  portion  of  the  trigeminal  nerve 
is  brought  into  physiologic  relation  with  the  sensor  portion  of  the  cerebral 
cortex  by  means  of  nerve-fibers  which  have  their  origin  in  the  cells  around 
which  the  terminal  branches  of  the  centrally  coursing  fibers  arborize.     The 
cells  situated  in  the  substantia  gelatinosa  give  off  axons,  which  after  a  short 
course  cross  the  median  line,  enter  the  fillet  and  then  ascend  in  the  general 


Fig.  273. — Ophthalmic  Branch  of  the 
Fifth,  i.  Ganglion  of  Gasser.  2.  Oph- 
thalniic  division  of  the  fifth.  3.  Lachrymal 
branch.  4.  Frontal  branch.  5.  Ivxtcrnal 
frontal.  6.  Internal  fro  nt  al.  7.  Sujjra- 
trochlear.  8.  Nasal  branch.  9.  External 
nasal.      10.     Internal  nasal. — {IlirscJiJeld:) 


THE  CRANIAL  NERVES. 


587 


Fig.  274. — I.  Superior  maxillary  nerve.  2,  3,  4,  5.  Dental  nerves.  6.  Spheno-palatine  gan- 
glion. 7.  Vidian  nerve.  8.  Large  superacial  petrosal,  g.  Carotid  branch  of  large  petrosal. 
10.  Oculo-motor.  11.  Superior  cervical  ganglion.  12.  Carotid  branches  of  this  ganglion.  13. 
Facial.  14.  Glosso-pharyngeal.  15.  Jacobson's  nerve,  and  16,  17,  18,  ig,  branches  to  the 
sympathetic,  fenestra  rotunda,  Eustachian  tube.  20.  Deep  external  petrosal.  21.  Deep  internal 
petrosal. — {Hirschfeld.) 


Fig.  275. — Inferior  M.A>aLLARY  Br.-\nch  of  the  Trigeminal  Nerve.  1.  Branch  to  the 
masseter  muscle.  2.  Filament  of  this  branch  to  the  temporal  muscle.  3.  Buccal  branch.  4. 
Branches  anastomosing  vi-ith  the  facial  nerve.  5.  Filament  from  the  buccal  branch  to  the  temporal 
muscle.  6.  Branches  to  the  external  pterygoid  muscle.  7.  Middle  deep  temporal  branch.  8. 
Auriculo-temporal  nerve,  g.  Temporal  branches.  10.  Auricular  branches.  11.  Anastomosis 
with  the  facial  nerve.  12.  Lingual  branch.  13.  Branch  of  the  small  root  to  the  mylo-hyoid 
muscle.  14.  Inferior  dental  nerve,  with  its  branches  (15,  15).  16.  Mental  branch.  17.  Anas- 
tomosis of  this  branch  with  the  facial  nerve. — {Hirschfeld.) 


588  TKXT-BOOK  OF  PHYSIOLOGY. 

sensor  tract  to  the  cortex  where  they  in  turn  arborize  around  sensor  nerve- 
cells. 

Properties. — Irritative  pathologic  lesions,  e.g.,  pressure  by  tumors, 
aneurysms,  neuritis,  degenerative  changes  in  the  ganglion  cells,  or  lesions 
which  in  any  way  gradually  impair  the  physical  or  chemic  integrity  of  the 
nerve-fibers,  give  rise  to  a  variety  of  painful  sensations  referable  to  the  seat 
of  the  lesion  or  to  one  or  more  regions  in  the  peripheral  distribution  of  the 
nerve.  Many  of  the  various  forms  of  trigeminal  neuralgia  are  caused  by 
lesions  of  this  character.  Exposure  of  the  dental  nerves  from  caries  of  the 
teeth,  the  presence  of  minute  foreign  bodies  in  the  conjunctiva,  operative 
procedures  in  the  nasal  chambers,  all  testify  to  the  extreme  sensibility  of  the 
nerve.  Division  of  the  large  root  within  the  cranium  is  followed  at  once  by 
complete  abolition  of  all  sensibility  in  the  head  and  face  to  which  its  branches 
are  distributed.  The  skin  and  mucous  membranes,  the  eye,  nose,  or  teeth 
may  be  experimentally  injured  without  any  evidences  of  pain  on  the  part  of 
the  animal.  Various  reflexes,  e.g.,  those  of  mastication,  insalivation,  degluti- 
tion, the  afferent  paths  of  which  are  formed  in  part  by  the  fifth  nerve,  are 
often  seriously  impaired.  At  the  same  time  the  lachrymal  secretion  dimin- 
ishes and  the  pupil  contracts.  The  same  results  are  observed  in  human 
beings  in  whom  the  nerve  has  been  divided  for  relief  from  severe  neuralgia. 
Anesthesia  or  a  loss  of  sensibility  may  also  be  caused  by  pathologic  lesions  of 
the  nerve-trunks  or  of  the  sensor  end-nuclei. 

Division  of  the  large  root  at  or  near  the  ganglion  of  Gasser  has  not  infre- 
quently been  followed  by  an  alteration  in  the  nutrition  of  the  eye  and  nose. 
In  the  course  of  twenty-four  hours  the  eye  becomes  vascular  and  inflamed; 
the  cornea  becomes  opaque;  and  ulceration  sets  in,  which  may  lead  to  com- 
plete destruction  of  the  eyeball.  The  mucous  membrane  of  the  nose  be- 
comes swollen,  vascular,  and  liable  to  hemorrhage  on  the  slightest  irritation. 
The  degenerative  changes  may  lead  to  a  complete  loss  of  smell.  These  results 
were  formerly  attributed  to  a  loss  of  trophic  influence  which  it  was  believed 
the  nerve  exercised  over  these  structures.  Modern  experimentation  and 
various  surgical  procedures  have  demonstrated  that  the  nutritive  disorders 
are  septic  in  origin,  made  possible  by  the  anesthetic  condition  and  by  the 
changed  vascular  supply  from  division  of  the  vasomotor  fibers  which  join  the 
nerve  at  or  near  the  ganglion. 

Origin  of  the  Efferent  Axons. — The  efferent  axons  arise  for  the 
most  part  from  nerve-cells  located  in  the  gray  matter  beneath  the  upper 
half  of  the  floor  of  the  fourth  ventricle.  A  group  of  cells  known  as  the 
superior  or  accessory  nucleus,  situated  posterior  to  the  corpora  quadrigem- 
ina,  gives  origin  to  axons  which  descend  and  join  the  axons  from  the  chief, 
motor  nucleus  (Fig.  272). 

Distribution. — From  their  origin  the  fibers  pass  forward  through  the 
pons  and  emerge  on  its  lateral  aspect,  forming  the  so-called  small  root  of 
the  fifth  nerve.  This  then  passes  forward  beneath  the  ganglion  of  Gasser, 
leaves  the  cavity  of  the  skull  through  the  foramen  ovale,  and  joins  the 
inferior  maxillary  division  already  described.  Its  axons  are  ultimately 
distributed  to  the  muscles  of  mastication:  viz.,  the  masseter.  the  temporal, 
the  external  and  internal  pterygoids,  the  mylohyoid,  and  the  anterior  portion 
of  the  digastric.     A  few  axons  are  also  distributed  to  the  tensor  tympani  and 


THE  CRANIAL  NERVES.  589 

tensor  palati  muscles.  The  efferent  or  peripherally  coursing  axons  thus  ser\^e 
to  bring  the  nerve-cells  from  which  they  arise  into  relation  with  the  muscles 
of  mastication. 

Cortical  Connections. — The  nuclei  of  origin  of  the  small  root  are  in 
histologic  and  physiologic  relation  with  the  lower  third  of  the  motor  area 
of  the  cerebral  cortex.  Nerve-cells  in  this  region  give  off  axons  which  enter 
the  pyramidal  tract,  descend  through  the  internal  capsule  and  the  crus 
cerebri,  after  which  they  cross  to  the  opposite  side.  Their  end-tufts  arbor- 
ize around  the  cells  of  nuclei  in  the  medulla  oblongata. 

Properties. — Stimulation  of  the  small  root  gives  rise  to  convulsive  move- 
ments of  the  muscles  of  mastication.  Division  of  the  nerve  is  followed  by 
a  paralysis  of  these  muscles.  Contraction  or  paralysis  of  the  tensor  tympani 
and  tensor  palati  muscles  would  also  be  observed  under  the  same  conditions. 

Functions. — The  function  of  the  aft'erent  fibers  of  the  fifth  nerve  is 
the  transmission  of  nerve  impulses  from  its  peripheral  distribution  to  (a) 
the  medulla  oblongata;  (b)  through  its  afferent  cortical  tracts  to  the  cerebral 
cortex  where  they  evoke  sensations.  The  nerve  therefore  endows  all  the 
parts  to  which  is  it  distributed  with  sensibility. 

The  function  of  the  efferent  fibers  is  the  transmission  of  nerve  impulses 
from  the  cells  from  which  they  take  their  origin,  to  the  muscles  of  mastication, 
which  are  excited  to  activity  by  them.  The  afferent  nerves  are  in  relation 
centrally  with  the  nuclei  of  origin  of  the  efferent  nerves,  hence  the  latter 
can  be  excited  not  only  voluntarily  but  reflexly  as  in  the  usual  acts  of  masti- 
cation. The  afferent  fibers  from  the  mouth  doubtless  assist  in  the  reflex 
secretion  of  saliva. 

Peripheral  stimulation  of  different  areas  in  the  distribution  of  the 
afferent  fibers,  e.g.,  conjunctiva,  nasal  and  oral  mucous  membranes,  teeth, 
etc.,  causes  a  variety  of  reflex  activities  in  the  muscles  associated  with  the 
eyes,  face,  the  respiratory  and  cardiac  mechanisms,  which  indicate  that  the 
afferent  fibers  are  centrally  in  relation  with  a  number  of  motor  nerve 
centers. 

SIXTH  NERVE.     THE  ABDUCENT. 

The  sixth  cranial  nerve,  the  abducent,  consists  of  peripherally  coursing 
axons  which  serve  to  bring  the  nerve-cells  from  which  they  arise  into  rela- 
tion with  the  external  rectus  muscle. 

Origin. — The  axons  arise  from  a  group  of  cells  located  in  the  gray  matter 
beneath  the  upper  half  of  the  floor  of  the  fourth  ventricle.  It  is  quite  prob- 
able that  a  few  fibers  in  each  nerve-trunk  come  from  the  nucleus  on  the 
opposite  side  of  the  middle  line. 

Distribution. — The  nerve-fibers  pass  forward  from  their  origin  through 
the  gray  and  white  matter  and  emerge  through  the  groove  between  the  med- 
ulla oblongata  and  the  pons  Varolii  just  external  to  the  anterior  pyramid. 
The  ner\'e  then  passes  through  the  sphenoid  fissure  into  the  orbit  cavity, 
where  it  is  distributed  to  the  external  rectus  muscle  (Fig.  276).  In  its  course 
the  nerve  receives  filaments  from  the  carotid  plexus  of  the  sympathetic. 

Cortical  Connections. — The  nucleus  of  the  sixth  nerve  is  in  histologic 
and  physiologic  connection  with  the  motor  area  of  the  cerebral  cortex.  From 
nerve-cells  in  this  region  axons  are  given  off  which  enter  the  pyramidal 


590 


TEXT-BOOK  OF  PHYSIOLOGY. 


tract,  descend  through  the  internal  capsule  and  crus  cerebri,  after  which 
they  cross  to  the  opposite  side,  where  their  end-tufts  arborize  around  the 
cells  of  the  nucleus  already  described. 

Properties. — Stimulation  of  the  nerve  is  followed  by  spasmodic  con- 
traction of  the  external  rectus  muscle  and  external  deviation  of  the  eyeball. 
Division  of  the  nerve  is  followed  by  paralysis  or  relaxation  of  the  muscle. 
As  a  result  of  the  unopposed  action  of  the  internal  rectus  the  anterior  pole 
of  the  eyeball  is  turned  toward  the  mid- 
dle line  (internal  strabismus).  In  con- 
sequence of  this  deviation  there  is 
homonymous  diplopia.  The  images  are 
on  the  same  level  and  parallel.  The 
image  of  the  paralyzed  eye  lies  external 
to  that  of  the  normal  eye. 

Function. — The  function  of  this 
nerve  is  to  transmit  nerve  impulses  to 
the  external  rectus  muscle  and  excite  it 
to  contraction. 

SEVENTH  NERVE.     THE  FACIAL. 

The  seventh  cranial  nerve,  the  facial, 
consists  of  peripherally  coursing  nerve- 
fibers,  which  serve  to  bring  the  nerve- 
cells  from  which  they  arise  into  relation 
with  most  of  the  superficial  muscles  of 
the  head  and  face. 

The  muscles  supplied  by  this  nerve, 
as  stated  by  the  general  anatomists,  are 
as  follows:  The  occipito-frontalis,  cor- 
rugator  supercilii,  orbicularis  palpebrarum,  levator  labii  superioris  alaeque 
nasi,  zygomatici,  the  pyramidaHs  nasi,  compressor  nasi,  depressor  alae  nasi, 
levator  anguli  oris,  buccinator,  orbicularis  oris,  depressor  anguli  oris,  de- 
pressor labii  inferioris,  levator  menti,  posterior  belly  of  the  digastric,  stylo- 
hyoid, and  platysma  myoides. 

Origin.- — The  nerve-fibers  or  axons  composing  the  seventh  nerve  arise 
for  the  most  part  from  a  nucleus  of  large  multipolar  nerve-cells  situated 
about  five  millimeters  beneath  the  upper  half  of  the  floor  of  the  fourth  ven- 
tricle toward  the  middle  line. 

From  this  nucleus,  which  is  about  four  millimeters  long,  axons  emerge 
which  at  first  pass  inward  and  backward  as  far  as  the  ependyma  of  the  ven- 
tricle; they  then  turn  on-  themselves,  forming  an  arch  that  encloses  the  nu- 
cleus of  the  sixth  nerve;  they  then  course  downward  and  outward,  emerging 
from  the  pons  at  its  lower  border  between  the  olivary  and  restiform  bodies. 
As  the  axons  approach  the  floor  of  the  ventricle  collateral  branches  are 
given  ofT  which,  crossing  the  median  line,  arborize  around  the  nerve-cells 
of  the  opposite  facial  nucleus. 

Clinic  observations  and  histologic  investigations,  however,  render  it 
probable  that  the  fibers  distributed  to  the  occipito-frontalis,  the  corrugator 


Fig.  276. — Distribution  of  the 
Motor  Oculi  Externus  or  Abducens. 
I.  Trunk  of  the  motor  oculi  communis, 
with  its  branches  (2,  3,  4,  5,  6,  7).  8. 
Motor  ocuU  externus,  passing  to  the  exter- 
nal rectus  muscle.  9.  Filaments  of  the 
motor  oculi  externus  anastomosing  with 
the  sympathetic.  10.  Ciliary  nerves. — 
{Hirschfeld.) 


THE  CRANIAL  NERVES. 


591 


supercilii,  and  the  upper  half  of  the  orbicularis  palpebrarum,  are  derived 
from  the  oculo-motor  nucleus,  and,  descending  the  posterior  longitudinal 
bundle,  enter  the  trunk  of  the  facial  as  it  turns  to  pass  forward  through  the 
pons.  It  is  also  probable,  for  similar  reasons,  that  the  fibers  distributed  to 
the  orbicularis  oris  are  derived  from  the  hypoglossal  nucleus. 

Cortical  Connections. — The  nucleus  of  the  facial  nerve  is  in  histologic 
and  physiologic  connection  with  the  facial  region  of  the  general  motor  area 
of  the  cerebral  cortex.  From  the  cells  of  this  region  axons  descend  through 
the  pyramidal  tract,  the  internal  capsule,  and  the  cms  cerebri,  beyond  which 
they  cross  to  the  opposite  side  and  arborize  around  the  cells  of  the  nucleus 
already  described. 

Distribution. — From  its  superficial  origin  the  trunk  of  the  nerv^e  passes 
into  the  internal  auditory  meatus  beside  the  auditory  nerve.  After  passing 
forward  and  outward  for  a  short  distance  through  the  bone  above  and  be- 
tween the  cochlea  and  vestibule,  the  nerve  makes  a  sharp  bend,  forming  the 
genu  facialis,  turns  backv/ard  and  enters  the  aqueduct  of  Fallopius,  the  gen- 
eral course  of  which  it  follows  as  far  as  the  stylo-mastoid  foramen.  After 
emerging  from  this  foramen  the  nerve  passes  downward  and  forward  as  far 
as  the  parotid  gland,  within  which  it  terminates  by  dividing  into  two  main 
branches,  the  temporo-facial  and  the  cervico-facial,  the  ultimate  branches 
of  which  are  distributed  as  previously  stated  to  the  superficial  muscles  of  the 
head  and  face  (Fig.  277). 

Properties. — Electric  stimulation  of  the  trunk  of  the  nerve  after  its  emer- 
gence from  the  stylo-mastoid  foramen  produces  convulsive  movements  in  all 
the  muscles  to  which  its  branches  are  distributed.  The  same  results  follow 
stimulation  of  the  intra-cranial  portion  of  the  nerve  in  an  animal  recently 
killed. 

Irritative  pathologic  lesions — e.g.,  tumors,  aneurysms,  etc. — situated 
along  the  course  of  the  nerve  or  at  its  nuclear  origin,  frequently  give  rise  to 
spasmodic  movements  of  the  facial  muscles  which  may  be  tonic  or  clonic  in 
character. 

Division  of  the  facial  nerve  after  its  emergence  from  the  stylo-mastoid 
foramen  is  followed  by  a  complete  relaxation  or  paralysis  of  the  superficial 
facial  muscles.  The  same  result  follows  compression  of  the  nerve-trunk  in 
any  part  of  its  intra-cranial  course. 

The  phenomena  presented  by  an  individual  suffering  from  division  or 
compression  of  the  facial  nerve,  and  which  collectively  constitute  facial  paraly- 
sis, are  as  follows:  A  relaxed  and  immobile  condition  of  the  side  of  the 
face  corresponding  to  the  lesion;  separation  of  the  eyelids  from  paralysis  of 
the  orbicularis  palpebrarum  and  the  unopposed  contraction  of  the  levator 
palpebrae  muscles;  abolition  of  the  ability  to  wink;  drooping  of  the  angle  of 
the  mouth;  an  escape  of  saliva  from  the  mouth;  contraction  of  the  muscles 
and  distortion  of  the  opposite  side  of  the  face;  on  attempting  to  laugh 
or  talk  the  distortion  of  the  face  is  increased;  during  mastication  the  food 
accumulates  between  the  teeth  and  cheek,  from  paralysis  of  the  buccinator; 
articulation  is  impaired  from  paralysis  of  the  orbicularis  oris  muscle,  the 
labial  sounds  especially  being  imperfectly  produced. 

Functions. — The  function  of  the  facial  nerve  is  the  transmission  of 
nerve  impulses  from  the  nerve-cells  in  which  it  arises  to  the  superficial 


592 


TEXT-BOOK  OF  PHYSIOLOGY 


muscles  of  the  face.  These  muscles  by  their  individual  and  cooperative 
contraction  express  ideas  and  feelings  and  are  therefore  termed  muscles  of 
expression.  By  reason  of  the  association  of  the  cortical  facial  area  and  the 
nucleus  of  origin  of  the  facial  nerve  the  latter  becomes  the  medium  of 
communication  between  the  cortical  area  and  the  facial  muscles  and  serves 
for  the  transmission  to  the  muscles  of  those  nerve  impulses  developed  by 


Fig.  277. — Superficial  Branches  of  the  Facial  and  the  Fifth. — i.  Trunk  of  the  facial. 
2.  Posterior  auricular  nerve.  3.  Branch  which  it  receives  from  the  cervical  plexus.  4.  Occipital 
branch.  5,  6.  Branches  to  the  muscles  of  the  ear.  7.  Digastric  branches.  8.  Branch  to  the 
stylo-hyoid  muscle.  9.  Superior  terminal  branch.  10.  Temporal  branches.  11.  Frontal 
branches.  12.  Branches  to  the  orbicularis  palpebrarum.  13.  Nasal  or  suborbital  branches. 
14.  Buccal  branches.  15.  Inferior  terminal  branch.  16.  Mental  branches.  17.  Cervical 
branches.  18.  Superficial  temporal  nerve  (branch  of  the  fifth).  iq,  20.  Frontal  nerves  (branches 
of  the  iifth).  21,  22,  23,  24,  25,  26,  27.  Branches  of  the  fifth.  28,29,30,31,32.  Branches  of  the 
cervical  nerves. — {Hirschfeld.) 

and  associated  with  psychic  states.  The  muscles  thus  excited  to  action 
individually  and  collectively  express  in  a  general  way  the  character  of 
the  psychic  state.  For  this  reason  the  facial  nerve  is  termed  the  nerve  of 
expression. 

Branches  of  the  Facial  Nerve ;  Their  Origin,  Properties  and  Func- 
tions.— Between  the  facial  and  the  acoustic  nerve  there  is  a  small  nerve  known 
as  the  pars  intermedia,  the  nervus  intermedins  or  the  nerve  of  Wrisberg.  The 
true  nature  of  this  nerve  has  long  been  a  subject  of  investigation.     The 


THE  CRANIAL  NERVES. 


593 


results  of  histologic  investigation  and  physiologic  experimentation  would 
indicate  that  it  is  composed  of  both  afferent  and  efferent  fibers.  The  afferent 
fibers  arise  from  nerve-cells  composing  in  large  part  the  ganglionic  enlarge- 
ment found  on  the  genu  of  the  facial  nerve  at  the  point  where  it  turns  back- 
ward to  enter  the  aqueduct  of  Fallopius.  The  cells  of  this  geniculate 
ganglion,  originally  bipolar  present  single  axons  which  soon  divide  into 
centrally  and  peripherally  coursing  branches.  The  centrally  coursing 
branches  constitute  in  part  the  nerve  of  Wrisberg,  which  entering  and  pass- 
ing through  the  pons  terminates  directly  or  indirectly  around  the  sensor 
end-nucleus  of  the  glosso-pharyngeal  nerve.  The  peripherally  coursing 
branches  enter  the  sheath  of  the  facial  nerve  and  accompany  it  as  far  as  a 
point  about  5  millimeters  above  the  stylo-mastoid  foramen. 

The  efferent  fibers  which  constitute  in  part  the  nerve  of  Wrisberg  have  their 
origin  in  a  group  of  cells  situated  beneath  the  floor  of  the  fourth  ventricle 
near  the  median  line  between  the 
nucleus  of  the  facial  and  the  nucleus 
of  the  motor  root  of  the  trigeminal 
nerve  and  known  as  the  nucleus  sali- 
va tortus.  From  its  mode  of  origin, 
the  nerve  of  Wrisberg  cannot  be  re- 
garded as  an  integral  part  of  the  facial 
nerve  proper,  but  must  be  considered 
as  an  independent  nen^e  composed 
of  both  afferent  and  efferent  fibers. 

At  the  beginning  and  in  the 
course  of  the  aqueduct  of  Fallopius 
the  facial  trunk  gives  off  the  following 
branches:  the  large  superficial  petro- 
sal, the  small  superficial  petrosal, 
the  stapedius  and  the  chorda  tym- 
pani,  (Fig.  278). 


Fig.  278. — Chorda  Tympani  Nerve,  i,  2, 
3,  4.  Facial  nerve  passing  through  the  aqueductus 
Fallopii.  5.  Ganglioform  enlargement.  6. 
Great    petrosal  nerve.     7.  Sphenopalatine  gan- 


I.    The  large  superficial  petrosal  ner\^e  ^lion.  8.  Small  petrosal  nerve.     9.  Chorda  tym- 

is  given  off  near  the  geniculate  P^^-    ^°'  "•  ''^  ^-  ^'=^"°"^  branches  of  the 


Glosso-phar}'ngeal  nerve. — ■ 


10,  II, 
,.  T^     .u  r  facial.     14,  14, 

ganglion.     It    then   passes   for-  {Hirschfdd.) 
ward  into  the  spheno-maxillary 

fossa  and  becomes  associated  with  the  spheno-palatine  or  Meckel's  gan- 
glion. In  its  course  it  receives  a  filament  known  as  the  deep  petrosal, 
from  the  carotid  plexus  of  the  sympathetic.  The  nerve-trunk  formed  by 
the  union  of  these  two  nerves  is  known  as  the  Vidian  nerve  and  terminates 
as  stated  above.  The  character  and  function  of  the  large  petrosal 
nerve  have  been  a  subject  of  much  discussion.  As  the  outcome 
of  modern  methods  of  investigation  it  may  be  concluded  that  it  is 
composed  mainly,  if  not  entirely,  of  fine  medullated  nerve-fibers  which  are 
the  continuations  of  corresponding  fibers  in  the  nerve  of  Wrisberg  and 
that  their  destination  is  the  spheno-palatine  ganglion,  around  the 
nerv^e-cells  of  which  their  terminal  branches  arborize. 
Stimulation  of  the  large  petrosal,  with  induced  electric  currents,  gives 
rise  to  a  dilatation  of  the  blood-vessels  of,  and  a  secretion  from  the 
mucous  membrane,  of  the  nose,  soft  palate,  upper  part  of  the  pharynx, 
38 


594  TEXT-BOOK  OF  PHYSIOLOGY. 

roof  of  the  mouth,  gums,  and  upper  lip — the  regions  of  distribution 
of  the  post-ganglionic  fibers  of  cells  of  the  spheno-palatine  ganglion 
(see  Sympathetic).  The  nerve  therefore  contains  both  vaso-dilatator 
and  secretor  fibers  which  belong  to  the  autonomic  system  of  nerves. 
As  after  the  administration  of  nicotine  stimulation  of  this  nerve  is 
without  effect,  and  as  stimulation  of  the  spheno-palatine  ganglion 
gives  rise  to  the  usual  vaso-dilatator  and  secretor  effects  it  may  be 
inferred  that  the  ganglion  is  the  way  station  between  the  pre-ganglionic 
fibers  and  the  blood-vessels  and  glands.  The  deep  petrosal,  which 
joins  the  large  petrosal  is  in  all  probability  a  vaso-constrictor  nerve 
coming  from  the  superior  cervical  ganglion  of  the  sympathetic.  There 
is  no  evidence  that  the  large  petrosal  contains  any  fibers  from  the 
facial  proper  for  the  innervation  of  any  striated  muscle  of  the  palate. 

2.  The  small  superficial  petrosal  nerve  is  given  off    from    the  facial  at  a 

point  somewhat  external  to  the  large  petrosal  nerve.  In  its  course 
it  is  joined  by  a  small  filament  derived  from  Jacobson's  branch  of  the 
glosso-pharyngeal.  Together  they  pass  into  the  otic  ganglion,  where 
the  fibers  arborize  around  the  nerve-cells  composing  it.  Experiments 
are  lacking  as  to  the  function  of  the  small  petrosal.  The  small  size  of 
its  nerve-fibers  and  their  termination  would  lead  to  the  conjecture 
that  they  are  probably  vaso-dilatator  and  secretor.  Stimulation  of  Jacob- 
son's  nerve  gives  rise  to  a  dilatation  of  the  blood-vessels  of,  and  secretion 
from,  the  mucous  membrane  of  the  check,  lips,  and  gums  and  of  the 
parotid  and  orbit  glands,  the  regions  of  distribution  of  the  post-ganglionic 
fibers  of  the  otic  ganglion.  This  nerve  therefore  contains  both  vaso- 
dilatator and  secretor  fibers,     (see  pages  152,  598.) 

3.  The  stapedius   nerve  or  tympanic  nerve  is  distributed  directly  to  the 

stapedius  muscle,  and  as  this  muscle  is  of  the  striated  or  skeletal 
variety  it  is  innervated  by  the  facial  proper. 

4.  The  chorda  tympani  nerve  is  given  off  from  the  facial  at  a  point  about 

5  millimeters  above  the  stylo-mastoid  foramen.     It  then  passes  upward 

and  forward  and  enters  the  tympanum  through  the  iter  chordae  posterius, 

crosses  the  tympanic  membrane  between  the  malleus  and  incus,  leaves 

the  tympanum  by  the  iter  chordae  anterius  or  canal  of  Huguier,  and 

finally  joins  the  lingual  branch  of  the  fifth  nerve.     Some  of  its  fibers  can 

be  traced  to  the  mucous  membrane  of  the  dorsum  of  the  tongue,  others 

to  the  submaxillary  and  sublingual  ganglia  with  which  they  become 

associated. 

The  determination  of  the  origin,  course,  and  functions  of  the  chorda 

tympani  nerve  has  given  rise  to  many  investigations  and  discussions,  and  it 

cannot  be  said  that  the  results  thus  far  attained  are  as  satisfactory  as  might 

be  desired. 

If  the  nerve  be  divided  as  it  crosses  the  tympanic  cavity  or  before  it  unites 
with  the  lingual  branch  of  the  fifth  ner\^e,  there  follows  a  loss  of  taste  in  the 
anterior  two-thirds  of  the  tongue  on  the  corresponding  side,  though  the  sensi- 
bility remains  unimpaired.  For  this  and  other  reasons,  the  chorda  tympani 
has  long  been  regarded  as  the  nerve  of  taste  for  this  region.  The  nerve-fibers 
subserving  the  sense  of  taste  are  believed  to  be  the  peripherally  coursing 
fibers  which  have  their  origin  in  the  nerve-cells  of  the  geniculate  ganglion 


THE  CRANIAL  NERVES.  595 

and  which  descending  in  the  aqueduct  of  Fallopius  are  continued  as  the 
chorda  tympani.  The  nerve  impulses  developed  in  the  peripheral  termina- 
tions of  this  nerve  by  the  action  of  organic  matter  in  solution  are  transmitted 
through  the  chorda  tympani,  along  the  facial  nerve  as  far  as  the  geniculate 
ganglion.  The  exact  pathway  for  these  q/fr^'w/ or  gustatory  fibers  beyond  the 
geniculate  ganglion  has  long  been  a  subject  of  much  discussion.  According 
to  some  observers  these  fibers  enter  the  great  petrosal  nerve,  pass  forward  as 
far  as  the  spheno-palatine  ganglion,  then  into  the  superior  maxillary  division 
of  the  trigeminal,  and  so  to  the  brain.  According  to  others,  these  fibers 
pass  into  the  pars  intermedia,  into  the  pons,  where  they  terminate  around 
the  sensor  end-nucleus  of  the  glosso-pharyngeal.  The  evidence  for  and 
against  either  of  these  two  vdews  is  most  conflicting  and  insufficient  to  justify 
positive  statements  one  way  or  the  other.  To  the  writer  the  weight  of  evi- 
dence seems  to  favor  the  view  that  the  gustatory  fibers  have  their  origin  in  the 
geniculate  ganglion;  that  they  pass  centrally  through  the  pars  intermedia; 
that  they  are  similar  in  function  to  the  glosso-pharyngeal;  and  that  they  are 
indeed  but  aberrant  branches  of  this  nerve. 

Division  of  the  chorda  tympani  nerve  is  also  followed  by  a  contraction  of 
the  blood-vessels  in  the  neighborhood  of  and  a  diminution  in  the  secretion 
from  the  submaxillary  and  sublingual  glands.  Stimulation  of  the  peripheral 
end  of  the  di\aded  nerv^e  gives  rise  to  a  dilatation  of  the  blood-vessels  and  an 
increased  production  and  discharge  of  saliva  from  these  glands.  (See  page 
150.)  From  these  results  it  is  certain  that  the  chorda  tympani  contains  both 
vaso-dilatator  and  secretor  fibers.  Nicotin  applied  to  the  submaxillary 
and  sublingual  ganglia  abolishes  the  effects  of  stimulation  of  the  chorda 
tympani.  It  does  not  prevent  the  same  effects  when  the  ganglia  themselves 
are  stimulated.  It  is  clear,  therefore,  that  the  vaso-dilatator  and  secretor 
fibers  arborize  around  the  cells  of  the  ganglia  and  are  not  distributed  directly 
to  the  gland  structures.  It  is  highly  probable  that  the  vaso-dilatator  and 
secretor  fibers  in  the  chorda  tympani  are  the  continuations  of  the  efferent 
fibers  found  in  the  pars  intermedia  and  that  they  too  have  their  origin  in  the 
nucleus  salivatorius. 

EIGHTH  NERVE.     THE  ACOUSTIC. 

The  eighth  cranial  nerve,  the  acoustic,  consists  of  the  centrally  coursing 
axons  of  neurons  which  connect  the  essential  organ  of  hearing  with  sensor 
end-nuclei  in  the  pons  Varolii.  This  nerve  consists  of  two  portions:  viz., 
a  cochlear  or  auditory  and  a  vestibular  or  equilibratory. 

Origin. — The  axons  comprising  the  cochlear  portion  have  their  origin 
in  the  bipolar  nerve-cells  of  the  spiral  ganglion  located  in  the  spiral  canal  near 
the  base  of  the  osseous  lamina  spiralis  (Fig.  279).  From  this  origin  they 
pass  centrally  into  the  central  canal  of  the  modiolus,  at  the  base  of  which 
they  emerge  in  well-defined  bundles  and  enter  the  internal  auditory  meatus. 
Dendritic  processes  from  these  cells  pass  peripherally  to  terminate  on  the 
ciliated  epithelial  cells  of  the  organ  of  Corti. 

The  axons  comprising  the  vestibular  portion  have  their  origin  in  the 
bipolar  ner^-e-cells  of  the  ganglion  of  Scarpa  located  in  the  internal  auditory 
meatus.     From  this  origin  they  pass  centrally  in  connection  with  the  cochlear 


50 


TEXT-BOOK  OF  PHYSIOLOGY. 


portion.  Dendritic  processes  from  these  cells  pass  peripherally  into  the 
internal  ear,  where  they  terminate  on  epithelial  cells  situated  on  the  inner 
surface  of  the  utricle  and  saccule  and  in  the  ampullae  of  the  semicircular 
canals. 

The  common  trunk  of  the  auditory  nerve,  consisting  of  both  cochlear 

and  vestibular  divisions  after  emerging  from  the  internal  auditory  meatus, 

,,  lo  passes  backward,  inward,  and  downward 

,.•'''  as  far  as  the  lateral  aspect  of  the  pons 

where  the  two  divisions  again  separate. 

The  cochlear  nerve,  the  external  root, 
passes  to  the  outer  side  of  the  restiform 
body  and  enters  the  ventral  acoustic 
nucleus  and  the  lateral  acoustic  nucleus, 
around  the  cells  of  which  its  end-tufts 
arborize.  The  vestibular  nerve,  the  in- 
ternal root,  passes  on  the  inner  side  of 
the  restiform  body  to  the  dorsal  portion 
of  the  pons,  where,  after  bifurcating, 
the  end-tufts  of  the  axons  arborize  around 
the  dorso-internal  or  chief  auditory  nu- 
cleus and  the  dorso-external  or  Deiters' 
nucleus.  Some  of  the  fibers  of  the  ves- 
tibular branch  descend  through  the  pons 
and  medulla  as  far  as  the  cuneate 
nucleus. 

Cortical  Connections. — The  coch- 
lear nerve  is  ultimately  connected  with 
the  cerebral  acoustic  area,  in  the  tem- 
poral lobe  of  the  opposite  side  through 
the  intermediation  of  the  auditory  tract. 
This  tract  is  complex  and  involved. 
In  a  general  way  it  may  be  said  to  con- 
sist in  part  of  fibers  which  come  direct 
from  the  cochlear  branch.  After  pass- 
ing through  the  ventral  nucleus  and  the 
trapezoid  body  they  cross  the  median 
line,  enter  the  lemniscus  or  fillet,  and 
finally  terminate  in  the  pre-  and  post- 
geminai  bodies.  In  their  course  they  give  off  collateral  branches  to  these 
various  nuclei  through  which  they  pass.  Other  fibers  taking  their  origin 
from  cells  in  these  various  nuclei  proceed  to  the  cortex  where  they  terminate. 
Properties.^ — Stimulation  of  the  cochlear  nerve  is  unattended  by  either 
motor  or  sensor  phenomena.  Division  of  the  nerve  is  followed  by  a  loss 
of  the  sense  of  hearing.  Irritative  pathologic  lesions  give  rise  to  sensations 
of  sound  of  varying  character  and  intensity.  Degeneration  of  the  nerve  or 
destruction  by  tumors,  etc.,  will  also  be  followed  by  a  loss  of  the  sense  of 
hearing. 

Experimental  lesions  of  the  semicircular  canals  involving  a  destruction 
of  the  physiologic  relations  of  the  vestibular  nerve  are  followed  by  a  loss  of 


Fig.  279. — Origin  and  Termination 
OF  THE  Auditory  Nerve,  i.  Cochlea. 
2.  Spiral  ganglion  (Corti).  3.  Cochlear 
nerve.  4.  Ventral  acoustic  nucleus.  5. 
Lateral  acoustic  nucleus.  6.  S  e  m  i- 
circular  canals.  7.  Ganglion  of  Scarpa. 
8.  Vestibular  nerve.  9.  Dorso-external 
nucleus  (D  eite  rs).  10.  Dorso-internal 
nucleus.— (^//fr  Moral  and  Doyon.) 


THE  CRANIAL  NERVES.  597 

the  coordinating  and  equilibratory  power.  Disordered  movements,  such  as 
rotation  to  the  right  or  left,  somersaults  backward  and  forward,  follow 
destruction  of  these  canals.  Pathologic  lesions  in  the  peripheral  distribution 
of  the  nerve  are  attended  in  man  by  disturbances  of  equilibrium,  e.g.,  vertigo, 
or  a  sense  of  swaying,  pitching,  and  staggering. 

Functions. — The  function  of  the  cochlear  nerve  is  to  convey  nerve 
impulses  from  its  origin  to  the  pons,  from  which  they  are  transmitted  by  the 
auditory  tract  to  the  acoustic  area  in  the  cerebral  cortex  where  they  evoke 
sensations  of  sound  and  its  different  qualities,  intensity,  pitch,  and  timbre. 
The  specific  physiologic  stimulus  to  the  development  of  these  impulses  is  the 
impact  of  atmospheric  undulations  on  the  tympanic  membrane,  received 
and  transmitted  by  the  chain  of  bones  to  the  structures  of  the  internal  ear — 
the  organ  of  Corti — with  which  the  peripheral  terminations  of  the  nerve  are 
connected. 

The  function  of  the  vestibular  nerve  is  the  transmission  of  nerve  impulses 
to  the  pons,  whence  they  are  transmitted  to  the  cortex  of  both  the  cerebrum 
and  cerebellum  and  to  other  centers.  The  specific  physiologic  stimulus  is 
supposed  to  be  a  variation  in  pressure  in  the  ampullae  of  the  semicircular 
canals  caused  by  inertia  of  the  endolymph  during  changes  in  the  position  of 
the  head  and  body.  The  impulses  carried  by  the  vestibular  nerve  give  rise 
reflexly  to  certain  adaptive  and  protective  movements  by  which  the  equi- 
librium of  the  body  in  both  dynamic  and  static  conditions  is  maintained. 

NINTH  NERVE.     THE  GLOSSO-PHARYNGEAL. 

The  ninth  cranial  nen-e,  the  glosso-pharyngeal,  consists,  as  shown  by 
both  histologic  and  experimental  methods  of  research,  of  both  aft'erent  and 
efferent  ner\'e-fibers,  of  which  the  former,  however,  are  by  far  the  more 
abundant.  Near  its  exit  from  the  cavity  of  the  skull  the  nerv'e  presents  two 
ganglionic  enlargements  known  as  the  petrosal  and  jugular  ganglia. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  serve  to  bring  cer- 
tain end-nuclei  in  the  medulla  oblongata  into  anatomic  and  physiologic 
relation  with  portions  of  the  mucous  membrane  of  the  tongue,  pharynx,  and 
middle  ear.  The  afferent  fibers  are  axons  of  the  monaxonic  cells  of  the 
petrosal  and  jugular  ganglia.  The  single  axon  from  each  of  these  cells  soon 
divides  into  two  branches,  one  of  which  passes  centrally,  the  other  peripher- 
ally. The  centrally  directed  branches  collectively  form  the  so-called  roots, 
four  or  five  in  number,  which  enter  the  medulla  between  the  olivary  and 
restiform  bodies.  The  peripherally  directed  branches  collectively  form  the 
two  main  divisions,  from  the  distribution  of  which,  to  the  tongue  and  pharynx, 
the  ner\-e  takes  its  name. 

Distribution. — The  axons  of  the  centrally  directed  branches  after 
entering  the  medulla  pass  toward  its  dorsal  aspect,  where  they  bifurcate, 
give  off  collateral  branches,  and  terminate  in  fine  end-tufts  in  the  immediate 
neighborhood  of  two  groups  of  nerve-cells,  the  sensor  end-nuclei.  The  axons 
of  the  peripherally  directed  branches,  after  emerging  from  the  base  of  the 
skull  through  the  jugular  foramen,  pass  forward  and  inw^ard  under  cover  of 
the  stylo-pharyngeal  muscle;  winding  around  this  muscle  they  divide  into 
terminal  branches  which  are  distributed  to  the  mucous  membrane  of  the 


598  TEXT-BOOK  OF  PHYSIOLOGY. 

posterior  one-third  of  the  tongue,  pharynx,  soft  palate,  uvula,  and  tonsils 
(Fig.  281). 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  serve  to  bring  the 
nerve-cells  from  which  they  arise  into  connection  with  a  portion  of  the  mus- 
culature of  the  fauces  and  pharynx.  These  nerve-cells  are  located  in  the 
lateral  portion  of  the  formatio  reticularis  at  some  distance  below  the  floor  of 
the  fourth  ventricle.  They  constitute  the  upper  portion  of  a  collection  of 
the  cells  known  as  the  nucleus  ambiguus. 

Distribution.- — ^From  this  origin  the  efferent  fibers  pass  dorsally  to  near 
sensor  end-nuclei,  then  turn  outward  and  forward  and  finally  emerge  from 
the  medulla  in  intimate  association  with  the  afferent  fibers.  They  are 
ultimately  distributed  to  the  stylo-pharyngeus,  and  to  the  middle  constrictor 
muscle  of  the  pharynx.  In  addition  to  the  foregoing  efferent  fibers  the 
glossopharyngeal  nerve  contains  at  its  emergence  from  the  medulla  both 
vaso-motor  and  secretor  fibers. 

Jacohsoii's  Nerve. — This  is  a  small  branch  which  leaves  the  glosso- 
pharyngeal at  the  petrous  ganglion.  After  passing  through  a  small  canal  in 
the  base  of  the  skull  it  enters  the  tympanic  cavity,  within  which  it  gives  off 
branches  to  the  great  and  lesser  petrosal  nerves,  to  the  mucous  membrane  of 
the  foramen  ovale,  the  foramen  rotundum,  and  to  the  Eustachian  tube. 

Cortical  Connections. — The  motor  nucleus  is  doubtless  connected  with 
the  general  motor  area  of  the  cortex  through  fibers  descending  in  the 
pyramidal  tract.  The  exact  location  of  the  cortical  area  for  the  pharynx 
is  not  well  determined,  but  is  most  likely  to  be  found  in  the  lower  part  of  the 
general  motor  area  near  the  termination  of  the  Rolandic  fissure.  The  exact 
cortical  connections  of  the  afferent  tract  are  unknown,  but  are  most  likely  to 
be  found  in  the  general  sensor  area. 

Properties. — Stimulation  of  the  glosso-pharyngeal  trunk  with  induced  el- 
ectric currents  calls  forth  evidence  of  pain  and  contraction  of  the  stylo-pharyn- 
geus and  middle  constrictor  muscles.  Peripheral  stimulation  of  the  termi- 
nals of  the  nerve  fibers  in  the  mucous  membrane  of  the  posterior  third  of  the 
tongue  with  different  kinds  of  organic  matter  in  solution,  develops  nerve 
impulses  which  transmitted  to  the  cortex  evoke  sensations  of  taste.  Division 
of  the  nerve  abolishes  sensibility  in  the  mucous  membrane  to  which  it  is 
distributed,  impairs  the  sense  of  taste  in  the  posterior  third  of  the  tongue, 
and  gives  rise  to  paralysis  of  the  above-mentioned  muscles. 

Stimulation  of  Jacobson's  nerve  is  followed  by  dilatation  of  the  blood- 
vessels of,  and  secretion  from,  the  mucous  membrane  of  the  lower  lip,  cheek, 
and  gums,  and  from  the  parotid  gland.  Divison  of  the  nerve  is  followed  by 
the  opposite  results.  The  course  of  the  fibers  which  give  rise  to  these  results 
is  by  way  of  the  lesser  petrosal  to  the  otic  ganglion,  around  the  cells  of  which 
the  fibers  arborize.  From  the  cells  of  this  ganglion  non-medullated  fibers 
pass  to  the  blood-vessels  and  gland  cells.  These  nerve-fibers  are  thus 
members  of  the  autonomic  system  of  nerves. 

Functions. — The  afferent  fibers  of  the  glosso-pharyngeal  transmit 
nerve  impulses  from  the  parts  to  which  they  are  distributed  to  the  cerebral 
cortex,  where  they  evoke  sensations  of  pain  and  sensations  of  taste;  they 
also  assist  in  all  probability  in  the  performance  of  certain  reflexes  connected 
with  deglutition.     The  afferent  fibers  are  therefore  divisable  into  nerves  of 


THE  CRANIAL  NERVES.  599 

general  sensibility  and  nerves  of  special  sense.  The  efferent  fibers  transmit 
impulses  to  muscles,  exciting  them  to  activity,  and  to  the  otic  ganglion, 
which  in  turn  dilates  blood-vessels  and  excites  secretion.  The  fibers  excit- 
ing secretion  have  in  all  probability  their  origin  in  the  nucleus  salivalorius. 

TENTH  NERVE.  THE  PNEUMOGASTRIC  OR  VAGUS. 

The  tenth  cranial  nerve,  the  pneumogastric  or  vagus,  consists,  as  shown 
by  histologic  methods  of  research,  of  both  afferent  and  efferent  fibers,  in- 
dependent of  those  derived  in  its  course  from  adjoining  motor  or  efi'erent 
nerves.  Near  the  exit  of  the  nerve  from  the  cavity  of  the  cranium  it  presents 
two  ganglionic  enlargements  known  respectively  as  the  ganglion  of  the  root 
(the  jugular)  and  the  ganglion  of  the  trunk  (the  plexiform). 

Origin  of  the  Afferent  Fibers. — -The  aft'erent  fibers  take  their  origin 
in  the  monaxonic  cells  of  the  ganglia  on  the  root  and  trunk.  The  single 
axon  from  each  of  these  cells  soon  divides  into  two  branches,  one  of  which 
passes  centrally,  the  other  peripherally.  The  centrally  directed  branches 
collectively  form  the  so-called  roots,  ten  to  fifteen  in  number,  which  enter  the 
medulla  between  the  restiform  body  and  the  lateral  column.  The  periph- 
erally directed  branches  collectively  form  a  portion  of  the  common  trunk 
of  the  nerve. 

Distribution. — The  axon  of  the  centrally  directed  branches  after  entering 
the  medulla  pass  toward  its  dorsal  aspect,  where  they  bifurcate,  give  collat- 
erals, and  terminate  in  fine  end-tufts  in  the  immediate  neighborhood  of  two 
groups  of  nerve-cells,  the  vagal  sensor  end-nuclei. 

The  axons  of  the  peripherally  directed  branches  unite  to  form  a  portion 
of  the  common  trunk,  which,  as  it  descends  the  neck  and  enters  the  thorax  and 
abdomen,  gives  oft"  a  number  of  branches  which  are  ultimately  distributed  to 
the  mucous  membrane  of  the  esophagus,  larynx,  lungs,  stomach,  and  in- 
testine, and  also  to  the  heart.  The  afferent  fibers  thus  serv'e  to  bring  into 
anatomic  and  physiologic  relation  the  mucous  membrane  of  these  organs 
and  certain  sensor  end-nuclei  in  the  medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  eft'erent  fibers  take  their  origin 
from  nerve-cells  located  in  the  lateral  portion  of  the  formatio  reticularis  at 
some  distance  below  the  floor  of  the  fourth  ventricle.  These  cells  constitute 
the  lower  portion  of  the  nucleus  ambiguus. 

Distribution. — From  their  origin  the  efferent  axons  pass  dorsally  to 
near  the  sensor  end-nuclei,  then  turn  outward  and  forward,  and  finally 
emerge  from  the  medulla  in  close  association  with  the  afferent  branches. 
They  are  ultimately  distributed  to  the  muscles  of  the  lower  two-thirds  of  the 
esophagus;  to  the  muscle-fibers  of  the  stomach  and  perhaps  the  intestines; 
to  the  walls  of  the  gall-bladder  and  to  the  sphincter  of  the  common  bile  duct; 
and  to  the  non-striated  muscle-fibers  of  the  bronchial  tubes,  and  to  the  heart. 
Among  the  efferent  fibers  are  some  which  are  distributed  to  the  gastric 
glands  and  to  the  pancreas  (?).  From  this  distribution  it  is  apparent 
that  the  eft'erent  fibers  in  the  vagus  are  largely  if  not  entirely  members  of 
the  autonomic  system  of  nerves. 

The  efferent  fibers  serve  to  bring  the  nerve-cells  from  which  they  arise 
into  anatomic  and  physiologic  connection  with  a  portion  of  the  musculature 


6oo 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  the  alimentary  canal  and  its  diverticulum,  the  lung  as  well  as  the  heart  and 
gastric  glands. 

Communicating  Branches. — At  or  near  the  ganglia  the  vagus  receives 
communicating  branches  from  the  eleventh  nerve,  the  spinal  accessory,  the 
facial,  the  hypoglossal,  and  the  anterior  branches  of  the  two  upper  cervical 
nerves.     Owing  to  this  manifold  origin  of  the  efferent  fibers  in  the  trunk  and 


Fig.  280. — Distribution  of  the  Pneumogastric. — i.  Trunk  of  the  left  pneumogastric. 
2.  Ganglion  of  the  trunk.  3.  Anastomosis  with  the  spinal  accessory.  4.  Anastomosis  with  the 
sublingual.  5.  Pharyngeal  branch  (the  auricular  branch  is  not  shown  in  the  figure) .  6.  Superior 
laryngeal  branch.  7.  External  laryngeal  nerve.  8.  Laryngeal  ple.xus.  9,  9.  Inferior  larj'ngeal 
branch.  10.  Cervical  cardiac  branch.  11.  Thoracic  cardiac  branch.  12,  13.  Pulmonary 
branches.  14.  Lingual  branch  of  the  fifth.  15.  Lower  portion  of  the  sublingual.  16.  Glosso- 
pharyngeal. 17.  Spinal  acessor}^  18,  19,  20.  Spinal  nerves.  21.  Phrenic  nerve.  22,  23. 
Spinal  nerves.     24,  25,  26,  27,  28,  29,  30.  Sympathetic  gangha. — (Hirsch/eld.) 

peripheral  branches  of  the  vagus,  it  is,  in  some  instances,  difficult,  if  not 
impossible,  to  determine  to  which  of  these  nerves  a  given  muscle  contraction 
is  to  be  referred. 

Vagal  Branches.- — As  the  vagus  passes  down  the  neck  it  gives  off  the 
following  main  branches  (Fig.  280) : 


THE  CRANIAL  NERVES.  6oi 

1.  The  pharyngeal  nerves,  which,  after  entering  into  the  formation  of  the 

pharyngeal  plexus,  are  distributed  to  the  mucous  membrane  and  to  the 
muscles  of  the  pharynx ;  e.g. ,  superior  and  inferior  constrictors,  the  levator 
palati,  and  the  azygos  uvulae ;  according  to  Beevor  and  Horsley  the  nerves 
for  these  muscles  are  branches  of  the  spinal  accessory. 

2.  The  esophageal  nerves,  which  after  entering  into  the  formation  of  the 

esophageal  plexus,  are  distributed  to  the  mucous  membrane,  and  to  the 
muscles  of  the  lower  two-thirds  of  the  esophagus. 

3.  The  superior  laryngeal  nerve  which,   entering  the  larynx  through  the 

thyro-hyoid  membrane,  is  distributed  to  the  mucous  membrane  lining 
the  interior  of  the  larynx  and  to  the  crico-thyroid  muscle.  From  the 
superior  laryngeal  and  the  main  trunk  small  branches  are  given  off 
which  in  the  rabbit  unite  to  form  a  single  nerve,  the  so-called  depressor 
nerve.  (See  page  375.)  It  is  distributed  to  the  heart-muscle.  Though 
this  anatomic  arrangement  is  not  found  in  man,  there  are  many  reasons  for 
believing  that  analogous  fibers  are  present  in  the  vagus  trunk  of  man  and 
other  animals. 

4.  The  inferior  laryngeal  nerve  which  is  distributed  utimately  to  all  the 

muscles  of  the  larynx  (except  the  crico-thyroid)  and  to  the  inferior  con- 
strictor of  the  pharynx.  These  motor  fibers  are  derived  from  the  spinal 
accessory. 

5.  The  cardiac  nerves  which,  after  entering  into  the  formation  of  the  cardiac 

plexus,  are  distributed  to  the  heart. 

6.  The  pulmonary  nerves  distributed  to  the  mucous  membrane  of  the  bronchial 

tubes  and  their  ultimate  terminations,  the  lobules  and  air-cells,  as  well 
as  to  their  non-striated  muscle-fibers. 

7.  The  gastric  and  intestinal  nerves,  distributed  to  the  mucous  membrane 

and  muscle  walls  of  the  stomach,  intestines,  and  gall-bladder.  Other 
fibers  in  all  probability  pass  to  the  liver,  spleen,  kidney,  and  suprarenal 
bodies. 

Properties  of  the  Pneiimogastric  or  Vagus  Nerve  and  its  Various 
Branches. — Faradization  of  the  vagus  nerve  close  to  the  medulla  oblongata 
gives  rise  to  sensations  of  pain  and  to  contraction  of  the  musculature  of  a 
portion  of  the  alimentary  tract,  viz. :  the  esophagus,  stomach,  and  possibly 
of  the  intestine  and  of  the  pulmonary  apparatus.  Division  of  the  nerve  is 
followed  by  a  loss  of  sensibility  in  the  mucous  membrane  of  the  alimentary 
tract  and  of  the  pulmonary  apparatus,  together  with  a  loss  of  motility  of 
the  structures  above  mentioned. 

Stimulation  of  the  trunk  of  the  nerve  in  dift'erent  parts  of  its  course  pro- 
duces a  variety  of  results  dependent  to  some  extent  on  the  presence  of  anas- 
tomosing branches  from  adjoining  nerves. 

The  Pharyngeal  Nerves. — Faradization  of  the  pharyngeal  nerves  consisting 
of  both  afferent  and  efferent  fibers,  gives  rise  to  sensations  of  pain,  contraction 
of  the  pharyngeal  muscles,  and  perhaps  to  vomiting.  Division  of  these  nerves 
is  followed  by  a  loss  of  sensibility  in  the  parts  to  which  they  are  distributed  and 
by  paralysis  of  the  muscles  with  a  consequent  impairment  of  deglutition. 

The  Esophageal  Nerves. — Faradization  of  the  esophageal  nerves,  gives 
rise  to  sensations  of  pain  and  to  contractions  of  the  muscle  coat  of  the  esoph- 
agus.    Division  of  these  nerves  is  followed  by  a  loss  of  sensibility  in  the  parts 


6o2  TEXT-BOOK  OF  PHYSIOLOGY. 

to  which  they  are  distributed,  a  partial  paralysis  of  the  muscle  coat  and  an 
impairment  of  deglutition. 

The  Superior  Laryngeal  Nerve. — Faradization  of  the  superior  laryngeal 
nerve  gives  rise  to  sensations  of  pain,  and  to  contraction  of  the  crico-thyroid 
muscle.  Through  reflected  impulses  it  causes  contraction  of  the  muscles 
of  deglutition,  and  of  the  muscles  concerned  in  the  act  of  coughing;  inhibi- 
tion of  the  inspiratory  movement  and  arrest  of  respiration  in  the  condition 
of  expiratory  standstill,  with  perhaps  a  tetanic  contraction  of  the  expiratory 
muscles,  and  contraction  of  the  laryngeal  muscles  with  closure  of  the  glottis. 
Peripheral  stimulation  of  this  nerve — e.g.,  the  contact  of  foreign 
particles — gives  rise  to  a  similar  series  of  phenomena.  Division  of  these 
nerves  is  followed  by  a  loss  of  sensibility  in  the  laryngeal  mucous  mem- 
brane, paralysis  of  the  crico-thyroid  muscle  with  a  consequent  lowering  of 
the  pitch,  and  a  diminution  in  the  clearness  of  the  voice.  In  consequence 
of  the  loss  of  the  sensibility  there  is  an  inability  to  perceive  the  entrance  of 
foreign  bodies  into  the  larynx. 

The  Depressor  Nerve. — Stimulation  of  the  peripheral  end  of  the  depres- 
sor nerve  is  without  effect;  stimulation  of  the  central  end  retards  and  even 
arrests  the  heart's  pulsations  and  lowers  the  general  blood-pressure.  These 
two  effects,  though  associated,  are  nevertheless  independent  of  each  other.  If 
the  vagus  nerves  be  divided  on  both  sides  between  the  origin  of  the  depres- 
sor and  the  origin  of  the  cardiac  nerves,  and  the  former  stimulated,  there 
will  be  a  fall  of  pressure  without  retardation  of  the  heart.  The  effect  on  the 
heart  is  attributed  to  a  stimulation  of  the  cardio-inhibitor  mechanism  in 
the  medulla  oblongata. 

The  fall  of  general  blood-pressure  was  formerly  attributed  to  a  sudden 
dilatation  of  the  splanchnic  blood-vessels  alone,  in  consequence  of  a  depres- 
sion of  that  portion  of  the  general  vaso-motor  center  which  maintains  through 
the  splanchnic  nerves  a  tonic  contraction  of  their  walls.  It  has  been  satis- 
factorily demonstrated  that  this  is  not  the  sole  cause;  for  after  division  of  the 
splanchnic  nerves,  stimulation  of  the  depressor  causes  a  still  further  fall  of 
from  30  to  40  per  cent,  in  the  general  pressure  (Porter  and  Beyer).  Evi- 
dently, not  any  one,  but  all  portions  of  the  vaso-motor  center  are  subject  to 
the  effects  of  depressor  stimulation. 

The  Inferior  Laryngeal  Nerves. — ^Faradization  of  the  inferior  laryngeal 
nerves  produces  effects  which  vary  in  accordance  with  the  strength  of  the 
stimulus,  with  different  animals,  and  with  the  same  animal  at  different  periods 
of  life.  In  the  adult  dog  and  in  man,  the  glottis  is  kept  widely  open  for 
respiratory  purposes  by  the  tonic  contraction  of  the  abductor  muscles  (the 
crico-arytenoids) ;  for  phonatory  purposes  the  glottis  is  closed  and  the 
vocal  membranes  approximated  by  the  contraction  of  the  adductor  muscles. 
It  has  been  shown  that  these  opposed  groups  of  muscles  have  independent 
nerve-supplies;  that  two  sets  of  fibers  in  the  common  trunk  can  be  separated 
and  stimulated  independently  of  each  other.  Feeble  stimulation  of  the  com- 
mon trunk  produces  a  still  further  abduction  of  the  vocal  cords.  With  an 
increase  in  the  strength  of  the  stimulus,  however,  the  reverse  obtains:  namely, 
adduction  which  increases  until  the  glottis  is  completely  closed.  Division 
of  the  nerves  is  followed  by  paralysis  of  both  the  phonatory  and  respiratory 
muscles,  the  abductors  and  adductors,  with  the  result  of  seriously  impairing 


THE  CRANIAL  NERVES.  603 

both  phonation  and  respiration  and  not  infrequently  causing  death.  The 
fibers  of  the  inferior  laryngeal  nerve  are  derived  from  the  eleventh  nerve, 
the  spinal  accessory. 

The  Cardiac  Nerves. — Faradization  of  the  trunk  of  the  vagus  or  of  the 
peripheral  end  of  the  divided  nerve  gives  rise  to  a  diminution  in  the  frequency 
and  force  of  the  heart's  contractions;  and  if  the  stimulation  be  sufficiently 
powerful,  completely  arrests  it  in  the  phase  of  diastole.  To  these  results  the 
term  inhibition  is  applied.  Division  of  the  vagi  or  of  the  cardiac  branches 
is  followed  by  an  increase  in  the  number  of  contractions  from  loss  of  inhibitor 
influences.  The  inhibitor  fibers  of  the  vagus  are  generally  believed  to  be 
derived  from  the  spinal  accessory,  though  this  has  been  questioned.  Accord- 
ing to  the  recent  investigations  of  SchaternikofT  and  Friedenthal,  they  come 
direct  in  the  vagus,  from  a  nucleus  near  the  vagal  motor  nucleus  in  the  med- 
ulla, the  spinal  accessory  sending  no  branches  to  the  heart.  In  the  frog 
and  other  batrachia  the  vagus  contains  also  accelerator  or  augmentor  fibers 
derived  from  the  sympathetic;  hence  stimulation,  especially  if  feeble,  may 
increase  the  heart's  action. 

The  Pulmonary  Nerves. — The  pulmonary  ner\TS,  given  off  from  the 
trunk  after  its  entrance  into  the  thorax,  do  not  lend  themselves  readily  to 
experimentation.  Division  of  both  vagi  in  the  neck  above  the  point  of  exit 
of  the  pulmonary  branches  is  followed  by  a  decrease  in  the  frequency  of  the 
respiratory  acts,  with  an  increase  in  their  depth.  At  the  same  time  there  is 
a  loss  of  sensibihty  of  the  mucous  membrane  of  the  trachea  and  lungs  and 
a  paralysis  of  non-striated  muscle-fibers. 

Stimulation  of  the  central  end  of  the  divided  vagus  with  weak  induced 
electric  currents,  increases  the  frequency,  but  decreases  the  amplitude,  of  the 
respiratory  movements.  This  would  indicate  that  in  the  physiologic  state 
these  nerve-fibers  conduct  afferent  nerve  impulses  that  inhibit  the  inspiratory 
discharge  and  lead  to  an  expiratory  movement  sooner  than  would  otherwise 
be  the  case.  If  the  stimulation  be  increased  in  intensity  the  inspiratory 
movement  gradually  so  exceeds  the  expiratory  that  the  inspiratory  muscles 
pass  into  the  condition  of  tetanus  and  the  chest  walls  come  to  rest  in  the 
condition  of  forced  inspiration. 

Feeble  stimulation  of  the  vagus  not  infrequently  inhibits  the  inspiratory 
movement  and  increases  the  expiratory  until  there  is  a  complete  cessation 
of  movement  in  the  condition  of  expiratory  standstill.  The  eff'ect  thus  pro- 
duced is  similar  to,  if  not  identical  with,  that  produced  by  stimulation  of  the 
superior  laryngeal  nerve.     (See  page  418.) 

Faradization  of  the  trunks  of  the  pulmonary  branches  or  stimulation  of 
their  peripheral  terminations  in  the  mucous  membrane  of  the  bronchial 
tubes  or  alveoli  by  the  inhalation  of  chemic  vapors  causes  arrest  of  respira- 
tory movements,  a  fall  of  blood-pressure,  and  a  reflex  inhibition  of  the  heart 
(Brodie). 

Gastric  Nerves. — Stimulation  of  the  peripheral  end  of  a  divided  vagus 
nerve  causes  a  distinct  contraction  of  the  right  half  of  the  stomach  and 
secretion  from  the  gastric  glands.  Division  of  the  nerve  abolishes  the  sen- 
sibility of  the  mucous  membrane  of  the  stomach,  impairs  motility,  and  inter- 
feres with  the  secretion  of  the  gastric  juice. 

Similar  experimentation  on  the  trunk  of  the  vagus  has  shown  that  the 


6o4  TEXT-BOOK  OF  PHYSIOLOGY. 

nerve  excites  contraction  of  the  upper  part  of  the  small  intestine  and  of  the 
gall-bladder,  the  secretion  of  the  pancreas,  the  renal  circulation,  the  secretion 
of  urine,  etc. 

Functions. — The  afferent  fibers  transmit  nerve  impulses  from  the  area 
of  their  distribution  to  the  medulla  and  thence  through  cortical  connections 
to  the  sensor  cerebral  areas,  where  they  evoke  sensations.  They  therefore 
endow  all  parts  to  which  they  are  distributed  with  sensibility. 

The  efferent  fibers  transmit  impulses  outward  which  excite  contraction 
of  the  muscle  of  the  lower  two-thirds  of  the  esophagus,  the  stomach,  the 
small  intestine,  and  the  gall-bladder,  and  the  muscles  of  the  bronchial  tubes; 
excite  secretion  from  the  glands  of  the  stomach,  pancreas,  and  kidney,  and 
exert  an  inhibitor  influence  on  the  activity  of  the  heart.  The  efferent  fibers 
belong  to  the  autonomic  system  of  nerves  and  are  not  connected  with  the 
ganglia  of  the  vagus,  but  with  local  peripheral  ganglia. 

The  afferent  fibers  also  assist  in  the  maintenance  of  certain  organic 
reflex  actions  which  are  highly  essential  to  the  life  of  the  individual,  e.g., 
respiration,  the  heart -beat,  blood-pressure,  etc.,  all  of  which  have  been  con- 
sidered in  foregoing  pages. 

ELEVENTH  NERVE.     THE  SPINAL  ACCESSORY. 

The  eleventh  cranial  nerve,  the  spinal  accessory,  consists  of  peripherally 
coursing  fibers  which  bring  the  nerve-cells  from  which  they  arise  into  relation 
with  separate  but  functionally  related  muscles,  such  as  those  entering  into 
the  formation  of  the  larynx.  It  consists  of  two  portions,  the  medullary  or 
bulbar  and  the  spinal. 

Origin. — The  axons  comprising  the  medullary  portion  arise  from  a  group 
of  nerve-cells  in  the  extreme  lower  part  of  the  nucleus  ambiguus,  known  as  the 
nidus  laryngei.  From  this  origin  the  axons  pass  forward  and  outward  to 
emerge  from  the  medulla  just  below  and  in  series  with  the  roots  of  the  vagus 
nerve. 

The  axons  comprising  the  spinal  portion  have  their  origin  in  nerve-cells 
in  the  lateral  margin  of  the  anterior  horn  of  the  gray  matter  in  the  cervical 
portion  of  the  cord  as  far  down  as  the  fifth  cervical  vertebra.  From  this 
origin  the  fibers  pass  to  the  surface  of  the  cord  to  emerge  between  the  ventral 
and  dorsal  roots  in  from  six  to  eight  filaments,  after  which  they  unite  from 
below  upward  to  form  a  distinct  nerve.  This  enters  the  cranial  cavity 
through  the  foramen  magnum,  where  it  joins  with  the  medullary  portion  to 
form  the  common  trunk,  which  then  passes  forward  to  emerge  from  the 
cranium  through  the  jugular  foramen.     (Fig.  281.) 

Distribution. — After  emerging  from  the  cranial  cavity  the  nerve  soon 
separates  into  two  branches: 

I.  An  internal  or  anastomotic  branch,  consisting  chiefly  of  filaments  coming 
from  the  medulla  oblongata.  It  soons  enters  the  trunk  of  the  vagus, 
from  which  fibers  pass  through  the  pharyngeal  plexus  to  the  superior  and 
inferior  constrictor  muscles  of  the  pharynx,  to  the  palato-pharyngeus, 
to  the  levator  palati  and  azygos  uvulae  muscles  (Beevor  and  Horsley) ;  to 
the  muscles  of  the  larynx  through  the  inferior  laryngeal  nerve,  and  to  the 
heart  according  to  some  authorities. 


THE  CRANIAL  NERVES. 


605 


2.  An  external  branch,  consisting  chiefly  of  the  accessory  fibers  from  the 

spinal  cord. 
It  is  distributed  to  the  sterno-cleido-mastoid  and  trapezius  muscles. 

Cortical  Connections. — The  nucleus  of  origin  of  the  medullary  branch 
at  least,  is  in  relation  with  nerve-cells  in 
the   lower  third  of  the  general  cerebral 
motor  area,  the  axons  of  which  descend 
in  the  pyramidal  tract. 

Properties. — Faradization  of  the  me- 
dullary portion  of  the  nerve  near  its 
origin  gives  rise  to  contraction  of  the 
muscles  to  which  it  is  distributed.  De- 
struction of  the  medullary  root  is  followed 
by  impairment  of  deglutition  from  a 
paralysis  of  the  muscles  of  the  pharynx 
and  palate  and  a  loss  of  the  power  of  pro- 
ducing vocal  sounds  on  account  of  pa- 
ralysis of  the  constrictor  muscles  of  the 
larynx.  According  to  some  authorities, 
there  is  also  an  acceleration  of  the  heart's 
action  from  a  loss  of  inhibitor  influences. 

Stimulation  of  the  external  branch 
gives  rise  to  contraction  of  the  sterno- 
cleido-mastoid  and  trapezius  muscles, 
though  division  of  the  branch  does  not 
give  rise  to  complete  paralysis,  as  they 
are  supplied  with  motor  fibers  also  from 
the  cervical  nerves.  In  consequence  of 
division  of  the  external  branch  animals 
experience  extreme  shortness  of  breath 
during  exercise,  from  a  want  of  coordina- 
tion of  the  muscles  of  the  fore-limbs  and 
the  muscles  of  respiration. 

Functions. — The  function  of  the 
fibers  of  the  spinal  accessory  nerve  is 
the  transmission  of  nerve  impulses  from 
the  cells  from  which  they  take  their 
origin  to  the  muscles  to  which  thev  are    pneumogastric.  i; 

*=>  .-  from    the    spinal    accessory  to  the  pneu- 

dlStributed.       1  hey    therefore     excite    to  mogastric.     ig.  Anastomosis  of  the  first 

action  some  of  the  muscles  of  deglutition ;  pair  of  cervical  nerves  w-ith  the  sublingual. 

,1  1  !_•    u  1    i.      ii-      i-         •  20.   Anastomosis  of  the  spinal  accessory 

the    muscles   which   regulate   the   tension  ,,iththesecondpairof  cervical  nerves.  21'. 

of  the  vocal  bands  during  phonation  and  Pharyngeal   plexus.      22.   Superior  laryn- 

the  muscles  which  control  the  respiratorv     §^^1  "^7f,-     ^3-  External  laryngeal  nerve. 
1       .  ,  .        ,      '      24.  Middle  cerncal  ganglion. — (Hirscli- 

movements  associated  with  sustained  or    .^/^ ) 

prolonged    muscle    efforts.     The    fibers 

may  also  convey  nerve  impulses  which  exert  an  inhibitor  influence  on  the 

heart. 


Fig.  281. — Spinal  .Accessory  Nerve. 
I.  Trunk  of  the  facial  nerve.  2.  2. 
Glosso-pharyngeal  nerve.  3,  3  Pneumo- 
gastric.  4,  4,  4.  Trunk  of  the  spinal  acces- 
sory. 5.  Sublingual  nerve.  6.  Superior 
cervical  ganglion.  7,  7.  Anastomosis  of 
the  first  two  cervical  nerves.  8.  Carotid 
branch  of  the  sympathetic.  9,  10,  11, 
12,  13.  Branches  of  the  glosso-pharyngeal. 
14,  15.  Branches  of  the  facial.  16.  Otic 
ganglion.  17.  Auricular  branch  of  the 
18.  Anastomosing  branch 


6o6 


TEXT-BOOK  OF  PHYSIOLOGY. 


TWELFTH  NERVE.     THE  HYPOGLOSSAL. 

The  twelfth  cranial  nerve,  the  hypoglossal,  consists  of  peripherally 
coursing  nerve-fibers  which  serve  to  connect  the  nerve-cells  from  which  they 
arise  with  the  musculature  of  the  tongue. 

Origin.— The  axons  composing  the  hypoglossal  nerve  arise  from  a  collec- 
tion of  nerve-cells  situated  beneath  the  floor  of  the  fourth  ventricle.  This 
nucleus  is  elongated  and  extends  from  the  medullary  striae  downward  as  far 
as  the  lower  border  of  the  olivary  body.     It  is  located  ventro-laterally  to  the 


Fig.  282. — Distribution  of  the  Hypoglossal  Nerve. — i.  Root  of  the  fifth  nerve.  2. 
Ganghon  of  Gasser.  3,  4,  5,  6,  7,  9,  10,  12.  Branches  and  anastomoses  of  the  fifth  nerve.  11. 
Submaxillary  ganglion.  13.  Anterior  beli/  of  the  Higastric  muscle.  14.  Section  of  the  mylo- 
hyoid muscle.  15.  Glosso-pharyngeal  Nerve,  j 6."  Ganglion  of  Andersch.  17,18.  Branches 
of  the  glosso-pharyngeal  nerve.  19,19.  Pneumogastric.  20,21.  Ganglia  of  the  pneumogastric. 
22,  22.  Superior  laryngeal  branch  of  the  pneumogastric.  23.  Spinal  accessory  nerve.  24. 
Sublingual  nerve.  25.  Descendens  noni.  26.  Thyro-hyoid  branch.  27.  Terminal  branches. 
28.  Two  branches  one  to  the  genio-hyo-glossus  and  the  other  to  the  genio-hyoid  muscle. — 
(Sappey.) 

spinal  canal.  After  leaving  the  cells  of  the  nucleus  the  axons  pass  forward 
and  outward  toward  the  surface  of  the  medulla,  from  which  they  emerge  in 
ten  or  twelve  small  bundles  or  filaments  in  the  groove  between  the  olivary 
body  and  the  anterior  pyramid.  Beyond  this  point  they  unite  to  form 
a  common  trunk. 

Distribution.- — The  common  trunk  thus  formed  passes  out  of  the  cranial 
cavity  through  the  anterior  condyloid  foramen.     In  its  course  it  receives 


THE  CRANIAL  NERVES.  607 

filaments  from  the  first  and  second  cervical  nerves,  the  sympathetic  and 
vagus.  It  is  finally  distributed  to  the  intrinsic  muscles  of  the  tongue  and  to 
the  genio-hyo-glossus,  hyo-glossus,  and  stylo-hyoid  muscles.  Branches 
derived  from  the  cervical  plexus  pass  to  muscles  which  elevate  and  depress 
the  hyoid  bone.     (Fig.  282.) 

Cortical  Connections.— The  hypoglossal  nerve  nuclei  are  connected 
with  nerve-cells  in  the  lower  third  of  the  general  motor  area  around  the  in- 
ferior termination  of  the  fissure  of  Rolando  by  axons  which  descend  in  the 
pyramidal  tract. 

Properties. — Faradization  of  the  nen-e  gives  rise  to  convulsive  move- 
ments of  the  muscles  to  which  it  is  distributed.  Division  of  the  nerve  is 
followed  by  a  loss  of  motion  and  an  interference  with  deglutition,  mastication, 
and  articulation,  especially  in  the  pronunciation  of  the  consonantal  sounds. 
In  hemiplegia,  complicated  with  paralysis  of  the  tongue  from  injury  to  the 
hypoglossal  tract,  the  opposite  side  of  the  tongue  is  involved  in  the  paralysis. 
On  protrusion  of  the  tongue  the  tip  is  deviated  to  the  paralyzed  side,  due  to 
the  unopposed  action  of  the  muscle  of  the  opposite  side. 

Function. — The  hypoglossal  nerve  transmits  nerve  impulses  from  its 
origin  to  the  intrinsic  and  extrinsic  muscles  of  the  tongue,  exciting  them  to 
activity.  The  coordinate  activity  of  these  muscles  favorably  assists  mastica- 
tion, articulation,  and  deglutition. 


CHAPTER  XXIV. 
THE  AUTONOMIC  OR  SYMPATHETIC  NERVE  SYSTEM. 

The  autonomic  nerve  system  consists  of  i,  the  sympathetic  ganglia  and 
their  branching  nerve-iibers,  and  2,  fine  medullated  nerve-libers  contained 
in  the  trunks  of  some  of  the  cranial  and  some  of  the  spinal  nerves,  w^hich 
serve  to  bring  the  nerve-cells  in  which  they  arise  into  relation  with  the  sym- 
pathetic ganglia.  The  tine-medullated  nerve-fibers  arising  in  cells  in  dif- 
ferent parts  of  the  central  nerve  system  and  passing  outward  in  the  trunks 
of  various  nerves  are  termed  from  their  relation  to  the  sympathetic  ganglia, 
pre-ganglionic  fibers;  the  non-medullated  fibers  arising  in  and  emerging  from 
the  sympathetic  ganglia  are  termed  post-ganglionic  fibers. 

This  system  of  nerves  is  distributed  almost  exclusively  to  the  epithelium 
of  secretor  organs  and  to  the  non-striated  muscle-fibers  in  the  walls  of  the 
blood-vessels,  including  the  striated  fibers  of  the  heart,  and  the  non-striated 
muscle-fibers  in  the  walls  of  the  hollow  viscera. 

Inasmuch  as  this  system  of  nerves  is  supposed  to  be  independent,  self- 
regulative,  or  autonomous  in  its  activity,  it  has  received  the  name  of  the 
autonomic  nerve  system. 

It  will  be  found  convenient  to  consider  first  the  sympathetic  ganglia  and 
the  distribution  of  their  post-ganglionic  fibers,  and  second,  the  origin,  course 
and  distribution  of  the  pre-ganglionic  fibers.  The  sympathetic  ganglia  may 
for  convenience  of  description  be  divided  into  three  groups:  viz.,  the  vertebral 
or  lateral,  the  pre-vertebral  or  collateral,  and  the  peripheral  or  terminal. 

The  vertebral  ganglia  are  arranged  in  the  form  of  chains,  one  on  each 
side  of  the  vertebral  column.  The  number  of  ganglia  in  the  chain  varies  in 
animals  of  different  and  in  animals  of  the  same  species.  In  man  the  number 
varies  from  20  to  22.  Each  chain  may  be  divided  into  a  cervical,  a  thoracic, 
a  lumbar,  a  sacral,  and  a  coccygeal  portion.  The  cervical  portion  is  usually 
described  as  consisting  of  three  ganglia — a  superior,  a  middle,  and  an  inferior. 
This  statement  is  open  to  question,  however,  as  the  middle  one  is  frequently 
absent  and  the  inferior  one  is  regarded  by  some  anatomists  as  belonging  to 
the  pre-vertebral  series.  The  thoracic  portion  consists  of  ten  or  twelve 
ganglia,  the  lumbar  and  sacral  portions  of  four  each  and  the  coccygeal 
portion  of  one,  the  so-called  ganglion  impar. 

The  pre-vertebral  ganglia  are  also  united  in  the  form  of  a  chain  situated 
in  the  abdominal  cavity.  The  ganglia  constituting  this  chain  are  known  as 
the  semilunar,  the  renal,  the  superior  and  inferior  mesenteric,  and 
hypogastric,  or  selvic. 

The  peripheral  ganglia  are  in  more  or  less  close  relation  with  the  tissues 
and  organs  in  different  parts  of  the  body.  As  members  of  this  group 
may  be  mentioned  the  ciliary  or  ophthalmic,  the  sphenopalatine,  the  otic, 
the  submaxillary  and  the  sublingual  ganglia;  the  ganglia  in  walls  of  the  heart, 
the  respiratory  organs,  the  stomach  and  intestines,  the  bladder,  etc. 

608 


THE  SYMPATHETIC  NERVE  SYSTEM. 


609 


Fig.  283. — Cervical  and  Thoracic  Portion  of  the  Sympathetic,  i,  i,  i.  Right  pneumo- 
gastric.  2.  Glosso-phanngeal.  3.  Spinal  accessory.  4.  Di\'ided  trunk  of  the  sublingual. 
5,  5,  5.  Chain  of  ganglia  of  the  sympathetic.  6.  Superior  cer\'ical  gangUon.  7.  Branches  from 
this  ganghon  to  the  carotid.  8.  Nerve  of  Jacobson.  9.  Two  filaments  from  the  facial,  one  to 
the  spheno-palatine  and  the  other  to  the  otic  ganghon.  10.  Motor  ocuh  e.xternus.  11.  Ophthal- 
mic ganglion,  recei\'ing  a  motor  filament  from  the  motor  oculi  communis  and  a  sensor}'  filament 
from  the  nasal  branch  of  the  fifth.  12.  Spheno-palatine  ganglion.  13.  Otic  ganghon.  14.  Lingual 
branch  of  the  fifth  nerve.  15.  Sub-ma.xillary  ganglion.  16,  17.  Superior  lar}mgeal  nerve.  18. 
External  laryngeal  nerve.  19,  20.  Recurrent  larj-ngeal  nerve.  21,  22,  23.  Anterior  branches 
of  the  upper  four  cer\'ical  nerves,  sending  filaments  to  the  superior  cervical  sympathetic  gan- 
glion.    24.  Anterior  branches  of  the  fifth  and  sixth  cervical  nerves,  sending  filaments  to  the  middle 


6io  TEXT-BOOK  OF  PHYSIOLOGY. 

The  general  arrangement  of  the  sympathetic  ganglia,  their  inter-connect- 
ing cords  and  branches,  is  shown  in  Figs.  285  and  286. 

Structure  of  the  Ganglia. — Each  ganglion  consists  of  a  capsule  or 
stroma  of  connective  tissue  in  which  are  contained  large  numbers  of  nerve- 
cells,  nerve-fibers,  medullated  and  non-medullated,  and  blood-vessels. 
The  nerve-cells  give  origin  to  two  or  more  dendrites,  which,  perforating  a 
nucleated  capsule  by  which  each  cell  is  surrounded,  branch  and  rebranch  and 
interlace  to  forma  pericapsular  plexus.  Each  cell  gives  origin  also  to  an  axon, 
which  as  it  leaves  the  cell  becomes  invested  with  a  sheath  continuous  with 
the  capsule  surrounding  the  cell-body.  It  is,  however,  wanting  in  a  medullary 
sheath,  and  hence  the  nerve  presents  a  gray  color.  Such  a  structure,  in  its 
entirety,  is  known  as  a  sympathetic  neuron. 

The  axonic  processes  as  they  emerge  from  the  cells  divide  and  sub- 
divide formirfg  ever  smaller  and  smaller  bundles  which  pass  in  different 
directions  to  regions  varying  in  position  according  to  the  situation  of  the 
ganglion  from  which  they  come.  The  branches  are  conventionally  termed 
rami,  communicantes  or  rami  viscerates  according  as  they  become  associated 
with  spinal  nerves  or  pass  directly  to  visceral  structures.  Whatever  the  route 
they  pursue,  it  has  been  shown  by  histologic  and  physiologic  methods  of 
investigation  that  they  are  ultimately  and  directly  distributed  to  but  two 
structures,  viz.,  non-striated  muscle  and  secretor  epithelium.  Moreover,  there 
is  no  evidence  to  warrant  the  assumption  that  these  structures  ever  re- 
ceive nerve  impulses  directly  from  the  spinal  or  cranial  nerves.  All  nerve 
impulses  that  influence  their  activities,  either  in  the  way  of  augmentation 
or  inhibition,  emanate  directly  though  not  primarily  or  originally  from  the 
sympathetic  ganglion  cells.  Since  non-striated  muscle-cells  are  found  in 
the  walls  of  the  blood-vessels,  in  the  walls  of  hollow  viscera  and  around 
hair  follicles,  and  since  secretor  epithelium  is  found  in  all  glands  there  is 
every  reason  to  believe  that  the  ganglia  in  some  way  are  associated  with 
vaso-augmentor  and  vaso-inhibitor,  viscero-augmentor  and  viscero-inhibitor , 
secreto-molor  and  s ecreto -inhibitor ,  and  pilo-motor  phenomena. 

Structure  of  the  Interconnecting  Cords. — The  interconnecting  cords 
are  composed  of  non-medullated  and  medullated  nerve-fibers.  The  former 
are  the  axons  of  cells  found  in  the  ganglia  more  centrally  located;  the  latter, 
as  will  be  stated  later,  are  derived  from  the  spinal  nerves,  from  the  fibers  of 
which,  however,  they  differ  in  character,  being  much  smaller  and  finer. 
The  fibers  of  the  interconnecting  cords,  as  a  rule,  transmit  nerve  impulses 
from  the  more  centrally  to  the  more  peripherally  located  ganglia,  and  are 
therefore  termed  rami  efferentes.     In  the  vertebral  chain  some  of  the  cords 


cervical  ganglion.  25,  26.  Anterior  branches  of  the  seventh  and  eighth  cervical  and  the  first 
dorsal  nerves,  sending  filaments  to  the  inferior  cervical  ganghon.  27.  Middle  cervical  ganglion. 
28.  Cord  connecting  tlie  two  gangUa.  29.  Inferior  cervical  ganghon.  30,  31.  Filaments  connect- 
ing this  with  the  middle  ganghon.  32.  Superior  cardiac  nerve.  t,t,.  Middle  cardiac  nerve. 
34.  Inferior  cardiac  nerve.  35,  35.  Cardiac  plexus.  36.  Ganglion  of  the  cardiac  plexus.  37. 
Nerve  following  the  right  coronary  artery.  38,  38.  Intercostal  nerves,  with  their  two  filaments 
of  communication  with  the  thoracic  gangha.  39,  40,  41.  Great  splanchnic  nerve.  42.  Lesser 
splanchnic  nerve.  43,  43.  Solar  plexus.  44.  Left  pneumogastric.  45.  Right  pneumogastric. 
46.  Lower  end  of  the  phrenic  nerve.  47.  Section  of  the  right  bronchus.  48.  Arch  of  the  aorta. 
49.  Right  auricle.  50.  Right  ventricle.  51,  52.  Pulmonary  artery.  53.  Right  half  of  the 
stomach.     54.  Section  of  the  diaphragm. — {Sappey.) 


THE  SYMPATHETIC  NEllVE  SYSTEM. 


6ii 


transmit  nerve  impulses  upward,  others  downward,  others  again  forward,  to 
the  pre-vertebral  and  peripheral  ganglia. 


Fig.  284. — Lumbar  axd  Sacral  Portions  of  the  Sympathetic,  i.  Section  of  the  dia- 
phragm. 2.  Lower  end  of  the  esophagus.  3.  Left  half  of  the  stomach.  4.  Small  intestine. 
5.  Sigmoid  flexure  of  the  colon.  6.  Rectum.  7.  Bladder.  8.  Prostate.  9.  Lower  end  of  the 
left  pneumogastric.  10.  Lower  end  of  the  right  pneuniogastric.  11.  Solar  plexus.  12.  Lower 
end  of  the  great  splanchnic  nerve.  13.  Lower  end  of  the  lesser  splanchnic  nerve.  14,  14.  Last 
two  thoracic  ganglia.  15,  15.  The  four  lumbar  gangha.  16,  16,  17,  17.  Branches  from  the 
lumbar  ganglia.  18.  Superior  mesenteric  plexus,  iq,  21,  22,  23.  Aortic  lumbar  plexiis.  20. 
Inferior  mesenteric  plexus.  24,  24.  Sacral  portion  of  the  sympathetic.  25,  25,  26,  26,  27,  27. 
Hypogastric  plexus.  28,  29,  30.  Tenth,  eleventh,  and  twelfth  dorsal  nerves.  31,  32,  33,  34,  35, 
36,  37,  38,  39.  Lumbar  and  sacral  nerves. — (Sappey.) 

Among   the  rami  efferentes  or  interconnecting  cords,  there  are  some 
which  possess  special  interest  for  the  physiologist,  \iz. : 


6x2  TEXT-BOOK  OF  PHYSIOLOGY. 

1.  The  cervical,   which  connects   the   thoracic  ganglia   with  the    superior 

cervical  ganglion.     It  is  composed  mainly  of  meduUatcd  nerve-fibers 
which  are  derived  originally  from  the  spinal  nerves. 

2.  The  great  splanchnic  nerve,  formed  by  the  union  of  branches  from  the  fifth 

to  the  tenth  thoracic  ganglia.     It  connects  these  ganglia  with  the  semi- 
lunar ganglion. 

3.  The  small  splanchnic  nerve,  formed  by  the  union  of  branches  from  the  ninth 

and  tenth  thoracic  ganglia.     It  connects  these  ganglia  with  the  solar 
and  renal  plexuses. 

THE  ANATOMIC  RELATIONS  OF  THE  SYMPATHETIC  GANGLIA  TO 
VISCERAL  STRUCTURES. 

The  Vertebral  Ganglia. — Each  ganglion  of  the  vertebral  chain  gives 
origin  to  one  or  more  gray  rami  communicantes,  which  pass  backward  and 
outward  and  enter  the  sheath  of  the  corresponding  spinal  nerve.  In  the 
cervical  region,  however,  where  the  ganglia  do  not  correspond  in  number 
with  the  cervical  spinal  nerves,  the  ganglia  give  off  two  or  more  gray  rami. 
Thus  in  man  the  superior  cervical  ganglion  sends  branches  to  the  first 
four  cervical  nerves.  The  middle  and  inferior  ganglia  send  a  branch  to  the 
fifth  and  sixth  and  the  seventh  and  eighth  cervical  nerves  respectively. 
In  the  thoracic,  lumbar,  and  sacral  regions,  the  ganglion  sends  at  least  one 
gray  ramus  into  the  sheath  of  the  corresponding  thoracic,  lumbar  or  sacral 
nerves. 

As  previously  stated,  the  gray  rami  which  thus  enter  the  sheath  of  the 
spinal  nerve  trunks,  pass  in  company  with  their  contained  efferent  fibers,  to 
the  periphery,  to  be  finally  distributed  to  structures  in  the  skin,  viz.,  non- 
striated  muscles  of  blood-vessels,  non-striated  muscles  of  hair-follicles, 
and  epithelium  of  sweat  glands.  Experimental  investigations  have  made  it 
apparent  that  these  post- ganglionic  fibers  may  be  regarded  as  having  vaso- 
motor and  secretor  functions.  The  blood-vessels  and  sweat  glands  of  the 
skin  of  the  neck  receive  their  ganglionic  nerve-supply  from  the  superior  and 
middle  cervical  ganglia;  those  of  the  skin  of  the  arm,  from  the  inferior  cervical 
and  first  thoracic  ganglia;  those  for  the  skin  of  the  trunk,  from  the  thoracic 
ganglia;  those  for  the  skin  of  the  hip  and  leg,  from, the  lumbar  and  upper 
sacral  ganglia;  those  for  the  skin  of  the  external  genital  organs,  from  the  lower 
sacral  ganglia. 

Most  if  not  all  the  vertebral  ganglia  give  origin,  in  addition  to  the  gray 
rami  communicantes  just  alluded  to,  other  branches  known  as  visceral 
'  branches  or  rami  viscerales  which  pass  to  regions  near  and  remote  though 
their  ultimate  distribution  is  not  in  all  instances  apparent. 

The  superior  cervical  ganglion  gives  off  from  its  cephalic  extremity  two 
visceral  branches,  which  subsequently  divide  and  subdivide  forming  the 
carotid  and  cavernous  plexuses ;  from  these  plexuses  slender  branches  follow 
the  course  of  the  more  superficial  arteries  at  least,  to  their  terminations, 
while  others  pass  into  the  trunks  of  the  trigeminal,  abducent,  and  the  superior 
and  deep  petrosal  branches  of  the  facial  nerve,  to  be  distributed  to  blood- 
vessels and  glands  of  special  regions  of  the  head  and  face.  Still  other 
branches  pass  down  the  neck  and  in  their  course  become  associated  with 
corresponding  branches  from  the  middle  and  inferior  cervical  ganglia.     In- 


THE  SYMPATHETIC  NERVE  SYSTEM.  613 

terlacing  in  an  intricate  manner  they  form  the  cardiac  plexuses.  With  the 
exception  of  fibers  arising  in  and  coming  from  the  inferior  cervical  gangUon 
there  is  no  reason  for  beUeving  that  the  branches  of  the  cardiac  plexus  are 
distributed  to  the  heart-muscle.  The  fibers  having  this  distribution  are 
derived  for  the  most  part  from  the  inferior  cervical  and  in  small  part  from 
the  first  thoracic  ganglion.  (See  page  305.)  The  thoracic,  lumbar,  and 
sacral  ganglia  also  give  off  visceral  branches  which  pass  for  the  most  part 
to  neighboring  structures,  though  from  the  lower  lumbar  and  sacral  ganglia 
branches  pass  to  viscera  in  the  lower  abdominal  and  pelvic  regions. 

In  accordance  with  the  law  of  distribution  and  relations  of  the  fibers  of  the 
sympathetic  ganglia  to  peripheral  organs,  it  can  be  assumed  that  to  whatever 
organ  the  visceral  branches  are  distributed  they  ultimately  terminate  in  the 
non-striated  muscle-cells  of  the  walls  of  the  blood-vessels  and  the  walls  of 
hollow  viscera,  and  in  some  situations  the  epithelium  of  glands  as  well. 

The  Pre-vertebral  Ganglia. — The  pre-vertebral  ganglia  are  located  in 
the  abdominal  cavity.  The  semilunar,  the  renal,  and  the  superior  mesenteric 
are  situated  in  the  neighborhood  of  the  coeliac  axis  and  on  a  level  with  the 
adrenal  bodies.  From  the  ganglia  an  enormous  number  of  visceral  branches 
are  given  off  which  interlace  in  a  very  intricate  manner  forming  what  is 
known  as  the  solar  plexus.  Subdivisions  of  this  plexus  taking  their  names 
from  the  regions  to  which  they  are  distributed  are  known  as  the  gastric, 
renal,  adrenal,  splenic,  hepatic,  and  superior  mesenteric.  The  terminals  of 
the  fibers  composing  these  plexuses  are  distributed  to  the  blood-vessels  of 
the  stomach,  kidney,  adrenal  body,  liver,  and  small  intestine;  to  the  muscle- 
walls  of  the  stomach  and  small  intestine  as  well  as  the  sphincter  muscles  sur- 
round the  gastro-duodenal,  the  pyloric  and  the  ileo-colic  orifices. 

From  the  inferior  mesenteric  ganglion  situated  close  to  the  origin  of  the 
inferior  mesenteric  artery  visceral  fibers  are  given  oft',  which  also  interlace 
to  form  the  hypogastric  plexus,  from  which  fibers  pass  to  the  muscle-walls 
of  the  colon,  bladder,  uterus,  vagina  and  to  the  blood-vessels  of  the  pelvic 
viscera. 

The  Peripheral  Ganglia. — The  peripheral  ganglia  as  previously  stated 
are  in  more  or  less  close  relation  with  tissues  and  organs  in  different  regions 
of  the  body.  Among  the  members  of  this  group  may  be  mentioned  the 
ciliary  or  opthalmic,  the  spheno-palatine,  the  otic  and  the  submaxillary 
ganglia;  the  ganglia  in  the  walls  of  the  heart,  the  respiratory  organs,  of  the 
stomach,  intestines  and  base  of  the  bladder  (the  pelvic).  The  situation 
of  the  first  four  of  the  ganglia,  (the  cephalic)  and  the  distribution  of  their 
visceral  branches  have  been  considered  in  connection  with  the  oculo-motor, 
facial,  and  glosso-pharyngeal  nerves.  The  ganglia  of  the  heart  and  intestines 
have  been  considered  in  connection  with  the  physiologic  action  of  these 
organs. 

THE    ANATOMIC    RELATION    OF    THE    CENTRAL    NERVE    SYSTEM  TO 
THE  SYMPATHETIC  GANGLIA. 

The  central  nerve  system  is  associated  with  the  sympathetic  ganglia 
through  the  intermediation  of  fine  medullated  nerve-fibers  which  have 
their  origin  in  nerve-cells  located  in  three  dift'erent  regions,  viz.:     i.  in 


6i4  TEXT-BOOK  OF  PHYSIOLOGY. 

the  lateral  portion  of  the  gray  matter  of  the  spinal  cord  from  the  level 
of  the  first  thoracic  nerve  to  the  level  of  the  third  or  fourth  lumbar 
nerves;  2.  in  the  gray  matter  beneath  the  aqueduct  of  Sylvius  just  where  it 
enlarges  to  form  the  cavity  of  the  third  ventricle  and  in  the  gray  matter  beneath 
the  floor  of  the  fourth  ventricle;  and  3.  in  the  gray  matter  of  the  spinal  cord 
at  the  levels  of  origin  of  the  second,  third,  and  fourth  sacral  nerves.  The 
nerve-fibers  which  thus  associate  the  spinal  cord  with  the  ganglia  are  termed 
pre -ganglionic  fibers  in  contradistinction  to  those  fibers  associating  the 
ganglia  with  the  tissues  and  organs  which  are  termed  post-ganglionic  fibers. 

1.  The  pre-ganglionic  fibers  that  have  their  origin  in  nerve-cells  in  the  lateral 

portion  of  the  gray  matter  of  the  spinal  cord  emerge  from  the  cord  in 
the  ventral  roots  of  the  spinal  nerves  from  and  including  the  first 
thoracic  to  the  third  lumbar  nerves.  In  association  with  the  large 
efferent  fibers  composing  these  roots,  the  fine  medullated  fibers  pass 
outward  to  the  point  at  which  the  spinal  nerve,  formed  by  the  union  of 
the  ventral  and  dorsal  roots,  separates  into  an  anterior  and  a  posterior 
division.  At  this  point  the  fine  medullated  fibers  leave  the  spinal  nerve, 
and  after  a  short  course  enter  the  ganglia  of  the  vertebral  chain  where 
some  of  the  fibers  terminate  in  ganglia  at  the  same  and  different  levels 
while  others  pass  forward  to  terminate  in  the  ganglia  of  the  pre-vertebral 
chain.  On  entering  the  ganglia  the  peripheral  branches  of  the  fibers 
arborize  around  the  nerve-cells  composing  them.  The  short  nerve- 
strands  that  pass  from  the  spinal  nerves  to  the  ganglia  are  termed 
from  their  color  white  rami  commiinicantes . 

In  accordance  with  their  distribution,  as  determined  by  histologic 
and  physiologic  methods  of  investigation,  these  pre-ganglionic  fibers 
may  be  divided  into  seven  groups,  viz. :  i.  those  passing  up  the  vertebral 
chain  to  the  superior  cervical  ganglion;  2.  those  passing  to  the  inferior 
cervical  ganglion;  3.  those  passing  to  the  first  thoracic  ganglion; 
4.  those  passing  directly  to  the  thoracic  and  upper  lumbar  ganglia;  5. 
those  passing  down  the  vertebral  chain  to  the  lower  lumbar  and  sacral 
ganglia;  6.  those  which  pass  through  the  thoracic  portion  of  the  verte- 
bral chain  and  forward  to  the  ganglia  of  the  pre-vertebral  chain, 
(the  semilunar,  renal,  and  superior  mesenteric)  which  in  their  course 
are  known  as  the  splanchnic  nerves;  7.  those  passing  through  the 
lumbar  portion  of  the  vertebral  chain  and  forward  to  the  inferior 
mesenteric  ganglion  in  which  for  the  most  part  they  terminate.  These 
nerves  are  sometimes  termed  the  inferior  splanchnics. 

2.  The  pre-ganglionic  fibers  that  arise  from  ner\''e-cells  in  the  gray  matter 

beneath  the  aqueduct  of  Sylvius  enter  the  trunk  of  the  third  or  oculo- 
motor nerve,  pass  forward  in  it,  as  far  as  the  interior  of  the  orbit  cavity, 
where  they  leave  the  nerve  and  terminate  around  the  cells  composing 
the  ciHary  ganglion.     (See  page  580.) 

The  pre-ganglionic  fibers  that  arise  from  nerve-cells  in  the  gray  matter 
beneath  the  floor  of  the  fourth  ventricle,  leave  by  three  routes,  viz.,  in 
the  trunks  of  the  pars  intermedia  or  nerve  of  Wrisberg,  the  glosso- 
pharyngeal, and  the  vagus. 

The  fibers  that  leave  in  the  pars  intermedia  enter  the  facial  nerve 
and  subsequently  pass  by  way  of  the  great  superficial  petrosal  nerve  to 


THE  SYMPATHETIC  NERVE  SYSTEM.  615 

the  spheno-palatine  ganglion  and  by  way  of  the  chorda  tympani  to  the 
submaxillary  and  sublingual  ganglia.     The  fibers  that  leave  the  glosso- 
pharyngeal nerve  pass  into  the  tympanic  branch  or  nerve  of  Jacobson 
and  ultimately  terminate  around  the  otic  ganglion.     The  fibers  that 
leave  by  the  vagus  nerve  pass  to  the  ganglia  in  the  heart,  stomach,  and 
small  intestine. 
3.  The  pre-ganglionic  fibers  that  arise  from  nerve-cells  in  the  gray  matter  of 
the   sacral   division  of   the   spinal   cord   enter  the  ventral  roots  of  the 
second,  third,  and  occasionally  fourth  sacral  nerves.     In  the  pelvis  these 
fibers  leave  the  sacral  nerves,  enter  the  pudendal  or  pelvic  nerv^e  and  are 
finally  distributed  to  ganglia  in  the  pelvic  cavity  associated  with  pelvic 
viscera  and  the  external  generative  organs. 
Afferent  Sympathetic  Fibers. — With  the  foregoing  groups  of  efi'erent 
fibers,  the  sympathetic  nerves,  in  the  thoracic  and  lumbar  regions  more 
especially,  contain  a  number  of  afferent  fibers  which  when  stimulated  give 
rise  to  sensations  of  pain  or  to  reflex  phenomena.     The  routes  by  which  these 
afferent  fibers  reach  the  spinal  cord  lead  through  the  white  rami  into  the 
spinal  nerve,  thence  into  the  dorsal  roots  to  the  spinal  ganglia,  where  they 
have  their  cells  of  origin.     The  number  of  afferent  fibers  in  any  trunk  in 
comparison  with  the  efferent  is  quite  small. 

FUNCTIONS  OF  THE  AUTONOMIC  OR  SYMPATHETIC  SYSTEM. 

The  view  according  to  which  the  sympathetic  ganglia  are  to  be  regarded 
as  independent  organs  endowed  with  functions  of  their  own  and  in  nowise 
directly  dependent  for  their  activities  on  the  spinal  cord,  is  at  the  present  time 
discarded.  Peripheral  structures  cease  to  exhibit  their  characteristic  func- 
tions after  division  of  the  spinal  nerves  in  connection  with  their  related 
ganglia.  This  does  not  exclude  the  possibility  of  the  sympathetic  cell-body, 
in  virtue  of  the  interchanges  between  it  and  the  blood  and  lymph  by  which 
it  is  surrounded,  maintaining  its  own  nutrition  and  exerting  a  favorable 
influence  over  the  nutrition  of  the  peripheral  tissues  to  which  its  post- 
ganglionic branches  are  distributed. 

The  nerve-tissue  in  its  entirety  may  be  regarded  as  a  single  system  which 
may  be  functionally  divided  into  a  ner^^e  system  of  animal  and  a  nerve 
system  of  vegetative  life,  according  as  the  nerve  energies  originating  in  and 
emanating  from  the  central  nervous  system  are  transmitted  directly  to  the 
skeletal  muscles  or  indirectly,  through  the  inter\^ention  of  a  sympathetic 
neuron,  to  visceral  muscles  and  glands.  In  the  former  system  but  one  neu- 
ron, the  spino-peripheral,  connects  the  spinal  cord  proper  with  the  muscle; 
in  the  latter  system  there  are  two,  the  spino-ganglionic  and  the  ganglio- 
peripheral. 

From  the  distribution  of  the  post-ganglionic  fibers  it  may  be  inferred  that 
the  activities  of  the  vascular  and  visceral  muscles,  either  in  the  way  of  aug- 
mentation or  inhibition,  the  activities  of  the  muscles  of  the  hair-follicles,  and 
of  the  epithelium  of  glands,  are  called  forth  by  the  ganglia  in  consequence  of 
the  arrival  of  nerve  impulses  coming  from  the  spinal  cord  through  the  pre- 
ganglionic fibers.  Experimental  observations  show  this  to  be  true.  The 
extent  to  which  these  difi"erent  modes  of  activity  manifest  themselves  in 


6i6  TEXT-BOOK  OF  PHYSIOLOGY. 

one  or  more  regions  of  the  body  will  depend  to  some  extent  on  the  portion 
of  the  sympathetic  system  subjected  to  ex])erimental  procedures. 

The  Functions  of  the  Cervical  Portion. — If  the  sympathetic  cord 
central  to  the  superior  cervical  ganglion  be  stimulated  with  the  induced 
electric  current,  among  the  resulting  phenomena  there  will  be  observed 
dilatation  of  the  pupil,  retraction  of  the  nictitating  membrane  in  animals 
possessing  it,  contraction  of  the  blood-vessels  of  the  skin  and  mucous  mem- 
brane in  different  parts  of  the  head,  neck,  and  face,  contraction  of  the  blood- 
vessels of  the  salivary  glands,  increase  of  secretion  from  the  submaxillary 
gland,  and  the  perspiratory  and  mucous  glands,  erection  of  hairs  in  different 
localities  of  the  head  and  neck,  and  in  the  dog  dilatation  of  the  blood-vessels 
of  the  lips,  gums,  and  hard  palate.  If  the  cervical  cord  be  divided,  opposite 
effects  will  be  observed:  viz.,  contraction  of  the  pupil,  dilatation  and  passive 
congestion  of  the  blood-vessels,  a  rise  in  temperature,  and  a  loss  of  the  power 
of  erecting  hairs.  Stimulation  of  the  peripheral  end  causes  a  disappearance 
of  the  latter  and  a  reappearance  of  the  former  phenomena.  These  facts 
indicate  that  the  cervical  portion  is  not  only  efferent  in  function  but  that 
it  transmits  both  vaso-constrictor  and  vaso-dilatator  fibers  for  blood-vessels, 
secretor  fibers  for  the  salivary  and  mucous  glands,  and  fibers  for  the  dilatator 
muscle  of  the  iris.  The  fibers  composing  it  are  pre-ganglionic  meduUated 
nerve-fibers  coming  from  the  spinal  cord  from  the  first  to  the  fourth  thoracic 
nerves.  From  these  several  sources  the  fibers  pass  by  way  of  the  white  rami 
into  the  vertebral  chain,  and  thence  without  interruption,  to  the  superior 
cervical  ganglion,  in  and  around  the  cells  of  which  their  end-tufts  arborize 
in  their  characteristic  manner. 

That  the  superior  cervical  ganglion  is  the  cell  station  between  the  spinal 
cord  and  the  peripheral  organs  is  shown  by  the  fact  discovered  and  applied 
by  Langley  that  the  intravenous  injection  of  nicotin  or  the  local  application 
of  it  to  the  ganglion  itself,  impairs  the  conductivity  of  the  terminals  of  pre- 
ganglionic fibers,  after  which  their  stimulation  has  no  effect  on  the  ganglion 
cells,  though  the  latter  retain  their  activity,  as  shown  on  direct  stimulation. 
Of  the  nerve-centers  in  the  spinal  cord  which  through  pre-ganglionic  fibers 
influence  peripheral  structures,  some  appear  to  be  in  a  state  of  constant 
activity:  e.g.,  the  vaso-constrictor  centers  and  the  pupillo-dilatator  centers. 
In  how  far  this  action  is  automatic  or  autochthonic,  or  reflex,  is  uncertain. 
The   Functions   of  the  Thoracic   Portion. — The  phenomena  which 
follow  stimulation  of  this  portion  of  the  sympathetic  system  resemble  in  a 
general  way  those  observed  in  the  head  when  the  cervical  portion  is  stimu- 
lated, viz.,  contraction  and  at  times  dilatation  of  the  blood-vessels  and  a 
secretion  of  sweat  and  in  some  animals  erection  of  hairs.     The  situation  of 
the  resulting  phenomena  will  vary  in  accordance  with  the  part  the  subject 
of  the  experiment.     For  an  understanding  of  the  results  of  experiment  the 
origin  and  distribution  of  the  following  nerve-branches  must  be  kept  in  view: 
(a)  The  cardiac  nerves  which  take  their  origin  in  part  in  cells  in  the  first 
thoracic  or  stellate  ganglion,  and  in  part  in  cells  in  the  inferior  cervical 
ganglion.     From  this  origin   they  pass  downward   and   forward  and 
reach  the  heart  by  way  of  the  cardiac  plexus.     Stimulation  of  these 
nerv^es  gives  rise  to  an  increased  frequency  and  an  augmentation  in  the 
force  of  the  heart-beat.     The  pre-ganglionic  fibers  by  which  these  cells 


THE  SYMPATHETIC  NERVE  SYSTEM.  617 

are  excited  to  activity  emerge  from  the  cord  by  the  first  and  second 
thoracic  nerves.  Stimulation  of  the  white  rami  of  these  nerves  gives 
rise  to  the  same  results. 
{h)  The  splanchnic  nerves,  the  roots  of  which  emerge  from  the  fourth  to  the 
tenth  or  eleventh  thoracic  ganglia.  The  fibers  composing  these  nerves 
are  for  the  most  part  pre-ganglionic  and  derived  from  the  corresponding 
spinal  nerves.  The  cell  stations  of  the  splanchnic  fibers  are  in  the 
semilunar,  superior  mesenteric,  and  renal  ganglia.  From  these  ganglia 
non-medullated  post-ganglionic  fibers  pass  peripherally  to  the  walls  of 
the  intestines,  the  blood-vessels  of  the  intestines,  liver,  kidneys,  spleen, 
etc.  Stimulation  of  the  great  splanchnic  produces  inhibition  of  the 
gastric  and  intestinal  movements  and  a  loss  of  tone,  though  occasionally 
there  is  a  slight  opposite  eft'ect,  namely  an  augmentation  of  the  move- 
ments, a  marked  primary  contraction  of  the  intestinal  blood-vessels  and 
other  viscera,  followed  by  dilatation,  coincidently  with  which  there  is  a 
primary  rise  succeeded  by  a  fall  of  blood-pressure  throughout  the  body. 
Division  of  the  nerve  is  followed  by  dilatation  of  the  intestinal  vessels 
and  a  fall  of  blood-pressure.  Stimulation  of  the  central  end  of  the  divided 
nerve  excites  the  activity  of  the  general  vaso-motor  center,  as  shown 
by  the  rise  of  the  general  blood-pressure.  Stimulation  of  the  smaller 
splanchnics  gives  rise  to  a  slight  primary  contraction  of  the  blood-vessels, 
soon  followed  by  a  marked  dilatation.  These  facts  indicate  that  the 
splanchnic  nerves  contain  visceral  nerves  which  inhibit  and  at  times 
augment  intestinal  movements,  vaso-motor  fibers  both  augmentor  and 
inhibitor,  secretor  nerves  for  the  intestinal  glands  and  for  the  adrenal 
glands, 
(c)  The  cutaneous  nerves  for  the  trunk  leave  the  lateral  ganglia  by  the  gray 
rami,  enter  the  thoracic  spinal  nerves,  and  pass  in  company  with  them 
to  their  terminations,  to  be  ultimately  distributed  to  the  walls  of  the 
blood-vessels,  the  arrectores  pilorum  muscles,  and  the  sweat-glands. 
The  pre-ganglionic  fibers  come  from  the  spinal  nerves  by  way  of  the 
white  rami.  Stimulation  of  either  the  white  or  gray  rami  gives  rise  to 
contraction  of  blood-vessels,  erection  of  hairs  and  a  secretion  of  sweat. 
Their  functions  are  therefore  vaso-motor,  pilo-motor,  and  secreto-motor. 
{d)  The  cutaneous  nerves  for  the  fore-limbs  have  their  origin  from  cells 
in  the  stellate  ganglion  (first  dorsal).  After  a  short  upward  course  they 
enter  the  trunks  of  the  nerve  composing  the  brachial  plexus.  The 
pre-ganglionic  fibers  come  from  the  white  rami  of  the  fourth  to  the 
ninth  thoracic  nerves.  After  entering  the  lateral  chain  they  take  an 
upward  direction  and  arborize  around  the  cells  of  the  stellate  ganglion. 
In  the  brachial  and  in  the  sciatic  nerves  as  well  vaso-motor  fibers  (con- 
strictors and  dilatators)  and  secretor  fibers  are  present,  as  shown  by 
experimental  methods  (see  page  488). 
The  Functions  of  the  Lumbo-sacral  Portion. — In  the  lumbar  region 
the  vertebral  chain  contains  a  number  of  pre-ganglionic  fibers  which  have 
descended  from  the  thoracic  region  as  well  as  fibers  which  have  come  into 
the  chain  by  the  white  rami  from  the  lumbar  nerves  themselves.  Some  of 
these  fibers  pass  through  the  chain  in  a  manner  similar  to  the  splanchnic 
nerves  in  the  thoracic  region  to  reach  the  inferior  mesenteric  ganglion,  in 


6i8  TEXT-BOOK  OF  PHYSIOLOGY. 

which  they  find  their  cell  station.  For  this  reason  these  nerves  are  sometimes 
termed  the  injerior  splanchnics.  Stimulation  of  these  nerves  as  well  as  of  the 
ganglion  and  its  branches  gives  rise  to  contraction  of  the  blood-vessels  of  the 
pelvic  viscera,  contraction  of  the  detrusor  muscle  of  the  bladder,  contraction 
of  the  muscle-fibers  of  the  uterus  and  vagina,  and  inhibition  of  the  circular 
and  longitudinal  fibers  of  the  large  intestine. 

The  cutaneous  nerves  for  the  hind-limbs  are  derived  from  the  lower 
lumbar  and  the  upper  sacral  ganglia.  They  also  enter  the  spinal  nerves 
by  the  gray  rami  and  pass  to  the  blood-vessels  and  glands  of  the  skin.  The 
pre-ganglionic  fibers  come  from  the  twelfth  thoracic  to  the  third  lumbar 
nerves. 

The  phenomena  that  follow  stimulation  of  this  portion  of  the  vertebral 
sympathetic  chain  resemble  in  a  general  way  those  that  follow  stimulation 
of  the  thoracic  portion  for  the  reason  that  the  post-ganglionic  fibers  are  dis- 
tributed to  similar  structures.  Thus  from  the  lumbar  and  upper  sacral 
ganglia  gray  rami  enter  the  lumbar  and  sacral  nerves  to  be  distributed 
ultimately  to  the  blood-vessels  and  sweat  glands  of  the  skin  of  the  hip  and 
leg.  Stimulation,  therefore,  of  these  nerves  gives  rise  to  contraction  of  the 
blood-vessels  and  a  secretion  of  sweat  of  the  corresponding  parts.  If  the 
stimulation  with  the  induced  current  be  slow,  dilatation  of  blood-vessels 
may  also  be  observed,  a  fact  that  indicates  that  these  nerves  also  carry 
vaso-dilatator  fibers.  The  pre-ganglionic  fibers  descend  the  vertebral  chain 
having  entered  it  mainly  from  the  lumbar  nerves.  Stimulation,  therefore, 
of  the  lumbar  chain  gives  rise  to  the  same  effects  as  stimulation  of  the 
post-ganglionic  fibers. 

The  Functions  of  the  Peripheral  Ganglia. — The  ganglia  situated  in  the 
head  are  usually  described  in  connection  with  and  as  constituent  parts  of  the 
cranial  nerve  system.  They,  however,  bear  the  same  relation  to  the  cranial 
nerves  that  the  ganglia  of  the  trunk  bear  to  the  spinal  nerves.  They  consist 
of  ganglion  cells  jrom  which  post-ganglionic  fibers  pass  to  glands,  blood- 
vessels, and  non-striated  muscles,  and  to  which  pre-ganglionic  fibers  pass 
from  the  cranial  nerves.  Motor  and  sensor  nerves  pass  through  one  or 
more  ganglia,  though  they  have  no  anatomic  connection  with  them.  In 
their  structure,  distribution,  and  functions  they  closely  resemble  the  col- 
lateral ganglia  of  the  abdominal  sympathetic: 

I.  The  ciliary  or  ophthalmic  ganglion  is  situated  in  the  orbital  cavity  posterior 
to  the  eyeball.  It  is  small  in  size,  gray  in  color,  and  consists  of  a  con- 
nective-tissue stroma  containing  nerve-cells.  From  these  cells  post- 
ganglionic fibers  emerge  which,  after  a  short  course  forward,  penetrate 
the  eyeball  and  terminate  in  the  circular  fibers  of  the  iris  and  the  ciliary 
muscle.  Pre-ganglionic  fibers  of  small  size,  and  similar  in  their  ana- 
tomic features  to  the  fibers  of  the  white  rami  of  the  spinal  nerves,  leave 
the  motor  oculi  by  a  short  root  from  the  inferior  division  and  arborize 
around  the  ganglion  cells.  Stimulation  of  the  pre-ganglionic  fibers 
gives  rise  to  contraction  of  the  circular  fibers  of  the  iris,  with  a  diminution 
in  the  size  of  the  pupil,  and  contraction  of  the  ciliary  muscle  with  accom- 
modation of  the  eye  for  near  vision.  Division  of  these  fibers  is  followed 
by  the  opposite  results.  Post-ganglionic  fibers  from  the  superior  cervical 
ganglion   which   come   through  the   cavernous   plexus   pass   through 


THE  SYMPATHETIC  NERVE  SYSTEM.  619 

the  ciliary  ganglion  to  the  blood-vessels  of  the  iris  and  retina  and  are 
vaso-constrictor  in  function.  Sensor  fibers  from  the  peripheral  division 
of  the  fifth  nerve  pass  to  the  cornea  and  endow  it  with  sensibility. 

2.  The  spheno-palatine  ganglion  is  situated  in  the  spheno-maxillary  fossa. 

Its  nerve-cells  send  non-medullated  post-ganglionic  fibers  to  the  blood- 
vessels and  glands  of  the  mucous  membrane  of  the  nasal  and  oral 
regions.  Stimulation  of  the  ganglion  gives  rise  to  dilatation  of  the  blood- 
vessels and  increase  of  secretion  in  this  entire  region.  The  pre-ganglionic 
fibers  are  derived  from  the  seventh  or  facial  nerve  by  way  of  the  great 
petrosal.  Sensor  fibers  from  the  superior  maxillary  division  of  the 
fifth  nerve  pass  through. the  ganglion  to  the  same  regions. 

3.  The  otic  ganglion  is  situated  just  below  the  foramen  ovale  and  internal 

to  the  third  division  of  the  fifth  nerve.  The  post-ganglionic  fibers 
pass  !o  the  parotid  gland  by  way  of  the  auriculo-temporal  division  of  the 
fifth  nerve,  and  to  the  blood-vessels  of  the  mucous  membrane  of  the 
lower  lip,  cheek,  and  gums.  The  preganglionic  fibers  are  derived  from 
the  efferent  fibers  in  the  glosso-pharyngeal  or  ninth  nerve,  by  way  of 
Jacobson's  nerve  and  the  small  petrosal.  Stimulation  of  these  nerves 
in  any  part  of  their  course  gives  rise  to  vascular  dilatation  and  increase 
of  secretion  in  the  region  of  the  distribution.  Motor  fibers  from  the 
small  or  motor  root  of  the  fifth  nerv^e  pass  through  this  ganglion  to  the 
tensor  tympani  muscle. 

4.  The  submaxillary  and  sublingual  ganglia  are  situated  close  to  the  corre- 

sponding glands.  Their  post-ganglionic  fibers  pass  to  the  blood-vessels 
and  gland-cells.  The  pre-ganglionic  fibers  are  derived  from  the  seventh 
or  facial  nerve  through  the  chorda  tympani  branch.  Stimulation  of  the 
chorda  or  of  the  ganglia  themselves  gives  rise  to  marked  dilatation  of 
the  blood-vessels  and  an  increased  flow  of  saliva.  It  therefore  contains 
vaso-dilatator  and  secretor  fibers  for  these  glands.  Vaso-constrictor 
and  a  few  secretor  nerves,  it  will  be  recalled,  come  to  these  glands  from 
the  superior  cervical  ganglion. 

5.  The  cardiac  ganglia  are  situated  in  diiferent  regions  in  the  walls  of  the 

heart;  their  visceral  branches  are  distributed  directly  to  the  heart 
muscle-cells.  Stimulation  of  these  ganglia  inhibit  the  action  of  the  heart. 
The  pre-ganglionic  fibers  for  these  ganglia  are  contained  in  the  trunk 
of  the  vagus  (pages,  305,  599).  Stimulation  of  the  vagus  has  a 
similar  inhibitor  action  on  the  heart. 

6.  The  pelvic  ganglia,  lying  at  the  base  of  the  bladder  are  by  reason  of  their 

position  and  relations  somewhat  inaccessible  to  direct  experimentation. 
The  preganglionic  fibers  in  connection  with  them  are  contained  in  part 
in  the  pudendal  or  pelvic  nerve.  Stimulation  of  the  post-ganglionic  and 
of  the  pre-ganglionic  nerves  gives  rise  to  a  marked  dilatation  of  the  blood- 
vessels of  the  penis,  clitoris,  vulva,  contraction  of  the  muscles  of  the 
bladder,  rectum,  etc. 


CHAPTER  XXV. 

PHONATION;  ARTICULATE  SPEECH. 

Phonation,  the  emission  of  vocal  sounds,  is  accomplished  by  the  vibration 
of  two  elastic  membranes  which  cross  the'  lumen  of  the  larynx  antero- 
posteriorly  and  whicli  are  thrown  into  vibration  by  a  blast  of  air  from  the 
lungs. 

Articulate  speech  is  a  modification  of  the  vocal  sounds  or«the  voice 
produced  by  the  teeth  and  the  muscles  of  the  lips  and  tongue  and  is  employed 
for  the  expression  of  ideas.  ^ 

The  larynx,  the  organ  of  the  voice,  is  situated  in  the  fore  part  of  the  neck, 
occupying  the  space  between  the  hyoid  bone  and  the  upper  extremity  of  the 
trachea.  In  this  situation  it  communicates  with  the  cavity  of  the  pharynx 
above  and  the  cavity  of  the  trachea  below.  From  its  anatomic  relations  and 
its  interna'  structure — the  interpolation  of  the  elastic  membranes — the 
larynx  subserves  the  two  widely  different  yet  related  functions,  respiration 
and  phonation. 

THE  ANATOMY  OF  THE  LARYNX. 

The  larynx  consists  primarily  of  a  series  of  cartilages  united  one  with 
another  in  such  a  manner  as  to  form  a  more  or  less  rigid  framework,  yet 
possessing  at  its  different  joints,  a  certain  amount  of  motion;  and  secondarily, 
of  muscles  and  nerves  which  conjointly  impart  to  the  cartilages  the  degree 
of  movement  necessary  to  the  performance  of  the  laryngeal  functions.  It 
is  covered  externally  by  fibrous  tissue  and  lined  throughout  by  mucous 
membrane  continuous  with  that  lining  the  pharynx  and  trachea. 

The  larynx  presents  a  superior  or  pharyngeal  and  an  inferior  or  tracheal 
opening.  The  pharyngeal  opening  is  triangular  in  shape,  the  base  being 
directed  forward,  the  apex  backward.  The  plane  of  this  opening  in  the 
living  subject  is  almost  vertical.  The  tracheal  opening  is  circular  in  shape 
and  corresponds  in  size  with  the  upper  ring  of  the  trachea.  Viewed  from 
above,  the  general  cavity  of  the  larynx  is  seen  to  be  partially  subdivided  by 
two  membranous  bands — the  vocal  hands  or  cords — which  run  from  before 
backward  in  a  horizontal  plane.  The  space  between  the  bands,  the  glottis, 
varies  in  size  and  shape  from  moment  to  moment  in  accordance  with  respira- 
tory and  phonatory  necessities.  The  average  width  of  the  glottis,  at  its 
widest  part,  during  quiet  respiration  is  about  13.5  mm.  in  men  and  11. 5  mm. 
in  women.  With  the  advent  of  phonation  the  vocal  membranes  are  at  once 
approximated,  and  to  such  an  extent  that  the  glottic  opening  is  reduced  to  a 
mere  slit.     It  is  then  spoken  of  as  the  rima  glottidis,  or  chink  of  the  glottis. 

The  space  above  the  vocal  bands,  the  supra-glottic  or  supra-rimal  space, 
is  triangular  in  shape  and  extends  from  the  pharyngeal  opening  to  the  plane 

620 


PHONATION;  ARTICULATE  SPEECH. 


621 


of  the  vocal  bands.  The  mucous  membrane  lining  the  walls  of  this  space, 
presents  on  either  side,  just  above  the  vocal  bands,  a  crescentic  fold  which 
runs  from  before  backward,  and  is  known  as  the  false  vocal  band  or  cord. 
Between  the  true  and  false  bands  there  is  a  cavity  or  space  prolonged  up- 
ward and  outward  for  some  distance,  forming  what  is  known  as  the  ventricle 
of  the  larynx.  The  space  below  the  vocal 
bands,  the  infra-glottic  or  infra-rimal  space, 
is  narrow  above  and  elongated  from  before 
backward,  but  wide  and  circular  below,  cor- 
responding to  the  lumen  of  the  trachea. 
(Fig.  285.) 

The  Laryngeal  Cartilages,  Articula- 
tions, and  Ligaments. — The  cartilages  which 
compose  the  framework  of  the  larynx  are  nine 
in  number,  three  of  which  are  single:  viz.,  the 
cricoid,  the  thyroid,  and  the  epiglottis,  while 
six  occur  in  pairs:  viz.,  the  arytenoids,  the 
cornicula  laryngis,  and  the  cuneiform.  (Figs. 
286  and  287.') 

The  cricoid  cartilage  is  the  foundation 
cartilage,  and  affords  support  to  the  remain- 
ing cartilages  and  the  structures  attached  to 
them.  In  shape  it  resembles  a  signet-ring, 
the  broad  quadrate  portion  of  which  is  directed 
backward,  while  the  narrow  circular  portion 
is  directed  forward.  It  rests  upon  the  upper 
ring  of  the  trachea,  to  which  it  is  firmly  at- 
tached by  fibrous  tissue.  The  posterior  upper 
border  of  the  quadrate  portion  presents  on 
either  side  an  oval  convex  facet  for  articula- 
tion with  the  arytenoid  cartilage.  The  long 
axis  of  this  facet  is  directed  downward,  out- 
ward, and  forward. 

The  thyroid,  the  largest  of  the  laryngeal 
cartilages,  is  composed  of  two  flat  quadrilat- 
eral plates,  united  anteriorly  at  an  angle  of 
about  90  degrees.  Each  plate  is  directed 
backward  and  outward  and   terminates  ina^       .      .,       ,,        ... 

,         ,  ,  .   ,     .  ,  ,  ,  1     Q-  oupenor    border  of   the   cricoid 

tree  border,  which  is  prolonged  upward  and    cartilage.    10.  Section  of  the  thy- 
downward  for  some  distance,  terminating  in    roid   cartilage,    n,   n.    Superior 
two  processes,  the  superior  and  inferior  cornua. 
The  upper  border  to  the  thyroid  is  deeply 
notched  in  front.     The  inferior  border  over- 


FiG.  285. — Longitudinal  Sec- 
tion OF  THE  Human  Larynx, 
Showing  the  Vocal  Bands,  i. 
Ventricle  of  the  larynx.  2.  Supe- 
rior vocal  cord.  3.  Inferior  vocal 
cord.  4.  Arytenoid  cartilage.  5. 
Section  of  the  arytenoid  muscle.  6, 
6.  Inferior  portion  of  the  cavity  of 
the  larynx.  7.  Section  of  the  pos- 
terior portion  of  the  cricoid  carti- 
lage. 8.  Section  of  the  anterior 
portion    of    the    cricoid     cartilage. 


portion  of  the  cav-ity  of  the  larynx. 
12,  13.  Arytenoid  gland.  14,  16. 
Epiglottis.  15,  17.  Adipose  tissue. 
18.  Section  of  the  hyoid  bone.     19, 

laps  laterally  the  cricoid.  ^^'  ^°'     ^^^  ea.— (  appey.) 

The  epiglottis  is  a  leaf-shaped  piece  of  cartilage  attached  to  the  thyroid 

at  the  median  notch.     It  is  firmly  united  by  membranes  and  ligaments  to 

the  thyroid  and  arytenoid  cartilages  and  to  the  base  of  the  tongue. 

The  arytenoid  cartilages  are  two  in  number  and  symmetric  in  shape. 

Each  cartilage  is  a  triangular  pyramid,  the  apex  of  which  is  recurved,  and 


622 


TEXT-BOOK  OF  PHYSIOLOGY. 


directed  backward  and  inward.  The  base  presents  three  angles — an  ante- 
rior, an  external,  and  an  internal.  The  anterior  angle  is  long  and  pointed 
and  projects  forward  in  a  horizontal  plane.  It  serves  for  the  attach- 
ment of  the  vocal  membranes  and  is  therefore  termed  the  vocal  process. 
The  external  angle  is  short,  rounded,  and  prominent,  and  serves  for  the 
attachment  of  muscles.  The  internal  angle  affords  a  point  of  insertion  for  a 
ligament.  The  inferior  surface  of  the  arytenoid  is  concave  for  articulation 
with  the  convex  surface  of  the  cricoid  facet.  Its  long  axis,  however,  is 
directed  from  before  backward  and  almost  at  right  angles  to  the  long  axis  of 
the  cricoid  facet. 


Fig.  286.  Fig.  287. 

Fig.  286. — Laryngeal  Cartilages  and  Ligaments,  Anterior  Surface,  i.  Hyoid  bone. 
2,  2,  3,  3.  Greater  and  lesser  cornua.  4.  Thyroid  cartilage.  5.  Thyro-hyoid  membrane.  6. 
Thyro-hyoid  ligaments.  7.  Cartilaginous  nodule.  8.  Cricoid  cartilage.  9.  The  crico-thyrtiid 
membrane.     10.  The  crico-thyroid  ligaments.     11.  Trachea. — (Sappey.) 

Fig.  287. — Laryngeal  Cartilages  and  Ligaments,  Posterior  Surface,  i,  i.  Thyroid 
cartilage.  2.  Cricoid  cartilage.  3,  3.  Arytenoid  cartilages.  4,  4.  Crico-arytenoid  articulations. 
5,5.  Crico-thyroid  articulations.  6.  Union  of  the  cricoid  cartilage  and  of  the  trachea.  7.  Epiglot- 
tis.    8.  Ligament  uniting  it  to  the  reentering  angle  of  the  thyroid  cartilage. — {Sappey.) 


The  cornicula  laryngis  and  the  cuneiform  cartilages  are  small  nodules  of 
yellow  elastic  cartilage  embedded  in  a  fold  of  membrane  which  unites  the 
arytenoid  and  the  epiglottis.  They  are  fragments  of  a  ring  of  cartilage  which 
in  some  animals — e.g.,  ant-eater — extends  between  these  two  cartilages. 

The  crico-thyroid  articulation  is  formed  by  the  opposition  of  the  tip  of  the 
inferior  cornu  of  the  thyroid  cartilage  and  an  articular  facet  on  the  side  of  the 
cricoid.  The  joint  is  provided  with  a  synovial  membrane  and  enclosed  by  a 
capsular  ligament.  The  movements  permitted  at  this  joint  take  place 
around  a  horizontal  axis  and  consist  of  an  upward  and  downward  movement 


PHONATION;  ARTICULATE  SPEECH.  623 

of  both  the  thyroid  and  cricoid,  combined  with  a  sliding  movement  of  the 
latter  upward  and  backward. 

The  crico-arytenoid  articulation  is  formed  by  the  apposition  of  the  articu- 
lating sufaces  of  the  cricoid  and  arytenoid  cartilages.  This  joint  is  provided 
with  a  synovial  membrane  and  enclosed  by  a  loose  capsular  ligament  which 
would  permit  of  an  extensive  sliding  of  the  arytenoid  cartilage  downward  and 
outward  were  it  not  prevented  by  the  posterior  crico-arytenoid  ligament, 
which  is  attached,  on  the  one  hand,  to  the  cricoid,  and,  on  the  other,  to  the 
inner  angle  of  the  arytenoid.  The  movements  permitted  at  this  joint  are: 
(i)  Rotation  of  the  arytenoid  around  a  vertical  axis  which  lies  close  to  its 
inner  surface.  (2)  A  sliding  motion  inward  and  forward  with  inward 
rotation  of  the  vocal  process,  or  a  sliding  motion  outward  and  backward 
with  outward  rotation  of  the  vocal  process.  In  either  case  the  process 
describes  an  arc  of  a  circle.  (3)  A  sliding  movement  toward  the  median 
line  in  consequence  of  which  the  inner  surfaces  of  the  arytenoids  are 
brought  almost  in  contact. 

The  crico-thyroid  membrane  is  composed  mainly  of  elastic  tissue.  It 
may  be  divided  into  a  mesial  and  two  lateral  portions.  The  mesial  portion 
is  well  developed,  triangular  in  shape,  and  unites  the  contiguous  borders  of 
the  cricoid  and  thyroid  cartilages.  The  lateral  portion  is  attached  below  to 
the  superior  border  of  the  cricoid.  From  this  attachment  it  passes  upward 
and  inward  under  cover  of  the  thyroid.  As  it  ascends  it  elongates  and  be- 
comes thinner,  and  is  finally  attached  anteriorly  to  the  thyroid  near  the 
median  line,  and  posteriorly  to  the  vocal  process  of  the  arytenoid,  thus  con- 
stituting the  inferior  thyro-arytenoid  ligament.  It  is  covered  internally  by 
mucous  membrane  and  externally  by  the  internal  thyro-arytenoid  muscle. 
The  free  edge  of  this  ligament  forms  the  basis  of  the  true  vocal  band.  A 
superior  thyro-arytenoid  ligament  forms  the  basis  of  the  false  vocal  band. 

The  thyro-hyoid  membrane,  composed  of  elastic  tissue,  unites  the 
superior  border  of  the  thyroid  to  the  hyoid  bone. 

The  mucous  membrane  lining  the  larynx  is  thin  and  pale.  As  it  passes 
downward  it  is  reflected  over  the  superior  thyro-arytenoid  ligament,  and 
assists  in  the  formation  of  the  false  vocal  band;  it  then  passes  into  and  lines 
the  ventricle,  after  which  it  is  reflected  inward  over  the  superior  border  of  the 
thyro-arytenoid  muscle  and  ligament,  and  assists  in  the  formation  of  the 
true  vocal  band;  it  then  returns  upon  itself  and  passes  downward  over  the 
lateral  portion  of  the  cricothyroid  membrane  into  the  trachea. 

The  thin,  free,  reduplicated  edge  of  the  mucous  membrane  constitutes 
the  true  vocal  band.  The  surface  of  the  mucous  membrane  is  covered  by 
ciliated  epithelium  except  in  the  immediate  neighborhood  of  the  vocal 
bands. 

The  vocal  bands  are  attached  anteriorly  to  the  thyroid  cartilage  near  the 
receding  angle  and  posteriorly  to  the  vocal  processes  of  the  arytenoid  cartil- 
ages. They  vary  in  length  in  the  male  from  20  to  25  mm.  and  in  the  female 
from  15  to  20  mm. 

The  Muscles  of  the  Larynx. — The  muscles  which  have  a  direct  action 
on  the  cartilages  of  the  larynx  and  determine  the  position  of  the  vocal  bands 
both  for  respiratory  and  phonatory  purposes,  and  which  regulate  their 
tension  as  well,  are  nine  in  number  and  take  their  names  from  their  points 


624 


TEXT-BOOK  OF  PHYSIOLOGY. 


of  origin  and  insertion:  viz.,  two  posterior  crico-arytenoids,  two  lateral  crico- 
arytenoids, two  thyroarytenoids,  one  arytenoid,  and  two  crico-thyroids 
(Figs.  288  and  289). 

The  posterior  crico-arytenoid  muscle  lies  on  the  posterior  surface  of  the 
quadrate  plate  of  the  cricoid  cartilage,  on  either  side  of  the  median  line, 
from  which  it  takes  its  origin.  The  fibers  of  the  muscle  pass  upward  and 
outward  and  in  their  course  converge  to  be  inserted  into  the  external  angle  of 
the  arytenoid  cartilage.  The  superior  and  more  horizontally  directed  fibers 
rotate  the  arytenoid  around  its  vertical  axis;  the  inferior  and  obliquely 


Fig.  288.  Fig.  289. 

Fig.  288. — Posterior  View  OF  THE  Muscles  OF  THE  Larynx,  i.  Posterior  crico-arytenoid 
muscle.  2,  3,  4.  Different  fasciculi  of  the  arytenoid  muscle.  5.  Aryteno-epiglottidean  muscle. — 
{Sappey.) 

Fig.  289. — Lateral  View  of  the  Muscles  of  the  Larynx,  i.  Body  of  the  hyoid  bone. 
2.  Vertical  section  of  the  thyroid  cartilage.  3.  Horizontal  section  of  the  thyroid  cartilage 
turned  downward  to  show  the  deep  attachment  of  the  crico-thyroid  muscle.  4.  Facet  of  articula- 
tion of  the  small  cornu  of  the  thyroid  cartilage  with  the  cricoid  cartilage.  5.  Facet  on  the  cricoid 
cartilage.  6.  Superior  attachment  of  the  crico-thyroid  muscle.  7.  Posterior  crico-arytenoid 
muscle.  8,  10.  Arytenoid  muscle.  9.  Thyro-arytenoid  muscle.  11.  Aryteno-epiglottidean 
muscle.     12.  Middle  thyro-hyoid  ligament.     13.  Lateral  thyro-hyoid  ligament. — {Sappey.) 

directed  fibers  draw  the  cartilage  downward  and  inward.  As  a  result  of  the 
action  of  the  muscle  in  its  entirety,  the  vocal  process  is  turned  upward  and 
outward,  and  as  the  vocal  band  is  carried  with  it  the  glottis  is  widened, 
a  condition  necessary  to  the  free  entrance  of  air  into  the  lungs  (Fig.  290). 
Since  the  contraction  of  the  crico-arytenoid  has  this  result,  it  is  generally 
spoken  of  as  the  abductor  or  respiratory  muscle. 

The  lateral  crico-arytenoid  muscle  arises  from  the  side  of  the  cricoid 
cartilage.  From  this  point  its  fibers  are  directed  upward  and  backward 
to  be  inserted  into  the  external  process  of  the  arytenoid.  Its  action  is  to 
draw  the  arytenoid  cartilage  forward  and  inward,  thus  approximating 
and  relaxino;  the  vocal  band. 


PRONATION;  ARTICULATE  SPEECH. 


62^ 


The  thyro-arytenoid  muscle  arises  from  the  inferior  two-thirds  of  the 
inner  surface  of  the  thyroid  cartilage  just  external  to  the  median  line.  From 
this  origin  the  libers  pass  backward  and  outward,  to  be  inserted  into  the 
anterior  surface  and  external  angle  of  the  arytenoid  cartilage.  The  inner 
portion  of  the  muscle  lies  close  to  and  supports,  if  it  does  not  constitute  a  part 
of,  the  vocal  band.  The  action  of  the  thyro-arytenoid  muscle  in  conjunction 
with  the  lateral  crico-arytenoid  is  to  rotate  the  arytenoid  cartilage  around  the 
vertical  axis  and  to  draw  the  vocal  process  forward  and  inward,  thus  carry- 
ing the  vocal  cord  toward  the  median  line.  When  the  muscles  of  the  two 
sides  simultaneously  contract,  the  vocal  bands  are  closely  approximated 
and  the  space  between  them,  the  rima  vocalis,  reduced  to  a  mere  slit,  one 
of /the  conditions  essential  to  phonation  (Fig.  291). 

The  arytenoid  muscle  consists  (i)  of  transversely  arranged  fibers  which 
ariseirom  and  are  inserted  into  the  outer  surface  of  the  opposite  arytenoid 


Fig.  290. — Glottis  Widely  Opened 
FROM  Simultaneous  Contractiox  of 
Both  Crico-arytenoid  Muscles,  b. 
Epiglottis,  rs.  False  vocal  band.  ri. 
True  vocal  band.  ar.  Arytenoid  car- 
tilages, a.  Space  between  the  arytenoids. 
c.  Cuneiform  cartilages,  ir.  Interarytenoid 
fold.  rap.  Aryepiglottic  fold.  cr.  Car- 
tilage rings. — (Maudl.) 


ra/>JS 


Fig.  291. — Position  of  the  Voc.a.l 
Bands  Due  to  the  Simultaneous 
Contraction  of  Both  Lateral  Crico- 
arytenoid Muscles  and  Both  Thyro- 
arytenoid Muscles,  h.  Epiglottis,  rs. 
False  vocal  band.  ri.  True  vocal  band. 
or.  Space  between  the  arytenoid  cartil- 
ages, the  glottis  respiratoria.  ar.  Ar}'- 
tenoid  cartilages,  c.  Cuneiform  carti- 
lages, rap.  Aryepiglottic  fold.  •/;-.  In- 
terarytenoid fold. — (Mandl.) 


cartilages,  and  (2)  of  obliquely  directed  fibers  which  arise  from  the  outer 
angle  of  one  arytenoid  to  be  inserted  into  the  apex  of  the  other.  In  their 
course  they  decussate  in  the  median  line.  The  action  of  this  muscle  is  to 
approximate  the  arytenoid  cartilages  and  thus  obliterate  that  portion  of  the 
glottis  between  the  vocal  processes,  the  rima  respiratoria,  and  so  direct  the 
expiratory  blast  of  air  toward  and  through  the  rima  vocalis. 

The  collective  actions  of  the  three  foregoing  muscles  is  to  close  or  con- 
strict the  glottis,  and  for  this  reason  they  are  spoken  of  as  the  adductor  or 
phonatory  muscles. 

The  crico-ihyroid  muscle  arises  from  the  side  and  front  of  the  cricoid 
cartilage  and  is  inserted  above  into  the  lower  border  of  the  thyroid  cartilage. 
The  action  of  this  muscle  is  to  draw  up  the  anterior  part  of  the  cricoid  car- 
tilage toward  the  thyroid,  which  remains  stationary,  and  to  swing  the 
quadrate  plate  of  the  cricoid  and  the  arytenoid  cartilages  downward  and 
backward.  This  movement  has  the  result  of  tensing  the  vocal  bands. 
The  cricoid  is  at  the  same  time  drawn  backward  by  the  action  of  the  more 
longitudinally  disposed  fibers. 
40 


626 


TEXT-BOOK  OF  PHYSIOLOGY. 


Nerves  of  the  Larynx. — The  nerves  which  innervate  the  muscles  of  the 
larynx  and  endow  the  mucous  membrane  with  sensibiUty  are  derived  from 
the  vagus  trunk.  The  superior  laryngeal  is  for  the  most  part  sensor  and 
distributed  to  the  mucous  membrane,  though  it  contains  motor  fibers  for 
the  crico-thyroid  muscle.  The  inferior  laryngeal  is  purely  motor  and  is 
distributed  to  all  the  muscles  with  the  exception  of  the  crico-thyroid. 

THE  MECHANISM  OF  PHONATION. 

Phonation,  the  production  of  vocal  sounds  in  the  larynx,  is  the  result  of 
the  vibration  of  the  vocal  bands  caused  by  an  expiratory  blast  of  air  from 
the  lungs.  That  a  sound  may  arise  it  is  essential  that  the  glottis  be  approxi- 
mately closed  and  the  vocal  bands  be  made  more  or  less  tense. 

The  closure  of  the  glottis — the  approximation  of  the  vocal  processes  and 
the  vocal  bands — is  accomplished,  it  will  be  recalled,  by  the  contraction 
of  the  lateral  crico-arytenoid,  the  arytenoid,  and  the  thyro-arytenoid  muscles. 
The  increase  in  tension  is  accomplished  by  the  contraction  of  the  crico-thyroid 
and  the  thyro-arytenoid  muscles,  the  former  by  the  backward  displacement 
of  the  cricoid  and  arytenoid  cartilages,  the  latter  by  converting  the  natural 
concave  edge  of  the  vocal  band  to  a  straight  line.  The  lengthening  and  tens- 
ing of  the  vocal  bands  by  the  crico-thyroid  muscle  is  regarded  by  some  inves- 


FiG.  292. — Position  of  the  Vocal 
Bands  Previous  to  the  Emission  of  a 
Sound,  b.  Epiglottis,  rs.  False  vocal 
band.  ri.  True  vocal  band.  ar.  Ary- 
tenoid cartilages. — {Mandl.) 


Fig.  293. — Position  of  the  Vocal 
Bands  in  the  Production  of  Notes 
of  Low  Pitch.  /.  Epiglottis,  or.  Glottis. 
lis.  False  vocal  cord.  ni.  True  vocal  cord. 
ar.  Arytenoid  cartilages. — {Mandl.) 


tigators  as  a  coarse  means,  the  approximation  of  the  free  edges  by  the  thyro- 
arytenoid, as  a  finer  means,  of  adjustment  for  the  producton  of  slight  changes 
in  the  pitch  of  sounds.  The  extent  to  which  the  glottis  is  closed  and  the 
membranes  tensed  will  depend,  however,  on  the  pitch  of  the  sound  to  be 
emitted.  The  appearance  presented  by  the  glottis  just  previous  to  the  emis- 
sion of  a  note  of  medium  pitch,  as  determined  by  laryngologic  examination,  is 
shown  in  Fig.  292.  When  the  foregoing  conditions  in  the  glottis  are  realized, 
the  air  stored  or  collected  in  the  lungs  is  forced  by  the  contraction  of 
the  expiratory  muscles,  through  the  narrow  space  between  the  bands. 
As  a  result  of  the  resistance  offered  by  this  narrow  outlet  and  the 
force  of  the  expiratory  muscles  the  air  within  the  lungs  and  trachea  is 
subjected  to  pressure,  and  as  soon  as  the  pressure  attains  a  certain  level 
the  vocal  bands  are  thrown  into  vibrations,  which  in  turn  impart  to  the 
column  of  air  in  the  upper  air-passages  a  corresponding  series  of  vibrations  by 
which  the  laryngeal  vibrations  are  reinforced.      The  degree  of  pressure  to 


PHONATION;  ARTICULATE  SPEECH. 


627 


•  f-  .^')' 


which  the  air  in  the  lungs  and  trachea  is  subjected  was  determined  by  Latour 
to  vary  from  160  mm.  of  water  for  sounds  of  moderate,  to  940  mm.  of  water 
for  sounds  of  highest  intensity.  With  the  escape  of  the  air  or  the  separation 
of  the  vocal  bands  the  vibration  ceases  and  the  sound  dies  away. 

The  Characteristics  of  Vocal  Sounds. — In  common  with  the  sounds 
produced  by  other  music  instruments,  all  vocal  sounds  are  characterized 
by  intensity,  pitch  and  quality,  tone  or  color. 

The  intensity  or  loudness  of  a  sound  depends  on  the  extent  or  amplitude 
of  the  to-and-fro  vibration  or  the  extent  of  the  excursion  of  the  vocal 
band  on  either  side  of  the  position  of  equilibrium  or  rest;  and  this  in  turn 
depends  on  the  force  with  which  the  blast  of  air  strikes  the  band.  The  more 
forceful  the  blast  of  air,  the  larger,  other  things 
'being  equal,  will  be  the  primary  vibrations  of 
the  bands,  and  hence  the  secondary  vibrations 
of  the  air  in  the  upper  air-passages. 

The  pitch  of  the  voice  depends  on  the  num- 
ber of  vibrations  in  a  unit  of  time,  a  second. 
This  will  be  conditioned  by  the  length  of  the 
bands  in  vibration  or  the  length  and  width  of 
the  aperture  through  which  the  air  passes  and 
the  degree  of  tension  to  which  the  bands  are  sub- 
jected. In  the  emission  of  sounds  of  highest 
pitch  the  tension  of  the  vocal  bands  and  the 
narrowing  of  the  glottis  attain  their  maximum. 
In  the  emission  of  sounds  of  lowest  pitch  the  re- 
verse conditions  obtain.  In  passing  from  the 
lowest  to  the  highest  pitched  sounds  in  the  range 
of  the  voice  peculiar  to  any  one  individua-1, 
there  is  a  progressive  increase  in  both  the  ten- 
sion of  the  vocal  bands  and  the  narrowing  of 
the  glottic  aperture.  In  the  production  of  low- 
pitched  notes  of  men,  those  due  to  vibrations  lying  between  80  and  240  per 
second,  the  tension  is  regulated  by  the  crico-thyroid  muscle;  the  aperture  of 
the  glottis  during  this  time  being  elliptic  in  shape  and  relatively  wide  (Fig. 
295).  In  the  production  of  notes  due  to  vibrations  lying  between  240  and 
512  vibrations  per  second,  the  anterior  fibers  of  the  crico-thyroid  muscle 
relax  and  the  thyroarytenoid  muscle  comes  into  play;  by  its  action  the  vocal 
bands  are  more  closely  approximated  and  the  vocal  aperture  reduced  to  a 
linear  slit.  In  the  high-pitched  notes  emitted  by  soprano  singers  the  vocal 
bands  are  so  closely  applied  to  each  other  that  only  a  very  small  portion  in 
front,  bounding  a  small  oval  aperture,  is  capable  of  vibrating  (Fig.  294). 
The  difference  in  the  pitch  of  the  voice  in  men  and  women  is  due  largely  to 
the  greater  size  and  development  of  the  vocal  bands  in  the  former  than  in  the 
latter. 

The  quality  of  the  voice,  the  timbre  or  tone-color,  depends  on  ih.t  Jorm 
combined  with  the  intensity  and  pitch  of  the  vibration.  As  with  sounds  pro- 
duced by  music  instruments,  the  primary  or  fundamental  vibration  of  the 
vocal  band  is  complicated  by  the  superposition  of  secondary  or  partial  vibra- 
tions (overtones).     The  form  of  the  vibration  will  therefore  be  a  resultant  of 


Fig.  294. — Glottis  Seen 
WITH  THE  Laryngoscope  dur- 
ing THE  Emission  of  High- 
pitched  Sounds,  i,  2.  Base 
of  the  tongue.  3,  4.  Epiglot- 
tis. 5,  6.  Pharynx.  7.  Ary- 
tenoid cartilages.  8.  Opening 
between  the  true  vocal  cords. 

9.  Aryteno-epiglottidean   folds. 

10.  Cartilage  of  Santorini.  11. 
Cuneiform  cartilage.  12.  Su- 
perior vocal  cords.  13.  In- 
ferior vocal  cords. — (Lc  Bon.) 


628  TEXT-BOOK  OF  PHYSIOLOGY. 

the  blending  of  a  number  of  different  vibrations.  The  quality  of  the  sound 
produced  in  the  larynx  is,  however,  modified  by  the  resonance  of  the  mouth 
and  nasal  cavities;  certain  of  the  overtones  being  reinforced  by  changes  in 
the  shape  of  the  mouth  cavity  more  especially,  thus  giving  to  the  voice  a 
somewhat  different  quality. 

The  Varieties  of  Voice. — The  region  of  the  music  scale,  comprising 
all  vibrations  between  32  and  2048  per  second,  with  which  laryngeal  sounds 
are  in  accord  will  vary  in  the  two  sexes  and  in  different  individuals  of  the 
same  sex.  It  is  customary  to  classify  voices,  especially  those  of  singers,  into 
bass,  baritone,  tenor,  contralto,  mezzo-soprano,  and  soprano,  in  accordance 
with  the  regions  of  the  music  scale  with  which  they  correspond.  Thus  the 
succession  of  notes  characteristic  of  the  bass  voice  vary  in  pitch  from  F,  faj, 
to  c',  do3,  or  from  85  to  256  vibrations  per  second;  those  of  the  baritone 
from  A,  la  J,  to  f,  fa^,  or  from  106  to  341  vibrations  per  second;  those  of  the 
tenor  from  c',  do,,  to  a',  kg,  or  from  128  to  427  vibrations  per  second;  those 
of  the  contralto  from  e,  mi,,  to  c",  do^,  or  from  160  to  512  vibrations  per 
second;  those  of  the  mezzo-soprano  from  g,  S0I2,  to  e",  mi^,  or  from  192  to 
640  vibrations  per  second;  those  of  the  soprano  from  b,  si2,  to  g",  sol^,  or 
from  240  to  768  vibrations  per  second.  The  range  of  the  voice  is  thus  seen 
to  embrace  from  one  and  three-quarters  to  two  octaves.  Some  few  individual 
singers  have  far  exceeded  this  range,  but  they  are  exceptional. 

Speech  is  the  expression  of  ideas  by  means  of  articulate  sounds.  These 
sounds  may  be  divided  into  vowel  and  consonant  sounds. 

The  vowel  sounds,  a,  e,  i,  0,  u,  are  laryngeal  sounds  modified  by  the 
superposition  and  reinforcement  of  certain  overtones  developed  in  the  mouth 
and  pharynx  by  changes  in  their  shapes.  The  number  of  vibrations  under- 
lying the  production  of  each  vowel  sound  is  a  matter  of  dispute.  According 
to  Konig,  the  sound  of  a  is  the  result  of  940  vibrations;  of  e,  1880  vibrations; 
of  i,  3760  vibrations;  of  o,  470  vibrations;  of  u,  235  vibrations. 

Consonant  sounds  are  produced  by  the  more  or  less  complete  interruption 
of  the  vowel  sounds  during  their  passage  through  the  organs  of  speech. 
These  may  be  divided  into : 

1.  Labials,  p,  b,  m. 

2.  Labio-dcntals,/,  v. 

3.  Linguo-dentals,  s,  2. 

4.  Anterior  linguo-palatals,  t,  d,  I,  n,  r. 

5.  Posterior  linguo-palatals,  k,  g,  h,  y. 

The  names  of  these  different  groups  of  consonants  indicate  the  region  of 
the  mouth  in  which  they  are  produced  and  the  means  by  which  the  air  blast 
is  interrupted. 

THE  NERVE  MECHANISM  OF  THE  LARYNX. 

The  ner\^e  mechanism  by  which  the  musculature  of  the  larynx  is  excited 
to  action  and  coordinated  so  as  to  subserve  both  respiration  and  phonation 
involves  the  fibers  contained  in  the  superior  and  inferior  laryngeal  nerves 
(both  branches  of  the  vagus)  and  their  related  nerve-centers  in  the  central 
nerve  system. 

For  respiratory  purposes  it  is  essential  that  the  lumen  of  the  glottis  shall 
be  sufficiently  large  to  permit  the  entrance  and  exit  of  air  without  hindrance. 


PHONATION;  ARTICULATE  SPEECH.  629 

Laryngoscopic  examination  of  the  larynx  in  the  human  being  shows  that 
during  quiet  respiration  the  vocal  bands  are  widely  separated  and  almost 
stationary,  moving  but  slightly  during  either  inspiration  or  expiration.  At 
this  time,  according  to  the  investigations  of  Semon,  the  area  of  the  glottis  is 
approximately  160  sq.  mm.,  somewhat  less  than  the  ar-ea  of  either  the 
supraglottic  or  infraglottic  regions,  which  is  about  200  sq.  mm.  This  con- 
dition of  the  glottis  is  maintained  by  the  steady  continuous  contraction  of  the 
posterior  crico-arytenoid  muscles,  the  abductors  of  the  vocal  bands. 

For  phonatory  purposes  it  is  essential  that  the  respiratory  function  be 
temporarily  suspended  and  the  vocal  bands  closely  approximated.  This  is 
accomplished  by  the  contraction  of  the  remaining  muscles  of  the  larynx, 
with  the  exception  of  the  crico-thyroid,  which  are  collectively  known  as  the 
adductors  of  the  vocal  bands.  During  phonation  the  adductor  muscles  over- 
come the  activity  of  the  abductors.  .  With  the  cessation  of  phonation  the 
abductors  immediately  restore  the  vocal  bands  to  their  former  respiratory 
position. 

The  activities  of  these  two  antagonistic  groups  of  muscles  are  under  the 
control  of  the  central  nerve  system.  The  only  pathway  for  the  excitator 
nerve  impulses  is  through  the  fibers  of  the  inferior  or  recurrent  laryngeal 
nerve.  The  relation  of  these  nerve-fibers  both  centrally  and  peripherally,  as 
well  as  their  physiologic  action,  has  been  the  subject  of  much  experimenta- 
tion. The  results  have  not  always  been  in  accord,  owing  to  the  choice  of 
animal,  the  use  of  anesthetics,  strength  of  stimulus,  etc. 

As  the  outcome  of  many  investigations  it  is  believed  that  each  muscle 
group  is  innervated  by  its  own  bundle  of  nerv-e-fibers,  both  of  which  are  con- 
tained in  the  inferior  laryngeal,  though  coming  from  two  separate  centers  in 
the  medulla  oblongata.  Russell  succeeded  in  separating  the  fibers  for  the 
abductors  from  the  fibers  for  the  adductors  in  the  inferior  laryngeal,  and  in 
tracing  them  to  their  terminations.  So  completely  was  this  done  that  it 
became  possible  to  produce  at  will,  through  stimulation,  either  abduction 
or  adduction,  without  contraction  of  the  muscle  of  opposite  function. 

The  laryngeal  respiratory  center  was  located  by  Semon  and  Horsley, 
in  the  cat,  in  the  upper  part  of  the  floor  of  the  fourth  ventricle.  Stimulation 
of  this  area  during  etherization  was  followed  by  abduction  of  the  vocal 
bands.  The  efferent  fibers  of  this  center  are  believed  by  some  investigators 
to  leave  the  central  nerve  system  in  the  spinal  accessory  nerve,  by  others  in 
the  lower  roots  of  the  vagus. 

From  the  continuous  activity  of  the  abductor  muscle,  and  the  stationary 
position  of  the  vocal  bands,  it  is  probable  that  the  medullary  center  is  in  a 
state  of  continuous  activity  or  tonus,  the  result  probably  of  reflex  influences, 

A  cortical  representation  for  laryngeal  respiratory  movements  has  been 
determined  by  Semon  and  Horsley  in  different  classes  of  animals.  In  the 
cat  especially,  stimulation  of  the  border  of  the  olfactory  sulcus  gives  rise  to 
complete  abduction  of  the  vocal  bands  on  both  sides.  The  representation  is 
therefore  bilateral. 

The  phonatory  center  was  located  by  the  same  investigators  in  the  medulla 
near  the  ala  cinerea  and  the  upper  border  of  the  calamus  scriptorius.  Stimu- 
lation of  this  area  was  invariably  followed  by  bilateral  adduction  of  the  vocal 
bands  and  closure  of  the  glottis. 


630  TEXT-BOOK  OF  PHYSIOLOGY. 

A  cortical  representation  for  phonatory  movements  also  was  located  in 
the  lower  portion  of  the  pre-central  convolution,  near  the  anterior  border. 
Stimulation  of  this  area  gives  rise  to  marked  adduction  of  both  vocal  bands, 
indicating  that  the  representation  is  therefore  bilateral. 

Faradic  stimulation  of  the  inferior  laryngeal  nerve  during  slight  ether 
anesthetization  gives  rise  to  closure  of  the  glottis;  the  same  stimulation, 
however,  during  deeper  anesthetization  gives  rise  to  opening  or  dilatation  of 
the  glottis,  a  fact  indicating  that  either  the  adductor  muscles  or  their  nerve 
terminals  are  depressed  by  the  action  of  the  ether  before  the  muscles  and 
nerves  of  opposite  function.  The  superior  laryngeal  nerves  contain  motor 
fibers  for  the  crico-thyroid  muscles.  Stimulation  of  the  nerve  gives  rise  to 
contraction  of  the  muscle  and  increased  tension  of  the  vocal  bands.  It  is 
believed  that  these  fibers  are  derived  originally  from  the  efferent  fibers  of  the 
glosso-pharyngeal  nerve.  The  remaining  fibers  of  the  superior  laryngeal 
endow  the  upper  portion  of  the  larynx  with  extreme  sensibility  which  to  a 
certain  extent  protects  the  air-passages  against  the  entrance  of  foreign 
bodies.  Irritation  of  the  terminal  filaments  of  this  nerve  by  particles  of 
food,  solid  or  liquid,  gives  rise  to  marked  reflex  spasm  of  the  adductor 
muscles  and  closure  of  the  glottis,  followed  by  a  strong  expiratory  blast  of  air 
from  the  lungs  by  which  the  offending  particles  are  removed.  Division  of 
this  nerve  on  both  sides  is  followed  by  a  paralysis  of  the  crico-thyroid  muscles, 
a  lowering  of  the  tension  of  the  vocal  bands,  and  a  loss  of  sensibility  of  the 
laryngeal  mucous  membrane. 


CHAPTER  XXVI. 
THE  SPECIAL  SENSES. 

It  is  one  of  the  functions  of  the  nerve  system  to  bring  the  individual  into 
conscious  relation  with  the  external  world.  This  is  accomplished  in  part 
through  the  intermediation  of  afferent  nerves,  connected  peripherally  with 
highly  specialized  terminal  organs,  and  centrally  with  specialized  areas  in 
the  cerebral  cortex. 

Excitation  of  the  terminal  organs  by  material  changes  in  the  environment 
develops  nerve  impulses  which,  transmitted  to  the  cortical  areas,  evoke 
sensations.  These  sensations,  differing  in  character  from  those  vague  ill- 
defined  sensations — e.g.,  fatigue,  well-being,  discomfort,  etc. — caused  by 
material  changes  occurring  within  the  body,  are  termed  special  sensations — 
e.g.,  touch;  pressure;  pain;  temperature;  taste;  smell;  light  and  its  varying 
qualities,  intensity,  hue,  and  tint;  sound  and  its  varying  qualities,  intensity, 
pitch,  and  timbre. 

The  terminal  organs  which  receive  the  impress  of  the  external  world  are  the 
skin,  tongue,  nose,  eye,  and  ear,  and  collectively  constitute  the  special  sense- 
organs.  The  physiologic  mechanisms  which  underlie  and  develop  these 
special  sensations  are  known  respectively  as  the  tactile,  gustatory,  olfactory, 
optic,  and  auditory.  Each  mechanism  responds  to  but  a  single  form  of 
stimulus  and  to  no  other.  Thus,  the  stimulus  for  the  skin  is  mechanic  pres- 
sure; for  the  tongue,  soluble  organic  and  inorganic  matter;  for  the  nose, 
volatile  or  gaseous  matter;  for  the  eye,  ether  vibrations;  for  the  ear,  atmos- 
pheric undulations.  These  stimuli  alone  are  adequate  to  the  physiologic 
excitation  of  the  different  mechanisms. 

The  factors  involved  in  the  production  of  the  sensations  include  (i)  a 
special  physical  stimulus;  (2)  a  specialized  terminal  organ;  (3)  an  afferent 
nerve  pathway,  and  (4)  a  specialized  receptive  sensor  cell  in  the  cerebral 
cortex. 

Though  the  resulting  sensations  in  each  instance  differ  widely  in  their 
characteristics,  it  is  difi&cult  to  present  a  satisfactory  explanation  for  these 
differences.  If  it  be  assumed  that  the  nerve  impulses  which  ascend  the 
different  nerves  of  special  sense  are  alike  in  quality,  then  it  must  be  ad- 
mitted that  the  character  of  the  sensation  is  the  expression  of  a  specialization 
and  organization  of  the  cortical  area.  If,  on  the  other  hand,  specialization 
of  the  cortex  is  denied,  then  there  must  be  admitted  a  specialization  of  the 
peripheral  organ — with  a  resulting  difference  in  quality  or  rapidity  of  the 
nerve  impulses  which  would  impress  or  excite  the  non-specialized  cortex  in 
such  a  way  as  to  call  forth  the  characteristic  sensation.  It  is  possible,  how- 
ever, that  neither  supposition  is  wholly  correct,  and  that  the  character  of  the 
sensation  depends  on  the  construction  and  adaptation  of  the  entire  sense 
apparatus  to  the  character  of  the  stimulus. 

631 


632  TEXT-BOOK  OF  PHYSIOLOGY. 

Whatever  the  conditions  for  their  origin  and  whatever  their  character- 
istics, sensations  in  themselves  do  not  constitute  knowledge;  they  are  but 
elementary  states  of  consciousness,  raw  materials  out  of  which  the  mind 
elaborates  conceptions  and  forms  judgments  as  to  the  character  of  any  given 
object  in  comparison  with  former  experiences. 

THE  SENSE  OF  TOUCH. 

The  physiologic  mechanism  involved  in  the  sense  of  touch  includes  the 
skin  and  the  mucous  membrane  lining  the  mouth,  the  afferent  nerves,  their 
cortical  connections,  and  nerve-cells  in  the  cortex  of  the  parietal  lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerv^e  impulses  which, 
transmitted  to  the  cortex,  evoke  sensations  of  touch  and  temperature.  To 
the  skin,  therefore,  is  ascribed  a  touch  sense  and  a  temperature  sense. 
Of  the  touch  sensations  two  kinds  may  be  distinguished:  viz.,  pressure 
sensations  and  place  sensations.  With  the  contact  of  an  external  body 
there  arises  the  perception  not  only  of  the  pressure,  but  also  the  perception 
of  the  place  or  locaHty  of  the  contact.  In  accordance  with  this,  it  is 
customary  to  attribute  to  the  skin  a  pressure  sense  and  a  location  sense. 

The  specific  physiologic  stimuli  to  the  terminal  organs  in  the  skin  and 
oral  mucous  membrane  are  mechanic  pressure  and  thermic  vibrations. 

The  Skin. — The  skin,  which  constitutes  the  basis  for  the  sense  of  touch, 
covers  and  closely  invests  the  entire  body.  It  varies  in  thickness  and  deli- 
cacy in  different  regions,  though  its  structure  is  everywhere  essentially  the 
same.  As  the  physiologic  anatomy  of  the  skin  has  elsewhere  been  detailed 
(page  486),  it  is  only  necessary  to  state  here  that  it  is  divided  into  a  deep 
and  a  superficial  layer.  The  former,  known  as  the  derma,  consists  of  an 
inner  layer  of  rather  loose  connective  tissue  and  an  outer  layer  of  condensed 
connective  tissue.  The  latter,  known  as  the  epidermis,  consists  of  an  inner 
layer  of  pigment  cells  and  a  thick  outer  layer  of  epithelial  cells.  The 
derma  is  characterized  by  the  presence  of  elevations  (papillae)  which  are 
everywhere  extremely  abundant.  Throughout  the  derma  ramify  blood- 
vessels and  nerves. 

The  Peripheral  or  Terminal  Organs. — Between  the  contact  surface 
and  the  afferent  nerves  specialized  structures  are  found  which  serve  as  inter- 
mediates between  the  stimulus,  on  the  one  hand,  and  the  afferent  nerves, 
on  the  other  hand.  By  virtue  of  their  structure  they  are  far  more  irritable 
than  the  nerve-fibers  and  hence  respond  more  quickly  to  the  physiologic 
stimulus  than  the  nerve-fiber  itself.  To  these  specialized  organs,  found  not 
only  in  the  skin  but  in  other  sense-organs  as  well,  the  term  peripheral  or 
terminal  organ  is  given.  It  is  these  structures  that  are  primarily  excited 
to  activity  by  the  physiologic  stimulus,  and  that  in  turn  arouse  the  nerve 
to  activity.  Peripheral  organs  are  to  be  regarded  as  special  modes  of  termi- 
nation of  afferent  nerv^es  adapted  for  the  impress  of  a  specific  stimulus. 
The  peripheral  organs  of  afferent  nerves  found  in  the  skin  and  oral  mucous 
membrane  present  a  variety  of  forms,  some  of  which  are  as  follows : 
I.  Free  Endings. — These  are  pointed  or  club-shaped  processes,  the  ultimate 
terminations  of  afferent  nerve-fibrils,  found  in  and  among  epidermic 
cells. 


THE  SENSE  OF  TOUCH. 


633 


2.  Tactile  Cells. — These  are  oval  nucleated  bodies  found  in  the  deeper 

layers  of  the  epidermis.  They  rest  upon  or  are  embraced  by  a  crescentic 
shaped  body,  the  tactile  meniscus,  which  in  turn  is  directly  connected 
with  the  nerve-fibril  and  probably  a  modification  of  it  (Fig.  295). 

3.  The  Corpuscles  of  Meissner  and  Wagner. — In  the  papillae  of  the  derma, 

especially  in  the  palm  of  the  hand  and  in  the  finger-tips,  are  found 
elliptical  bodies  consisting  of  a  connective-tissue  capsule  containing  a 
number  of  tactile  discs  with  which  the  nerve-fibrils  are  connected.  If 
the  afferent  nerve  is  traced  to  the  capsule,  it  is  found  to  lose  both  its 
neurilemma  and  its  medulla,  after  which  the 
naked  fibril  penetrates  the  capsule,  breaks  up 
into  a  number  of  branches,  and  after  pursu- 
ing a  more  or  less  spiral  course  becomes  con- 
nected with  the  tactile  discs  (Fig.  296). 

4.  Hair  Wreaths. — Just  below  the  openings  of  the 

sebaceous  glands  the  hair-follicles  are  sur- 
rounded by  naked  axis-cylinder  fibrils  in  the 
form  of  a  wreath,  which  in  all  probability 
terminate   in   the    cells    of  the  external  root- 


FiG.  295. — Tactile  Cells  from  Snout 
OF  Pig.  a.  Tactile  cell.  m.  Tactile  disc. 
n.  Nerve-fiber. — (Stirling.) 


Fig.  296. — Touch-cor- 
puscle OF  Meissner  and 
Wagner,  b.  Papilla  of 
cutis.  d.  Ner\^e-fiber  of 
touch-corpuscle,  e,  f. 
Nerve-fiber  in  touch-cor- 
puscles, g.  Cells  of  Mal- 
pighian  laye  v.— -{From 
Stirling.) 


sheath.     These,  too,  are  to  be  regarded  as  part  of  the  touch  apparatus. 

5.  Corpuscles  of  Vater  or  Pacini. — These  are  oval-shaped  structures  found 
along  the  nerves  distributed  to  the  palms  of  the  hands  and  the  soles 
of  the  feet,  on  the  nerves  distributed  to  the  external  genital  organs,  to 
joints  and  other  structures.  They  consist  of  a  thick  capsule  of  lamel- 
lated  connective  tissue  in  the  interior  of  which  is  a  bulb  resembling 
granular  protoplasm.  The  axis-cylinder  of  the  nerve-fiber  enters  the 
capsule  and  becomes  connected  with  the  bulb  (Fig.  297). 
Other  forms  of  peripheral  organs  are  found  in  special  regions  of  the  skin 

as  well  as  in  different  animals. 


634 


TEXT-BOOK  OF  PHYSIOLOGY. 


Touch  Sense. — The  area,  stimulation  of  which  evokes  sensations  of 
touch  is  coextensive  with  the  skin  and  that  limited  portion  of  the  mucous 
membrane  lining  the  mouth.  Careful  stimulation  of  the  skin  by  means  of  a 
fine  stiff  bristle  has  revealed  the  fact,  however,  that  the  touch  area  is  not 
continuous,  but  discrete,  presenting  itself  under  the  form  of  small  areas  or 
spots,  separated  by  relatively  large  areas  insensitive  to  the  same  agent. 
Stimulation  of  these  spots  always  calls  forth  a  sensation  of  touch.  For  this 
reason  they  are  known  as  "touch  spots."  The  number  of  such  spots  in  any 
given  area  of  skin  varies  considerably.  Thus,  in  the  skin  of  the  calf  fifteen 
such  spots  have  been  counted  in  a  square  centimeter.     In  the  palm  of  the 

hand  from  forty  to  fifty  have  been  counted  in  an 
area  of  the  same  extent.  They  are  also  especi- 
ally abundant  in  the  immediate  neighborhood  of 
the  hair-follicles. 

The  peripheral  end-organ  associated  with  the 
touch  spots  in  the  neighborhood  of  a  hair-follicle  is 
in  all  probability  the  wreath  of  nerve-fibrils  sur- 
rounding the  follicle.  In  regions  devoid  of  hairs 
the  end-organ  is  the  Meissner  corpuscle,  for  in  the 
palmar  surface  of  the  distal  phalanx  of  the  index- 
finger,  where  the  touch  sense  is  quite  acute,  about 
20  corpuscles  are  present  in  each  square  millimeter 
of  surface.  The  specific  stimulus  necessary  to 
evoke  the  sensation  of  touch  is  a  deformation  of  the 
skin;  and  the  greater  this  is,  within  physiologic 
limits,  the  more  pronounced  is  the  sensation. 

Pressure  Sense. — The  contact  of  an  external 
body  is  attended  by  a  certain  amount  of  pressure, 
which,  however,  must  attain  a  certain  degree  before 
the  sensation  can  be  evoked.  This  is  known  as  the 
threshold  value,  or  the  degree  of  liminal  intensity. 
Since  the  sensations  are  the  result  of  pressure,  they  are  termed  pressure 
sensations,  and  their  intensity  may  be  expressed  in  terms  of  pressure. 

The  sensitivity  of  the  skin  as  determined  by  the  pressure  sense  varies  in 
different  regions  of  the  body  and  in  accordance  with  the  size  of  the  area 
pressed.  Thus,  the  liminal  intensity  of  a  stimulus  for  an  area  of  nine  square 
millimeters  for  the  skin  of  the  forehead  is  0.002  gram;  for  the  flexor  aspect 
of  the  forearm,  0.003  gram;  and  for  the  hips,  thigh,  and  abdomen,  0.005 
gram;  for  the  palmar  surface  of  the  finger,  0.019  gram;  for  the  heel,  i  gram. 
The  delicacy  of  the  sense  of  touch  is  measured  by  the  slight  increase  or 
decrease  in  the  intensity  of  the  stimulus  that  will  produce  an  appreciable 
change  in  the  intensity  of  the  sensation.  Not  all  changes  in  the  stimulus, 
however,  are  attended  by  a  change  in  the  sensation.  It  has  been  deter- 
mined that  the  latter  will  change  only  when  the  former  changes  in  a  definite 
ratio,  which  for  the  volar  surface  of  the  third  phalanx  of  the  index-finger  is  as 
29  is  to  30.  Thus,  other  things  being  equal,  a  sensation  caused  by  a  given 
weight  will  only  change  with  moderate  stimulation  when  one-thirtieth  of  the 
weight  is  either  added  or  subtracted.  The  ratio  of  change,  however,  varies 
in  different  regions  of  the  body:  thus,  for  the  back  of  the  hand  the  ratio 


Fig.  297. — Pacinian 
Corpuscles,  c.  Capsules. 
d.  Endothelial  lining  sepa- 
rating the  latter,  n.  Nerve. 
/.  Funicular  sheath  of  nerve. 
in.  Central  mass.  n'.  Ter- 
minal fiber;  and  a.  Where 
it  splits  up  into  finer  fibrils. 
— (Stirling.) 


THE  SENSE  OF  TOUCH.  635 

varies  from  one-tenth  to  one-twentieth;  for  the  tongue,  one-thirtieth  to  one- 
fortieth.  The  difference  of  stimulus  necessary  to  evoke  a  sensation  is 
known  as  the  threshold  difference.  It  seems  to  be  a  law  not  only  for  the 
skin,  but  for  other  senses  as  well,  that  a  change  in  the  intensity  of  a  sensa- 
tion, to  an  appreciable  extent,  will  occur  only  when  the  objective  stimulus 
changes  in  a  definite  ratio.  This  ratio,  however,  will  vary  not  only  in  dif- 
ferent regions  of  the  skin,  in  different  individuals,  but  with  the  sense-organ 
investigated. 

Place  Sense. — The  sensation  evoked  by  stimulation  of  the  skin  is  always, 
under  normal  conditions,  referred  to  the  place  stimulated.  This  holds  true 
not  only  for  two  or  more  points  near  or  widely  separated  on  the  same  side, 
but  also  for  corresponding  points  on  opposite  sides  of  the  body,  even  when 
the  stimuli  have  the  same  intensity  and  are  simultaneously  applied.  The 
cause  for  this  localizing  power  is  to  be  found  in  a  difference  in  the  quality  of 
the  sensation  related  in  some  way  to  the  part  stimulated.  Each  cutaneous 
area  is  supposed  to  give  to  the  tactile  sensation  a  quality  or  local  sign  by 
virtue  of  which  the  mind  is  enabled  to  localize  the  point  of  contact. 

Each  cutaneous  area  which  has  a  local  sign  of  its  own  is  known  as  a 
sensor  circle,  for  the  reason  that  the  mind  does  not  refer  the  sensation  to  a 
point,  but  to  an  area  more  or  less  circular  in  outline.  The  skin  may  there- 
fore be  regarded  as  composed  of  myriads  of  such  circles  varying  in  size  in 
different  regions  of  the  body. 

The  delicacy  of  the  localizing  power  in  any  part  of  the  skin  is  determined 
by  testing  the  power  which  the  part  possesses  of  distinguishing  the  sensa- 
tions produced  by  the  contact  of  the  points  of  a  pair  of  compasses  placed 
close  together.  The  distance  to  which  the  points  must  be  separated  in 
order  to  evoke  two  separate  recognizable  sensations  is  a  measure  of  the 
diameter  of  the  sensor  circle.  Within  this  circle  the  two  sensations  become 
fused  into  one  sensation.  The  discriminative  sensibility  of  different  regions 
as  determined  by  compass  points  is  shown  in  the  following  table;  the  numbers 
represent  the  distances  at  which  two  sensations  are  recognized: 

mm. 

Tip  of  tongue i .  i 

Palmar  surface  of  third  phalanx  of  index  finger 2.2 

Red  surface  of  Ups 4.5 

Palmar  surface  of  first  phalanx  of  finger 5.5 

Tip  of  nose 6.8 

Palm  of  hand 8.9 

Lower  part  of  forehead 22.6 

Dorsum  of  hand 31-6 

Dorsum  of  foot 40 . 6 

Middle  of  the  back 67.7 

The  discriminative  sensibility  of  any  portion  of  the  body  is  a  function  of 
its  mobility.  This  is  shown  by  the  fact  that  it  increases  rapidly  from  the 
shoulders  to  the  fingers  and  from  the  hips  to  the  toes. 

The  Temperature  Sense. — The  sensations  of  heat  and  cold  which  are 
experienced  from  time  to  time  are  caused  by  changes  in  the  temperature  of 
the  skin  produced  in  a  variety  of  ways.  As  these  sensations  are  specifically 
different  from  those  of  touch,  as  well  as  different  from  each  other,  it  is  highly 
probable  that  for  each  sensation  there  are  special  nerve-endings  distributed 
throughout  the  skin.     Investigations  have  shown  that  all  over  the  skin  there 


636 


TEXT-BOOK  OF  PHYSIOLOGY. 


are  innumerable  spots  of  varying  size  which  if  stimulated  evoke  sensations  of 
heat  or  cold.  Such  points  are  termed  heat  and  cold  spots.  Each  responds 
to  but  one  kind  of  stimulus.  A  warm  object  applied  to  a  heat  spot  will 
evoke  a  sensation  of  warmth.  It  will  have  no  effect  on  the  cold  spot.  The 
reverse  is  also  true.  Between  the  cold  and  heat  spots  there  are  areas  that 
are  neutral,  insensitive  to  either  heat  or  cold.  The  cold  spots  are  more 
numerous  than  the  heat  spots  in  almost  all  regions  of  the  body.  (See  Fig. 
298.) 

The  sensitivity  of  the  skin  to  temperature  changes  is  very  acute,  as  shown 
by  the  fact  that  even  0.05°  C.  is  readily  appreciable.  This  holds  true, 
however,  only  when  the  temperature  of  the  object  lies  between  27°  and  33°  C. 
This  capability  varies  in  different  regions  of  the  skin,  and  depends  on  the 


Fig.  298. — Cold  and  Hot  Spots  from  the  Anterior  Surface  of  the  Forearm,  a.  Cold 
spots,  h.  Hot  spots.  The  dark  parts  are  the  most  sensitive,  the  hatched  the  medium,  the  dotted 
the  feebly,  and  the  vacant  spaces  the  non-sensitive. — {Landois  and  Stirling.) 

number  of  heat  and  cold  spots  present,  the  thickness  of  the  epidermis,  the 
thermal  conductivity  of  the  object  touching  it,  and  the  extent  to  which  it  is 
habitually  exposed  or  protected. 

The  physiologic  stimulus  to  the  thermic  end-organs  is  the  passage  of 
heat  through  the  skin  from  "the  interior  of  the  body  to  the  surrounding  air. 
If  the  radiation  is  continuous  and  uniform,  the  end-organs  soon  adapt  them- 
selves to  the  temperature  of  the  surrounding  air  and  the  sensation  of  heat, 
under  physiologic  conditions,  is  not  evoked.  If  there  is  a  sudden  rise  in  the 
external  temperature  caused  by  natural  or  artificial  means,  which  diniinishes 
the  radiation,  the  temperature  of  the  skin  will  at  once  rise,  the  end-organs 
will  be  stimulated,  and  a  sensation  of  warmth  developed.  If,  on  the  other 
hand,  there  is  a  sudden  fall  in  temperature  and  an  increased  radiation,  the 
temperature  of  the  skin  will  fall,  the  end-organs  will  be  stimulated,  and  a 
sensation  of  cold  developed.  Experiment  also  teaches  that  the  intensity  of 
a  warm  or  cold  sensation  will  depend  on  the  existing  temperature  of  the  skin, 
and  not  upon  the  absolute  temperature  of  the  object.     Thus,  water  at  20°  C. 


THE  SENSE  OF  TOUCH.  637 

will  evoke  a  sensation  of  heat  or  cold  respectively  according  as  the  skin 
has  previously  been  cooled  below  or  warmed  above  this  temperature. 

The  Muscle  Sense. — As  a  result  of  the  activities  of  the  musculature  of 
the  body  or  even  of  its  individual  parts,  there  arises  in  consciousness  a 
series  of  sensations  which  are  termed  muscle  sensations.  These  sensations 
give  rise  to  the  perception — 

1.  Of  the  direction  and  duration  of  both  passive  (due  to  external  causes) 

and  active  movements  (due  to  internal,  volitional  efforts)  which  take 
place  without  hindrance; 

2.  Of  the  resistance  offered  to  movements  by  external  bodies;  and 

3.  Of  the  posture  of  the  body  or  of  its  individual  parts. 

As  to  the  seat  of  the  physiologic  processes  which  precede  and  underlie 
the  development  of  the  sensations  two  views,  at  least,  may  be  advanced,  viz. : 

1.  That  the  processes  are  central  in  origin  and  partake  of  the  nature  of  a 

discharge  of  nerve  impulses  from  the  nerve-cells  through  the  motor 
nerves  to  the  muscles,  the  entire  process  being  accompanied  by  sensation. 
This  is  known  as  the  innervation  theory. 

2.  That  the  processes  are  peripheral  in  origin,  initiated  by  stimulation  of 

specialized  end-organs  in  the  muscles  and  tendons  which  are  connected 
through  the  intermediation  of  afferent  nerves  with  nerve-cells  in  the 
cerebral  cortex. 
The  physiologic  mechanism  subserving  the  muscle  sense,  according  to  the 
second  theory,  now  held  by  many  physiologists,  thus  involves  peripheral  end- 
organs,  afferent  ners'es,  their  cortical  connections  and  nerve-cells  in  the 
cerebral  cortex  at  or  near  the  junction  of  the  superior  and  inferior  parietal 
convolutions. 

The  End-organs. — These  are  small  fusiform  structures  found  in  and 
among  the  muscle  bundles  of  all  the  muscles  of  the  body  with  the  exception 
of  the  diaphragm  and  eye  muscles.     In  the  muscles  of  the  arm  and  in  the 


Fig.  299. — A  Neuro-muscle  Spindle  of  a  Cat.     (Ruffini.)     c.  Capsule,    pr.  e.  Primary 
ending,     s.  e.  Secondary  ending,     pi.  e.  Plate  ending.     (All  these  are  probably  sensor  in  function.) 

— (Starling's  " Physiology.") 

small  muscles  of  the  hand  they  are  especially  abundant.  From  their  shape 
they  are  known  as  muscle  spindles.  They  vary  in  length  from  2  to  12  mm. 
and  in  breadth  from  0.15  to  0.4  mm.  Each  spindle  (Fig.  299 j  consists  of  a 
connective-tissue  capsule  containing  from  two  to  ten  longitudinally  arranged 
striated  muscle  fibers  of  fine  diameter.  In  the  middle  or  equatorial  region  of 
these  intra-fusal  fibers  there  is  frequently  found  a  quantity  of  non-striated 
protoplasmic  matter.  The  spindle  is  supplied  with  both  sensor  and  motor 
nerves.  The  sensor  fiber  loses  its  external  investments  as  it  approaches  the 
capsule.     The  naked  axis-cylinder  then  penetrates  the  capsule,  and  after 


638  TEXT-BOOK  OF  PHYSIOLOGY. 

dividing  several  times  terminates  in  a  ribbon-like  or  spiral  manner  around  the 
intra-fusal  muscle  fiber.  This  ending  was  described  by  and  is  known  as 
Ruffini's  "  annulo-spiral  ribbon."  The  motor  nerve  also  penetrates  the 
capsule  and  terminates  in  the  polar  extremities  of  the  intra-fusal  fiber. 
Sensor  end-organs  supposed  to  be  connected  with  the  muscle  sense  are  also 
found  in  the  tendons  of  muscles. 

Afferent  Nerves. — That  muscles  are  abundantly  supplied  with  afferent 
nerves  has  been  proved  by  different  methods  of  investigation.  With  histo- 
logic methods  Sherrington  has  traced  afferent  fibers  from  the  muscle  spindles 
directly  into  the  spinal  nerve  ganglia.  The  contractions  of  muscles  from 
electric  stimulation  as  well  as  the  contractions  known  as  muscle  cramp, 
due  to  unknown  agents,  give  rise  to  sensations  of  pain,  a  fact  which  in- 
dicates the  presence  in  muscles  of  afferent  or  sensor  nerves. 

Cortical  Area. — Pathologic  findings  have  shown  that  an  impairment 
or  a  loss  of  the  muscle  sense  is  associated  with  destructive  lesions  of  perhaps 
the  super-  and  sub-parietal  convolutions  (Figs.  252,  253).  In  a  case  reported 
by  Starr  the  removal  of  a  small  tumor  in  the  pia  mater  situated  over  the 
junction  of  the  superior  and  inferior  parietal  lobules  was  followed  by  a  loss 
of  the  muscle  sense  and  marked  ataxia  in  the  right  hand  for  a  period  of  six 
weeks,  after  which  recovery  took  place.  These  symptoms  were  attributed 
to  injury  of  the  cortex  from  unavoidable  surgical  procedures. 

The  muscle  sensations,  as  stated  in  foregoing  paragraphs,  form  the 
basis  of  the  perception  not  only  of  the  direction  and  the  duration  of  a  body 
movement  and  the  resistance  experienced,  but  also  of  the  position  and  the 
tension  of  the  muscle  groups.  The  latter  fact  more  especially  makes  it 
possible  for  the  mind  to  direct  the  muscles  and  to  graduate  the  energy 
necessary  to  the  accomplishment  of  a  definite  purpose. 

Active  Touch. — Active  touch  or  the  application  of  the  fingers  to  the 
surfaces  of  external  objects  implies  the  cooperation  of  the  skin  and  the  muscles. 
The  sensations  which  are  evoked  are  combinations  of  contact  and  muscle 
sensations.  The  union  of  these  sensations  forms  the  basis  of  the  perception 
of  hardness,  softness,  smoothness,  and  roughness  of  bodies. 

THE  SENSE  OF  TASTE. 

The  physiologic  mechanism  involved  in  the  sense  of  taste  includes  the 
tongue,  the  gustatory  nerves  (the  chorda  tympani  and  the  glosso-pharyngeal), 
their  cortical  connections  and  nerve-cells  in  the  gray  matter  of  the  fourth 
temporal  convolutions.  The  peripheral  excitation  of  this  apparatus  gives 
rise  to  nerve  impulses  which  transmitted  to  the  brain  evoke  the  sensations  of 
taste.  The  specific  physiologic  stimulus  is  matter,  organic  and  inorganic, 
in  a  state  of  solution. 

The  Tongue.— The  tongue  consists  of  both  intrinsic  and  extrinsic  mus- 
cles, in  virtue  of  which  it  is  susceptible  of  a  change  both  in  shape  and  in 
position.  The  movements  of  the  tongue,  though  not  essential  to  taste,  are 
made  use  of  in  the  finer  discrimination  of  tastes. 

The  tongue  is  covered  over  by  mucous  membrane  continuous  with  that 
lining  the  oral  cavity.  The  dorsum  of  the  tongue  presents  a  series  of  papillae 
richly  supplied  with  blood-vessels  and  nerves.  Of  these  there  are  three 
varieties,  the  filiform,  the  fungiform,  and  the  circumvallate  (Fig.  300). 


THE  SENSE  OF  TASTE. 


639 


2. 


Fig.  300. — The  Tongue. 

1.  PapillcT    circumvallaiie. 

2.  Papillae  fungiformes. 


The  filiform  papillcE,  the  most  numerous,  cover  the  anterior  two-thirds  of 
the  tongue;  they  are  conical  or  fiUform  in  shape  and  covered  with 
horny  epithehum  which  is  often  prolonged  into  filamentous  tufts. 
The  fungiform  papillcB,  found  chiefly  at  the  tip 
and  sides  of  the  tongue  are  less  numerous  but 
larger  than  the  preceding  and  of  a  deep  red 
color. 
3.  The  circumvallate  papillcB,  from  eight  to  ten  in 
number,  are  situated  at  the  base  of  the  tongue  ~ 
arranged  in  the  form  of  the  letter  V.  They  con- 
sist of  a  central  projection  surrounded  by  a 
wall  or  circumvallation  from  which  they  take 
their  name. 

The  Peripheral  End-organs.  The  Taste- 
buds. — Embedded  in  the  epithelium  covering  the 
mucous  membrane  not  only  of  the  tongue  but  of 
the  palate  and  posterior  surface  of  the  epiglottis  are 
small  ovoid  bodies  which  from  their  relation  to  the 
gustatory  nerv^es  are  regarded  as  their  peripheral 
end-organs  and  known  as  taste-buds  or  taste-beakers. 
Each  bud  is  ovoid  in  shape  (Fig.  301).  Its  base  rests 
on  the  tunica  propria ;  its  apex  comes  up  to  the  epi- 
thelium, where  it  presents  a  narrow  funnel-shaped 
opening,  the  taste-pore.  The  wall  of  the  bud  is  composed  of  elongated 
curved  epithelium.  The  interior  contains  narrow  spin- 
dle shaped  neuro-epithelial  cells  provided  at  their  outer 
extremity  with  stiff  hair-like  filaments  which  project 
into  the  taste-pore. 

The  neuro-epithelial  cells  are  in  physiologic  relation 
with  the  nerv-es  of  taste.  The  terminal  branches,  after 
entering  the  bud  at  its  base,  develop  fine  tufts  which 
come  into  contact  with  the  cells.  That  the  taste-buds 
are  connected  with  the  nervTS  of  taste  is  rendered  prob- 
able from  the  fact  of  their  degeneration  after  division  of 
the  nerves. 

The  Taste  Area. — ^The  taste  area,  though  confined 
for  the  most  part  to  the  tongue,  extends  in  different  in- 
dividuals to  the  mucous  membrane  of  the  hard  palate, 
to  the  anterior  surface  of  the  soft  palate,  to  the  uvula, 
the  anterior  and  posterior  half  arches,  the  tonsils,  the 
posterior  wall  of  the  pharynx,  and  the  epiglottis. 

The  Taste  Sensations. — The  sensations  which  arise 
in  consequence  of  impressions  made  by  different  sub- 
stances on  the  peripheral  apparatus  of  this  area  are  in  so 
many  instances  combinations  of  taste,  touch,  temperature, 
and  smell  that  they  are  extremely  dilffcult  of  classifica- 
tion. Nevertheless  six  primary  tastes  can  be  recognized: 
bitter,  sweet,  acid  or  sour,  salt  or  saline,  alkaline  and  metallic.  Though 
the  contact  of  any  bitter,  sweet,  acid,  salt,  etc.,  substance  with  any  part  of 


Fig.  301. — Taste- 
bud  FROM  Circum- 
vallate Papilla  of 
A  Child.  The  oval 
structure  is  limited  to 
the  epithelium  (e) 
lining  the  furrow, 
encroaching  slightly 
upon  the  adjacent 
connective  tissue  (/) ; 
o,  taste-pore  through 
which  the  taste-cells 
communicate  \\'ith 
the  mucoi's  surface. 
— {Ajicr  Pier  sol.) 


640  TEXT-BOOK  OF  PHYSIOLOGY. 

the  tongue  will,  if  the  substance  be  present  in  suflficient  quantity  or  con- 
centration, develop  a  corresponding  sensation,  some  regions  of  the  tongue 
are  more  sensitive  and  responsive  than  others.  Thus,  the  posterior  por- 
tion is  more  sensitive  to  bitter  substances  than  the  anterior;  the  reverse  is 
true  for  sweet  substances  and  perhaps  for  acids  and  salines. 

The  intensity  of  the  resulting  sensation  in  any  given  instance  will  depend 
on  the  degree  of  concentration  of  the  substance,  while  its  massiveness  will 
depend  on  the  area  affected. 

THE  SENSE  OF  SMELL. 

The  physiologic  mechanism  involved  in  the  sense  of  smell  includes  the 
nasal  fossse,  the  olfactory  nerves,  the  olfactory  tracts,  and  nerve-cells  in  those 
areas  of  the  cortex  known  as  the  uncinate  convolution  and  anterior  part  of 
the  gyrus  fornicatus.  Peripheral  stimulation  of  this  mechanism  develops 
nerve  impulses  which,  transmitted  to  the  cortex,  evoke  the  sensations  of  odor. 
The  specific  physiologic  stimulus  is  matter  in  the  gaseous  or  vaporous  state. 

The  Nasal  Fossae. — -The  nasal  fossae  are  irregularly  shaped  cavities 
separated  by  a  vertical  septum  formed  by  the  perpendicular  plate  of  the 
ethmoid  bone,  the  vomer,  and  the  triangular  cartilage.  The  outer  wall 
presents  three  recesses  separated  by  the  projection  inward  of  the  turbinated 
bones.  Each  fossa  opens  anteriorly  and  posteriorly  by  the  anterior  and 
posterior  nares,  the  latter  communicating  with  the  pharynx.  Both  fossae  are 
lined  throughout  by  mucous  membrane.  The  upper  part  of  the  fossa  is 
known  as  the  olfactory,  the  lower  portion  as  the  respiratory  region.  In  the 
former,  the  mucous  membrane  over  the  septum  and  superior  turbinated 
bone  is  somewhat  thicker  than  elsewhere  and  covered  with  a  neuro-epithelium 
which  constitutes 

The  Peripheral  End-organ. — This  consists  of  a  basement  membrane 
supporting  two  kinds  of  cells,  the  olfactory  and  the  sustentacular.  The 
olfactory  cells  are  bipolar  nerve-cells,  the  center  of  which  contains  a  large 
spheric  nucleus.  The  peripheral  pole  is  cylindric  or  conic  in  shape  and 
provided  at  its  extremity  with  several  hair-like  processes.  The  central 
pole  becomes  the  axon  process  and  passes  directly  to  the  olfactory  bulb. 

The  sustentacular  cells  are  epithelial  in  character  and,  as  their  name 
implies,  support  or  sustain  the  olfactory  cells. 

For  the  appreciation  of  odorous  particles  the  air  must  be  drawn  through 
the  nasal  fossae  with  a  certain  degree  of  velocity.  If  the  particles  are  widely 
diffused  in  the  air,  they  must  be  drawn  not  only  more  quickly  but  more 
forcibly  into  contact  with  the  olfactory  hairs,  as  in  the  act  of  sniffing,  the 
result  of  short  energetic  inspirations.  To  many  substances  the  olfactory 
apparatus  is  extremely  sensitive.  Thus,  it  has  been  shown  that  a  particle 
of  mercaptan  the  actual  weight  of  which  was  calculated  to  be  t'sit'o  o  o  o  o  o 
of  a  milligram  gives  rise  to  a  distinct  sensation. 

The  Olfactory  Sensations. — The  sensations  which  arise  in  consequence 
of  the  excitation  of  the  olfactory  apparatus  are  very  numerous  and  their 
classification  is  extremely  difficult.  For  this  reason  it  is  customary  to  divide 
them  into  two  groups:  viz.,  agreeable  and  disagreeable,  in  accordance  with 
the  feelings  they  excite  in  the  individual.     As  the  olfactory  sensations  give 


THE  SENSE  OF  SMELL.  641 

rise  to  feelings  rather  than  ideas,  this  sense  plays  in  man  a  subordinate  part 
in  the  acquisition  of  knowledge.  In  lower  animals  this  sense  is  employed 
for  the  purpose  of  discovering  and  securing  food,  for  detecting  enemies  and 
friends,  and  for  sexual  purposes.  In  land  animals  the  entire  olfactory  appa- 
ratus is  well  developed  and  the  sense  keen;  in  some  aquatic  animals,  as  the 
dolphin,  whale,  and  seal,  the  apparatus  is  poorly  developed  and  the  sense 
dull. 


41 


CHAPTER  XXVII. 
THE  SENSE  OF  SIGHT. 

The  physiologic  mechanism  involved  in  the  sense  of  sight  includes  the 
eyeball,  the  optic  nerve,  the  optic  tracts,  their  cortical  connections,  and  nerve- 
cells  in  the  cuneus  and  adjacent  gray  matter.  Peripheral  stimulation  of  this 
mechanism  develops  nerve  impulses  which  transmitted  to  the  cortex  evoke 
(i)  the  sensation  of  light  and  its  different  qualities — colors;  (2)  the  perception 
of  light  and  color  under  the  form  of  pictures  of  external  objects;  and  (3)  in 
connection  with  the  ocular  muscles,  the  production  of  muscle  sensations  by 
which  the  size,  distance,  and  direction  of  objects  may  be  judged. 

The  specific  physiologic  stimulus  to  the  terminal  end-organ,  the  retina, 
is  the  impact  of  ether  vibrations.  In  general,  it  may  be  said  that,  at  least 
for  the  same  color,  the  intensity  of  the  objective  vibration  determines  the 
intensity  of  the  sensation. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EYEBALL. 

The  eyeball  is  situated  at  the  fore  part  of  the  orbit  cavity,  and  in  such  a 
position  as  to  permit  of  an  extensive  range  of  vision.  It  is  loosely  held  in 
position  by  a  fibrous  jnembrane,  the  capsule  of  Tenon,  which  is  attached, 
on  the  one  hand,  to  the  eyeball  itself,  and,  on  the  other,  to  the  walls  of  the 
orbit  cavity.  Thus  suspended,  the  eyeball  is  susceptible  of  being  turned  in 
any  direction  by  the  contraction  of  the  muscles  attached  to  it. 

The  ball  is  spheroid  in  shape,  measuring  about  24  millimeters  in  its 
antero-posferior  diameter  and  a  little  less  in  its  transverse  and  vertical 
diameters.  When  viewed  in  profile,  it  is  seen  to  consist  of  the  segments  of 
two  spheres,  of  which  the  posterior  is  the  larger,  occupying  five-sixths,  and 
the  anterior  is  the  smaller,  occupying  one-sixth  of  the  ball.  It  is  composed 
of  several  concentrically  arranged  membranes  enclosing  various  refracting 
media  essential  to  vision. 

The  membranes,  enumerating  them  from  without  inward,  are  as  follows: 
the  sclera  and  cornea,  the  chorioid  and  iris,  and  the  retina.  The  refracting 
media  are  the  aqueous  humor,  the  crystalline  lens,  and  the  vitreous  humor. 

The  Sclera  and  Cornea. — The  sclera  is  the  thick  opaque  membrane 
covering  the  posterior  five-sixths  of  the  ball.  It  is  composed  of  layers  of 
connective  tissue  which  are  arranged  transversely  and  longitudinally.  It 
is  pierced  posteriorly  by  the  optic  nervx  about  3  or  4  millimeters  internal  to 
the  optic  axis.  By  virtue  of  its  firmness  and  density  the  sclera  gives  form  to 
the  eyeball,  protects  delicate  structures  enclosed  by  it,  and  serves  for  the 
attachment  of  the  muscles  by  which  the  ball  is  moved  (Figs.  302,  307).  The 
cornea  is  the  transparent  membrane  forming  the  anterior  one-sixth  of  the  ball. 
It  is  nearly  circular  in  shape,  measuring  in  its  horizontal  meridian  12  mm., 
in  its  vertical  meridian  11  mm.     The  curvature  is  therefore  sharper  in  the 

642 


THE  SENSE  OF  SIGHT. 


643 


latter  than  in  the  former.  The  radius  of  curvature  of  the  anterior  surface 
at  that  central  portion  ordinarily  used  in  vision  is  7.829  mm.;  that  of  the 
posterior  surface  about  6  mm. 

The  substance  of  the  cornea  is  made  up  of  thin  layers  of  delicate  trans- 
parent fibrils  of  connective  tissue  continuous  with  those  found  in  the  sclera. 
Lymph-spaces  are  present  throughout  the  cornea,  in  which  are  to  be  found 
lymph-corpuscles.  The  anterior  surface  of  the  cornea  is  covered  with  several 
layers  of  nucleated  epithelium  supported  by  a  structureless  membrane,  the 
anterior  elastic  lamina.  The  posterior  surface  also  is  covered  by  a  layer  of 
epithelium  supported  by  a  similar  membrane,  the  posterior  elastic  lamina  or 
the  membrane  of  Descemet, 

which   at   its  periphery  be-  — ?«  l^J2J^i5  o   t. 

comes  continuous  with  the 
iris.  At  the  junction  of  the 
cornea  and  sclera  there  is  a 
circular  groove,  known  as 
the  canal  of  Schlemm. 

The  posterior  elastic  lam- 
ina, near  the  margin  of  the 
cornea,  breaks  up  into  fibers 
to  form  a  network  structure, 
the  intervals  between  the 
fibers  of  which  are  known 
as  the  spaces  of  Fontana. 
These  spaces  are  in  com- 
munication with  the  canal  of 
Schlemm. 

The  Chorioid,  Iris, 
Ciliary  Muscle,  and  Ciliary 
Processes. — The  chorioid  is 
the  dark  brown  membrane 
which  extends  forward  nearly 
to  the  cornea,  where  it  ter- 
minates in  a  series  of  folds,  the 
ciliary  processes.  Posteri- 
orly, it  is  pierced  by  the  optic 
nerve.  It  is  composed  largely 
of  blood-vessels,  arteries,  capillaries,  and  veins,  supported  by  connective 
tissue.  Externally  it  is  loosely  connected  to  the  sclera;  internally  it  is  lined 
by  a  layer  of  hexagonal  cells  containing  black  pigment  which,  though  usually 
described  as  a  part  of  the  chorioid,  are  now  known  to  belong,  embryolog- 
ically  and  physiologically,  to  the  retina.  Lying  within  the  outer  layer  ,of 
arteries  and  veins  there  is  a  thick  layer  of  small  arterioles  and  capillaries, 
known  as  the  chorio-capillaris.  The  chorioid  with  its  contained  blood- 
vessels bears  an  important  relation  to  the  nutrition  and  function  of  the  eye. 
It  provides  a  free  supply  of  lymph  and  presents  a  uniform  tempera'ture  to 
the  retina  in  contact  vv^ith  it. 

The  iris  is  the  circular,  variously  colored  membrane  in  the  anterior  part 
of  the  eye  just  behind  the  cornea.     It  presents  a  little  to  the  nasal  side  of  the 


Fig.  302. — Chorioid  Coat  of  the  Eye.  i.  Optic- 
nerve.  2,  2,  2,  2,  3,  3,  3,  4.  Sclerotic  coat  divided  and 
turned  back  to  show  the  chorioid.  5,  5,  5,  5.  The 
cornea,  divided  into  four  portions  and  turned  back. 
6,  6.  Canal  of  Schlemm.  7.  External  surface  of  the 
chorioid,  traversed  by  the  ciliary  nerves  and  one  of  the 
long  ciliary  arteries.  8.  Central  vessel  into  which 
open  the  vasa  vorticosa.  9,  q,  10,  10.  Chorioid  zone. 
II,  II.  Ciliar}'  nerves.  12.  Long  ciliary  artery.  13, 
13,  13,  13.  Anterior  ciliary  arteries.  14.  Iris.  15,  15. 
Vascular  circle  of  the  iris.     1 6.  Pupil. — (Sappey.) 


644 


TEXT-BOOK  OF  PHYSIOLOGY. 


center  a  circular  opening,  the  pupil.  The  outer  or  circumferential  border 
is  united  by  connective  tissue  to  the  cornea,  sclera,  and  ciliary  muscle;  the 
inner  border  forms  the  boundary  of  the  pupil.  The  iris  consists  of  a  frame- 
work of  connective  tissue  supporting  blood-vessels,  muscle-fibers,  and  pig- 
mented connective-tissue  cells.  The  anterior  surface  is  covered  by  a  layer 
of  cells  continuous  with  those  covering  the  posterior  surface  of  the  cornea. 
The  posterior  surface  is  formed  by  a  thin  structureless  membrane  supporting 
a  layer  of  pigment  cells  continuous  with  those  lining  the  chorioid.  The 
color  which  the  iris  presents  in  different  individuals  depends  on  the  relative 
amount  of  pigment  in  the  connective-tissue  corpuscles.  In  blue  eyes  the 
pigment  is  wanting.  In  gray,  brown,  and  black  eyes  the  pigment  is  present 
in  progressively  increasing  amounts.  The  blood-vessels  are  connected  with 
those  of  the  chorioid  coat. 

The  muscle-fibers  are  of  the  non-striated  variety  and  arranged  in  two  sets, 
one  circularly,  the  other  radially,  disposed. 

The  circular  fibers  are  found  close  to  the  pupil  near  the  posterior  surface 
of  the  iris.  Contraction  of  this  band  of  fibers  diminishes,  relaxation  increases, 
the  size  of  the  pupil.  This  muscle  is  known  as  the  sphincter  pupillcB  or 
sphincter  i  rid  is. 


Fig.  303.- — Section  through  the  Ciliary  Region  of  the  Human  Eye.  a.  Radiating 
bundles  of  the  ciliary  muscle,  b.  Deeper  bundles,  c.  Circular  network,  d.  Annular  muscle  of 
Miiller.  e.  Tendon  of  ciliary  muscle.  /.  Muscle-fibers  on  posterior  side  of  the  iris.  g.  Muscles 
on  the  ciliary  border  of  the  same.     h.  Ligamentum  pectinatum. — (After  Iwanoff.) 

The  radial  fibers  form  a  more  or  less  continuous  layer  in  the  posterior 
part  of  the  iris,  extending  from  the  margin  of  the  pupil,  where  they  blend 
with  the  circular  fibers,  to  the  outer  border.  Contraction  of  the  fibers  in- 
creases the  size  of  the  pupil.     The  muscle  is  known  as  the  dilatator  pupillcB. 

The  nerves  exciting  the  sphincter  pupillce  to  action  are  the  ciliary  nerves, 
axons  of  nerve-cells  located  in  the  ciliary  or  ophthalmic  ganglion.  Stimula- 
tion of  these  fibers  gives  rise  to  contraction  of  the  sphincter  and  diminution  in 
the  size  of  the  pupil.  The  nerves  exciting  the  dilatator  pupillm  to  action  are 
axons  of  nerve-cells  located  in  the  superior  cervical  ganglion.  They  reach 
the  iris  by  way  of  the  cervical  sympathetic,  the  ophthalmic  division  of  the 
fifth,  and  the  long  ciliary  nerve.  Stimulation  of  these  nerv^es  is  followed  by 
contraction  of  the  dilatator  and  an  increase  in  the  size  of  the  pupil.  Both 
the  ciliary  and  superior  cervical  ganglia  are  in  relation  with  pre-ganglionic 
fibers  coming  from  the  central  nerve  system.     (See  pages  582,  618.) 

The  ciliary  muscle  is  a  gray  circular  band  about  two  millimeters  in 
width,   consisting  of  non-striated   muscle-fibers.     The  majority  of  these 


THE  SENSE  OF  SIGHT. 


64s 


Layer  of  rods  and  cones. 


3.  External  limiting  membrane. 


fibers  pursue  a  radial  or  meridional  direction.  Taking  their  origin  from  the 
junction  of  the  sclera,  cornea,  and  iris,  they  pass  backward  to  be  inserted 
into  the  chorioid  coat  opposite  the  ciliary  processes.  The  inner  portion  of 
the  muscle  is  interrupted  by  bundles  of  libers  which  pursue  a  circular 
direction.  (Fig.  303.)  They  collectively  constitute  the  annular  or  ring 
muscle  of  Miiller.  The  ciliary  muscle  in  common  with  the  circular  fibers 
of  the  iris  receives  its  nerv-e-supply  direct  from  the  nerve-cells  in  the  ciliary 
ganglion.     Contraction  of  the  .„.         ,       .       ,       ^ 

*-".,,  ,11  I-  Pigment-layer  (not  shown). 

ciliary  muscle  tenses  the  cho- 
rioid coat,  and  for  this  reason 
it  is  frequently  termed  the  ten- 
sor chorioidecE. 

The  Retina. — The  retina 
is  the  internal  coat  of  the  eye, 
extending  forward  almost  to 
the  ciliary  processes,  where  it 
terminates  in  an  indented  bor- 
der, known  as  the  ora  serrata. 
In  the  living  condition  it  is 
clear,  transparent  and  pink 
in  color.  After  death  it  be- 
comes opaque.  The  retina  is 
abundantly  supplied  with 
blood-vessels,  derived  from 
the  arteria  centralis  retince,  a 
branch  of  the  ophthalmic, 
which  pierces  the  optic  nerve 
near  the  sclera,  runs  forward 
in  its  center,  to  the  retina,  in 

which  its  terminal  branches  are  distributed.  The  veins  arising  from  the 
capillary  plexus  leave  the  retina  by  the  same  route. 

In  the  posterior  portion  of  the  retina,  at  a  point  corresponding  with  the 
axis  of  vision,  there  is  a  small  oval  area  about  2  mm.  in  its  transverse  and 
about  0.8  mm.  in  its  vertical  diameter.  From  the  fact  that  it  presents  a 
yellow  appearance,  it  is  known  as  the  macula  lutea.  This  area  presents  in 
its  center  a  depression  with  sloping  sides,  known  as  the  fovea  centralis. 
About  3.5  mm.  to  the  nasal  side  of  the  macula  is  the  point  of  entrance  of  the 
optic  nen^e. 

The  retina  is  remarkably  complex  in  structure,  presenting  an  appearance, 
when  viewed  microscopically,  something  like  that  represented  in  Fig.  304, 
indicating  that  it  is  composed  of  different  cellular  elements  arranged  in 
layers.     These  have  been  named,  from  behind  forward,  as  follows: 


Outer  nuclear  layer. 


5.  Outer  molecular  layer. 

6.  Inner  nuclear  layer. 

7.  Inner  molecular  layer. 

8.  Layer  of  ganglion  cells. 

9.  Layer  of  nerve-fibers. 


Fig. 


304. — Vertical  Section  of  Human  Retina. 
— (Schaper.) 


The  layer  of  pigment  cells. 

The  layer  of  rods  and  cones,  or  Jacobson's  layer. 

The  external  limiting  membrane. 

The  outer  nuclear  or  granular  layer. 

The  outer  molecular  or  reticular  layer. 

The  inner  nuclear  or  granular  layer. 

The  inner  molecular  or  reticular  laver. 


646 


TEXT-BOOK  OF  PHYSIOLOGY. 


8.  The  layer  of  ganglion  cells. 

9.  The  layer  of  nerve-fibers. 

Modern  histologic  methods  of  research  have  made  it  possible  to  reduce  the 
retina,  exclusive  of  the  pigment  cells,  to  three  successive  layers  of  nerve-cells, 
supported  by  a  highly  developed  neuroglia,  forming  what  has  been  termed  the 
fibers  of  Miiller.     These  nerve-cells  are  as  follows: 

1.  The  visual  cells. 

2.  The  bipolar  cells. 

3.  The  ganglion  cells. 

The  relation  of  these  nerve-cells  one  to  another  and  to  the  supporting  neurog- 
lia tissue  and  the  manner  in  which  they  unite  to  form  the  above-mentioned 
layers  are  schematically  shown  in  Fig.  305. 

The  pigment  layer  is  composed  of 
hexagonal  cells.  Though  formerly 
described  as  forming  a  part  (the  inner 
layer)  of  the  chorioid,  these  cells  belong 
embryologically  to  the  retina.  From 
their  retinal  surface  delicate  pigmented 
processes  extend  into  and  between 
the  rods  and  cones.  On  exposure  to 
light  these  procesess  elongate  and 
push  themselves  between  the  rods. 
In  the  dark  they  retract  and  withdraw 
into  the  cell-body. 

The  visual  cells  which  form  the 
layer  of  rods  and  cones  are  of  two  varie- 
ties, the  rod-shaped  and  the  cone- 
shaped. 

The  rod-shaped  visual  cell  con- 
sists of  a  straight  elongated  cylinder 
extending  through  the  entire  thickness 
of  Jacobson's  membrane  and  a  fine 
fiber  containing  a  nucleus,  which, 
after  piercing  the  external  limiting 
membrane,  passes  into  the  outer  molec- 
ular layer,  where  it  terminates  in  a 
spheric  enlargement.  The  outer  por- 
tion of  the  rod  is  clear  and  homo- 
geneous, though  containing  a  pigment 
known  as  visual  purple  or  rhodopsin; 
the  inner  portion  of  the  rod  is  slightly  granular. 

The  cone-shaped  visual  cells  also  consist  of  two  portions,  a  conic  portion 
situated  in  Jacobson's  membrane  between  the  rods,  and  a  fine  fiber,  contain- 
ing a  nucleus,  which,  after  piercing  the  external  limiting  membrane,  passes 
into  the  outer  riiolecular  layer,  where  it  terminates  in  a  fine  tuft.  The 
inner  portion  of  the  cone  is  thicker  than  the  rod  and  rests  on  the  limiting 
membrane;  the  outer  portion  tapers  to  a  fine  point  and  is  known  as  the 
cone-style.  The  cones,  as  a  rule,  are  shorter  than  the  rods.  The  proportion 
of  rods  to  cones  varies  in  different  parts  of  the  retina,  though  there  are  on 


Fig.  305. — Cross-section  of  the  Retina 
PROM  A  Mammal.  A.  Layer  of  rods  and 
cones.  B.  Visual  cells  (outer  granules). 
C.  Outer  molecular  layer.  E.  Eipolar  cells 
(inner  granules).  F.  Inner  molecular  layer. 
G.  Ganglion  cells.  H.  Layer  of  nerve- 
fibers,  a.  Rods.  b.  Cones.  e.  Bipolar 
rod.  f.  Bipolar  cone.  r.  Lower  ramifica- 
tion of  a  bipolar  rod.  f.  Lower  ramification 
of  a  bipolar  cone,  g,  h,  i,  j,  k.  Ganglion 
cells  in  various  stages,  branching  from  F. 
X,  z.  Bipolar  contact  of  rods  and  cones,  t. 
Miiller's  supporting  fibers.  S.  Centrifugal 
nerve-fibers. — {After  Ramon  y  Cajal.) 


THE  SENSE  OF  SIGHT. 


647 


the  average  about  fourteen  rods  to  one  cone.  In  the  macula  the  rods  are 
entirely  absent,  cones  alone  being  present. 

The  layer  of  visual  cells  together  with  the  neuroglia  constitutes  the  first 
of  the  three  layers  of  the  retina  proper.  The  external  limiting  membrane  is 
formed  by  the  bending  of  the  ends  of  neuroglia  cells. 

The  bipolar  cells  consist  of  a  central  portion,  found  in  the  inner  nuclear 
layer,  from  which  are  given  off  two  processes  which  pass  in  opposite  direc- 
tions, one  toward  the  \dsual  cells,  the  other  toward  the  ganglion  cells.  The 
former  terminate  in  tufts  which  arborize  around  the  tufts  and  spheric  en- 
largements of  the  visual  cells,  and  assist  in  the  formation  of  the  outer  molec- 
ular layer;  the  latter  terminate  in  similar  tufts  in  the  inner  molecular 
layer. 

The  ganglion  cells  are  arranged  in  a  single  layer,  as  a  rule.  They 
are  large  and  nucleated.  From  the  inner  side  of  each  cell  there  is  given 
off  a  single  axon  which  passes  toward  the  center  of  the  retina  (forming  the 
nerv'e-fiber  layer),  where  it  enters  and  assists  in  forming  the  optic  nerve. 


Fig.  306. — Horizontal  Section  through  the  ALacula  and  Fovea  of  a  Man  Sixty  Years 
Old.  The  section  is  not  through  the  e.xact  center  of  the  fovea,  for  there  are  only  cone  \isual  cells 
and  no  remnants  of  the  confluence  of  the  inner  granule  and  ganglion  cell  layers  are  present,  i. 
Cones.  2.  External  limiting  membrane.  3.  Outer  nuclear  layer.  4.  Henle's  fiber  layer.  5. 
Outer  molecular  or  reticular  layer.  6.  Inner  nuclear  layer.  7.  Inner  molecular  or  reticular 
layer.     8.  Layer  of  gangUon  cells.     9.  Nerve-fiber  layer. — {After  S chap er,  St ohr's  "Histology.") 

From  the  outer  side  of  the  ganglion  cell  dendrites  pass  into  and  assist  in 
forming  the  inner  molecular  layer.  These  dendrites  come  into  physiologic 
relation  with  those  of  the  inner  processes  of  the  bipolar  cells. 

Horizontally  disposed  nerve-cells  are  also  present  in  the  outer  molecular 
layer  in  relation  with  the  visual  cells.  Spongioblasts  or  amacrine  cells  are 
also  present  at  the  border  of  and  in  the  inner  molecular  layer. 

From  the  relation  of  the  ganglion  cells,  in  which  the  optic  nerve-fibers 
take  their  origin,  to  the  visual  cells  and  the  bipolar  cells,  the  former  may 
be  regarded  as  the  terminal  visual  organ,  the  intermediary  between  the 
ether  vibrations  and  the  ganglion  cell.  The  visual  cells  are  directed  toward 
the  chorioid,  away  from  the  entering  light,  dipping  into  the  pigment  cells. 


648 


TEXT-BOOK  OF  PHYSIOLOGY. 


They,  with  the  pigment  layer,  are  the  elements  by  which  the  ether  vibrations 
are  transformed  into  nerve  energy. 

In  the  fovea  most  of  the  retinal  elements  are  wanting  or  are  reduced  in 
thickness.  The  cones  alone  are  present.  The  cone-fibers  with  their  nuclei 
are  directed  obliquely  upward  and  outward  along  the  slope  of  the  fovea, 
to  end  in  tufts  which  come  into  physiologic  relation  with  the  dendrites  of 
the  ganglion  cells,  which  at  the  top  of  the  fovea  are  generally  increased  in 
number  (Fig.  306). 

It  is  estimated  that  the  optic  nevxe  contains  about  500,000  nerve-fibers, 
and  that  for  each  fiber  there  are  about  7  cones,  100  rods,  and  7  pigment 
cells.  In  accordance  with  this  estimate  there  would  be  about  3,500,000 
cones,  50,000,000  rods,  and  3,500,000  pigment  cells.  The  distance  be- 
tween the  centers  of  two  adjacent  cones  in  the  fovea  is  4  micromillimeters. 

Media.  The  vitreous  humor  is  the  largest  of  the  refracting  media  and 
occupies  by  far  the  largest  portion  of  the  interior  of  the  eyeball.  From  its 
position  it  gives  support  to  the  retina.  Anteriorly  it  presents  a  concavity, 
in  which  the  crystalline  lens  is  lodged.     The  vitreous  humor  consists  of 


Fig.  307. — Horizontal  Section  of  the  Eyeball,  i.  Sclera.  2.  Cornea.  3.  Chorioid. 
4.  Iris.  5.  Ciliary  muscle.  6.  Retina.  7.  Lens.  8.  Suspensory  ligament.  9.  Canal  of  Schlemm. 
10.  Canal  of  Petit.     11.  Optic  nerve. — {Deaver). 

water  (97  per  cent.),  organic  matter  and  salts,  enclosed  in  a  transparent 
membrane,  the  tunica  hyaloidea.  The  mass  of  the  vitreous  humor  is  pene- 
trated by  a  species  of  connective  tissue. 

The  aqueous  humor  is  small  in  amount  in  comparison  with  the  vitreous 
and  is  found  in  the  space  bounded  by  the  cornea,  the  ciliary  body,  the  sus- 
pensory ligament,  and  the  lens.  The  projection  of  the  iris  into  this  space 
partially  divides  into  an  anterior  and  posterior  portion  or  chamber.  The 
aqueous  humor  is  a  clear,  watery,  alkaline  fluid  derived  from  or  secreted 


THE  SENSE  OF  SIGHT.  649 

by  the  capillary  blood-vessels  of  the  ciHary  body.  From  this  origin  it  passes 
through  the  pupil  into  the  anterior  chamber.  It  serves  to  keep  the  cornea 
tense  and  smooth.  The  ocular  tension  depends  partly  on  the  presence  of 
this  fluid  in  the  eyeball.  There  is  every  reason  for  believing  that  there  is  a 
constant  stream  of  fluid  from  the  blood-vessels  into  the  eye  and  from  the 
eye  through  the  spaces  of  Fontana  at  the  base-  of  the  iris  into  the  canal  of 
Schlemm,  and  so  into  the  blood.  Any  interference  with  the  exit  of  this  fluid 
rapidly  increases  the  intra-ocular  tension. 

The  lens  is  the  transparent  biconvex  body  situated  just  behind  the  iris, 
in  the  concavity  of  the  vitreous.  The  thickness  of  the  lens  is  3.6  mm., 
the  diameter  about  9  mm.  It  consists  of  a  transparent  capsule  containing 
elongated  hexagonal  fibers  which,  having  their  origin  near  the  anterior 
central  portion  of  the  lens,  pass  out  toward  the  margin,  where  they  bend 
around  to  terminate  in  a  triradiate  figure  on  the  opposite  side.  Chemically 
the  lens  consists  of  water,  a  globuhn  body  (crystallin),  and  salts. 

The  Suspensory  Ligament. — The  lens  is  held  in  position  by  the  suspensory 
ligament,  formed  in  part  by  the  hyaloid  membrane  and  in  part  by  fibers 
derived  from  the  ciliary  processes.  The  former  becomes  attached  to  the  pos- 
terior surface,  the  latter  to  the  anterior  surface  of  the  lens  near  the  equator. 
The  space  between  the  two  layers  of  the  ligament  is  the  canal  of  Petit.  The 
anterior  surface  of  the  ligament  presents  a  series  of  plications  conforming 
to  corresponding  plications  on  the  surface  of  the  ciliary  processes. 

The  relations  of  all  the  parts  entering  into  the  structure  of  the  eye  are 
shown  in  Fig.  307. 

THE  PHYSIOLOGY  OF  VISION. 

The  Retinal  Image. — The  general  function  of  the  eye  is  the  formation 
of  images  of  external  objects  on  the  free  ends  of  the  percipient  elements  of 
the  retina,  the  rods  and  cones.  The  existence  of  an  image  on  the  retina 
can  be  readily  seen  in  the  excised  eye  of  an  albino  rabbit,  when  placed 
between  a  lighted  candle  and  the  eye  of  an  observer.  Its  presence  in  the 
human  eye  can  be  demonstrated  with  the  ophthalmoscope.  It  is  this  image, 
composed  of  focal  points  of  luminous  rays,  that  stimulate  the  rods  and 
cones,  which  is  the  basis  of  our  sight-perceptions,  and  out  of  which  the  mind 
constructs  space  relations  of  external  objects.  In  only  two  essential  re- 
spects as  far  as  space  relations  go,  does  the  image  on  the  retina  differ  from 
the  appearance  of  the  object,  aside  from  the  fact  that  the  object  has  usually 
three,  the  image  only  two,  dimensions — viz.,  in  size  and  position.  What- 
ever the  distance,  the  image  is  generally  smaller  than  the  object;  it  is  also 
reversed,  the  upper  part  of  the  object  becoming  the  lower  part  of  the  image, 
and  the  right  side  of  the  object  the  left  of  the  image. 

The  Dioptric  Apparatus. — The  formation  of  an  image  is  made  pos- 
sible by  the  introduction  of  a  complex  refracting  apparatus  consisting  of  the 
cornea,  aqueous  humor,  lens,  and  vitreous  humor.  Without  these  agencies 
the  ether  vibrations  would  give  rise  only  to  a  sensation  of  diilused  luminosity. 
Rays  of  light  emanating  from  any  one  point — that  is,  homocentric  rays — 
arriving  at  the  eye  must  traverse  successively  the  different  refracting 
media.     In  their  passage  from  one  to  the  other,  they  undergo  at  the  surfaces 


650  TEXT-BOOK  OF  PHYSIOLOGY. 

changes  in  direction  before  they  are  finally  converged  to  a  focal  point.  In 
order  to  follow  mathematically  the  rays  in  all  their  deviations  through  the 
media,  to  determine  their  focal  points  and  to  construct  an  image,  a  knowl- 
edge of  the  form  of  the  refracting  surfaces,  the  refractive  indices  of  the  dif- 
ferent media,  and  the  distance  of  the  surfaces  from  one  another  must  be 
known. 

The  following  constants  are  now  accepted:  The  radius  of  curvature 
of  that  portion  of  each  refracting  surface  used  for  distinct  vision  is  for  the 
cornea  7.829  mm.,  for  the  anterior  and  posterior  surfaces  of  the  lens  10  and 
6  mm.,  respectively.  The  indices  of  refraction  of  the  different  media  are 
as  follows:  cornea  and  aqueous  humor,  1.3365;  lens,  1.437 1;  vitreous  body, 
1.3365.  The  distance  from  the  vertex  of  the  cornea  to  the  lens  is  3.6  mm.; 
the  thickness  of  the  lens,  3.6  mm.;  the  distance  from  the  posterior  surface 
of  the  lens  to  the  retina,  15  mm.  As  the  two  surfaces  of  the  cornea  are  prac- 
tically parallel,  and  as  the  index  of  refraction  of  the  aqueous  humor  is  the 
same  as  that  of  the  cornea,  they  may  be  regarded  as  but  one  medium.  The 
refracting  surfaces  may  therefore  be  reduced  to  the  anterior  surface  of  the 
cornea,  the  anterior  surface  of  the  lens,  and  the  posterior  surface  of  the 
lens.^ 

Parallel  rays  of  light  entering  the  eye  pass  from  air,  with  an  index  of  re- 
fraction of  1.00025,  into  the  cornea,  with  an  index  of  refraction  of  1.3365. 
In  passing  from  the  rarer  into  the  denser  medium  they  undergo  refraction 
in  accordance  with  the  laws  of  optics  and  are  rendered  somewhat  convergent. 

The  extent  of  this  first  refraction 
and  convergence  is  sufficiently 
great  to  bring  parallel  rays,  if  con- 
tinued, to  a  focus  about  10  mm, 
behind  the  retina.  This  would  be 
the  condition  in  aphakia  whether 
the  lens  is  congenitally  absent  or 
has  been  removed  by  surgical  pro- 
cedures.    Perfect  vision,  however, 

Fig.      308.— Refraction      of      Homocextric     rRnMirr^'i    fViat    thp  rnnvprapnrp  of 

Rays  and  the  Formation  of  an  Image.  requires  tnat  tne  convergence  01 

the  light  must  be  great  enough  to 
bring  the  focal  point,  the  image,  on  the  retina.  This  is  accomplished  by 
the  introduction  of  an  additional  refracting  body,  the  lens.  On  entering 
the  lens  the  rays  are  for  the  same  reason — i.e.,  the  passage  from  a  rarer  into 
a  denser  medium — again  refracted  and  converged,  and  if  continued  would 
come  to  a  focus  about  6.5  mm.  behind  the  retina.  On  passing  from  the  lens 
into  the  \itr eons— i.e.,  from  a  denser  into  a  rarer  medium- — the  rays  are 
once  more  converged  and  to  an  extent  sufficient  to  focalize  them  on  the 
retina  (Fig.  308). 

While  it  is  thus  possible  to  follow  the  rays  geometrically  through  these 
media  by  means  of  the  above-mentioned  factors,  the  procedure  is  attended 
with  many  difficulties.     Moreover,  as  the  relations  all  change  when  rays 

'  Strictly  speaking,  the  posterior  surface  of  the  cornea  is  not  parallel  to  the  anterior  surface, 
and  the  index  of  refraction  of  the  cornea  is  a  trifle  greater  than  that  of  the  aqueous  humor,  viz,. 
1.377.  ^ut  as  the  increase  in  the  corneal  refraction  due  to  the  higher  index  is  almost  exactly 
counteracted  by  a  decrease  in  refraction  due  to  the  higher  curvature  of  the  posterior  corneal 
surface,  the  usual  assumptions  furnish  quite  accurate  results. 


THE  SENSE  OF  SIGHT.  651 

enter  the  eye  from  objects  situated  progressively  nearer  the  eye,  a  separate 
calculation  is  necessitated  for  each  distance  for  the  determination  of  the 
size  of  the  image. 

A  method  by  which  these  difficulties  are  much  reduced  was  suggested 
by  Gauss  and  developed  by  Listing.  It  was  demonstrated  by  Gauss  that 
in  every  complicated  system  of  refracting  media  separated  by  centered 
spheric  surfaces  there  may  be  assumed  certain  ideal  or  cardinal  points,  to 
which  the  system  may  be  reduced,  and  which,  if  their  relative  position  and 
properties  be  known,  permit  of  the  determination,  either  by  calculation 
or  geometric  construction,  of  the  path  of  the  refracted  ray,  and  the  position 
and  size  of  the  image  in  the  last  medium,  if  those  of  the  object  in  the  first 
medium  be  known. 

Every  dioptric  system  can  be  replaced,  as  Gauss  showed,  by  a  single 
system  composed  of  six  cardinal  points  and  six  planes  perpendicular  to  the 
common  axis — e.g.,  two  focal  points,  two  principle  points,  two  nodal  points, 
two  focal  planes,  two  principal  planes,  and  two  nodal  planes. 

Properties  of  the  Cardinal  Points. — The  first  focal  point,  F^,  in  Fig. 
309,  has  the  property  that  every  ray  which  before  refraction  passes  through 
it,  after  refraction  is  parallel  to  the  axis. 

The  second  focal  point,  F^,  has  the  property  that  every  ray  which  before 
refraction  is  parallel  to  the  axis,  passes  after  refraction  through  it. 

The  second  principal  point,  H^,  is  the  image  of  they/>5/,  H^;  that  is, 
rays  in  the  first  medium  which  go  through  the  first  principal  point  pass  after 


^ 


jr, 


firA    \^ 


J> 


Fig.  309. — Diagram  showing  the  Position  and  Relation  of  the  Cardinal  Points. 

the  last  refraction  though  the  second.  Planes  at  right  angles  to  the  axis  at 
these  points  are  principal  planes.  The  second  principal  plane  is  the  image 
of  the  first.  Every  point  in  the  first  principal  plane  has  its  image  after 
refraction  at  a  corresponding  point  in  the  second  principal  plane  at  the  same 
distance  from  the  axis  and  on  the  same  side. 

The  seco^id  nodal  point,  N .^,  is  the  image  of  l)ie  first,  N^:  a  ray  which  in 
the  first  medium  is  directed  to  the  first  nodal  point  passes  after  refraction 
through  the  second  nodal  point,  and  the  direction  of  the  rays  before  and 
after  refraction  are  paralled  to  each  other.  In  Fig.  309  let  A  B  represent 
the  axis.  The  distance  of  the  first  focal  point,  F^,  from  the  first  principal 
plane,  H^,  is  the  anterior  focal  distance.  The  distance  of  the  second  focal 
point,  F^,  from  the  second  principal  plane,  H.^,  is  the  posterior  focal  distance. 
The  distance  of  the  first  nodal  point,  N^,  from  the  first  focal  point,  F^,  is  equal 
to  the  posterior  focal  distance  H.^  F^.  The  distance  of  the  second  nodal 
point,  N^,  from  the  second  focal  point,  F^,  is  equal  to  the  anterior  focal 
distance,  H^F^^.     It  is  evident,  therefore,  that  the  distance  of  the  corre- 


6S2 


TEXT-BOOK  OF  PHYSIOLOGY. 


spending  principal  and  nodal  points  from  each  other  is  equal  to  the  differ- 
ences between  the  two  focal  distances.  Also  the  distance  of  the  two 
principal  points  from  each  other  is  equal  to  the  distance  of  the  two  nodal 
points  from  each  other.  Finally,  the  focal  distances  are  proportional  to  the 
refractive  indices  of  the  first  and  last  media.  Planes  passing  through  the 
focal  points  vertically  to  the  axis  are  known  as  focal  planes. 

From  these  properties  of  the  cardinal  points  the  position  of  an  image  in 
the  last  medium  of  a  luminous  point  in  the  first  may  be  determined,  and  the 


Fig.  310  — Diagram  to  Find  the  Image  in  Last  Medium  of  a  Luminous  Point  in 

THE  First, 


course  of  a  refracted  ray  in  the  last  medium  be  constructed  if  its  direction  in 

the  first  be  given  according  to  the  following  rules : 

I.  To  find  the  image  in  the  last  medium  of  a  luminous  point  in  the  first:  Let 
A  (Fig.  310)  be  this  given  point.  Draw  A  B  parallel  to  the  axis  until  it 
meets  the  second  principal  plane  in  B;  then  B  F^  will  be  this  ray  after 
refraction.  Draw  a  second  ray  from  A  to  the  first  nodal  point;  then 
draw  another  ray,  D  E,  from  the  second  nodal  point  parallel  to  A  C. 
This  will  be  the  refracted  ray  in  the  last  medium.  Where  the  two  re- 
fracted rays,  BF^  and  D  E,  intersect,  the  image  of  A  will  be  A^^ 


Fig.  311. — Diagram  to  Find  the  Refracted  Ray  in  the  Last  Medium  of  a  Given 
Ray  in  the  First  Medium. 


2.  To  find  the  refracted  ray  in  the  last  medium  of  a  given  ray  in  the  first 
medium:  Let  A  B  (Fig.  311)  be  the  given  ray.  Continue  this  ray  until 
it  meets  the  first  principal  plane  in  C.  Draw  C  D  parallel  to  the  axis. 
Now  assume  any  point,  such  as  E,  in  the  given  ray,  and  find  its  image  £j 
by  the  Rule  i.     Then  D  E^  becomes  the  course  of  the  refracted  ray. 

'  If  the  point  A  is  infinitely  far  from  the  eye,  all  the  rays  striking  the  eye  will  be  parallel  to 
each  other.  The  nodal  ray  must  therefore  be  drawn,  and  the  point  where  this  nodal  ray  meets 
the  second  focal  plane  will  be  the  image  of  ^,  or  ^  1  where  all  rays  parallel  to  the  nodal  ray  will 
meet. 


THE  SENSE  OF  SIGHT. 


653 


The  Schematic  Eye. — Accepting  the  system  of  cardinal  points,  Listing, 
Bonders,  and  v.  Helmholtz  have  constructed  "schematic"  eyes  to  be  sub- 
stituted for  the  refracting  system  of  the  natural  eye. 

For  this  purpose  it  is  necessary  to  make  use  of  the  various  estimates  of 
the  indices  of  refraction  of  the  different  media,  of  the  radii  of  curvatures 
of  the  different  refracting  surfaces,  and  of  the  distances  separating  them,  to 
deduce  an  average  eye  as  a  basis  for  calculation.  The  most  widely  accepted 
attempt  is  that  of  v.  Helmholtz.  The  data  he  assumed  are  as  follows :  The 
refractive  index  of  air  =  i;  of  the  cornea  and  aqueous  humor,  1.3365;  of  the 
lens,  1. 4371;  of  the  vitreous  humor,  1.3365;  the  radius  of  curvature  of  the 
cornea,  7.829  mm. ;  of  the  anterior  surface  of  the  lens,  10  mm. ;  of  the  posterior 
surface,  6  mm.;  the  distance  from  the  apex  of  the  cornea  to  the  anterior 
surface  of  the  lens,  3.6  mm.;  thickness  of  lens,  3.6  mm.     From  the  above- 


FiG.  312. — Diagram  showing  the  Position  of  the  Caiidinal  Points  in  the  "Schematic 
Eye."  The  continuous  lines  in  the  upper  half  of  the  figure  show  their  position  in  the  passive 
emmetropic  eye.  The  dotted  Hnes  indicate  the  change  in  their  position  in  an  eye  accommodated 
for  the  object  A  at  the  distance  a  from  the  cornea,  or  152  mm.  The  lower  half  of  the  figure  shows 
the  formation  of  a  distinct  image  on  the  retina  of  an  eye  accommodated  for  the  object  A  at  the 
distance  a  from  the  cornea. 


mentioned  data  v.  Helmholtz  calculated  the  position  of  the  cardinal  points 
for  the  eye  as  follows  (see  Fig.  312) :  The  first  focal  point  is  situated  13.745 
mm.  before  the  anterior  surface  of  the  cornea;  the  second  focal  point  is 
situated  15.619  mm.  behind  the  posterior  surface  of  the  lens;  the  first 
principal  point,  1.753  ^^-  behind  the  cornea;  the  second  principal  point, 
2.106  mm.  behind  the  cornea;  the  first  and  second  nodal  points,  6.968  and 
7.321  mm.  behind  the  apex  of  the  cornea,  respectively.  The  anterior  focal 
distance  of  this  schematic  eye,  the  distance  between  F^  and  H^,  there- 
fore amounts  to  15.498  mm.,  and  the  posterior  focal  distance,  H^  to  F^,  to 
20.713  mm. 


6S4 


TEXT-BOOK  OF  PHYSIOLOGY. 


When  the  eye,  however,  is  accommodated  for  near  vision,  the  relations 
of  the  cardinal  points  are  changed  and  will  be  as  follows,  if  the  point  accom- 
modated for  lies  152  mm.  from  the  cornea:  Anterior  focal  distance,  13.990 
mm.;  posterior  focal  distance,  18.689  mm.;  distance  from  cornea  of  the  first 
and  second  principal  points,  1.858  and  2.257  ^^^-  respectively;  distance  of 
the  posterior  focus,  20.955  ^"^-  from  cornea.  Given  this  schematic  eye  in  the 
accommodated  state,  the  course  of  the  rays  and  the  determination  of  the 
position  of  an  image  in  the  last  medium  of  a  luminous  point  in  the  first  can 
easily  be  determined  by  the  rules  already  given. 

The  Reduced  Eye. — As  suggested  by  Listing,  this  schematic  eye  may 
be  yet  further  simplified  or  reduced  to  a  single  refracting  surface  bounded 
anteriorly  by  air  and  posteriorly  only  by  aqueous  or  vitreous  humor.  Without 
introducing  any  noticeable  error  in  the  determination  of  the  size  of  the  retinal 
image,  the  anterior  principal  and  the  anterior  nodal  points  may  be  disre- 
garded, owing  to  the  minuteness  of  the  distances  (0.39  mm.)  separating  the 
two  systems  of  points.  There  is  thus  obtained  one  principal  point  and  one 
nodal  point,  which  latter  becomes  the  center  of  curvature  of  the  single  re- 
fracting surface.  The  dimensions  of  this  ''reduced"  eye  are  as  follows  (see 
Fig.  313).  From  the  anterior  surface  of  the  cornea,  corresponding  to  the 
principal  plane  H,  to  the  nodal  point  A^,  5.2x5  mm.,  from  the  anterior  focal 
point  Pp  to  the  principal  plane  H,  i.e.,  the  anterior  focal  distance/',  15.498 


Fig.  313. — The  Reduced  Eye. 


Fig.  314. — The  Formation  of  an  Image  in  the 
Reduced  Eye. 


mm.;  from  the  principal  plane  H  to  the  posterior  focal  point  F^,  i.e.,  the 
posterior  focal  distance/",  20.713  mm.;  the  index  of  refraction  is  1.3365. 
There  is  thus  substituted  for  the  natural  eye  a  single  refracting  surface  with 
a  radius  of  curvature,  r,  of  5.125  mm.  In  such  an  eye  luminous  rays  emanat- 
ing from  the  anterior  focal  point  are  parallel  to  the  axis  after  refraction  in 
the  interior  of  the  eye.  Also  rays  parallel  to  the  axis  before  refraction  unite 
at  the  posterior  focal  point. 

By  means  of  this  reduced  eye  the  construction  of  the  refracted  ray,  the 
various  calculations  as  to  the  size  of  the  image,  the  size  of  diffusion  circles, 
etc.,  are  greatly  facilitated:  e.g.. 

In  Fig.  314  let  A  B  represent  an  object.  From  A  a  pencil  of  rays  falls 
on  the  single  refracting  surface.  One  of  the  rays,  the  nodal  ray,  falling  on 
the  surface  perpendicularly,  passes  unrefracted  through  the  single  nodal  point, 
N,  to  the  posterior  focal  plane.  The  remaining  rays,  partially  represented 
in  the  figure,  falling  on  this  surface  under  varying  degrees  of  incidence, 
undergo  corresponding  degrees  of  refraction,  by  which  they  form  a  converg- 
ing cone  of  rays  which  unite  at  a  point  situated  on  the  nodal  ray.     These 


THE  SENSE  OF  SIGHT.  655 

two  points,  A,  a,  are  known  as  conjugate  foci.  The  same  holds  true  for 
a  pencil  of  rays  emanating  from  B  or  any  other  point  of  the  object. 

The  Size  of  the  Retinal  Image. — The  size  of  the  retinal  image,  /  (in 
Fig.  316  a  b),  may  now  be  easily  calculated,  when  the  size  of  the  object,  O 
(in  Fig.  T,i6  AB),  and  its  distance,  D,  from  the  refracting  surface  with  radius 
of  curv'ature,  r,  are  known,  by  the  following  formula : 

O:  I=D  +  r:f"-r. 

For,  as  the  triangles  A  N  B  and  a  N  b  are  similar,  we  have 

A  By.  N  s  Oi  {"—r  \ 

A  B:ab  =/ N:N  g,oxab=  — ^% — ^;  and  therefore  7  =  — ^■^---O 
•^  *'  fN        '  D+r 

Independent  of  the  foregoing  method,  the  size  of  the  retinal  image  may  be 
calculated  if  it  is  remembered  that  the  eye,  like  any  optic  system,  has  a  point 
of  such  a  quality  that  a  ray  of  light  which  before  entering  the  eye  was  directed 
toward  it,  after  refraction  continues  as  if  it  came  from  this  point.  In  other 
words,  there  is  in  the  eye  a  point  which  allows  a  ray  of  light  to  pass  unre- 
fracted  as  would  a  pinhole  instead  of  a  lens.  This  point,  termed  the 
nodal  point  of  the  eye,  determines  the  size  of  the  image;  for  if  a  Hne  be 
drawn  from  both  the  upper  and  lower  ends  of  an  object  through  this  nodal 
point,  it  is  clear  that  the  images  of  the  respective  points  must  lie  on  these 
two  rays  where  they  intersect  the  retina.  The  distance  of  this  nodal  point 
from  the  retina  is  15.498  mm.  It  is  clear,  therefore,  that  the  size  of  the 
object  is  to  the  size  of  the  image,  as  the  distance  of  the  object  from  the 
nodal  point  is  to  the  distance  of  the  - 

nodal  point  from  the  retina ;  or,  in     ^ A  y^      ^^'^' 

other  words,  to  find  the  size  of  the  ~    ~- — W^-^-A/^      --"^llilli^' 

retinal  image :  multiply  the  diameter  ___-_----i-^t^^^^^rW^^^^~^^ 

of  the  object  by  15.5  mm.  and  divide     r  fe-- "     \W      ""~-..,  7B' 

by  the  distance  of  the  object  from  °  \^^^      ^y^ 

the  eye. 

TlipVi<;iifl1  Ano-lp  The  vicinal  ^^^-    315 —Drawing    Designed    to    show 

1  ne  V  ISUai  Angle.       i  ne  \  isuai      ^^^^,  ^^^  visual  Angle  and  Size  of  Retinal 

angle  is  defined  as  the  angle  formed  Image  Varies  with  the  Distance  of  an 
by  the  intersection  of  two  lines  Object  or  Given  Size.  For  the  distant  position 
J  r  ,1  ,  •.•  r  of    A-B    the  casual    anele    is  a;   for    the    near 

drawn  from  the  extremities  of  an     ^^^^^^^^  ^^^^^^^^  u^es)  ^.    (From  Stewart.) 
object  to  the  nodal  point  of  the  eye. 

Beyond  the  nodal  point,  however,  the  lines  again  diverge  and  form  an  in- 
verted or  reversed  image  of  the  object  on  the  retina.  The  size  of  the 
visual  angle  increases  with  the  nearness  and  decreases  with  the  remote- 
ness of  the  object;  the  retinal  image  correspondingly  increases  and  de- 
creases in  size.  These  facts  will  become  apparent  from  an  examination  of 
Fig.  315.  As  the  size  of  the  retinal  image  diminishes  when  the  visual  angle 
diminishes  either  as  a  result  of  the  removal  of  a  given  object  from  the  eye,  or 
of  a  diminution  of  the  size  of  the  object,  there  comes  a  limit  in  the  size  of  the 
visual  angle,  beyond  which  it  is  impossible  to  see  the  two  end  points  {A  and  B) 
of  the  object  separately.  When  this  limit  is  reached  the  size  of  the  angle  ex- 
pressed in  degrees  of  the  circle,  may  be  determined  if  the  distance  between 
the  two  points  and  their  distance  from  the  eye  be  known.  Thus  it  has  been 
experimentally  determined  that  at  a  distance  of  5  meters,  the  smallest  object  or 
the  smallest  interval  between  two  points  which  permits  the  eye  to  distinguish 


656  TEXT-BOOK  OF  PHYSIOLOGY. 

them  as  such,  is  about  1.454  mm.  Lines  drawn  from  the  extremities  of 
such  an  object  or  interv'al,  to  the  nodal  point,  subtend  an  angle  of  60  seconds.^ 
Beyond  this  the  two  points  are  indistinguishable.  In  other  words  the 
emmetropic  eye  possesses  the  power  of  distinguishing  the  correspondingly 
small  interval  between  the  two  images  on  the  retina  of  the  two  objective 
points.  The  size  of  the  image  or  the  interval  between  the  two  retinal  points, 
determined  from  the  foregoing  factors  by  the  formulae  on  page  655  is  0.004 
mm.,  which  would  correspond  to  a  visual  angle  of  60  seconds.  If  the 
retinal  distance  is  less  than  this  the  two  sensations  fuse  into  one.  The  reason 
assigned  for  this  is,  that  the  distance  between  the  centers  of  two  adjoining 
cones  in  the  macula  is  0.004  nirn.  With  a  visual  angle  not  less  than  60 
seconds,  the  two  foci  fall  on  separate  cones.  With  a  smaller  visual  angle 
the  two  foci  fall  on,  and  excite  but  a  single  cone  and  hence  there  arises  the 
sensation  of  but  a  single  point.  The  acuteness  of  vision,  therefore,  of  the 
emmetropic  eye  depends  on  its  power  of  distinguishing  the  smallest  retinal 
image  or  the  smallest  interval  between  two  cones  on  the  retina,  correspond- 
ing to  a  visual  angle  of  60  seconds. 

In  ophthalmic  practice  it  is  customary  in  testing  the  acuteness  of  vision 
to  employ  test  letters  of  specific  sizes  for  specific  distances.  The  letters  are 
so  proportioned  that  when  they  are  placed  at  the  specified  distances,  the 
extremities  of  the  letters  subtend  an  angle  of  5  minutes.  The  letters  have 
been  constructed  on  the  following  basis:  Since  to  an  angle  of  60  seconds 
there  corresponds  an  object  of  1.454  mm.  at  the  distance  of  5  meters  as 
shown  before  and  as  the  object  decreases  in  proportion  to  the  distance 
(for  the  same  \dsual  angle)  it  is  evident  that  the  object  would  have  to  be  one- 
fifth  of  1.454  mrh.  or  0.2908  mm.  in  order  to  subtend  an  angle  of  60  seconds 
at  one  meter.  From  this  the  size  for  any  other  distance  in  meters  is  found 
simply  by  multiplying  0.2908  mm.  by  the  distance.  The  standard  letters 
are  so  constructed  that  each  is  inscribed  within  a  square  the  sides  of  which 
at  a  specific  distance  subtend  an  angle  of  5  minutes  and  which  is  again  sub- 
divided into  25  small  squares  each  side  of  which  subtends  an  angle  of  i 
minute.  These  partial  little  squares  correspond  to  the  details  of  the  letter 
while  the  whole  letter  of  course,  embraces  an  angle  of  5  minutes  both  as  to 
height  and  to  breadth.     The  letter  that  could  be  distinctly  seen  at  a  dis- 

^  The  size  of  the  visual  angle,  under  which  an  object  of  this  size  and  situated  at  a  distance 
of  5  meters  is  distinctly  seen,  can  be  determined  from  the  following  Fig.  3 16,  in  which  A  B  represents 

the  size  of  the  object  i  .454  mm. ; 
-^pT----^^^  N,    the  nodal  point;  CN,  the 

line   which   bisects  the  object, 

represents  the  distance  of  the 

object  from   the  nodal   point; 

a,  the  visual  angle  subtended 

and  whose  value  it  is  desired 

to  know,  and  b  one-half  of  the 

angle  a.     By  trigonometry  the 

size  of  the  angle  a  can  be  de- 

Fig.  316.-F1GUEE  SHOWING  the  Method  of  Obtainixg    termined  in  the  following  way: 

THE  Visual  Angle  Expressed  in  Degrees  or  Fraction    T«     ^,     /".^t    •.^•°^^ 

OF  A  Degree  of  an  arc  f  ^'  ^1  ^^^,^1  ^^.  ^^  distance 

from  the  nodal  pomt;  the  quo- 
tient is  the  tangent  of  half  the  angle.  Thus  0.727-^5000=0.0001454.  By  reference  to  tables  of 
natural  tangents,  it  wiU  be  found  that  the  angle  or  fraction  of  the  circle  corresponding  to  this 
tangent  is  30  seconds,  and  that  therefore  the  whole  angle  is  60  seconds. 


THE  SENSE  OF  SIGHT. 


657 


tance  of  5  meters,  would  have,  therefore,  a  vertical  and  a  horizontal  dimen- 
sion of  5  times  1.454  mm.  or  7.27  mm.  (Fig.  317  A),  and  at  10  meters  corre- 
sponding dimension  of  14.54  mm.,  etc.     (Fig.  317  B.) 

If  with  the  accommodation  suspended,  the  emmetropic  eye  could 
clearly  distinguish  a  letter  7.27  mm.  in  size  at  a  distance  of  5  meters  and 
which  would,  therefore,  subtend  an  angle  of  5  minutes,  the  acuity  of  the 
vision  would  be  normal  and  could  be  expressed  as  follows:  V=|-  or  V  =  i. 


E    T 


£1    u 


-/' 


B. 
Fig.  317. — St.\xdard  Test  Letters. 

If  on  the  contrary  at  this  distance  the  smallest  letter  that  could  be  clearly 
seen  is  one  that  would  subtend  an  angle  of  5  minutes  at  a  distance  of  10  meters 
then  the  visual  acuity  would  be  only  one-half  the  normal  and  could  be 
expressed  as  follows  V=yV  orV=^,  etc.  The  acuity  of  vision  is  expressed, 
therefore,  by  a  fraction  the  numerator  of  which  is  the  distance  at  which  the 
test  is  made  and  whose  denominator  is  the  distance  at  which  the  smallest 
letters  distinguished  by  the  patient  subtend  an  angle  of  5  minutes,  or  in 
other  words  the  distance  at 
which  the  patient  reads  di- 
vided by  the  distance  at  which 
he  ought  to  read  the  smallest 
letters  seen  by  him  on  the 
chart. 

Acconimodation.  —  Ac- 
commodation may  be  defined 
as  the  power  which  the  eye 
possesses  of  adjusting  itself  to 
vision  at  different  distances; 
or  in  other  words,  the  power 
of  focusing  rays  of  light  on  the 
retina,  which  come  from 
different  distances  at  different 
times.  That  such  a  power 
is  a  necessity  is  apparent  from 
the  fact  that  it  cannot  focus 
rays  coming  from  a  distant 
and  a  near  object  at  the  same  time.-  Thus,  if  an  object  is  held  before 
one  eye  at  a  distance  of  22  centimeters,  for  example,  and  the  vision 
is  directed  to  a  distant  object  it  is  evident  that  the  near  object  is  indistinctly 
seen;  but  if  the  vision  is  then  directed  to  the  near  object,  it  in  turn  becomes 
clear  and  distinct,  while  the  distant  object  becomes  blurred  and  indistinct. 
It  is  evident,  therefore,  that  rays  of  light  coming  from  a  distant  and  a  near 
object  cannot  be  simultaneously,  but  only  alternately,  focused  on  the  retina. 
The  obser\^er  at  the  same  time  becomes  conscious,  as  the  vision  is  directed 

43 


Fig.  318. — The  Refraction  of  Parallel  and 
Divergent  Rays  ln  the  Emmetropic  Eye  in  the 
Passive  and  in  the  Actwe  or  Accommodated 
Condition. 


658  TEXT-BOOK  OF  PHYSIOLOGY. 

from  the  distant  to  the  near  object,  of  a  change  in  the  eye  itself,  a  change 
that  involves  time  and  effort.  The  reasons  for  these  facts  will  become 
apparent  from  a  consideration  of  the  following  facts: 

In  a  normal  or  emmetropic  eye,  parallel  rays  of  light  (Fig.  318,  a,  h) 
after  passing  through  the  optic  media  are  converged  and  brought  to  a 
focus  on  the  retina,  /.  Rays,  however,  which  come  from  a  luminous 
point  situated  near  the  eye,  P,  and  are  therefore  divergent,  passing  through 
the  optic  media  at  the  same  time,  are  intercepted  by  the  retina  before  they 
are  focused,  and  give  rise  to  the  formation  of  diffusion- circles  and  indis- 
tinctness of  vision.  The  reverse  is  also  true.  When  the  eye  is  adjusted 
for  the  refraction  and  focusing  of  divergent  rays  (Fig.  318,  P)  parallel  rays 
will  be  brought  to  a  focus  before  reaching  the  retina,  and,  again  diverging, 
will  form  diffusion-circles.  It  is  evident,  therefore,  that  it  is  impossible 
to  focus  simultaneously  both  parallel  and  divergent  rays,  and  to  see  dis- 
tinctly at  the  same  time,  two  objects  which  are  situated  at  different  distances. 
The  eye  must  be  alternately  adjusted  first  to  one  object  and  then  to  another. 
To  this  adjustment  the  term  accommodation  has  been  given. 

The  following  table  of  Listing  shows  the  size  of  the  diffusion-circles 
formed  of  objects  situated  at  different  distances  when  the  accommodative 
power  is  suspended  in  an  emmetropic  eye: 

Distance  of  the  Focal 
Distance  of  Luminous  Point.  Point  behind  the  Posterior  Diameter  of  the  Diffusion-circle. 

Surface  of  the  Retina. 

°°  0.0     mm.  0.0        mm. 

65  m.  0.005  mm.  o.ooiimm. 

25  m.  0.012  mm^.  0.0027  mm. 

12  m.  0.025  mm.  0.0050  mm. 

6  m.  0.050  mm.  0.0112  mm. 

3  m.  o.  100  mm.  0.0222  mm. 

1.500m.  0.20    mm.  0.0443  mm. 

0.750  m.  0.40    mm.  0.0825  mm. 

0.375  m.  0.80    mm.  o.i6i6mm. 

0.188  m.  1.60    mm.  0.3122  mm. 

0.094  m.  3 -20    mm.  0.5768  mm. 

0.088  m.  3-42    mm.  0.6484  mm. 

From  the  foregoing  table  it  is  evident  that  between  infinity  and  65  meters, 
the  diffusion-circles  are  so  slight  that  no  perceptible  accommodative  effort 
is  required  to  ehminate  them.  From  65  meters  to  6  meters  the  diffusion- 
circles  gradually  become  larger,  though  they  are  yet  so  faint  as  to  require 
for  their  correction  an  accommodative  effort  which  is  scarcely  measurable. 
From  6  meters  up  to  6  centimeters,  however,  a  progressive  increase  in  accom- 
modative power  is  demanded  for  distinct  vision. 

The  normal  eye  when  adjusted.for  distant  vision  is  in  a  passive  condition, 
and  hence  vision  of  distant  objects  is  unattended  with  fatigue.  In  the  act 
of  adjustment,  however,  for  near  vision  the  eye  passes  into  an  active  state, 
the  result  of  a  muscle  effort,  the  energy  of  which  is  proportional  to  the  near- 
ness of  the  object  toward  which  the  eye  is  directed. 

Mechanism  of  Accommodation.^ — Inasmuch  as  neither  the  corneal 
curvature  nor  the  shape  of  the  eyeball  undergoes  any  change  during  accom- 
modation, the  necessary  change,  whatever  it  may  be,  is  to  be  sought  for  in 
the  lens.  As  to  the  character  of  the  changes  in  this  body,  two  views  are  held, 
based  largely  on  the  fact  and  its  interpretation,  that  images  of  a  luminous 


THE  SENSE  OF  SIGHT. 


659 


point  reflected  from  the  anterior  surface  of  the  cornea  and  the  anterior  and 
posterior  surfaces  of  the  lens,  change  their  relative  positions  during 
accommodation. 

Thus,  if  in  a  darkened  room  a  lighted  candle  be  placed  in  front  of  and 
to  the  side  of  an  individual  whose  eye  is  directed  to  a  distant  object,  an 
obser\^er  placed  in  the  same  relative  position  as  the  candle  will  observe  three 
images  in  the  eye,  one  at  the  surface  of  the  cornea  and  two  at  the  pupillary 
margin  (Fig.  319).  Of  the  two  latter,  one  is  quite  large 
and  situated  apparently  in  front  of  the  third,  which  is 
faint,  small,  and  inverted.  The  middle  image  is  reflec- 
ted from  the  anterior  surface  of  the  lens,  the  last  from 
the  posterior  surface.  These  images  of  reflection  are 
known  as  catoptric  images.  If  now  the  individual  be 
directed  to  fix  the  gaze  on  a  near  object,  the  second  im- 
age changes  its  position,  advances  toward  the  corneal 
image  and  at  the  same  time  becomes  smaller,  a  change 
which,  in  accordance  with  the  laws  of  optics,  could  only 
be  due  to  an  increase  in  the  convexity  of  the  anterior 
surface  of  the  lens.  A  slight  displacement  of  the  third 
image  sometimes  observed  indicates  a  possible  increase 
in  the  convexity  of  the  posterior  surface  of  the  lens. 

According  to  Helmholtz,  during  accommodation  the 
entire  anterior  surface  of  the  lens  becomes  more  convex, 
while  at  the  same  time  it  slightly  advances,  possibly  as 
much  as  0.4  mm.  in  extreme  efforts.  This  change  is  represented  in  Fig. 
320.  According  to  Tscherning,  the  increase  in  convexity  of  the  anterior 
surface  is  confined  to  the  central  portion,  the  remainder  of  the  surface 
becoming  somewhat  flattened.  There  is,  moreover,  no  evidence  that  there 
is  any  advance  of  the  surface  or  any  increase  in  the  thickness  of  the  lens. 
A  series  of  new  and  ingenious  experiments  lend  support  to  Tschering's 


Fig.  319. — Catop- 
tric Images  ix  the 
Eve.  a.  Upright 
image  of  reflection, 
from  the  cornea,  b. 
Upright  image  from 
the  anterior  surface 
of  the  lens.  c.  In- 
verted image,  from 
the  posterior  surface 
of  the  lens. — {Helm- 
holtz.) 


forne^t  proper- 
J/ejremef  Memirana 

-        ^=s=S3 Spcricterjrixiis 

\"~ V\^  dlLoryJfiis^ 


Fig.  320. 


-The  Left  Half  Represents  the  Eve  in  a  State  of  Rest. 
Half  in  State  of  Accommodation. 


The  Right 


view,  though  of  late  Hess  has  brought  forward  definite  experimental  evi- 
dence in  favor  of  the  view  of  Helmholtz.  The  radius  of  curvature  in  either 
case  approximates  6  mm.  in  extreme  efforts  of  accommodation.  The  in- 
crease in  convexity  naturally  increases  the  refracting  power. 

Whicheverview  is  accepted,  the  nearer  the  object — that  is,  the  greater  the 
degree  of  divergence  of  the  light  rays — the  more  pronounced  must  be  the 
increase  in  convexity  in  order  that  they  may  be  sufficiently  converged  and 


66o  TEXT-BOOK  OF  PHYSIOLOGY. 

focalized  on  the  retinal  surface.  Changes  in  the  convexity  of  the  lens, 
either  of  increase  or  decrease,  are  attended  by  changes  in  the  distinctness  of 
images.  Coincidcntly  with  the  lens  change,  the  pupillary  margin  advances 
and  the  pupil  itself  becomes  smaller.  By  this  means  an  indistinctness  of  the 
image  is  prevented  by  cutting  off  the  rays  which  would  give  rise,  owing  to 
the  angle  at  which  they  fall  on  the  surface,  to  diffusion-circles,  from  spheric 
aberration. 

The  Function  of  the  Ciliary  Muscle. — Though  it  is  generally  admit- 
ted that  the  increase  in  the  convexity  of  the  lens  is  caused  by  the  contrac- 
tion of  the  ciliary  muscle,  the  exact  manner  in  which  this  is  accomplished 
is  not  clearly  understood.  According  to  Helmholtz,  when  the  eye  is  in 
repose  and  directed  to  a  distant  object  the  lens  is  somewhat  flattened  from 
a  traction  exerted  by  the  suspensory  ligament.  When  the  eye  is  directed 
to  a  near  object,  the  ciliary  muscle  contracts,  thereby  relaxing  the  ligament^ 
as  a  result  of  which  the  lens,  by  virtue  of  an  inherent  elasticity,  bulges  for- 
ward and  becomes  more  convex.  In  consequence  of  this  latter  fact  the 
refracting  power  is  proportionally  increased.  In  extreme  efforts  of  accom- 
modation it  is  believed  by  some  observers  that  the  circularly  arranged 
fibers,  the  so-called  annular  muscle,  contract  and  exert  a  pressure  on  the 
periphery  of  the  lens  and  thus  aid  other  mechanisms  in  relaxing  the  liga- 
ment and  in  increasing  the  convexity.  This  view  appears  to  be  supported 
by  the  fact  that  in  hypermetropia,  where  a  constant  effort  is  required  to 
obtain  a  distinct  image  of  even  distant  objects,  the  annular  muscle  becomes 
very  much  hypertrophied,  thus  reinforcing  the  meridional  fibers.  In 
myopia,  on  the  contrary,  where  the  accommodative  effort  is  at  a  minimum, 
the  entire  muscles  possesses  less  than  its  average  size  and  development. 

According  to  Tscherning,  a  different  explanation  of  the  action  of  the 
ciliary  muscle  must  be  given.  Thus,  when  it  contracts,  the  antero-internal 
angle,  that  portion  in  close  relation  with  the  suspensory  ligament,  recedes 
and  exerts  on  the  ligament  a  pressure  which  in  turn  exerts  a  traction  on  the 
peripheral  portions  of  the  anterior  surface  of  the  lens,  which  produces  the 
deformation  observed.  At  the  same  time  the  postero-external  portion  of 
the  muscle  exerts  traction  on  the  chorioid,  thus  sustaining  the  vitreous 
and  indirectly  the  lens. 

The  reason  for  the  flattening  of  the  periphery  of  the  lens  from  zonular 
compression  and  the  sharpening  of  the  central  convexity  is  to  be  found  in 
the  fact  that  the  convexity  of  the  more  solid  central  portion,  the  nucleus, 
is  greater  than  that  of  the  lens  itself.  Hence  it  is  easily  understood  why  a 
zonular  traction  would  give  rise  to  peripheral  flattening. 

There  is,  however,  one  point  which  seems  difficult  to  harmonize  with 
Tscherning's  view;  that  is,  the  fact  that  during  accommodation  the  lens 
appears  to  be  slightly  tremulous,  thus  showing  relaxation,  and  not  increased 
tension,  of  the  suspensory  ligament. 

Range  of  Accommodation. — It  has  been  stated  that  rays  of  light 
coming  from  a  luminous  point  situated  at  any  distance  beyond  65  meters 
are  so  nearly  parallel  that  no  accommodative  effort  is  required  for  their 
localization.  So  long  as  the  luminous  point  remains  between  infinity  and 
65  meters,  the  eye,  directed  toward  it,  remains  completely  relaxed.  The 
point  at  which  the  object  can  be  distinctly  seen  without  accommodation 


THE  SENSE  OF  SIGHT.  66i 

is  termed  the  far  point  or  the  puncium  remotum.  This  for  the  normal  eye 
is  at  a  distance  of  65  meters  or  beyond.^  If  the  luminous  point  gradually 
approaches  the  eye  from  a  point  65  meters  distant,  the  accommodative 
power  comes  into  play  and  gradually  increases  until  it  attains  its  maximum. 
The  nearest  point  up  to  which  the  eye  is  able  to  form  distinct  images  of 
objects  is  called  its  near  point  or  punctum  proximum.  This  near  point  in  a 
healthy  boy  of  twelve  years  will  lie  at  2f  Inches  or  7  cm.  from  the  eye, 
while  the  same  point  lies  only  8  inches  or  20  cm.  distant  in  a  man  of  forty 
years.  Of  objects  which  lie  nearer  than  the  punctum  proximum  the  eye 
cannot  form  distinct  images.  The  distance  between  the  punctum  remotum 
and  the  punctum  proximum  is  termed  ih.&  range  of  accommodation. 

Force  of  Accommodation. — The  increase  in  curvature  of  the  lens 
necessary  to  focaUze  rays  when  the  eye  is  directed  from  the  far  to  the  near 
point  necessitates  the  expenditure  of  energy  on  the  part  of  the  ciliary  muscle. 
The  force  expended  in  the  act  of  accommodation  may  be  measured  by 
a  lens  the  refracting  power  of  which  is  such  as  to  enable  it  to  produce  the 
same  result — that  is,  to  give  the  diverging  rays  coming  from  the  near  point, 
e.g.,  20  cm.,  a  parallel  direction.  A  lens,  therefore,  which  has  a  focal  dis- 
tance of  20  cm.  would  be  a  measure  of  the  force  expended;  for  such  a  lens 
placed  in  front  of  the  crystalline  lens,  when  in  a  state  of  repose,  would, 
with  the  assistance  of  the  latter,  bring  diverging  rays  coming  from  the  near 
point  to  a  focus  on  the  retina.  A  lens  of  this  character  is  said  to  have  a 
refracting  power  of  5  dioptrics. 

Since  lenses  of  the  same  curvature  made  from  different  materials  have 
different  refracting  powers,  it  becomes  necessary  to  have,  for  purposes 
of  comparison,  some  unit  of  measurement.  The  unit  now  accepted  is  the 
refracting  power  of  a  glass  lens  which  is  sufhcient  to  focalize  parallel  rays 
at  a  distance  of  100  cm.  or  i  meter.  This  amount  of  refracting  power  is 
termed  a  dioptry.  Lenses  which  would  focalize  parallel  rays  at  a  distance 
of  50,  20,  or  10  cm.  are  said  to  have  a  refractive  power  of  2,  5,  or  10  dioptrics 
respectively,  obtained  by  dividing  too  cm.  by  the  focal  distance.  The  re- 
fracting power  of  a  biconcave  lens  is  determined  by  prolonging  backward 
in  the  direction  the  parallel  rays  have  come,  the  rays  which  have  been 
rendered  divergent  by  the  lens,  and  using  a  corresponding  negative  figure. 
Thus  a  lens  which  diverges  parallel  rays  in  such  a  way  as  to  make  them 
appear  to  radiate  from  a  point  20  centimeters  behind  itself  is  said  to  have 
a  refractive  power  of  minus  5  dioptrics. 

The  refracting  media  of  the  human  eye  in  repose  have  collectively  a 
refracting  power  of  about  64  dioptrics,  the  reciprocal  of  its  anterior  focal 
distance.  The  refracting  power  of  the  corneal  surface  alone  is  equivalent  to 
42  dioptrics.  The  crystalline  lens  by  reason  of  its  relations  and  situation 
in  the  optic  media  has  a  refracting  power  of  about  20  dioptrics. 

The  capability  of  the  lens  to  increase  its  refraction  during  accommodative 
efforts  beyond  the  20  dioptrics  varies  considerably  at  different  periods  of  life. 
At  ten  years  the  increase  is  14  dioptrics,  as  the  near  point  is  7  cm. ;  at  thirty 
years  the  increase  is  but  7  dioptrics,  as  the  near  point  is  14  cm.;  at  sixty  the 
increase  is  but  i  dioptry  and  the  near  point  100  cm.;  at  seventy  it  is  zero. 

'  In  practical  ophthalmic  work  a  point  six  meters  distant  is  taken  as  the  far  point  for  the 
reason  that  the  rays  at  this  distance  are  practically  parallel. 


662  TEXT-BOOK  OF  PHYSIOLOGY. 

From  youth  to  old  age,  the  elasticity  of  the  lens  steadily  declines,  and  the 
range  of  accommodation  diminishes  from  the  recession  of  the  near  point. 

Convergence  of  the  Eyes  during  Acconimodation. — In  binocular 
vision  of  near  objects  the  eyes  are  turned  inward  and  the  optic  axis  of  each 
— a  line  passing  through  the  center  of  the  cornea  and  the  center  of  the  eye 
— turned  toward  the  median  line  during  accommodation.  So  long  as  the 
eyes  are  directed  toward  the  far  point,  65  meters  or  beyond,  the  optic  axes 
are  parallel.  When  the  eyes  are  directed  to  any  point  within  65  meters  the 
optic  axes  are  converged,  the  convergence  increasing  steadily  as  the  near 
point  is  approached.  In  this  way  the  fovea  of  each  eye  is  directed  to  the 
same  point  and  single  vision  made  possible.  Were  this  not  the  case,  double 
vision  would  result. 

Functions  of  the  Iris. — For  purposes  of*  distinct  vision  it  is  essen- 
tial that  the  quantity  of  light  entering  the  interior  of  the  eye  shall  be  so 
adjusted  that  the  formation  and  subsequent  perception  of  the  image  shall  be 
sharp  and  distinct.  This  is  accomplished  by  the  iris,  the  circular  fibers 
of  which  respectively  contract  and  relax  with  increasing  and  decreasing  in- 
tensities of  the  light.  The  size  of  the  pupil,  therefore,  through  which  the 
light  passes,  will  vary  from  moment  to  moment  and  in  accordance  with 
variation  in  the  light  intensity.  The  quantity  of  light  necessary  to  distinct 
vision  is  thus  regulated. 

In  the  total  absence  of  light  the  sphincter  pupillae  muscle  is  relaxed  and 
the  pupil  widely  dilated.  With  the  appearance  of  light  and  an  increase 
in  its  intensity  the  muscle  again  contracts  and  the  pupil  progressively  narrows. 
With  a  given  intensity  in  the  light,  the  sphincter  contraction  is  greater  when 
the  light  falls  directly  upon  the  fovea.  Contraction  of  this  muscle  is  an 
associated  movement  in  the  convergence  of  the  eyes  during  accommodation 
and  in  consensus  with  the  other  eye. 

In  addition  to  this  function  of  the  iris,  it  constitutes,  by  virtue  of  the 
sphincter  muscle  contraction,  an  important  corrective  apparatus.  Being 
non-transparent,  it  serves  as  a  diaphragm  intercepting  those  rays  which 
would  otherwise  pass  through  the  peripheral  portions  of  the  lens  and  by 
spheric  aberration  give  rise  to  indistinctness  of  the  image.  The  movements 
of  the  iris  by  which  the  size  of  the  pupil  is  determined  are  caused  by  the  con- 
tractions and  relaxations  of  the  sphincter  pupillce  and  dilatator  pupillcB 
muscles.  The  contraction  of  the  sphincter  is  entirely  reflex  and  involves 
those  structures  necessary  to  the  performance  of  any  reflex  act,  viz. :  a  recep- 
tive surface,  the  retina;  afferent  nerves,  the  pupillary  fibers  of  the  optic  nerve; 
a  central  emissive  center  situated  in  the  gray  matter  beneath  the  aqueduct 
of  Sylvius;  and  efferent  nerves,  the  motor  oculi  and  the  ciliary  nerves.  The 
stimulus  requisite  to  the  excitation  of  this  mechanism  is  the  impact  of  light 
waves  or  ether  vibrations  on  the  rods  and  cones.  According  to  the  intensity 
of  these  vibrations  will  be  the  resulting  contraction  of  the  muscle.  The 
contraction  of  the  dilatator  pupillse  muscle  is  determined  by  the  activity 
of  a  continuously  active  nerve-center  in  the  medulla  oblongata  which  trans- 
mits its  nerve  impulses  through  the  spinal  cord,  along  the  first  and  second 
dorsal  nerves  to  the  superior  cer\dcal  ganglion,  and  thence  to  the  iris  by 
way  of  the  fifth  nerve.  (See  Fig.  270,  page  582.)  These  two  muscles 
appear  to  bear  an  antagonistic  relation  to  each  other,  for  section  of  the  motor 


THE  SENSE  OF  SIGHT.  663 

oculi  is  followed  by  relaxation  of  the  sphincter  muscle  and  dilatation  of  the 
pupil.  Stiniulation  of  the  sympathetic  is  followed  by  a  more  pronounced 
dilatation.  The  size  of  the  pupil  is  the  resultant  of  a  balancing  of  these 
two  forces. 

OPTIC  DEFECTS. 

Presbyopia. — ^Presbyopia  may  be  defined  as  a  condition  of  the  normal 
eye  in  which  the  accommodation  has  become  so  reduced  by  age  that  reading  has 
become  impossible  at  ordinary  distances.  As  age  advances  the  lens  loses 
its  elasticity  and  the  power  to  increase  its  refraction,  and  vision  at  the  normal 
reading  distance  becomes  impossible.  The  near  point,  the  punctum  prox- 
imum,  therefore,  advances  toward  the  far  point,  or  recedes  from  the  indi- 
vidual. The  range  of  accommodation  is  also  diminished.  At  forty  years 
the  near  point  is  about  22  cm.;  at  forty-five  years  it  has  receded  to  28  cm. 
This  would  indicate  that  the  lens  in  these  five  years  has  lost  i  dioptry  of  refracting 
power;  at  fifty  years  the  near  point  recedes  to  43  cm.,  and  at  sixty  to  200 
cm.,  indicating  a  loss  in  refracting  power  on  the  part  of  the  lens  of  2  and  4 
dioptrics  respectively.  Convex  lenses  placed  before  the  eyes  having  a 
refracting  power  of  i,  2,  and  4  dioptrics  would  in  the  three  instances  return 
the  near  point  to  its  normal  position.  At  the  age  of  seventy  the  lens  is 
incapable  of  any  increase  during  an  accommodative  efi'ort.  A  lens  of  4 
dioptrics  would  therefore  be  required  by  such  a  man,  for  clear  vision  at  10 
inches  or  250  centimeters. 

Myopia. — Myopia  may  be  defined  as  a  condition  of  the  eye  characterized 
by  un  increase  in  the  antero-posterior  diameter  or  a  hypernormal  refracting 


Fig.  321. — Myopia.     Parallel  rays  Fig.  322. — Correction  of  Myopi.\  by 

focus  at  F,  cross  and  form  diffusion-  A  Concave  Lens. 

circles;  divergent  rays  from  A  focus 
on  the  retina. 

power  of  the  lens.  The  former  is  the  usual  condition.  Parallel  rays  of 
light  brought  to  -a  focus  in  front  of  the  retina  again  diverge,  giving  rise  to 
diffusion-circles  and  indistinctness  of  the  image.  Divergent  rays  alone 
are  capable  of  being  focalized  on  the  retina  in  its  new  position.  The  distant 
point,  the  punctum  remotum  is  always  at  a  finite  distance,  but  approaches 
the  eye  as  the  myopia  increases.  The  near  point  is  usually  much  nearer  the 
eye  than  20  cm.  For  this  reason  the  condition  is  termed  near  sight. 
(Fig.  321). 

The  increase  in  the  length  of  the  antero-posterior  diameter  may  range 
from  a  fraction  of  a  milHmeter  up  to  10  mm.  With  an  increase  of  0.16  mm. 
the  far  point  is  but  200  cm.  distant;  and  wath  an  increase  of  3.2  mm.  it  is  but 
10  cm.  distant.  Inasmuch  as  only  divergent  rays  can  be  focalized  by  the 
myopic  eye  normal  \asion  can  be  restored  by  the  use  of  a  biconcave  lens  with 


664  TEXT-BOOK  OF  PHYSIOLOGY. 

a  diverging  power  in  the  first  instance  of  0.5  dioptry  and  the  second  of  10 
dioptrics.     (Fig  322.) 

Hypermetropia. — Hypermetropia  may  be  defined  as  a  condition  of  the 
eye  characterized  by  decrease  of  the  normal  antero-posterior  diameter  or 
by  a  subnormal  refracting  power  of  the  lens.  The  former  is  the  usual  con- 
dition. Parallel  rays  of  light  do  not,  therefore,  come  to  a  focus  when  the 
accommodation  is  suspended.  Falling  on  the  retina  previous  to  focalization, 
they  give  rise  to  diffusion-circles  and  indistinctness  of  the  image.  As  no 
object  can  be  seen  distinctly  no  matter  how  remote,  there  is  no  positive  far 


T^sC— ~—  ss»->K 


Fig.  323. — The  Hypermetropic  Eye.  Parallel  rays  (.4,  B)  can  be  focused  only  at  a  point 
behind  the  eye,  as  at/;  rays  of  light  coming  from  the  retina  take,  on  emerging  from  the  eye,  a 
divergent  direction,  C,  D.     K.  The  negative  punctum  remotum. 

point.  The  near  point  is  abnormally  distant — sometimes  as  far  as  200  cm. 
For  this  reason  the  condition  is  termed  far  sight.  A  hypermetropic  eye 
without  accommodative  effort  can  focalize  only  converging  rays  on  the  retina. 
If  rays  of  light  were  to  come  from  the  retina  of  such  an  eye,  they  would,  on 
emerging,  take  a  divergent  direction,  as  shown  in  Fig.  323,  dotted  line  C 
and  D.  If  there  same  rays  were  to  be  prolonged  backward,  they  would 
meet  at  the  point  K,  which  is  the  punctum  remotum;  and  as  it  is  behind  the 
eye,  it  is  termed  negative.  Since  rays  coming  from  the  retina  take  a  divergent 
direction  on  emerging  from  the  eye,  it  is  evident  that  only  converging  rays 


Fig.  324. — Hypermetropia.     Par-  Fig.  325. — Correction  of  Hyper- 

allel  Rays   Focused  behind  the  metropia  by  a  Convex  Lens. 

Retina. 

can  be  focalized  by  a  passive  hypermetropic  eye.  As  there  are  no  convergent 
rays  in  nature,  it  is  necessary  for  distinct  vision  that  all  rays,  parallel  and 
divergent,  shall  be  given  a  convergent  direction  before  entering  the  eye. 
The  hypermetropic  person  attempts  to  focalize  the  rays  by  increasing  the 
convexity  of  the  lens  though  an  increased  accommodative  effort  which  often 
gives  rise  to  accomodation  fatigue  and  headache.  The  convergence  of  the 
rays  of  light  before  they  enter  the  hypermetropic  eye  is  accomplished  by 
the  placing  before  the  eye  convex  lenses  the  converging  power  of  which  is 
proportional  to  the  degree  of  hypermetropia.      (Figs.  324,  325). 


THE  SENSE  OF  SIGHT. 


665 


Astigmatism. — Astigmatism  may  be  defined  as  a  condition  of  the  eye 
characterized  by  an  inquahty  of  curvature  of  its  refracting  surfaces  in  con- 
sequence of  which  not  all  of  a  homocentric  bundle  of  rays  are  brought  to 
the  same  focus.  The  inequality  may  be  either  in  the  cornea  or  lens,  or 
both,  though  usually  in  the  cornea. 

In  the  normal  cornea  the  radius  of  curvature  in  the  vertical  meridian 
is  a  trifle  shorter,  7.6  mm.,  than  that  of  the  horizontal,  7.8  mm.,  and  hence 
its  focal  distance  is  slightly  shorter.  The  difference,  however,  in  the  focal 
distances  is  so  slight  that  the  error  in  the  formation  of  the  image  is  scarcely 
noticeable.  A  transverse  section  of  a  cone  of  light  coming  from  the  cornea  is 
practically  a  circle.  If,  however,  the  vertical  curvature  exceeds  the  normal 
to  any  marked  extent,  the  rays  passing  in  the  vertical  plane  will  be  more 
sharply  refracted  and  brought  to  a  focus  much  sooner  than  the  rays  passing 
through  the  horizontal  plane.  The  result  will  be  that  the  cone  of  light  will 
be  no  longer  circular,  but  more  or  less  elliptic.     The  variations  of  the  shape 


Fig.  326. — Refraction  by  .\x  Astigmatic  Surface. — {Hansell  and  Sweet.) 


of  this  cone  are  shown  in  Fig.  326,  which  represents  the  appearance  pre- 
sented on  cross-section  both  before  and  after  focalization  of  each  set  of  rays. 
Though  the  vertical  plane  has  usually  the  sharper  curvature,  it  not  infre- 
quently happens  as  illustrated  in  this  figure,  that  the  reverse  is  true.  For 
the  reason  that  the  rays  from  one  point  do  not  all  come  to  the  same  focus 
or  point,  the  condition  is  termed  astigmatism. 

Spheric  Aberration. — When  the  rays  of  light  which  emanate  from  a 
point  fall  upon  a  spheric  lens,  they  do  not  after  passing  through  it  reunite 
at  one  point  because  of  the  fact  that  the  more  peripheral  rays  have  a 
shorter  focus  than  the  central  rays.  To  this  condition  the  term  spheric 
aberration  is  given.  Spheric  aberration  can  be  demonstrated  in  the  human 
eye.  That  this  condition  is  present  to  but  a  slight  extent  in  the  normal 
eye  is  due  to  the  presence  of  the  iris,  which  intercepts  those  rays  which  would 
otherwise  pass  through  the  marginal  portions  of  the  refracting  media. 
In  widely  dilated  eyes  the  spheric  aberration  of  the  peripheral  parts  may 
amount  to  as  much  as  4  or  5  dioptrics. 

I  Chromatic  Aberration. —  When  a  beam  of  light  is  made  to  pass 
through  a  prism,  it  is  decomposed  into  the  primary  colors  owing  to  a  difference 
in  the  refrangibility  of  the  rays.  In  passing  through  the  refracting  media  of 
the  eye  the  different  rays  composing  white  light  also  undergo  unequal  refrac- 
tion and  those  rays  which  give  rise  to  one  color  are  brought  to  a  focus  at  a 
point  somewhat  different  from  those  which  give  rise  to  other  colors.     If  the 


666  TEXT-BOOK  OF  PHYSIOLOGY. 

eye  is  accommodated  for  one  set  of  rays,  it  is  not  for  another,  and  the  result  is  a 
fringe  of  colors  around  the  image.  This  defect  in  the  normal  eye  is  so  slight 
that  the  mind  fails  to  take  cognizance  of  it.  That  the  eye  is  incapable  simul- 
taneously focalizing  rays  of  widely  different  refrangiblity,  as  those  which 
give  rise  to  the  blue  and  red  colors,  is  shown  by  the  following  experiment: 
The  eye  being  directed  to  a  luminous  point,  a  plate  of  cobalt-glass  is 
placed  between  the  light  and  the  observer  close  to  the  eye.  This  substance 
has  the  property  of  intercepting  all  rays  but  the  red  and  the  blue  and  hence 
these  alone  will  be  seen.  The  center  of  the  image  produced  will  be 
red  and  clearly  defined,  the  periphery  blue  and  ill-defined.  The  reason 
for  this  is  clear.  The  eye  more  readily  accommodates  itself  for  the 
red  rays,  and  hence  their  focal  point  is  distinct.  The  blue  rays,  having  a 
higher  degree  of  refrangibility,  come  to  a  focus,  cross  and  diverge,  and  give 
rise  to  diffusion-circles.  If  a  biconcave  glass  be  placed  before  the  cobalt, 
the  blue  rays  can  be  focalized  on  the  retina,  while  the  red  will  fall  on  the  retina 
without  focalization.  The  image  will  now  be  blue  and  distinct  in  the  center, 
the  periphery  red  and  ill-defined.  With  the  removal  of  the  minus  glass  the 
reverse  condition  again  obtains. 

Imperfect  Centering. — From  a  purely  physical  point  of  view,  the  eye 
is  not  a  perfect  optic  instrument.     In  addition  to  the  defects  noticed  in  the 


Hem^raZ  Sc^rle 


Nasal  Sute- 

Fig.  327. — DiAGR.'VM  showing  the  Corneal  Axis  D  D,  the  Optic  Axis  O  A,  the  Visual 
Axis  V  L,  and  the  Line  of  Fixation  V  C;  also  the  Three  Angles,  a,  /?,  u. 

foregoing  paragraphs,  there  is  yet  another,  viz.:  an  imperfect  centering  of 
the  refracting  surfaces.  In  first-class  optic  instruments  the  lenses  are 
centered — that  is,  their  optic  centers  are  situated  on  the  same  axis.  In 
viewing  an  object  through  such  a  system  the  visual  line  corresponds  with  the 
axis  of  the  lens  system.  This  is  not  the  case  with  the  refracting  system  of  the 
eye.  A  line  passing  through  the  center  of  the  cornea  and  the  center  of  the 
eye,  the  optic  axis  (O  A  in  Fig.  327),  does  not  pass  exactly  through  the 
center  of  the  lens  and  does  not  fall  into  the  point  for  most  distinct  vision,  the 
fovea.  This  has  lead  to  the  recognition  of  other  lines  the  relations  of  which 
must  be  kept  in  mind  in  all  optic  discussions,  viz. : 

I.  The  visual  axis  or  visual  line  {V  L),  the  line  connecting  the  point  viewed, 
the  nodal  point,  and  the  fovea  centralis. 


THE  SENSE  OF  SIGHT.  667 

2.  The  line  oj  fixatimi  or  line  of  regard  (F  C),  the  Hne  connecting  the  point 

viewed  with  the  center  of  rotation,  the  latter  being  situated  6  mm.  behind 

the  nodal  point  of  the  eye  and  9  mm.  before  the  retina.     The  relation 

of  these  lines  and  certain  angles  connected  with  them  are  shown  in  Fig. 

327.     The  angle  included  between  the  line  D  D  (the  major  axis  of  the 

corneal  ellipse)  and  the  visual  line  is  the  angle  alpha,  amounting  on  the 

average  to  5°.     The  angle  incuded  between  the  optic  axis  and  the  line  of 

fixation  or  regard  is  the  angle  gamma,  while  the  angle  between  the  optic 

axis  and  the  line  of  vision  is  the  angle  beta .     In  emmetropia  the  angle  alpha 

is  about  5°.     In  hypermetropia  it  is  greater,  amounting  to  7°  or  8°, 

giving  to  the  eye  an  appearance  of  divergence.     In  myopia  it  is  much 

smaller — 2° — or  in  extreme  cases  may  be  abolished,  the  line  of  vision 

corresponding  with  the  optic  axis  or  even  passing  beyond  it.     The 

angle  gamma  is  of  value  in  determining  the  actual  deviation  of  the  eye 

in  squint. 

Functions  of  the  Retina. — Of  all  the  layers  of  the  retina,  the  rods  and 

cones  appear  to  be  the  most  essential  to  vision.     It  is  only  this  layer  that  is 

capable  of  receiving  the  light  stimulus  and  of  transforming  it  into  some 

specific  form  of  energy,  which  in  turn  arouses  in  the  fibers  of  the  optic  nerve 

the  characteristic  nerve  impulses.     A  ray  of  light  entering  the  eye  passes 


Fig.  328. — Diagram  for  Observing  the  Situation  of  the  Blind  Spot. — 

{HelmhoUz.) 

entirely  through  the  various  layers  of  the  retina,  and  is  arrested  only  upon 
reaching  the  pigmentary  epithelium  in  which  the  rods  and  cones  are  embedded. 
As  to  the  manner  in  which  the  objective  stimuli — light  and  color,  so  called — 
are  transformed  into  nerve  impulses,  but  little  is  known.  It  is  probable 
that  the  ether  vibrations  are  transformed  into  heat,  which  excites  the  rods 
and  cones.  These,  acting  as  highly  specialized  end-organs  of  the  optic 
nerve,  start  the  impulses  on  their  way  to  the  brain,  where  the  seeing  process 
takes  place.  As  to  the  relative  function  of  the  rods  and  cones,  it  has  been 
suggested,  from  the  study  of  the  facts  of  comparative  anatomy,  that  the  rods 
are  impressed  only  by  differences  in  the  intensity  of  light,  while  the  cones,  in 
addition,  are  impressed  by  quahtative  differences  in  color.  The  nerve- 
fibers  themselves  are  insensible  to  the  impact  of  the  ether  vibrations,  and 
require  for  their  excitation  some  intermediate  form  of  energy.  That  this  is 
the  case  was  shown  by  Bonders,  who  reflected  a  beam  of  light  on  the  optic 
nerv'e  at  its  entrance  without  the  individual  experiencing  any  sensation  of 
light.  This  region,  occupied  only  by  the  optic-nerve  fibers  and  devoid  of  any 
special  retinal  elements,  is  therefore  an  insensitive  or  blind  spot.  The 
diameter  of  this  spot  is  about  1.5  mm.,  and  occupies  in  the  field  of  vision  a 
space  of  about  6°.  It  is  situated  about  3.5  mm.  to  the  nasal  side  of  the 
visual  axis.     Its  existence  can  be  demonstrated  by  the  familiar  experiment 


668  TEXT-BOOK  OF  PHYSIOLOGY. 

of  Mariotte,  which  consists  in  placing  before  the  eye  two  objects  having  the 
relation  to  each  other  shown  in  Fig,  328.  With  the  left  eye  closed  and  the 
right  eye  directed  to  the  cross,  both  objects  may  be  visible.  But  by  moving 
the  figure  away  from  or  toward  the  eye,  there  will  be  found  a  distance, 
about  30  cm.,  when  the  circle  will  be  invisible.  This  occurs  when  the 
image  falls  on  the  optic  nerve  at  its  entrance.  The  experiment  of  Purkinje 
as  described  in  the  following  paragraph  demonstrates  also  the  fact  that  the 
sensitive  portion  of  the  retina  is  to  be  found  only  in  the  layer  of  rods  and 
cones. 

It  is  well  known  that  the  blood-vessels  of  the  retina  are  situated  in  its 
innermost  layers  a  short  distance  behind  the  optic-nerve  fibers.  Owing  to 
this  anatomic  arrangement,  a  portion  of  the  light  coming  through  the  pupil 
will  be  intercepted  by  the  vessels  and  a  shadow  projected  on  the  layer  of 
rods  and  cones.  Ordinarily,  these  shadows  are  not  perceived,  for  the  reason 
that  the  shaded  parts  are  more  sensitive,  so  that  the  small  amount  of  light 
passing  through  the  vessels  produces  as  strong  an  impression  on  this  part 
as  does  the  full  amount  of  light  on  the  unshaded  parts  of  the  retina,  and 
perhaps  because  the  mind  has  learned  to  disregard  them.  But  if  light  be 
made  to  enter  the  eye  obliquely,  the  position  of  the  shadows  will  be  changed, 
when  at  once  they  become  apparent.  This  can  be  shown  in  the  following 
way:  If  in  a  darkened  room  a  lighted  candle  be  held  several  inches  to  the 
side  and  to  the  front  of  the  eye,  and  then  moved  up  and  down,  there  will 
be  perceived,  apparently  in  the  field  of  vision,  an  arborescent  figure  corres- 
ponding to  the  retinal  blood-vessels.  This  is  due  to  the  falling  of  the 
shadows  on  unusual  portions  of  the  layer  of  rods  and  cones. 

Excitability  of  the  Retina. — The  retina  is  not  equally  excitable  in  all  parts 
of  its  extent.  The  maximum  degree  df  sensibility  is  found  in  the  macula 
lutea,  and  especially  in  its  central  portion,  the  fovea.  In  this  region  the 
layers  of  the  retina  almost  entirely  disappear,  the  layers  of  rods  and  cones 
alone  remaining,  and  in  the  fovea  only  the  cones  are  present.  That  this 
area  is  the  point  of  most  distinct  vision  is  shown  by  the  observation  that 
when  the  eye  is  directed  to  any  given  point  of  light,  its  image  always  falls 
in  the  fovea.  z\ny  pathologic  change  in  the  fovea  is  attended  by  marked 
indistinctness  of  vision.  The  sensibility  of  the  retina  gradually  but  irregu- 
larly diminishes  from  the  macula  toward  the  periphery.  This  diminution 
in  insensibility  holds  true  for  monochromatic  as  well  as  white  light. 

As  stated  above,  the  nature  of  the  molecular  processes  which  take  place  in 
the  retinal  tissue,  caused  on  one  hand  by  the  light  vibrations,  and  on  the  other 
hand  developing  nerve  impulses,  is  entirely  unknown.  The  discovery  of 
the  visual  purple  in  the  outer  segment  of  the  rods  gave  promise  of  some 
explanation  of  the  process,  especially  when  it  was  shown  to  undergo 
changes  when  exposed  to  the  action  of  light.  But  as  the  pigment  is  wanting 
in  the  cones,  and  especially  in  the  fovea,  it  cannot  be  considered  essential 
to  distinct  vision,  although  that  it  plays  some  important  role  in  the  visual 
process  is  highly  probable.  It  was  observed  by  Van  Genderen  Stort, 
that  when  an  animal  is  kept  in  darkness  some  time  before  death,  the  cones 
are  long  and  filiform;  but  if  the  animal  has  been  exposed  to  light,  they  are 
short  and  swollen.  It  was  discovered  by  Boll  that  if  an  animal  is  kept  in 
darkness  an  hour  or  two  before  death  the  pigment  is  massed  at  the  ends  of 


THE  SENSE  OF  SIGHT. 


669 


the  rods  and  cones,  but  after  exposure  to  light  it  becomes  displaced  and 
extends  over  and  between  the  rods  almost  to  the  external  limiting  mem- 
brane.    These  conditions  are  represented  in  Fig.  329. 


Fig.  329. — Sectiox  of  the  Retina  of  a  Frog.     A.  In  darkness.     B.  In  light.— 
(After  Van  Genderen  Start,  from  Tsclierniiig's  "Physiologic  Optics.") 

The  Eye  a  Living  Camera, — In  its  construction,  in  the  arrangement 
of  its  various  parts,  and  in  their  mode  of  action  the  eye  may  be  compared 
to  a  camera  ohscura.  Though  the  comparison  may  not  be  absolutely  exact, 
yet  in  a  general  way  it  is  true  that  there  are  many  striking  points  of  simil- 
arity between  them;  e.g.,  the  sclera  and  chorioid  may  be  compared  to  the 
walls  of  the  camera;  the  combined  refracting  media  to  the  component  glasses 
of  the  lens,  the  action  of  which  results  in  the  focusing  of  the  light  rays;  the 
retina  to  the  sensitive  plate  receiving  the  image  formed 
at  the  focal  point;  the  iris  to  the  diaphragm  for  the 
regulation  of  the  amount  of  light  to  be  admitted,  and 
for  the  partial  exclusion  of  those  marginal  rays  which 
give  rise  to  spheric  aberration;  the  ciliary  muscle  to  the 
adjusting  screw,  by  means  of  which  the  image  is  brought 
to  a  focus  on  the  sensitive  plate,  notwithstanding  the 
varying  distances  of  the  object  from  the  lens.  The 
presence  of  the  visual  purple  in  the  rods  of  the  retina  a  Rabbit.  Optogr.am 
capable  of  being  altered  by  light  makes  the  compari-  "jj-^ers  dTstant^^o^ 
son  still  more  striking.  Yellow    spot,    b,  b. 

Kuhne  even  succeeded  in  obtaining  a  fixed  image    Streak    of    Medullated 

,  ,      ,  .     °    .  nerve-fibers. — (Kuhne.) 

or  an  optogram  01  an  external  object  m  a  manner 

similar  to  that  by  which  an  image  is  fixed  on  the  sensitive  plate  of  a 
camera.  An  animal  is  kept  in  the  dark  for  about  ten  minutes  in 
order  to  permit  the  retinal  pigment  to  be  completely  regenerated. 
The  animal,  with  the  eyes  covered,  is  then  brought  into  a  room  with 
a   single   window.     While   the   head   is   steadily  directed   to  the  window, 


Fig.  330. — Retina  of 


670  TEXT-BOOK  OF  PHYSIOLOGY. 

the  eye  is  exposed  for  several  minutes.  The  eyes  are  again  covered,  the 
animal  killed,  and  the  eyes  removed  by  the  light  of  a  sodium  flame.  The 
retina  is  then  placed  in  a  4  per  cent,  solution  of  alum.  In  a  short  time  the 
image  of  the  window,  the  optogram,  will  be  fixed  (Fig.  330).  That  portion 
of  the  image  corresponding  to  the  window  lights  will  be  quite  bleached  in 
appearance  from  the  action  of  the  light  on  the  pigment,  while  that  corres- 
ponding to  window  bars  will  have  the  usual  color  of  the  retina.  Dur- 
ing life  the  regeneration  of  the  visual  purple  must  take  place  with  extreme 
rapidity  if  a  similar  change  takes  place  with  the  formation  of  each  image. 
The  visual  purple  is  believed  to  be  derived  from  a  pigment  seci^eted  by  the 
layer  of  pigment  cells. 

Binocular  Vision.— Though  two  images  are  formed,  one  on  each 
retina,  when  the  eyes  are  directed  to  a  given  object,  there  results  but  one 
sensation.  If  the  direction  of  either  visual  axis  be  changed  by  pressure  on 
the  eyeball,  there  arise  two  sensations,  and  the  object  appears  to  be  doubled. 
The  reason  assigned  for  this,  in  the  first  instance,  is  that  the  two  images 
fall  into  the  foveae,  two  corresponding  points;  while  in  the  second  instance 
they  fall  on  non-corresponding  points.  It  would  appear,  therefore,  that  for 
the  purpose  of  seeing  an  object  singly  when  the  eyes  are  directed  toward 
it,  the  rays  emanating  from  it  must  fall  on  corresponding  parts  of  the  retina. 

As  all  portions  of  the  retina  are  sensitive 
to  light,  though  in  varying  degrees,  it  is 
not  essential  that  the  images  always  fall 
in  the  foveae.  The  parts  of  the  retinae 
which  correspond  physiologically  are 
shown   in  Fig.  331.     In  this  figure  the 

,  retinal  area  is  divided   into  quadrants 

Fig.  ^^i. — Corresponding  Areas  OF  THE  ,  .     ,  ,   ,       .        ,    1  i-  r 

Retina.  by  vertical  and  horizontal  fines  01  sepa- 

ration, as  they  are  termed.  If  one  retina 
is  placed  in  front  of  or  over  the  other,  it  will  be  found  that  the  quadrants 
bearing  similar  letters  cover  each  other.  So  long  as  the. rays  of  light, 
entering  the  eye,  fall  on  corresponding  areas  the  sensation  of  but  one 
object  arises.  If,  however,  they  fall  on  non-corresponding  areas,  two  sen- 
sations arise.  Normal  binocular  vision  enlarges  very  considerably  the  area 
of  the  visual  field,  permits  of  a  better  estimation  of  the  size  and  distance 
of  objects,  enables  the  mind  to  form  more  readily  a  perception  of  depth, 
and  increases  the  intensity  of  sensations. 

The  Horopter.— When  the  eyes  are  in  a  so-called  secondary  position 
— that  is,  in  a  position  in  which  the  visual  axes  are  converged  and  directed 
to  a  point  in  front  of  and  in  the  middle  plane  of  the  body — it  will  be  found 
on  examination  that  rays  of  light  from  a  number  of  other  objects  enter  the 
eye,  pass  through  the  nodal  point,  and  fall  on  corresponding  parts  of  the 
two  retinae  and  give  rise  to  but  single  images.  All  such  points  lie,  for  the 
horizontal  line  of  separation,  on  a  line  termed  the  horopter.  The  form  of 
this  line  is  that  of  a  circle  which  passes  through  the  fixation  point  and  the 
two  nodal  points.  Any  object  on  the  horopter  will  give  rise  to  but  a  single 
image.  This  is  shown  in  Fig.  332,  in  which  the  objects  I,  II,  III  project 
their  rays  into  both  eyes  and  upon  corresponding  areas. 

In  addition  to  the  horopter  for  the  horizontal  line  of  separation,  there 


THE  SENSE  OF  SIGHT. 


671 


is  also  an  horopter  for  the  vertical  Hne  of  separation.  At  a  distance  of  two 
meters  the  vertical  horopter  is  a  plane.  Within  this  distance  it  is  concave  to 
the  face;  beyond  this  distance  it  is  convex. 

An  object  which  lies  either  in  front  of  or  behind  the  fixation  point  will 
project  its  rays  on  parts  of  the  retinae  which  do  not  correspond,  and  hence 
give  rise  to  double  images.  This  is  evident  from  examination  of  Fig.  333, 
While  the  eyes  are  directed  to  figure  2,  of  which  there  is  but  a  single  image, 
the  objects  B  and  A  give  rise  to  double  images,  for  reasons  already  given. 
It  the  eyes  are  now  directed  to  B,  double  images  will  be  formed  of  2  and  A. 

At  all  times,  therefore,  double  images  are  formed  on  the  retinae  the 
existence  of  which  is  scarcely  noticed  unless  the  attention  is  directed  to  them. 


B 


Fig.  332. — Horopter  for  the 
Secondary  Position,  with  Con- 
vergence OF  THE  Visual  Axes. 
— {Landois.) 


Fig.  333. — Scheme  of  Identical  and 
Non-identical  Points  of  the  Retina. — 
(Landois.) 


This  is  due  to  the  fact  that  many  of  the  images  fall  on  the  peripheral,  less 
sensitive  parts  of  the  retinae.  At  the  same  time,  from  a  want  of  accommo- 
dation and  the  formation  of  diffusion-circles,  they  are  indistinct.  For  these 
reasons  they  are  readily  neglected. 

In  the  primary  position  of  the  eyes — that  is,  a  position  in  which  the 
visual  axes  are  parallel — the  horopter  is  a  plane  at  infinity.  In  the  tertiary 
positions  the  horopter  is  a  curve  of  complex  form. 

Movements  of  the  Eyeball. — The  almost  spheric  eyeball  lies  in  the 
correspondingly  shaped  cavity  of  the  orbit,  like  a  ball  placed  in  a  socket, 
and  is  capable  of  being  rotated  to  a  considerable  extent  by  the  six  muscles 
which  are  attached  to  it.  These  muscles  are  the  superior  and  inferior  recti, 
the  external  and  internal  recti,  and  the  superior  and  inferior  oblic|ui  (Fig. 
334).  The  four  recti  muscles  arise  from  the  apex  of  the  orbit  cavity,  from 
which  point  they  pass  forward  to  be  inserted  into  the  sclera  about  7  to  8 
mm.  from  the  corneal  border.  The  superior  oblique  muscle  having  a  similar 
origin  passes  forward  to  the  upper  and  inner  angle  of  the  orbit  cavity,  at 


672 


TEXT-BOOK  OF  PHYSIOLOGY. 


which  point  its  tendon  passes  through  a  cartilaginous  pulley,  after  which 
it  is  reflected  backward  to  be  inserted  into  the  superior  surface  of  the  sclera 
about  16  mm.  behind  the  corneal  border.  The  inferior  oblique  muscle  arises 
from  the  inner  and  inferior  angle  of  the  orbit  cavity.  It  then  passes  outward, 
upward,  and  backward,  to  be  inserted  into  the  upper,  posterior,  and  temporal 
portion  of  the  sclera  about  4  or  5  mm.  from  the  optic  nerve  entrance. 

The  movements  of  each  eye  are  referred  to  three  fixed  lines  or  axes, 
which  have  their  origin  at  the  point  of  rotation  of  the  eyeball,  this  point 
lying  about  1.7  mm.  behind  the  center  of  the  globe.  If  the  eye  looks  straight 
forward  in  the  horizontal  plane  (the  head  being  erect),  the  line  joining  the 
center  of  rotation  with  the  object  looked  at  is  the  line  oj  fixation  or  line  of 
regard.  Around  this  antero-posterior  axis  the  eye  may  be  regarded  as  per- 
forming its  circular  rotation  or  torsion.  At  right  angles  to  this  line,  and 
joining  the  centers  of  rotation  of  both  eyes,  is  the  horizontal  or  transverse 
axis,  around  which  the  movements  of  elevation  (up  to  34  degrees)  and  de- 
pression (down  to  57  degrees) 
take  place.  At  right  angles  to 
both  of  these  lines  there  is 
the  vertical  axis,  around 
which  the  movements  of  ad- 
duction (toward  the  nose  up 
to  45  degrees)  and  abduction 
(toward  the  temple  up  to  42 
degrees)  occur.  The  six  mus- 
cles may  be  divided  into  three 
pairs,  each  of  which  has  a 
common  axis  around  which  it 
tends  to  move  the  eyeball. 
But  only  the  common  axis  of 
the  internal  and  external  recti 
coincides  with  one  of  three 
axes  before  mentioned — name- 
ly, with  the  vertical  axis — 
thus  moving  the  ball  only  in- 
wardly or  outwardly — respec- 
tively. The  other  two  pairs, 
however,  have  their  own  axes  of  action,  and  their  movements  of  the 
ball  must  be,  therefore,  analyzed  with  regard  to  all  the  three  axes,  each 
of  these  four  muscles  producing  rotation,  elevation,  and  depression,  and 
abduction  or  adduction.  The  superior  and  inferior  recti  muscles,  form- 
ing one  pair,  move  the  eye  around  a  horizontal  axis  which  intersects  the 
median  plane  of  the  body  in  front  of  the  eyes  at  an  angle  of  63  degrees, 
and  the  superior  and  inferior  oblique  muscles  forming  the  third  pair  rotate 
the  globe  around  a  horizontal  axis  which  cuts  the  median  plane  of  the  body 
behind  the  eyes  at  an  angle  of  39  degrees.  Thus  it  is  that  each  muscle 
moves  the  eye  as  follows,  the  movement  for  practical  purposes  being  referred 
to  the  cornea :  The  rectus  externus  draws  the  cornea  simply  to  the  temporal 
side,  the  rectus  internus  simply  to  the  nose;  the  superior  rectus  displaces  the 
cornea  upward,    slightly  inward,  and  turns  the  upper  part  toward  the  nose 


Fig.  334. — Muscles  of  the  Eye  and  Tendon 
OK  Ligament  of  Zinn.  i.  Tendon  of  Zinn.  2.  Ex- 
ternal rectus  divided.  3.  Internal  rectus.  4.  In- 
ferior rectus.  5.  Superior  rectus.  6.  Superior 
oblique.  7.  Pulley  for  superior  oblique.  8.  In- 
ferior oblique.  9.  Levator  palpebrae  superioris. 
10,10.  Its  anterior  expansion.  11.  Optic  nerve. — 
{Sappey.) 


THE  SENSE  OF  SIGHT. 


673 


(medial  torsion);  the  inferior  rectus  moves  the  cornea  downward,  sHghtly 
inward,  and  twists  the  upper  part  away  from  the  nose  (lateral  torsion); 
the  superior  oblique  displaces  the  cornea  downward,  slightly  outward,  and 
produces  medial  torsion;  while  the  inferior  oblique  moves  the  cornea  upward, 
slightly  outward,  and  produces  lateral  torsion.  These  facts  show  that  for 
certain  movements  of  the  eye  at  least  three  muscles  are  necessary  (see 
following  table) : 


Inward Rectus  internus. 

Outward Rectus  externus. 

^r  ,  /  Rectus  superior. 

^       \  Obliquus  inferior 

J   Rectus  inferior. 
\  Obliquus  superior. 
Rectus  internus. 

upward \   Rectus  superior. 

Obliquus  inferior. 


Downward . 
Inward  and 


Inward  and  f  Rectus,  internus. 

downward -j  Rectus  inferior. 

\  Obliquus  superior. 
Outward  and  I  Rectus  externus. 

upward <  Rectus  superior. 

[  Obliquus  inferior. 
Outward  and  i  Rectus  externus. 

downward \  Rectus  inferior. 

[  Obliquus  superior. 


If  both  eyes  have  their  line  of  vision  in  the  horizontal  plane  parallel 
with  each  other  and  with  the  median  plane  of  the  body,  they  are  said  to  be 
in  the  primary  position.  All  other  positions  are  called  secondary  and  tertiary. 
Both  eyes  always  move  simultaneously,  which  is  called  the  associated  move- 
ment oj  the  eyes.  There  are  three  forms  of  associated  movements:  (i)  move- 
ment of  both  eyes  in  the  same  direction;  (2)  movements  of  convergence  by 
which  the  visual  lines  are  converged  on  a  point  in  the  middle  line  of  the  body; 
(3)  movements  of  divergence,  by  which  the  eyes  are  brought  back  from 
convergence  to  parallelism,  or  even  to  divergence,  as  in  certain  stereoscopic 
exercises.  A  combination  of  (i)  and  (2)  or  of  (i)  and  (3)  takes  place  for 
certain  positions  of  the  object  looked  at. 

Color-perception. — ^A  beam  of  sunlight  passed  through  a  glass  prism 
is  decomposed  into  a  series  of  colors — red,  orange,  yellow,  green,  blue,  and 
violet — the  so-called  spectral  colors,  so  well  exemplified  in  the  rainbow. 
The  spectral  colors  are  termed  simple  colors,  because  they  cannot  be  any 
further  decomposed  by  a  prism.  Objectively,  the  spectral  colors  consist  of 
very  rapid  transverse  electro-magnetic  vibrations  of  the  ether,  from  about 
400  millions  of  millions  per  second  for  red  to  about  760  millions  of  millions 
for  violet,  but  subjectively  they  are  sensations  caused  by  the  impact  of  the 
ether-waves  on  the  percipient  layer  of  the  retina. 

It  is  possible  to  mix  or  blend  these  spectral  color-sensations  in  the  eye ' 
by  stimulating  the  same  area  of  the  retina  by  different  spectral  colors,  either 
at  the  same  time  or  in  rapid  succession.     The  following  table  shows  the 
results  of  such  experiments  as  performed  by  v.  Helmholtz   (Dk.  =dark; 
\Vh.=  whitish): 


Violet 


Indigo 


Cyan-blue 


Bluish-green  i        Green 


Greenish- 
yellow 


Yellow 


Red. 
Orange. 

Yellow. 
Gr. -yellow. 
Green. 


Purple. 
jDk.-rose. 

Wh.-rose. 

iWhite. 

i  White-blue. 


Dk.-rose. 
Wh.-rose. 

White. 
Wh. -green. 
Water-blue. 


Bluish-green.   Water-blue,  i Water-blue. 
Cyan-blue.         Indigo.  


Wh.-rose. 
'White. 
Wh. -green 
Wh. -green. 
Bl. -green. 


I  White. 
Wh.-yellow. 
Wh. -yellow. 
Green. 


1  Wh.-yellow. 
i  Yellow. 

■Gr. -yellow. 


|Gold-yellow.    Orange. 
Yellow. 


43 


674  TEXT-BOOK  OF  PHYSIOLOGY. 

These  are  the  mixed  colors.  But  it  is  to  be  observed  that  only  two  new  color- 
sensations  can  be  produced,  white  and  purple,  the  remaining  mixed  colors 
already  finding  their  equivalent  in  the  spectrum.  White  and  .purple, 
therefore,  are  color-sensations  which  have  no  objective  equivalent  in  a 
simple  number  of  ether-vibrations  like  the  spectral  colors. 

Two  spectral  colors  which  by  their  mixture  produce  the  sensation  of 
white  are  called  complementary  colors.  Such  are  red  and  green-blue,  golden 
yellow  and  blue,  green  and  violet.  The  mixture  of  all  the  spectral  colors 
produces  white  again.  This  is  the  result  of  adding  two  or  more  color- 
sensations.  Different  results  are  obtained,  however,  by  adding  color  pig- 
ments. Yellow  and  blue,  for  example,  produce  in  the  eye  white,  but  on  the 
painter's  palette  green.  The  colors  of  nature  are  usually  mixtures  of  simple 
colors,  as  can  be  shown  by  spectroscopic  analysis  or  by  a  synthesis  of  spectral 
colors. 

In  all  color-sensations  we  must  distinguish  three  primary  qualities:  (i) 
hue;  (2)  purity  or  tint;  (3)  brightness  or  luminosity.  The  first  quality  gives 
the  main  name  to  the  color — e.g.,  red  or  blue — this  depending  on  the  spectral 
color  or  the  mixture  of  two  spectral  colors  with  which  it  can  be  matched. 
The  second  quality,  the  tint,  depends  on  the  admixture  of  white  with  the 
ground  color;  and  the  third  quality,  brightness,  depends  on  the  objective 
intensity  of  the  light  and  the  subjective  sensitiveness  of  the  retina.  Color- 
perception  thus  far  refers  only  to  the  most  sensitive  part  of  the  retina.  At 
the  more  peripheral  parts  of  the  retina  the  colors  are  seen  somewhat  differ- 
ently, as  is  shown  by  the  following  table  giving  the  limits  up  to  which  the 
colors  are  recognized: 

White.  Blue.  Red.  Green. 

Externally 90°  80°  65°  50° 

Internally 60°  55°  50°  40° 

Superiorly .    45°  40°  35°  3°° 

Inferiorly 70°  60°  45°  35° 

Theories  of  Color-perception. — The  theory  oj  v.  Hehnholtz,  originated 
by  Thomas  Young  (1807),  assumes  in  its  latest  form  the  existence  in  the 
human  retina  of  three  different  kinds  of  end-organs,  each  of  which  is  loaded 
with  its  own  photo-chemical  substance  capable  of  being  decomposed  by  a 
certain  color,  and  thus  exciting  the  fiber  of  the  optic  nerve. 

In  the  first  group  these  end-organs  are  loaded  with  a  red-sensitive  sub- 
stance, which  is  affected  mainly  by  the  red  part  of  the  spectrum;  the  second 
group  has  its  end-organs  provided  with  a  green-sensitive  substance,  which 
is  mainly  excited  by  the  green  color;  while  the  third  group  is  provided  with 
a  blue-sensitive  substance,  this  latter  being  mainly  affected  and  decomposed 
by  the  blue- violet  portion  of  the  spectrum.  All  these  three  different  end- 
organs  are  present  in  every  part  of  the  most  sensitive  area  of  the  retina,  and 
are  connected  by  separate  nerve-fibers  with  special  parts  of  the  brain,  in  the 
cells  of  which  each  calls  up  its  separate  sensation  of  red  or  green  or  blue. 

Out  of  these  three  primary  color-sensations  all  other  color-sensations 
arise.  If  a  light  mainly  excites  the  red-  or  green-  or  blue-sensitive  substance 
of  a  retinal  area,  we  term  it  red,  green,  or  blue,  respectively.  But  if  two  of 
these  photo-chemical  substances  are  stimulated  simultaneously,  quite  differ- 
ent sensations  arise.     Thus  simultaneous  stimulation  of  the  red  and  green 


THE  SENSE  OF  SIGHT.  675 

substances  gives  rise  to  the  sensation  of  yellow,  that  of  red  and  blue  to  the 
sensation  of  purple,  and  that  of  blue  and  green  to  the  sensation  of  blue-green. 
Simultaneous  stimulation  of  all  three  substances  of  a  certain  area  produces 
the  sensation  of  white.  According  to  this  theory,  complementary  colors 
are  any  two  which  together  excite  all  three  substances.  Color-blindness 
is  explained  by  this  theory,  on  the  assumption  that  two  of  the  photo-chemical 
substances  have  become  similar  or  equal  in  composition  to  each  other. 

The  theory  oj  Hering,  brought  forward  in  1874,  has  the  underlying 
assumption  that  the  process  of  restitution  in  a  nerve-element  is  capable  of 
exciting  a  sensation.  This  theory  asserts  that  there  are  three  visual  sub- 
stances in  the  retina — a  white-black,  a  red-green,  and  a  yellow-blue  visual 
substance.  A  destructive  process  in  the  white-black  substance,  such  as  is 
induced  not  only  by  white  light,  but  also  by  any  other  simple  or  mixed  color, 
produces  the  sensation  of  white,  while  the  process  of  restitution  or  assimila- 
tion in  this  substance  produces  the  sensation  of  black.  Similarly,  red  light 
produces  dissimilation  or  decomposition  in  the  red-green  substance,  and 
this,  again,  the  sensation  of  red.  Green  light,  however,  favors  the  process 
of  restitution  or  assimilation  in  the  red-green  substances,  and  thus  gives  rise 
to  the  sensation  of  green.  In  the  same  way  the  sensation  of  yellow  has  its 
cause  in  the  decomposition  of  yellow-blue  substance  induced  by  yellow  light, 
while  the  sensation  of  blue  is  produced  by  an  assimilative  process  in  the 
same  substance.  Simultaneous  processes  of  dissimilation  and  assimila- 
tion in  the  same  visual  substance  antagonize  each  other,  and  consequently 
produce  no  color-sensation  by  means  of  this  substance,  but  only  the 
sensation  of  white,  by  reason  of  decomposition,  by  both  colors,  in  the 
white-black  substance.  Thus,  yellow  and  blue,  impinging  on  the  same 
retinal  area,  have  no  effect  on  the  yellow-blue  substance,  because  they  are 
antagonistic  in  their  action  on  this  substance,  but  produce  only  the  sensation 
of  white,  as  both  yellow  and  blue  decompose  the  white-black  material. 
Color-blindness  is  explained  by  the  assumption  of  the  absence  of  either  the 
red-green  or  the  yellow-blue  visual  substance  in  the  retina. 

Accessory  Structures. — The  eyeball  is  protected  anteriorly  by  the 
eyelids  and  their  associated  structures,  the  Meibomian  glands,  the  lachrymal 
glands,  and  tears. 

The  eyelids  consist  of  a  central  framework  of  connective  tissue  support- 
ing muscle  tissue  (the  orbicularis  palpebrarum  muscle)  and  glands,  and 
covered  externally  by  skin  and  internally  by  a  modified  skin,  the  conjunctiva. 
The  free  border  of  each  lid  is  strengthened  by  a  semilunar  plate  of  dense 
fibrous  tissue,  the  tarsus.  The  cutaneous  edge  of  the  lid  is  bordered  with 
short  stiff  hairs.  At  the  inner  extremity  each  eyelid  presents  a  small  opening, 
the  punctiim  lacrimale,  the  beginning  of  the  lachrymal  duct.  The  two  ducts 
after  uniting  open  into  the  nasal  duct. 

The  Meibomian  glands  are  modified  sebaceous  glands  imbedded  in  the 
posterior  portion  of  the  lids  (Fig.  335).  Their  ducts  open  on  the  free  border 
of  the  lid.  These  glands  secrete  an  oleaginous  material  resembling  sebace- 
ous matter,  which  accumulates  along  the  margin  of  the  lid  and  prevents  the 
tears  from  flowing  down  the  cheek. 

The  lachrymal  gland  is  situated  at  the  upper  and  outer  part  of  the  orbit 
cavity.     It  consists  of  a  series  of  compound  tubules  lined  by  epithelium. 


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TEXT-BOOK  OF  PHYSIOLOGY. 


The  secretion  (the  tears)  is  conducted  from  the  gland  to  the  outer  part  of  the 
conjunctiva  by  seven  or  eight  ducts.  The  lachrymal  secretion  consists  of 
water  and  inorganic  salts.  It  is  distributed  over  the  corneal  surface  during 
the  act  of  winking,  thus  keeping  it  moist  and  free  from  foreign  particles. 


Fig.  335. — The  Lacrim.\l  and  Meibomian  Glands,  and  Adjacent  Organs  of  the  Eye. 
I,  I.  Inner  wall  of  orbit.  2,  2.  Inner  portion  of  orbicularis  palpebrarum.  3,  3.  Attachment  to 
circumference  of  base  of  orbit.  4.  Orifice  for  transmission  of  nasal  artery.  5.  Muscle  of  Horner 
(tensor  tarsi).  6,  6.  Meibomian  glands.  7,  7.  Orbital  portion  of  lacrimal  gland.  8,9,  10. 
Palpebral  portion.     11,  11.  Mouths  of  excretory  ducts.     12,  13.  Lacrimal  puncta. — {Sappey). 

It  eventually  passes  into  the  lachrymal  ducts  and  then  into  the  nose.  The 
lachrymal  glands  receive  secretory  fibers  by  way  of  the  fifth  nerve  and  the 
cervical  sympathetic.  The  secretion  can  be  excited  refiexly  from  stimulation 
of  sensor  nerves  as  well  as  by  emotional  states. 


CHAPTER  XXVIII. 
THE  SENSE  OF  HEARING. 

The  physiologic  mechanism  involved  in  the  sense  of  hearing  includes  the 
ear,  the  auditory  nerve,  its  cortical  connections,  and  nerve-cells  in  the  cortex 
of  the  temporal  lobe. 

Peripheral  excitation  of  this  mechanism  develops  nerve  impulses  which, 
transmitted  to  the  cortex,  evoke  the  sensation  of  sound  and  its  varying 
quahties — intensity,  pitch,  and  timbre. 

The  specific  physiologic  stimulus  to  the  terminal  organ,  the  organ  of 
Corti,  is  the  impact  of  atmospheric  undulations  of  varying  energy  and 
rapidity. 

THE  PHYSIOLOGIC  ANATOMY  OF  THE  EAR. 

The  ear,  the  organ  of  hearing,  is  lodged  within  the  petrous  portion  of  the 
temporal  bone.  It  may,  for  convenience  of  description,  be  divided  into  three 
portions:  viz.,  the  external,  the  middle,  and  the  internal  portion  (Fig.  336). 

The  external  ear  consists  of  the  pinna  or  auricle  and  the  external  audi- 
tory canal.  The  pinna  is  composed  of  a  thin  layer  of  cartilage  which  presents 
a  series  of  elevations  and  depressions.  It  is  attached  by  fibrous  tissue  to  the 
outer  edge  of  the  auditory  canal  and  covered  by  a  layer  of  skin  continuous 
with  that  covering  adjacent  structures.  The  general  shape  of  the  pinna  is 
concave.  Its  anterior  surface  presents,  a  little  below  the  center,  a  deep 
depression — the  concha. 

The  external  auditory  canal  extends  from  the  concha  inward  for  a  dis- 
tance of  from  25  to  30  mm.  It  is  directed  at  first  upward,  forward,  inward, 
and  then  somewhat  downward  to  its  termination.  It  is  composed  partly 
of  bone  and  partly  of  cartilage  and  lined  by  a  reflection  of  the  skin  covering 
the  pinna.  At  the  external  portion  of  the  canal  the  skin  contains  a  number 
of  tubular  glands,  the  ceruminous  glands,  which  resemble  in  their  con- 
formation the  perspiratory  glands.     They  secrete  cerumen  or  ear-wax. 

The  middle  ear,  or  tympanum,  is  an  irregularly  shaped  cavity  hollowed 
out  of  the  temporal  bone  and  situated  between  the  external  auditory  canal 
and  the  internal  ear.  It  is  narrow  from  side  to  side,  though  wider  above  than 
below.  It  is  relatively  long  in  its  antero-posterior  and  vertical  diameters. 
The  upper  portion  is  known  as  the  attic.  The  middle  ear  is  in  communica- 
tion posteriorly  with  the  mastoid  cells,  anteriorly  with  the  pharynx  through 
the  Eustachian  tube. 

The  Eustachian  Tube. — The  passageway  between  the  tympanic  cavity  and 
the  naso-pharynx  is  known  from  its  discoverer  as  the  Eustachian  tube.  It 
is  composed  internally  of  bone,  externally  of  cartilage,  and  is  lined  by  mucous 
membrane  covered  with  ciliated  epithelium.  Near  the  middle  of  its  course 
the  tube  is  contracted,  though  expanded  at  either  extremity  (Fig.  336).     It 

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TEXT-BOOK  OF  PHYSIOLOGY. 


measures  about  40  mm.  in  length.  Its  general  direction  from  the  pharyn- 
geal orifice  is  outward,  backward,  and  upward  at  an  angle  of  about  45 
degrees. 

The  middle  ear  cavity  is  separated  from  the  external  ear  by  a  membrane 
— the  membrana  tympani — and  from  the  internal  ear  by  an  osseo-mem- 
branous  partition  which  forms  a  common  wall  for  both  cavities.  The 
interior  of  the  cavity  is  crossed  from  side  to  side  by  a  chain  of  bones  and 
lined  by  a  mucous  membrane  continuous  with  that  lining  the  pharynx. 

The  membrana  tympani  is  a  thin,  translucent,  nearly  circular  membrane, 
measuring  about  10  mm.  in  diameter,  placed  at  the  inner  termination  of  the 
external  auditory  canal.     It  is  inclosed  in  a  ring  of  bone  which  in  the  fetal 


Fig.  336. — ^The  Ear.  i.  Pinna,  or  auricle.  2.  Concha.  3.  External  auditory  canal. 
4.  Membrana  tympani.  5.  Incus.  6.  Malleus.  7.  Manubrium  mallei.  8.  Tensor  tympani. 
9.  Tympanic  cavity.  10.  Eustachian  tube.  11.  Superior  semicircular  canal.  12.  Posterior  semi- 
circular canal.  13.  External  semicircular  canal.  14.  Cochlea.  15.  Internal  auditory  canal. 
16.  Facial  nerve.  17.  Large  petrosal  nerve.  18.  Vestibular  branch  of  auditory  nerve.  19. 
Cochlear  branch. — (Sappey.) 


condition  can  be  easily  removed,  but  in  the  adult  condition  cannot  be  re- 
moved, owing  to  its  consolidation  with  the  surrounding  bone.  This  mem- 
brane consists  primarily  of  a  layer  of  fibrous  tissue  which  is  covered  extern- 
ally by  a  thin  layer  of  skin  continuous  with  that  lining  the  auditory  canal, 
and  internally  by  a  thin  mucous  membrane.  The  tympanic  membrane  is 
placed  obliquely  at  the  bottom  of  the  auditory  canal,  inclining  from  above 
and  behind  downward  and  forward  at  an  angle  of  about  forty-five  degrees. 
The  external  surface  of  this  membrane  presents  a  funnel-shaped  depression, 
the  sides  of  which  are  slightly  convex. 

The  Ear-bones. — Running  across  the  tympanic  cavity  and  forming  an 
irregular  line  of  joined  levers  is  a  chain  of  bones,  which  articulate  one  with 
another  at  their  extremities.     These  bones  are  known  as  the  malleus,  incus, 


THE  SENSE  OF  HEARING. 


679 


and  stapes.     The  form  and  arrangement  of  these  bones  are  shown  in  Figs. 

The  malleus,  or  hammer  bone,  consists  of  a  head,  neck,  and  handle, 
•  of  which  the  latter  is  atttached  to  the  inner  surface  of  the  membrana  tympani. 
The  incus  or  anvil  bone  presents  a  concave  articular  surface  which  receives 
the  head  of  the  malleus.  The  stapes,  or  stirrup  bone,  articulates  externally 
with  the  long  process  of  the  incus,  and  internally,  by  its  oval  base,  with  the 
edges  of  an  oval  opening,  the  foramen  ovale.  The  entire  chain  is  partially 
supported  by  a  ligament  attached  to  the  short  process  of  the  incus  and 
to  the  walls  of  the  tympanic  cavity. 

The  Tensor  Tympani  Muscle. — This  is  a  delicate  muscle,  about  15  mm. 
in  length,  situated  in  a  narrow  groove  just  above  the  Eustachian  tube  (Fig. 

339).  It  arises  from  the  car- 
tilaginous portion  of  the  Eusta- 
chian tube  and  the  adjacent 
portion  of  the  sphenoid  bone. 
From  this  origin  it  passes  nearly 
horizontally  backward  to  the 
tympanic    cavity;  just  opposite 


Fig.  337. — Tympanic  Membrane  and  the  Audi- 
tory Ossicles  (Left)  seen  from  within,  i.  e., 
FROM  THE  Tympanic  Cavity.  M.  Manubrium 
or  handle  of  the  malleus.  T.  Insertion  of  the 
tensor  tympani.  h.  Head.  IF.  Long  process  of 
the  malleus,  a.  Incus,  with  the  short  {K)  and  the 
long  (/)  process.  S.  Plate  of  the  stapes.  Ax, 
Ax,  is  the  common  axis  of  rotation  of  the  auditory 
ossicles.  5'.  The  pinion-wheel  arrangement  be- 
tween the  malleus  and  incus. — {Landois.) 


Fig.  338. — Audi- 
tory Ossicles,  i. 
Head  of  malleus.  2. 
Processus  brevis.  3. 
Processus  gracilis. 
4.  Manubrium.  5. 
Long  process  of  in- 
cus. 6.  Articulation 
between  incus  and 
stapes.  7.  Stapes. 
—{Sappey.) 


the  foramen  ovale  its  tendon  bends  at  a  right  angle  over  the  processus 
cochleariformis  and  then  passes  outward  across  the  tympanic  cavity  to  be 
inserted  into  the  handle  of  the  malleus  near  the  neck. 

The  stapedius  muscle  emerges  from  the  ca\'ity  of  a  pyramid  of  bone  which 
projects  from  the  posterior  wall  of  the  tympanum.  Its  tendon  passes  forward 
to  be  inserted  into  the  neck  of  the  stapes  bone  near  its  point  of  articulation 
with  the  incus. 

The  internal  ear,  or  labyrinth,  is  located  within  the  petrous  portion 
of  the  temporal  bone.  It  consists  of  an  osseous  and  a  membranous  portion, 
the  latter  contained  within  the  former. 

The  osseous  labyrinth,  is  subdivided  into  vestibule,  semicircular 
canals,  and  cochlea. 

The  vestibule  is  a  small,  triangular-shaped  cavity  between  the  semicir- 


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TEXT-BOOK  OF  PHYSIOLOGY. 


cular  canals  and  the  cochlea.  It  is  separated  from  the  cavity  of  the  middle 
ear  by  an  osseous  partition  which  presents  near  its  center  an  oval  opening, 
the  joramen  ovale.  In  the  living  condition  this  opening  is  closed  by  the  base 
of  the  stapes  bone,  which  is  held  in  position  by  an  annular  ligament.  The 
4  inner  wall  presents  a  number  of  openings  for 
the  passage  of  nerve-fibers  (Fig.  340). 

The  semicircular  canals  are  three  in  number, 
each  at  right  angles  to  the  other  two,  a  superior 
vertical,  an  inferior  vertical,  and  a  horizontal, 
each  of  which  opens  by  two  orifices  into  the 
cavity  of  the  vestibule,  with  the  exception  of  the 
two  vertical,  which  unite  at  one  extremity  and 
then  open  by  a  single  orifice.  Each  canal  near 
its  vestibular  orifice  in  enlarged  to  almost  twice 
the  size  of  the  rest  of  the  canal,  forming  what 
is  known  as  the  ampulla. 

The  cochlea,  the  anterior  portion  of  the  laby- 
rinth, is  a  gradually  tapering  canal,  about  35 
mm.  in  length,  wound  spirally  two  and  a  half 
times  around  a  central  bony  axis,  the  modiolus.  The  cavity  of  the  cochlea 
is  partially  subdivided  into  two  cavities  by  a  thin  spiral  plate  of  bone  which 
projects  from  the  inner  wall,  known  as  the  lamina  ossea  spiralis.  In  the 
natural  condition  this  partition  is  completed  by  a  connective  tissue  mem- 
brane, so  that  the  two  passages  are  completely  separated  from  each  other. 
The  upper  passage  or  scala  is  in  free  communication  with  the  vestibule, 
and  is  known  as  the  scala  vestibuli;  the  lower  passage  or  scala  in  the  deadcon- 


FiG.  339. — M,  The  Tensor 
Tympani  Muscle — the  Eus- 
tachian Tube  (Left). — 
(Landois.) 


Fig.  340. — Bony  Cochlea,  i. 
Ampulla  of  superior  semicircular 
canal.  2.  Horizontal  canal.  3. 
Junction  of  superior  and  posterior 
semicircular  canals.  4.  The  pos- 
terior semicircular  canal.  5.  Fora- 
men rotundum.  6.  Foramen  ovale. 
7.  Cochlea. 


Fig.  341. —  I.  Utricle.  2. 
Saccule.  3.  Vestibular  end  of 
cochlea.  4.  Canalis  reuniens. 
5.  Membranous  cochlea.  6. 
Membranous  semicircular 

canals. 


dition  communicates  with  the  tympanum  by  means  of  a  round  opening,  the 
foramen  rotundum,  and  is  therefore  known  as  the  scala  tympani.  In  the  liv- 
ing condition  this  opening  is  completely  closed  by  a  membrane,  a  second 
membrana  tympani.  Both  the  scalse  vestibuli  and  tympani  communicate 
at  the  apex  of  the  cochlea  by  a  small  opening,  the  helicotrema.  The 
modiolus,  the  central  bony  axis,  is  perforated  from  base  to  apex  by 
a  canal  for  the  passage  of  the  auditory  nerve-fibers;  lateral  canals,  diverg- 


THE  SENSE  OF  HEARING. 


68i 


ing  from  the  central  canal,  pass  through  the  osseous  lamina  spiralis  and 
transmit  fibers  of  the  auditory  nerve.  The  interior  of  the  bony  labyrinth  is 
lined  by  periosteum  covered  by  epithelium  and  in  communication  with  lymph- 
spaces  at  the  base  of  the  skull  by  means  of  the  aqueduct  of  the  vestibule. 

The  membranous  labyrinth,  lying  within  the  osseous  labyrinth,  cor- 
responds with  it  in  form,  though  it  is  smaller  in  size.  It  may  be  subdivided 
into  vestibule,  semicircular  canals,  and  cochlea  (Fig.  341). 

The  vestibular  portion  consists  of  two  small  sacs,  the  utricle  and  the  saccule, 
which  communicate  with  each  other  by  means  of  the  two  branches  of  a  duct 
passing  through  the  aqueduct  of  the  vestibule — the  ductus  endolymphaticus. 

The  semicircular  canals  communicate  with  the  utricle  in  the  same  manner 
as  the  bony  canals  communicate  with  the  vestibule.  The  saccule  communi- 
cates with  the  membranous  cochlea  by  a  short  canal,  the  canalis  reuniens. 
The  walls  of  the  utricle,  saccule,  and  semi- 
circular canals  are  composed  of  connective- 
tissue  lined  by  epithelium.  At  the  points  of 
entrance  of  the  auditory  nerve,  the  maculce 
aciistic'X,  in  all  three  structures,  the  epithe- 
lium undergoes  a  marked  change  in  appear- 
ance and  structure.  It  becomes  columnar 
in  shape  and  provided  with  stiff  hair-like 
processes  or  threads,  which  project  into  the 
cavity.  In  the  saccule  and  utricle  the  hair- 
like processes  are  covered  by  a  layer  of 
small  crystals  of  calcium  carbonate  held 
together  by  a  gelatinous  material.  The 
crystals  are  knowm  as  otoliths  (Fig.  342). 

The  fibers  of  the  vestibular  nerve,  arising 
from  the  cells  of  the  ganglion,  of  Scarpa  in 
the  internal  auditory  meatus,  send  their 
peripherally  directed  branches  through  the 
foramina  in  the  inner  wall  of  the  vestibule, 
through  the  walls  of  the  utricle  and  semi- 
circular canals  near  the  ampulla.  As  the  fibers  approach  the  maculae 
acusticae  they  subdivide  into  delicate  fibrillce,  which  ultimately  become 
histologically  and  physiologically  related  to  the  neuroepithelium.  From 
the  relation  of  the  nerve-fibers  to  the  epithelium,  the  latter  must  be  re- 
garded as  the  highly  specialized  terminal  organ  of  the  vestibular  portion  of 
the  auditory  nerve. 

The  cochlea  is  a  closed  membranous  tube  situated  between  the  osseous 
lamina  spiralis  and  the  outer  bony  wall.  A  trans\'erse  section  of  the  entire 
cochlea  shows  the  relation  of  the  osseous  and  membranous  portions  (Fig.  343). 
The  cochlear  tube  is  triangular  in  shape.  The  base  is  attached  to  the  bony 
wall,  the  apex  to  the  edge  of  osseous  lamina  spiralis.  One  side  of  the  tube 
forms  in  part  the  membrane  of  Reissner,  the  other  side  forms  in  part  the 
basilar  membrane.  The  sides  of  the  cochlea  toward  the  scala  vestibuli  and 
scala  tympani  are  covered  with  epithelium.  The  triangular  cavity  of  the 
cochlear  tube  is  known  as  the  scala  media.  The  inner  surface  of  the  cochlear 
tube  is  lined  by  epithelium,  which  becomes  extraordinarily  modified  and 


Hi 

'# ^,^, 

Fig.  342. —  Section  of  Wall  of 
Utricle  of  the  Internal  Ear, 
through  macular  region,  from 
rabbit,  showang  otoliths  (o),  em- 
bedded within  granular  substance 
(g).  h.  Ciliated  cells  with  proc- 
esses. (/»),  extending  between 
sustentacular  elements  (s).  m. 
Basement  membrane,  n.  Nerve- 
fibers  within  fibrous  tissue  (/) 
passing  toward  hair-cells  and 
becoming  non-meduUated  at  base- 
ment-membrane.— {After  Piersol.) 


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TEXT-BOOK  OF  PHYSIOLOGY. 


specialized  along  the  surface  of  the  basilar  membrane,  to  constitute  what  is 
known  as 

The  Organ  of  Corti. — In  Fig.  343  this  organ  is  represented  as  it  appears 
on  cross-section  of  the  cochlea.  It  consists  primarily  of  an  arch  composed 
of  two  modified  epithelial  cells  known  as  the  rods  or  pillars  of  Corti,  which 
rest  below  on  the  basilar  membrane,  but  meet  and  interlock  above;  it  con- 
sists secondarily  of  a  series  of  columnar  epithelial  cells  provided  with  hair- 
like processes  which  rest  upon  and  are  supported  by  the  rods  both  on  the 
inner  and  outer  aspects  of  the  arch.  The  space  beneath  the  arch  is  known 
as  the  tunnel.  The  inner  hair  cells  are  not  nearly  so  numerous  as  the  outer 
hair  cells.  The  epithelial  cells  external  to  the  outer  and  inner  hair  cells  are 
supporting  or  sustentacular  in  character. 

The  organ  of  Corti  extends  the  entire  length  of  the  cochlea.     The  num- 
ber of  rods  which,  standing  side  by  side,  form  the  inner  limb  of  the  arch  is 
estimated  at  5600;  the  number  which  form  the  outer  limb  is  estimated  at 
3850.     The  outer  rods  are  broader  than  the  inner  and  at  some  places  articu- 
late with  two  or  three  inner  rods.   The 
upper  edges  of  the  rods  are  flattened, 
elongated,  and  project  outward,  form- 
ing a  reticulated  membrane  through 
the  meshes  of  which  the  hair-like  pro- 
cesses of  the  cells  project. 

From  the  connective- tissue  thicken- 
ing on  the  upper  surface  of  the  os- 
seous lamina  spiralis  there  extends 
outward  over  the  organ  of  Corti  a  thin 
membrane,  the  membrana  tectoria. 
The  common  cavity  between  the  walls 
of  the  osseous  and  membranous 
labyrinth  in  the  vestibule,  the  semi- 
circular canals,  in  the  scala  vestibuli 
and  scala  tympani  of  the  cochlea,  is 
filled  with  a  clear  fluid- — the  perilymph;  the  common  cavity  within  the  walls 
of  the  entire  membranous  labyrinth  is  also  filled  with  a  similar  fluid — the 
endolymph.  The  hair-like  processes  of  the  epithelial  cells  covering  the 
maculae  acusticae  and  the  rods  of  Corti  are  consequently  bathed  by  endo- 
lymph. Both  fluids  are  in  relation  with  the  subarachnoid  lymph-spaces  at 
the  base  of  the  brain,  the  perilymph  through  the  aqueduct  of  the  vestibule, 
the  endolymph  through  the  endolymphatic  duct. 

The  fibers  of  the  cochlear  nerve,  arising  from  the  ganglion  cells  of  the 
spiral  ganglion  situated  in  the  osseous  lamina  spiralis  near  the  modiolus, 
send  their  peripheral  branches  to  the  saccule  and  to  the  organ  of  Corti.  As 
they  approach  this  structure  they  lose  their  medullary  sheath  and  become 
naked  axis-cylinders.  The  fibers  then  divide  into  two  parts,  of  which  one 
passes  to  the  inner  hair  cells;  the  other  passes  between  the  inner  rods  and 
crosses  the  tunnel  between  the  outer  rods  to  the  outer  hair  cells.  The  exact 
method  of  termination  of  these  fibers  in  the  hair  cells  is  unknown. 

From  the  relation  of  the  nerve-fibers  to  the  organ  of  Corti  the  latter  must 
be  regarded  as  the  highly  specialized  terminal  organ  of  the  cochlear  division 
of  the  auditory  nerve. 


Fig.  343, — A  Transverse  Skction  of  a 
Turn  of  the  Cochlea. 


THE  SENSE  OF  HEARING.  683 

THE  PHYSIOLOGY  OF  HEARING. 

The  general  function  of  the  ear  is  the  reception  and  transmission  of  at- 
mospheric vibrations  from  the  concha  to  the  percipient  elements — the  hair 
cells — of  the  organ  of  Corti.  The  vibratory  excitation  of  these  end-organs 
thus  caused,  is  the  basis  of  auditory  perceptions.  The  atmospheric  vibra- 
tions are  collected  by  the  pinna  and  concha,  conveyed  by  the  auditory  canal 
to  the  tympanic  membrane,  transmitted  by  the  chain  of  bones  to  the  laby- 
rinth to  pass  successively  through  the  perilymph,  the  membranous  walls,  and 
the  endolymph,  to  the  hair  cells.  The  nerve  impulses  generated  by  these 
vibrations  are  then  transmitted  by  the  cochlear  nerve  to  the  auditory  centers 
of  the  cerebrum,  where  the  sensations  of  sound  are  evoked.  In  order  to 
appreciate  the  function  of  the  individual  structures  concerned  in  this  gen- 
eral function  there  must  be  kept  in  mind  a  few  of  the  characteristics  of 
atmospheric  vibrations. 

Atmospheric  Vibrations. — The  vibrations  of  the  atmosphere  which  are 
the  objective  causes  of  the  sensations  of  sound  are  communicated  to  it  by  the 
vibrations  of  elastic  bodies  such  as  tuning-forks,  rods,  strings,  membranes, 
etc.  These  produce  in  the  air  a  to-and-fro  movement  of  its  particles,  re- 
sulting in  a  succession  of  alternate  condensations  and  rarefactions  which  are 
propagated  in  all  directions.  The  impact  of  a  rhythmic  succession  of  such 
condensations  on  the  ear  gives  rise  to  musical  sounds;  the  impact  of  an 
arrhythmic  or  irregular  succession  gives  rise  to  noises. 

If  a  writing  point  attached  to  a  tuning-fork  in  vibration  be  placed  in  con- 
tact with  a  traveling  recording  surface,  each  vibration  will  be  recorded  in  the 
form  of  a  wave.  For  this  reason  atmospheric  vibrations  are  generally 
spoken  of  as  sound-waves.  A  line  drawn  horizontally  through  such  a  curve 
indicates  the  position  of  rest  of  the  fork;  the  extent  of  the  curve  on  each  side 
of  this  line  indicates  the  excursion  of  the  fork  or  the  amplitude  of  its 
movement. 

The  sounds  which  physiologically  result  from  the  impact  and  transmis- 
sion of  the  effects  of  sound-waves,  possess  intensity,  pitch,  and  quality  or 
timbre. 

The  intensity  or  loudness  of  a  sound  depends  on  the  amplitude  of  the 
vibration  which  causes  it.  The  greater  the  amplitude  or  swing  of  the  vibrat- 
ing body,  the  greater  is  the  energy  with  which  it  strikes  the  ear. 

The  pitch  of  a  sound  depends  on  the  number  of  vibrations  which  strike 
the  ear  in  a  unit  of  time — a  second.  The  greater  the  number,  the  higher  the 
pitch.  Thus  while  the  pitch  of  the  sound  caused  by  the  note  C,  on  the  first 
leger  line  below  the  G  clef,  of  the  music  scale,  corresponds  to  256  vibrations, 
the  pitch  of  the  sound  caused  by  the  note  C  an  octave  above,  corresponds  to 
512  vibrations.  The  lowest  rate  of  vibration  which  can  produce  a  distinct 
sound  varies  in  different  individuals  from  14  to  18;  the  highest  rate  varies 
from  35,000  to  40,000  per  second.  Between  these  two  extremes  lies  the  range 
of  audibility,  which  embraces  about  11  octaves.  Vibrations  less  than  14  per 
second  cannot  be  perceived  as  a  continuous  sound;  vibrations  beyond 
40,000  also  fail  to  be  so  perceived.  In  the  ascent  of  the  music  scale  from  the 
lowest  to  the  highest  regions  there  is  a  gradual  increase  in  the  vibration 
frequency. 


684  TEXT-BOOK  OF  PHYSIOLOGY. 

The  qualUy  of  a  sound  depends  on  the  Jorm  of  the  vibration.  It  is  this 
feature  which  gives  rise  to  those  differences  in  sensations  which  permit  one  to 
distinguish  one  instrument  from  another  when  both  are  emitting  the  same 
note.  The  form  of  the  sound-wave  in  any  given  instance  is  the  resuhant  of  a 
combination  of  a  fundamental  vibration  and  certain  secondary  vibrations  of 
subdivisions  of  the  vibrating  body.  These  secondary  vibrations  give  rise 
to  what  is  known  as  overtones.  By  their  union  with  and  modification  of  the 
fundamental  vibration  there  is  produced  a  special  form  of  vibration  which 
gives  rise  not  to  a  simple  but  a  composite  sensation.  It  is  for  this  reason  that 
the  same  note  of  the  piano,  the  violin,  and  the  human  voice  varies  in  quality. 

The  Function  of  the  Pinna  and  External  Auditory  Canal. — In  those 
animals  possessing  movable  ears  the  pinna  plays  an  important  part  in  the 
collection  of  sound-waves.  In  man  it  is  doubtful  if  it  plays  a  part  at  all 
necessary  for  hearing.  Nevertheless  an  individual  with  defective  hearing 
may  have  the  perception  of  sound  increased  by  placing  the  pinna  at  an  angle 
of  90  degrees  to  th*e  side  of  the  head  or  by  placing  the  hand  behind  it.  The 
external  auditory  canal  transmits  the  sonorous  vibrations  to  the  tympanic 
membrane.  From  the  obliquity  of  this  canal  it  has  been  supposed  that  the 
vibrations,  after  passing  the  concha,  undergo  a  series  of  reflections  on  their 
way  to  the  tympanic  membrane,  which,  owing  to  its  inclination,  would  be 
struck  by  them  in  a  much  more  effective  manner. 

The  Function  of  the  Tympanic  Membrane. — The  function  of  the 
tympanic  membrane  is  the  reception  of  the  atmospheric  vibrations  which 
are  transmitted  to  it.  This  it  does  by  vibrating  In  unison  with  them.  The 
vibrations  which  the  membrane  exhibits  correspond  in  amplitude,  in  fre- 
quency, and  in  form  to  those  of  the  atmosphere.  That  this  membrane 
actually  reproduces  all  vibrations  within  the  range  of  audibility  has  been 
experimentally  demonstrated.  The  membrane,  not  being  fixed  as  far  as  its 
tension  is  concerned,  does  not  possess  a  fixed  fundamental  note,  like  a  station- 
ary fixed  membrane,  and  is  therefore  just  as  well  adapted  for  the  reception  of 
one  set  of  vibrations  as  another.  This  is  made  possible  by  variations  in  its 
tension  in  accordance  with  the  pitch  or  frequency  of  the  atmospheric  vibra- 
tions. In  the  absence  of  vibration  the  membrane  is  in  a  condition  of  re- 
laxation; with  the  advent  of  sound-waves  possessing  a  gradual  increase  of 
pitch,  as  in  the  ascent  of  the  music  scale,  the  tension  of  the  membrane  in- 
creases until  its  maximum  is  reached  at  the  upper  limit  of  the  range  of 
audibility.  By  this  change  in  tension  certain  tones  become  perceptible  and 
distinct,  while  others  become  imperceptible  and  indistinct. 

The  Function  of  the  Tensor  Tympani  Muscle. — The  function  of  this 
muscle  is,  as  its  name  indicates,  to  change  and  to  fix  the  tension  of  the  tym- 
panic membrane,  so  that  it  can  most  readily  vibrate  in  unison  with  vibrations 
of  varying  degrees  of  rapidity.  The  tendon  of  this  muscle  playing  around 
the  processus  cochleariformis  is  attached  almost  at  a  right  angle  to  the  handle 
of  the  malleus.  Hence  as  the  muscle  contracts  it  exerts  its  traction  from  the 
process  and  draws  the  handle  of  the  malleus  inward,  thus  increasing  the 
convexity  of  the  tympanic  membrane  and  at  the  same  time  its  tension. 
With  the  relaxation  of  the  muscle  the  handle  of  the  malleus  passes  outward, 
and  the  convexitv  and  tension  diminish. 


THE  SENSE  OF  HEARING.  685 

In  the  ascent  of  the  music  scale,  each  note  corresponding  to  an  increase 
in  vibration  frequency  requires  for  its  perception  an  increase  in  tension  and 
an  increase  in  the  force  of  the  contraction  of  the  tensor  muscle.  In  the 
descent  of  the  music  scale  the  reverse  conditions  obtain.  The  contraction  of 
the  muscle  is  of  the  nature  of  a  single  twitch,  and  of  just  sufficient  force  and 
duration  to  tense  the  membrane  for  a  given  rate  of  vibration. 

The  contraction  of  the  muscle  is  excited  reflexly.  The  afferent  path  is 
through  fibers  of  the  trigeminal  nerve  distributed  to  the  tympanic  mem- 
brane; the  efferent  path  is  through  fibers  in  the  small  root  of  the  trigeminal. 
The  stimulus  is  sudden  pressure  on  the  tympanic  membrane.  The  more 
frequently  and  forcibly  the  stimulus  is  applied,  the  greater  is  the  muscle 
response.  The  tensor  tympani  muscle  may  therefore  be  regarded  as  an 
accommodative  apparatus  by  which  the  tympanic  membrane  is  adjusted 
for  the  reception  of  \abrations  of  varying  degrees  of  frequency. 

The  Function  of  the  Chain  of  Bones. — The  function  of  the  chain  of 
bones  is  to  transmit  the  effects  of  the  atmospheric  vibrations  to  the  fluid  of  the 
labyrinth.  The  manner  in  which  this  is  accomplished  becomes  evident 
from  the  relation  which  the  bones  of  this  chain  bear  to  one  another  and  to  the 
tympanic  membrane  on  the  one  hand  and  to  the  fluid  of  the  labyrinth  on 
the  other. 

When  pressure  is  made  on  the  outer  surface  of  the  tympanic  membrane 
it  is  at  once  pushed  inward,  carrying  with  it  the  handle  of  the  malleus,  the 
head  at  the  same  time  rotating  outward  around  an  axis  corresponding  to  its 
ligamentous  attachments.  As  the  handle  moves  inward  a  small  ledge  of  bone 
just  below  the  malleo-incudal  joint  locks  with,  and  hence  pushes  inward, 
the  long  process  of  the  incus.  Since  this  process  is  united  at  almost  a 
right  angle  to  the  stapes  bone,  the  latter  is  forced  toward  and  into  the 
foramen  ovale,  thus  producing  a  pressure  on  the  perilymph.  With  the 
cessation  of  the  pressure  the  elastic  forces  of  the  membrane  and  of  the  liga- 
ments return  the  handle  of  the  malleus  to  its  former  position;  by  the  unlock- 
ing of  the  malleo-incudal  joint  the  entire  chain  also  returns  to  its  former  posi- 
tion without  exerting  undue  traction  on  the  basal  attachment  of  the  stapes. 

As  the  long  process  of  the  incus  is  shorter  than  the  handle  of  the  malleus, 
and  as  the  movement  between  them  takes  place  around  an  axis  from  before 
backward,  it  follows  that  the  excursion  of  the  incus  and  stapes  will  be  less 
than  that  of  the  malleus,  while  the  force  will  be  greater.  Hence  as  the 
vibrations  are  transferred  from  the  tympanic  membrane  of  large  area  to  the 
base  of  the  stapes  of  small  area  (20  to  1.5),  they  lose  in  amplitude  but  in- 
crease is  force.  Their  pressure  on  the  perilymph  is  therefore  13 . 3  times 
greater  than  on  thc-membrana  tympani.  In  addition  to  its  function  as  a 
transmitter  of  vibrations,  the  chain  of  bones  serves  as  a  point  of  attachment 
for  muscles  which  regulate  the  tension  of  the  tympanic  membrane  and 
the  pressure  on  the  labyrinth. 

The  Function  of  the  Stapedius  Muscle. — The  function  of  the  stapedius 
muscle  is  a  subject  of  much  discussion.  According  to  Henle,  its  function 
is  so  to  adjust  the  stapes  bone  that  it  will  be  prevented  from  exerting  an  undue 
pressure  on  the  perilymph  during  the  inward  excursions  of  the  incus  process. 
According  to  Toynbee,  its  function  is  to  press  the  posterior  part  of  the  stapes 


686  TEXT-BOOK  OF  PHYSIOLOGY. 

inward,  make  it  a  fixed  point,  and  place  the  anterior  part  in  such  a  position 
that  it  will  vibrate  freely  and  accurately. 

The  Function  of  the  Eustachian  Tube. — In  order  that  the  tympanic 
membrane  may  vibrate  freely  it  is  essential  that  the  air  pressure  on  both 
sides  shall  be  equal  at  all  times.  This  is  made  possible  by  the  Eustachian 
tube.  Were  it  not  for  this  passageway,  with  each  inward  swing  of  the  mem- 
brane the  air  in  the  tympanic  cavity  would  be  condensed  and  its  pressure 
raised,  in  consequence  of  which  the  movement  of  the  membrane  would  be 
retarded;  with  each  outward  swing,  the  air  would  be  rarefied  and  its  pressure 
lowered  below  that  of  the  atmosphere,  and  in  consequence  the  movement 
outward  would  be  retarded;  the  maximum  response,  therefore,  of  the  mem- 
brane to  a  given  vibration  could  not  be  attained  and  the  resulting  sound 
would  be  muffled  and  indistinct.  But  as  with  each  vibration  of  the 
membrane  the  air  can  pass  into  and  out  of  the  tympanum  through  this 
partially  closed  tube,  inequalities  of  pressure  are  prevented  and  a  free 
vibration  permitted. 

The  impairment  in  the  acuteness  of  hearing  which  is  caused  by  either 
a  rise  or  fall  of  pressure  in  the  middle  ear  can  be  shown — 

1.  By  closing  the  mouth  and  nose  and  then  forcing  air  from  the  lungs 

through  the  Eustachian  tube  into  the  tympanum,  thus  increasing  the 
pressure. 

2.  By  closing  the  mouth  and  nose  and  then  making  an  effort  of  deglutition. 

As  this  act  is  attended  by  an  opening  of  the  pharyngeal  end  of  the 
Eustachian  tube,  the  air  in  the  tympanum  is  partly  withdrawn  and  the 
pressure    lowered.     In    each    instance    hearing    is    impaired.     After 
either  experiment  the  normal  condition  is  restored  by  swallowing  with 
the  nasal  passages  open. 
The  Functions  of  the  Internal  Ear.^ — ^From  the  anatomic  arrange- 
ment of  the  structures  of  the  internal  ear  it  is  evident  that  if  the  vibrations 
of  the  stapes  bone  are  to  reach  the  peripheral  organs — the  hair  cells — of 
both  the  vestibular  and  cochlear  nerves,  they  must  traverse  successively 
the  perilymph,  the  membranous  walls,  and  the  endolymph.     As  the  perilymph 
is  incompressible,  the  inward  movement  of  the  stapes  would  be  prevented 
were  it  not  for  the  elastic  character  of  the  membrane  closing  the  foramen 
rotundum.     The  pressure  wave  occasioned  by  each  inward  movement  of  the 
stapes  is  transmitted  through  the  scala  vestibuli,  the  helicotrema,  and  the 
scala  tympani,  to  this  membrane,  which  by  virtue  of  its  elasticity  is  pressed 
into  the  tympanic  cavity.      With  the  outward  movement  of  the  stapes, 
equilibrium  is  at  once  restored. 

The  Functions  of  the  Cochlea. — The  cochlea  is  the  portion  of  the  in- 
ternal ear  which  is  concerned  in  the  perception  of  tones.  The  arrangement 
of  the  histologic  elements  of  the  organ  of  Corti  indicates  that  they  in  some 
way  respond  to  the  vibrations  of  varying  frequency  and  form,  and  through 
the  development  of  nerve  impulses,  evoke  the  sensations  of  pitch  and  quality. 
The  manner  in  which  this  is  accomplished  is  largely  a  matter  of  speculation. 
While  many  theories  have  been  offered  in  explanation  of  the  power  to  distin- 
guish the  pitch  and  the  quality  or  timbre  of  a  tone,  most  physiologists  prefer 
that  of  Helmholtz,  who  regarded  the  transverse  fibers  of  the  basilar  mem- 
brane as  the  elements  immediately  concerned,  and  compared  them,  both  in 


THE  SENSE  OF  HEARING.  687 

their  arrangement  and  power  of  sympathetic  vibration,  with  the  strings  of  a 
piano.  He  said:  "  If  we  could  so  connect  every  string  of  a  piano  with  a  nerve- 
fiber  that  the  nerve-fiber  would  be  excited  as  often  as  the  string  vibrated, 
then,  as  is  actually  the  case  in  the  ear,  every  musical  note  which  affected  the 
instrument  would  excite  a  series  of  sensations  exactly  corresponding  to  the 
pendulum-like  vibrations  into  which  the  original  movements  of  the  air  can 
be  resolved;  and  thus  the  existence  of  each  individual  overtone  would  be 
exactly  perceived,  as  is  actually  the  case  with  the  ear.  The  perception  of 
tones  of  different  pitch  would,  under  these  circumstances,  depend  upon 
different  nerve-fibers,  and  hence  would  occur  quite  independently  of  each 
other.  Microscopic  investigation  shows  that  there  are  somewhat  similar 
structures  in  the  ear.  The  free  ends  of  all  the  nerve-fibers  are  connected 
with  small  elastic  particles  which  we  must  assume  are  set  into  sympathetic 
vibration  by  sound-waves."  (Stirling.) 

The  mechanism  might  be  regarded,  therefore,  somewhat  as  follows: 
The  sound-waves  received  by  the  membrana  tympani  and  transmitted  by  the 
chain  of  bones  to  the  fenestra  ovalis  produce  variable  pressures  in  the 
fluids  of  the  internal  ear;  these  pressures  vary  in  intensity,  in  number, 
and  in  quality,  and  correspond  with  the  intensity,  pitch,  and  quality  of  the 
tones.  If,  therefore,  a  compound  wave  of  pressure  be  communicated  by 
the  base  of  the  stapes,  it  will  be  resolved  into  its  constituents  by  the  different 
transverse  fibers  of  the  basilar  membrane,  each  picking  out  its  peculiar 
portion  of  the  wave  and  thus  stimulating  corresponding  nerve  filaments. 
Thus  different  nerve  impulses  are  transmitted  to  the  brain,  where  they  are 
fused  in  such  a  manner  as  to  give  rise  to  a  sensation  of  a  particular 
quahty,  but  still  so  imperfectly  fused  that  each  constituent,  by  a  strong 
effort  of  attention,  may  be  still  recognized.  The  transverse  fibers  of  the 
basilar  membrane  vary  in  length  from  0.04155  mm.  at  the  base  of  the 
cochlea  to  0.495  J^^^-  ^^  the  apex,  and,  according  to  Retzius,  are  about  24,000 
in  number.  As  the  human  ear  usually  cannot  distinguish  more  than  11,000 
tones,  it  is  evident  that  there  is  a  sufficient  anatomic  basis  for  this  theory. 

The  functions  of  the  semicircular  canals  have  already  been  stated 
in  connection  with  the  chapter  relating  to  the  functions  of  the  cerebellum. 


CHAPTER  XXIX. 
REPRODUCTION. 

Reproduction  is  the  process  by  which  a  new  individual  is  initiated  and 
developed  and  the  species  to  which  it  belongs  is  preserved.  Reproduc- 
tion is  the  result  of  the  union  and  subsequent  development  of  germ-  and 
sperm-cells.  These  cells  are  produced  and  their  union  accomplished  by  the 
cooperation  of  the  reproductive  organs  characteristic  of  the  two  sexes. 

Embryology  is  a  department  of  anatomic  science  which  has  for  its 
object  the  investigation  of  the  successive  stages  that  the  new  being  passes 
through  during  its  gradual  development  prior  to  birth. 

THE  REPRODUCTIVE  ORGANS  OF  THE  FEMALE. 

The  reproductive  organs  of  the  female  comprise  the  ovaries,  Fallopian 
tubes,  uterus,  and  vagina  (Fig.  344). 

The  Ovaries. — The  ovaries  are  two  small,  flattened  bodies,  measuring 
about  40  mm.  in  length  and  20  in  breadth.  They  are  situated  in  the  cavity 
of  the  pelvis,  one  on  either  side,  and  embedded  in  a  fold  of  the  peritoneum, 


Fig.  ^44. — Uterus,  Fallopian  Tubes  and  Ovaries;  Posterior  View,  i,  i.  Ovaries. 
2,2.  Fallopian  tubes.  3,3.  Fimbriated  extremity  of  the  left  Fallopian  tube  seen  from  its  concavity. 
4.  Opening  of  the  left  tube.  5.  Fimbriated  extremity  of  the  right  tube,  posterior  view.  6,  6. 
Fimbriae  vi^hich  attach  the  extremity  of  each  tube  to  the  ovary.  7,  7.  Ligaments  of  the  ovary. 
8,  8,  9,  9.  Broad  ligament.     10.  Uterus.     11.  Cervix  uteri.     12.  Os  externum.     13,  13.  Vagina. 

known  as  the  broad  ligament.  A  section  of  the  ovary  shows  that  it  consists 
externally  of  a  thin,  firm,  connective-tissue  membrane  and  internally  of  a 
fine  connective-tissue  stroma,  supporting  blood-vessels,  non-striated  muscle- 
fibers  and  nerves,  and  containing  in  its  meshes  a  very  large  number  of  spheric 
sacs  named  after  their  discoverer,  de  Graaf,  the  Graafian  sacs  or  follicles. 
These  foUicles  are  very  numerous  and  are  present  in  all  portions  of  the  ovary, 


REPRODUCTION. 


689 


r 


o         a 


though  they  are  most  abundant  toward  its  peripheral  portions.  It  is  esti- 
mated that  each  human  ovary  contains  from  20,000  to  40,000  follicles.  The 
follicles  vary  considerably  in  size;  while  many  are  visible  to  the  unaided  eye, 
others  require  for  their  detection  high  powers  of  the  microscope.  Although 
the  follicles  are  present  in  the  ovary  at  the  time  of  birth,  it  is  not  until  the 
period  of  puberty  that  they  assume  functional  activity. 

From  this  time  on  to  the  catamenial  period  there  is  a  constant  growth 
and  development  of  these  follicles.  Each  follicle  consists  of  an  external 
investment  of  fibrous  tissue  and  blood-vessels,  and  an  internal  investment  of 
cells,  the  memhrana  granulosa.  At  the  lower  portion  of  this  membrane  there 
is  an  accumulation  of  cells, 
the  proligerous  disc  (Fig.  345). 
The  cavity  of  the  follicle  con- 
tains a  slightly  yellowish, 
alkaline,  albuminous  fluid,  a 
transudate  in  all  probabihty 
from  the  blood-vessels.  The 
Graafian  follicle  is  of  especial 
interest,  for  it  is  in  this  struc- 
ture, and  more  especially  in 
the  proligerous  disc,  that  the 
true  germ-cell  or  ovum  is 
developed. 

The  ovum  is  a  spheric 
body  measuring  about  0.3 
mm.  in  diameter.  It  consists 
of  a  mass  of  living,  proto- 
plasmic material,  cytoplasm, 
a  nucleus  or  germinal  vesicle, 
and  a  nucleolus  or  germinal 
spot.  The  cytoplasm  presents 
toward  Its  central  portion  a 
quantity  of  granular  material, 
partly  fatty  in  character,  the 
deutoplasm  or  vitellus.  The 
peripheral  portion  of  the  cyto- 
plasm is  surrounded  by  a 
clear  thick  membrane,  the 
zona  pellucida,  external  to 
which  is  a  layer  of  radially 
placed  columnar  epithelium,  the  corofta  radiata  (Fig.  346). 

The  nucleus  consists  of  a  nuclear  membrane  enclosing  contents.  The 
latter  consist  of  an  amorphous  material  in  which  is  embedded  a  network, 
some  of  the  threads  of  which  have  a  strong  afl&nity  for  certain  staining 
materials,  and  hence  are  known  as  chromatin,  in  the  meshes  of  which  lies  a 
material  that  stains  less  deeply  and  known  as  achromatin. 

The  Fallopian  Tubes. — The  Fallopian  tubes  are  about  12  centimeters 
in  length  and  extend  from  the  upper  angles  of  the  uterus  to  the  ovaries. 
Each  tube  is  somewhat  trumpet-shaped,  the  narrow  portion  being  close  to 
44 


'"^■^iyW^m^^^Mf^. 


Fig.  345. — Section  of  Cortex  of  Cat's  Ovasy, 
ExHiBiTixG  Large  Gr-^afi.ant  Follicles,  a.  Per- 
ipheral zone  of  condensed  stroma,  h.  Groups  of  im- 
mature follicles,  c.  Theca  of  follicle,  d.  Membrana 
granulosa,  e.  Discus  proligerus.  /.  Zona  pellucida. 
g.  \'itellus.  h.  Germinal  vesicle.  /.  Germinal  spot. 
k.  Cavity  of  liquor  foUiculi. — {After  Piersol.) 


690 


TEXT-BOOK  OF  PHYSIOLOGY. 


the  uterus,  the  wide  portion  close  to  the  ovary.  The  outer  extremity  of  the 
tube  is  expanded  and  subdivided,  and  presents  a  series  of  processes  termed 
fimbriae,  one  of  which  is  attached  to  the  ovary.  The  tube  consists  of  three 
coats — an  external  or  fibro-serous;  a  middle  or  muscle,  the  fibers  of  which 
are  arranged  longitudinally  and  circularly ;  and  an  internal  or  mucous,  which 
is  folded  longitudinally.  The  surface  of  the  mucous  coat  is  covered  with  a 
layer  of  ciliated  epithelial  cells,  the  direction  of  motion  of  which  is  toward 
the  uterus. 

The  Uterus. — The  uterus  is  pyriform  in  shape  and  divided  into  a  body 
and  neck.  It  measures,  before  the  first  pregnancy,  about  7  cm.  in  length, 
5  cm.  in  breadth  and  2^  cm.  in  thickness.     A  frontal  section  of  the  uterus 


Fig.  346.  —  Ovum  of  a  Cow.  i.  Zona 
pellucida.  2.  Cytoplasm,  vitellus.  3.  Nu- 
cleus, germinal  vesicle.  4.  Nucleolus,  germ- 
inal spot.  5.  Corona  radiata.  The  radial 
striation  of  the  zona  pellucida  can  not  be 
seen. — {Stohr.) 


Fig.  347. — Frontal  Sec- 
tion of  the  Uterus.  I.  Cav- 
ity of  the  body.  2,  3.  Lateral 
walls.  4,4.  Comua.  5.  Os 
internum.  6.  Cavity  of  the 
cervix.  7.  Arbor  vitse  of  the 
cer\ax.  8.  Os  externum.  9. 
Vagina. — {Sappey.) 


shows  a  central  cavity  which  in  the  body  is  triangular  in  shape,  in  the  neck 
oval  or  fusiform  (Fig.  347).  At  the  upper  angles  of  the  uterus  the  cavity  is 
continuous  with  the  cavity  of  each  Fallopian  tube.  At  the  junction  of  the 
body  and  the  neck,  the  cavity  presents  a  constriction,  the  internal  os.  The 
constriction  at  the  end  of  the  neck  is  known  as  the  external  os.  The  walls  of 
the  uterus  are  extremely  thick  and  composed  of  non-striated  muscle-fibers 
arranged  in  a  very  complicated  manner.  The  interior  of  the  uterus  is  lined 
by  mucous  membrane  covered  with  cylindric  ciliated  epithelial  cells,  the 
motion  of  which  is  toward  the  external  os.  Tubular  glands  are  found  in 
large  numbers  in  the  mucous  membrane  lining  the  cavity,  while  racemose 
glands  are  found  in  the  mucous  membrane  lining  the  neck.  Owing  to  the 
flattening  of  the  uterus  from  before  backward  the  walls  are  almost  in  contact 
and  the  cavity  almost  obliterated. 


REPRODUCTION.  691 

The  Vagina. — -The  vagina  is  a  musculo-membranous  canal,  from  12  to 
18  cm.  in  length,  situated  between  the  rectum  and  bladder.  It  extends 
from  the  surface  of  the  body  to  the  brim  of  the  pelvis,  and  embraces  at  its 
upper  extremity  the  neck  of  the  uterus. 

Ovulation. — After  the  establishment  of  puberty  a  Graafian  follicle 
develops  and  ripens  or  matures  periodically,  usually  every  twenty-eight  days. 
During  the  time  of  maturation  the  follicle  increases  in  size,  from  an  augmen- 
tation of  its  fluid  contents,  and  approaches  the  surface  of  the  ovary,  where  it 
forms  a  projection  varying  from  6  to  12  mm.  in  size.  When  maturation  is 
complete  the  vesicle  ruptures,  and  the  ovum  and  liquid  contents  are  discharged. 
The  ovum,  by  a  mechanism  not  fully  understood,  is  received  by  the  fimbriated 
extremity  of  the  Fallopian  tube  and  enters  its  cavity.  The  ovum  is  then 
transferred  through  the  tube  by  the  peristaltic  contraction  of  its  muscle- 
fibers  and  by  the  action  of  the  cilia  of"  its  lining  epithelium.  The  time 
occupied  in  the  transference  of  the  ovum  from  the  ovary  to  the  interior  of  the 
uterus  has  been  estimated  to  be  from  four  to  ten  days. 

Either  at  the  time  of,  or  very  shortly  after,  its  discharge  from  the  follicle, 
the  ovum,  and  more  especially  the  nucleus,  undergoes  a  series  of  histologic 
changes  which  eventuates  in  an  extrusion  of  a  portion  of  the  chromatin 
material.  The  extruded  portions  are  known  as  the  polar  bodies.  The  non- 
extruded  portion  of  the  chromatin  material  is  known  as  the  female  pronu- 
cleus or  germ  nucleus.  The  chromosomes  are  reduced  to  one-half  the  somatic 
number.  The  succession  of  changes  which  the  nucleus  undergoes  is  termed 
maturation.  As  the  nucleus  is  regarded  as  the  part  of  the  ovum  which 
transmits  parental  characteristics  it  is  assumed  that  the  extrusion  of  a  por- 
tion of  the  nuclear  material  is  a  means  by  which  an  excess  of  inherited 
substance  is  prevented. 

Menstruation. — Menstruation  is  a  periodic  discharge  of  blood  and 
mucus  from  the  surface  of  the  mucous  membrane  of  the  uterus,  and  occurs 
about  every  twenty-eight  days.  The  duration  of  the  menstrual  period  ex- 
tends over  four  or  five  days  and  the  amount  of  blood  discharged  varies 
from  180  c.c.  to  200  c.c.  Menstruation  is  usually  an  accompaniment  of 
ovulation,  though  the  latter  process  may  take  place  independently  of  the 
former.  It  is  characterized  by  both  local  and  systemic  changes.  The 
local  changes  are  most  marked  in  the  uterus,  the  mucous  membrane  of  which 
increases  in  thickness  from  a  proliferation  of  the  connective  tissue  and  a 
hyperemic  condition  of  the  blood-vessels.  Subsequently  to  these  changes 
the  epithelial  surface,  as  well  as  the  more  superficial  portions  of  the  connec- 
tive tissue,  undergo  degeneration  and  exfoHation,  after  which  the  finer 
blood-vessels  rupture  and  permit  of  an  escape  of  blood  into  the  uterine 
cavity.  At  the  end  of  the  menstrual  period  regenerative  changes  set  in 
which  continue  until  the  normal  condition  of  the  mucous  membrane  is 
reestablished. 

The  Corpus  Luteum. — With  the  rupture  of  the  Graafian  follicle  there  is 
an  effusion  of  blood  into  the  follicular  cavity  which  soon  coagulates,  loses  its 
color  and  assumes  the  characteristics  of  fibrin.  The  walls  of  the  follicle, 
which  have  become  thickened  from  the  deposition  of  a  reddish-yellow  glutin- 
ous substance,  now  become  convoluted  and  undergo  a  still  further  hypertrophy, 
until  they  encroach  upon  and  almost  obliterate  the  follicular  cavity.     In  a 


692 


TEXT-BOOK  OF  PHYSIOLOGY. 


few  weeks  the  mass  loses  its  red  color  and  becomes  decidedly  yellow,  when  it 
is  known  as  the  corpus  luteum.  With  the  continuance  of  reparative  changes 
this  body  gradually  disappears  until  at  the  end  of  two  months  nothing 
remains  but  a  small  cicatrix  on  the  surface  of  the  ovary.  Such  are  the 
changes  in  the  follicle  if  the  ovum  has  not  been  impregnated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes  a 
much  slower  development,  becomes  larger,  and  continues  during  the  entire 
period  of  gestation.  The  difference  between  the  corpus  luteum  of  the  un- 
impregnated  and  pregnant  condition  is  expressed  in  the  following  table  by 
Dalton: 


Corpus  Luteum  of  Menstruation. 


Corpus  Luteum  of  Pregnancy 


At  the  end  of 

weeks. 
One  month. . . . 


three 


Two  months. 
Four  months . 
Six  months. . 
Nine  months. 


Smaller;  convoluted  wall  bright 
yellow;  clot  still  reddish. 

Reduced  to  the  condition  of  an 
insignificant  cicatri.x. 

Absent  or  unnoticeable 


Absent . 


Absent. 


Three-quarters  of  an  inch  in  diameter;  central  clot  reddish;  convoluted 
wall  pale. 

Larger;  convoluted  wall  bright  yellow; 
clot  still  reddish. 

Seven-eighths  of  an  inch  in  diameter;  con- 
voluted wall  bright  yellow ;  clot  perfectly 
decolorized. 

Seven-eighths  of  an  inch  in  diameter;  clot 
pale  and  fibrinous;  convoluted  wall  dull 
yellow. 

Still  as  large  as  at  the  end  of  second 
month;  clot  fibrinous;  convoluted  wall 
paler. 

Half  an  inch  in  diameter;  central  clot  con- 
verted into  a  radiating  cicatrix;  external 
wall  tolerably  thick  and  convoluted,  but 
without  any  bright  yellow  color. 


THE  REPRODUCTIVE  ORGANS  OF  THE  MALE. 


The  reproductive  organs  of  the  male  comprise  the  testicles,  vasa  deferen- 
tia,  vesiculas  seminales,  and  penis. 

■  The  Testicles. — The  testicles  are  oblong  glands,  about  40  mm.  in 
length,  30  mm.  in  breadth  and  20  mm.  in  thickness,  and  contained  within 
the  cavity  of  the  scrotum.  A  section  of  the  testicle  (Fig.  348)  reveals  the 
presence  externally  of  a  dense  fibrous  membrane,  the  timica  albuginea,  and 
internally  a  connective-tissue  framework  consisting  mainly  of  septa,  which 
enter  the  organ  on  its  posterior  aspect  at  the  mediastinum  testis,  passing  in- 
ward in  a  diverging  manner.  The  spaces  between  the  septa  are  occupied 
by  the  true  gland  substance,  the  seminiferous  tubules. 

The  seminiferous  tubules  are  very  numerous,  the  estimate  as  to  their 
number  varying  from  800  to  1000.  When  unraveled  they  measure  from  30 
to  40  cm.  in  length  and  0.3  mm.  in  diameter.  At  their  peripheral  extremities 
the  tubules  are  very  much  convoluted,  but  as  they  pass  toward  the  mediasti- 
num testis,  the  convolutions  disappear,  and  after  uniting  with  one  another 
terminate  in  from  twenty  to  thirty  straight  tubes,  of  small  diameter, 
the  vasa  recta,  which  pass  through  the  mediastinum  and  form  the  rete  testis. 
At  the  upper  part  of  the  mediastinum  the  tubules  unite  to  form  from  nine 
to  thirty  small  ducts,  the  vasa  ejferentia,  which  soon  become  very  much  con- 
voluted. After  a  short  course  they  unite  to  form  a  single  tortuous  tube, 
about  7  meters  in  length  and  0.4  mm.  in  diameter,  which  descends  behind 


REPRODUCTION. 


693 


the  testicle  to  its  lower  border.  This  tube  is  known  as  the  epididymis. 
The  seminal  tubule  consists  of  a  basement  membrane  lined  by  granular 
nucleated  epithelium. 

The  vas  deferens,  the  excretory  duct  of  the  testicle,  is  about  60  cm.  in 
length  and  from  2  to  3  mm.  in  diameter,  and  extends  upward  from  the 
epididymis  to  the  inguinal  canal,  through  which  it  passes  into  the  abdominal 
ca\dty  and  then  to  the  under  surface  of  the  base  of  the  bladder,  where  it 
unites  with  the  duct  of  the  vesicula  seminalis  to  form  the  ejaciilatory  duct. 

The  vesiculae  seminales  are  two  lobulated  pyriform  bodies,  about  40 
mm.  in  length,  situated  on  the  under  surface  of  the  bladder.  Each  vesicula 
seminalis  consists  of  an  external  fibrous  coat,  a  middle,  muscular  coat,  and  an 
internal  mucous  coat.  The  mucous  coat  contains  a  number  of  small 
tubular   albumin-producing   glands   which   secrete   a   characteristic   fluid. 


Fig.  348. — DiAGR.\ii  of  a  Ver- 
tical Skction  through  a  Tes- 
ticle. I.  Mediastinum  testis.  2, 
2.  Trabeculse.  3.  One  of  the 
lobules.  4,  4.  Vasa  recta.  5. 
Globus  major  of  the  epididymis. 
6.  Globus  minor.  7.  Vas  def- 
erens.— {H  olden.) 


Fig.  349. — \'as  Deferens,  Vesicul.e 
.Semixales,  axd  Ejaculatory  Ducts. 
a.  Vas  deferens,  b.  Seminal  vesicle. 
c.  Ejaculatorj'  duct.  d.  Termination  of 
the  ejaculator}'  duct.  e.  Opening  of  the 
prostatic  utricle.  /,  g.  Veru  montanum. 
h,  I.  Prostate. — (Liege'ois.) 


The  ejaculatory  duct,  formed  by  the  union  of  the  vas  deferens  and  the 
duct  of  the  vesicula  seminalis,  opens  into  the  prostatic  portion  of  the  urethra 
(Fig.  349). 

The  prostate  gland  is  a  musculo-glandular  mass  surrounding  the 
posterior  extremity  of  the  urethra.  It  contains  a  large  number  of  tubules, 
more  or  less  branched  and  convoluted,  and  lined  by  columnar  epithelium. 
They  secrete  a  fluid  which  is  poured  into  the  urethra  at  the  time  of  the  ejac- 
ulation   of    semen  and    impart  motility  to  the  spermatozoa  or   spermia. 

The  penis  consists  of  three  parts:  the  corpus  spongiosum  below,  through 
which  passes  the  urethra,  and  the  two  corpora  cavernosa,  one  on  either  side 
and   above.     The   corpus   spongiosum   terminates   anteriorly   in   a   conic- 


694 


TEXT-BOOK  OF  PHYSIOLOGY 


shaped  structure,  the  glans  penis;  the  corpora  cavernosa  consist  externally 
of  a  fibrous  investment  and  internally  of  a  fibrous  investment  and  internally 
of  erectile  tissue.  These  bodies  are  abundantly  supplied  with  blood,  which 
after  entering  their  susbtance  by  the  arteries,  passes  into  sinuses  or  reser- 
voirs, from  which  it  is  carried  away  by  veins.  These  vessels  pass  to  the 
dorsum  of  the  penis  and  unite  to  form  a  large  vein  by  which  the  blood  is  re- 
turned to  the  general  circulation.  By  virtue  of  the  erectile  tissue  in  the  cor- 
pora cavernosa  the  penis  becomes  erect  and  rigid  when  the  blood  supply 
is  increased.  This  takes  place  in  response  to  peripheral  stimulation  or 
emotional  states,  or  both  combined.  When  these  conditions  are  established 
nerve  impulses  pass  outward  through  nerves,  the  nervi  erigentes,  which  have 
their  origin  in  the  lumbar  segment  of  the  spinal  cord,  and 
bring  about  an  active  dilatation  of  the  arteries  and  a  re- 
laxation of  the  non-striated  muscle-fibers  in  the  corpora 
cavernosa.  (See  page  619.)  With  these  events  there  is 
a  rapid  influx  of  blood  and  a  distention  and  an  erection 
of  the  organ.  This  condition  is  furthered  and  main- 
tained by  a  partial  compression  of  the  dorsal  vein  by  the 
fibrous  capsule. 

Semen. — The  semen  is  a  complex  fluid  composed  of 
the  secretions  of  the  testicles,  the  vesiculse  seminales,  the 
prostatic  tubules,  and  urethral  or  Cowper's  glands.  It 
is  grayish-white  in  color,  mucilaginous  in  consistence, 
characteristic  in  odor,  and  somewhat  heavier  than  water. 
In  response  to  appropriate  stimulation  the  muscle-fibers 
in  the  walls  of  the  vasa  deferentia,  vesiculae  seminales, 
and  prostatic  tubules  contract  and  discharge  their  con- 
tents into  the  urethra,  from  which  they  are  forcibly 
ejected  by  the  rhythmic  contraction  of  the  ejaculatory 
muscles,  the  ischio-  and  bulbo-cavernosi.  The  amount  of 
semen  discharged  at  each  ejaculation  varies  from  i  to 
5  c.c. 

Spermatozoa. — The  spermatozoa  or  spermia  are 
peculiar  morphologic  elements  which  arise  within  the 
seminiferous  tubules  as  a  result  of  complex  histologic 
changes  in  the  lining  epithelium.  An  adult  spermatozoon 
consists  of  a  conoid  slightly  flattened  head,  from  the 
posterior  part  of  which  there  projects  a  short  straight 
rod,  provided  with  a  long  filamentous  tail  or  cilium  and  an  end-piece 
(Fig.  350).  The  head  contains  a  nucleus  of  chromatin  material.  The 
lotal  length  of  a  spermatozoon  varies  from  50  to  80  micro-millimeters. 
The  characteristic  physiologic  feature  of  spermatozoa  is  incessant 
locomotion  when  in  a  suitable  medium.  So  long  as  they  are  confined 
to  the  vas  deferens  they  are  quiescent,  but  with  their  advent  into  the 
vesicula  seminalis  and  dissemination  in  its  contained  fluid,  they  become 
extremely  active  and  move  around  with  considerable  rapidity.  The  power 
of  locomotion  depends  on  the'  possession  of  the  tail  which,  by  lashing  the 
surrounding  fluid  now  in  this  and  now  in  that  direction,  propels  the  head 
from  place  to  place.     The  vitality  of  spermatozoa  is  such  as  to  enable 


Fig.  350. — ^HtJMAN 
Spermatozoon,  i  . 
Front  view,  2,  side 
view,  of  the  head. 
k.  Head.  m.  mid- 
dle piece.  /.  Tail. 
e.  Terminal  fila- 
m  e  n  t. — (After  Ret- 
ziiis.) 


REPRODUCTION.  695 

them  to  retain  their  physiologic  activities  in  the  uterus  for  more  than  eight 
days. 

The  development  of  spermatozoa  from  testicular  cells  as  observed  in  lower 
animals  indicates  that  each  cell  gives  rise  to  four  embryonic  forms — spermatids 
— which  subsequently  develop  into  adult  spermatozoa.  In  this  process  the 
primary  nuclear  chromatin  undergoes  a  division,  so  that  each  spermatozoon 
receives  but  a  fractional  amount  representing  one-half  the  number  of  somatic 
chromosomes.  The  changes  by  which  this  condition  is  brought  about  are 
comparable  to  the  changes  exhibited  by  the  ovum,  and  have  for  their  result 
a  reduction  in  the  quantity  of  hereditary  substance  to  be  transmitted. 

Fecundation. — Fecundation  is  the  union  of  the  spermatozoon  (the 
sperm-cell)  with  the  ovum  (the  germ-cell)  and  takes  place  in  the  great 
majority  of  instances  in  the  Fallopian  tube.  After  the  introduction  of  the 
spermatozoa  into  the  vagina  during  the  act  of  copulation,  they  soon  begin  to 
pass  upward,  into  and  through,  the  uterine  cavity  and  out  into  the  Fallopian 
tube,  where  they  accumulate  in  large  numbers  and  retain  their  vitality  for 
some  days.  The  migration  is  effected  by  the  propelling  power  of  the  fila- 
mentous tail  and  by  the  action  of  the  cilia  of  the  uterus  and  tubes. 

From  observations  made  on  the  behavior  of  the  spermatozoa  toward 
the  ovum  in  lower  animals,  and  on  the  manner  in  which  their  union  is 
effected,  the  inference  may  be  drawn  that  a  similar  procedure  takes  place  in 
mammals.  In  lower  animals  the  spermatozoa  on  approaching  an  ovum 
take  on  increased  activity,  swimming  around  it  in  all  directions  and  appar- 
ently seeking  a  point  of  entrance.  In  fish  and  molluscs  the  zona  pellucida 
presents  a  distinct  opening,  the  micropyle,  through  which  the  spermatozoon 
passes.  Inasmuch  as  the  mammalian  ovum  is  devoid  of  such  an  opening, 
the  mechanism  of  entrance  of  the  spermatozoon  is  not  clearly  understood. 
Notwithstanding  their  enormous  numbers  it  is  generally  believed  that  but  a 
single  spermatozoon  effects  an  entrance  into  the  ovum.  With  the  accom- 
plishment of  this,  however,  the  spermatozoon  loses  its  mobility,  after  which 
the  tail  disappears. 

The  germ  nucleus  proceeds  to  the  middle  of  the  ovum  where  it  is  followed 
by  head  and  middle-piece  of  the  spermium;  the  middle-piece  forms  a  central 
spindle  while  the  germ  nucleus  and  head  of  the  spermium  each  resolves 
itself  into  one-half  the  number  of  chromatic  loops  of  a  somatic  cell.  In  this 
condition  the  fertilized  ovum  represents  a  parent  cell,  that  possesses  the 
physiologic  activities  and  characters  of  both  ancestral  cells.  From  this 
parent  cell  the  offspring  develops  through  successive  division,  multiplication 
and  differentiation  of  the  resulting  cells.  The  chromatic  material  of  the 
germ  nucleus  and  head  of  the  spermium  represent  the  transmitters  of  in- 
herited characters. 

The  Fixation  of  the  Ovum. — The  ovum,  after  fertilization  in  the 
oviduct,  continues  to  divide  and  pass  slowly  to  the  uterus  (8  to  10  days) 
where  it  is  retained  until  the  end  of  gestation.  A  menstrual  mucosa  having 
developed  the  ovum  lodges  on  a  smooth  thick  area  and  gradually  sinks 
beneath  the  surface.  During  the  passage  down  the  o^dduct  the  zona  pellu- 
cida has  become  attenuated  and  has  been  finally  replaced  by  a  thick  layer 
of  ameboid  and  phagocytic  cells  called  the  trophoderm.  Upon  lodgment 
of  the  ovum  these  cells  destroy  the  underlying  mucosa  and  produce  a  cavity 


696  TEXT-BOOK  OF  rilYSIOLOGY. 

into  which  the  ovum  sinks.  As  the  ovum  increases  in  size  the  mucosa 
gradually  covers  it;  that  portion  of  the  mucosa  toward  the  uterine  cavity  is 
called  the  decidiia  capsularis  (d.  reflexa),  that  beneath  the  ovum  the  decidua 
basilaris  {placental  d.),  while  the  remainder  constitutes  the  decidua  parietalis 
id.  vera).  As  development  proceeds  the- decidua  basilaris  becomes  greater, 
ultimately  developing  into  the  placenta. 

Segmentation  of  the  Ovum. — Immediately  after  fertilization  the  ovum 
divides  and  rcdivides,  within  the  diminishing  zona  pellucida,  forming  an 
irregular  mass  called  the  morula.  The  peripheral  cells  form  a  layer,  the 
trophoderm,  beneath  the  attenuated  zona  pellucida  ultimately  replacing 
that  structure.  The  remaining  cells  of  the  morula  differentiate  into  three 
masses — ectodermal,  entodermal  and  mesodermal;  the  central  cells  of  these 
masses  liquefy  and  disappear  forming  thus  the  ectodermal  or  amniotic 
cavity,  limited  by  the  ectoderm;  the  entodermal  cavity,  limited  by  the  ento- 
derm; and  the  mesodermal  or  celomic  cavity,  limited  by  the  extra-embryonic 
mesoderm.  Meanwhile  cells  in  various  parts  of  the  thickened  trophoderm 
have  disappeared  leaving  this  layer  in  the  form  of  dehcate  trophodermal 
villi,  the  future  chorionic  and  placental  villi. 

The  Embryonic  Shield. — The  floor  of  the  amniotic  cavity,  consisting 
of  ectoderm  and  entoderm,  constitutes  the  embryonic  shield  or  disk.  As 
the  shield  increases  in  size  a  median  longitudinal  thickening  is  seen 
occupying  the  caudal  half  of  the  area.  This  is  the  primitive  streak,  a  tem- 
porary structure  that  is  soon  overshadowed  by  changes  in  the  areas  just  in 
front  of  it.  Here  is  formed  a  median,  longitudinal,  grooved  ridge  of  ecto- 
derm, that  develops  rapidly  in  length.  This  is  the  neural  groove  and  folds. 
The  dorsal  lips  of  the  groove  approach  each  other  in  the  mid-line  and  fuse, 
separating  from  the  original  ectoderm  which  closes  over  the  ectodermal 
tube.  This  ectodermal  tube  is  the  neural  tube  from  which  the  nerve  system 
is  developed. 

In  the  immediate  vicinity  of  the  head  end  of  the  primitive  streak  is  seen 
a  darkened  area,  Hensen's  node  that  represents  the  beginning  invagination 
of  the  ectoderm  in  the  formation  of  the  embryonic  mesoderm  and  notochord 
to  be  considered  later.  That  portion  of  the  embryonic  shield  that  gives 
rise  to  the  embryo  itself  becomes  distinctly  outlined  laterally  and  in  the  head 
and  tail  regions  of  the  neural  groove.  Just  external  to  this  area,  the  embry- 
onic area  proper,  is  a  transparent  area,  the  area  pellucida,  beyond  which  is  the 
area  opaca  in  which  the  first  blood-vessels  appear. 

Mesoderm  and  Notochord. — So  far  in  the  embryonic  area  only  ecto- 
derm and  entoderm  exist.  Hensen's  node  at  the  head  end  of  the  primitive 
streak  represents  an  invagination  (gastrulation)  of  ectoderm  between  ecto- 
derm and  entoderm.  This  invagination  elongates  headward  in  the  embry- 
onic area  constituting  a  tube  of  ectodermal  cells,  the  chordal  canal.  Later 
the  ventral  wall  of  the  canal  and  the  adjacent  entoderm  disappear  so  that 
the  chordal  ectoderm  temporarily  forms  the  dorsal  median  boundary  of  the 
entodermal  cavity.  By  this  process  a  communication  is  established  between 
the  entodermal  cavity  and  neural  groove,  called  the  neurenteric  canal.  The 
chordal  ectoderm  separates  from  the  entoderm  and  then  forms  a  solid  cord 
of  cells,  the  notochord,  between  entoderm  and  neural  groove,  the  neurenteric 
canal,  however,  persisting  for  some  time.     In  the  meantime  other  ecto- 


REPRODUCTION. 


697 


dermal  cells  in  the  region  of  the  chordal  invagination  spread  between  ecto- 
derm and  entoderm  and  form  the  anlage  of  the  mesoderm.  These  cells  by 
rapid  proliferation  soon  separate  ectoderm  and  entoderm  and  join  the 
extra-embryonic  mesoderm.  The  separation  of  ectoderm  and  entoderm  is 
complete  except  in  the  regions  of  the  bucco-pharyngeal  and  cloacal  membranes. 

Upon  each  side  of  the  neural  groove  the  mesoderm  becomes  transversely 
grooved  on  its  ectodermal  surface,  forming  a  number  of  successive  block- 
like masses  called  primitive  somites  or  segments.  Of  these  there  are  thirty- 
eight  of  the  trunk  and  possibly  four  for  the  head  region.  Each  segment 
consists  of  three  parts— the  sclerotome,  the  myotome  and  the  dermatome. 
Lateral  to  the  somite  is  a  thickened  mass  of  mesoderm,  the  intermediate  cell- 
mass,  that  laterally  splits  into  two  layers,  the  outer  accompanies  the  ecto- 
derm forming  the  somatopleiire  which  gives  rise  to  the  body  wall,  the  inner 
joins  the  entoderm  forming  the  splanchnopleure  from  which  the  gut-tract, 
vitelline  duct  and  yolk-sac  are  derived. 

Fetal  Membranes. — As  the  primitive  streak  and  neural  groove  are  form- 
ing, the  extra-embryonic  mesoderm  that  lies  beneath  the  trophoderm  invades 
the  trophodermal  villi  forming  thus 
the  chorion  with  its  villi.  Gradually 
the  mesoderm  of  the  roof  of  the 
amniotic  cavity  splits  into  two  layers, 
the  upper  constituting  chorionic  meso- 
derm while  the  under  one  attached  to 
the  ectoderm  of  the  amniotic  forms 
with  the  latter  the  amnion.  In  the 
chick  and  some  mammals,  the  amnion 
is  derived  from  the  somatopleure  in  the 
folding  off  of  the  body.  In  amniotes 
the  amniotic  cavity  is  at  first  small 
but  rapidly  increases  in  size.  It  con- 
tains a  clear,  transparent  liquid,  the 
amniotic  fluid ,  which  amounts  to  about 
one  liter  at  term;  it  serves  to  protect 
the  fetus  during  gestation  and  at  parturition  it  dilates  the  os  cer\'icis,  and 
flushes  the  birth  canal.  This  liquid  is  derived  mainly  from  the  blood  as  it 
contains  albumin,  sugar,  fat  and  inorganic  salts.  Traces  of  urea  indicate 
that  some  of  its  constituents  are  derived  from  the  embryo  itself. 

The  caudal  end  of  the  embryonic  area  is  left  connected  with  the  chorion 
by  a  heavy  band  of  mesoderm  termed  the  belly-stalk,  to  which  the  caudal 
part  of  the  amnion  is  attached.  The  entoderm  is  invaginated  into  the 
belly-stalk  for  a  short  distance  constituting  the  allantois  of  higher  forms. 
In  oviparous  forms  the  allantois  grows  out  between  the  closing  somatopleuric 
folds  that  form  the  body-wall  and  constitutes  a  free  sac  upon  which  vessels 
(allantoic  arteries  and  veins)  develop  from  the  embryo.  This  sac  then 
spreads  beneath  the  white  shell  membrane  forming  the  organ  of  nutrition 
and  respiration  of  these  forms  during  the  last  half  of  their  incubation  periods. 
In  mammals  the  extra-embryonic  portion  of  the  allantois  is  of  little 
importance.     (Fig.  351.) 


Fig.  351. — HxJMAX  Embryo  and  its  En- 
velopes AT  THE  EXD  OF  THE  ThIRD  MoXTH. 
— {D  alt  on.) 


698 


TEXT-BOOK  OF  PHYSIOLOGY. 


Placenta  Formation.— The  chorionic  villi  increase  rapidly  in  size  and 
numbar  and  usually  surround  the  whole  fetal  sac,  giving  it  a  peculiar  shaggy 
appearance.  Blood-vessels  now  proceed  from  the  embryo  along  the  belly- 
stalk  (not  the  allantois  in  higher  forms  as  formerly  stated).  These,  the 
umbilical  arteries  and  veins,  pass  to  the  chorionic  villi  and  send  branches  to 
those  of  the  placental  area;  these  vascularized  villi  constitute  the  chorion 
frondosum,  while  the  avascular  villi  form  the  chorion  leve.  The  villi  of  the 
latter  disappear  during  the  second  month,  leaving  the  chorionic  membrane 
smooth.  The  villi  of  the  chorion  frondosum  now  penetrate  the  uterine  glands 
of  the  decidua  basilaris,  which  by  this  time  have  been  denuded  of  epithelium, 
and  have  gained  connection  with  the  blood-vessels  of  the  mucosa;  in  this 

Amnion.  B».'ii..i»ij»ii»mijiiiuiiic, 
Chorion 


rt  f  Compact 
•£        layer. 


Cavernou.s  f 
^  ^      layer.      '( 


Muscularis. 


■  Chorionic  villi. 

Intervillous  spaces. 
Floating  villus. 

Attached  villi. 
\'ein. 


FiC'-  352. — DrAGR,\M  OF  Human  Placexta  .at  the  Close  of  Pregx.axcy. — {Schaper.) 


manner  these  uterine  glands  have  become  converted  into  blood  sinuses.  The 
chorionic  villi  either  attach  themselves  to  the  tunica  propria  of  the  mucosa 
( fixed  villi)  or  remain  free  {floating  villi) .  At  the  edge  of  the  placental  area 
very  few  villi  develop,  leaving  a  circular  channel  called  the  marginal  sinus. 
This  attachment  of  villi  becomes  marked  from  the  third  month  on  and 
this  is  considered  the  beginning  of  placentation.  From  this  time  on  to  term 
there  is  merely  an  increase  in  number  of  villi  and  vessels  with  thus  a  cor- 
responding increase  in  the  size  of  the  placenta.    (Fig.  352.) 

The  Placenta. — Of  all  the  embryonic  structures  the  placenta  is  the 
most  important.  It  is  formed  by  the  end  of  the  third  month,  after  which  it 
gradually  increases  in  size  up  to  the  end  of  the  eighth  month,  by  which 
time  it  is  fully  developed.  It  then  measures  from  18  to  24  cm.  in  diameter 
and  weighs  from  400  to  600  grams.  It  is  most  frequently  attached  to  the 
upper  and  back  part  of  the  uterine  wall.  Though  exceedingly  complex 
in  structure  it  consists  essentially  of  two  portions,  a  fetal  and  a  maternal. 

TYiQ  fetal  portion  consists  primarily  of  those  villi  on  the  chorion  in  relation 
with  the  decidua  basilaris.     These  structures  gradually  increase  in  size  and 


REPRODUCTION.  699 

number,  and  receive  the  ultimate  branches  of  the  umbihcal  arteries.  The 
maternal  portion  consists  primarily  of  the  decidua  basilaris.  As  gestation 
advances  the  placental  villi  rapidly  increase  in  size  and  number,  and  re- 
ceive the  branches  of  the  umbilical  arteries.  At  the  same  time  the  decidua 
basilaris  becomes  hypertrophied  and  vascular.  With  the  continued  growth 
and  development  of  these  two  structures  they  gradually  fuse  together  and 
finally  become  inseparable.  In  accordance  with  the  needs  of  the  embryo, 
the  decidua  basilaris  and  its  contained  blood-vessels  undergo  certain  histo- 
logic changes  which  result  in  the  formation  of  large  cavities,  sinuses,  or 
lakes,  into  which  the  blood  of  the  uterine  vessels  is  emptied.  As  the  pla- 
centa develops,  the  structures  separating  the  blood  of  the  mother  from  that 
of  the  child  gradually  become  modified  until  they  are  represented  by  a  thin 
cellular  or  homogeneous  membrane.  The  conditions  now  are  such  as  to 
permit  of  a  free  exchange  of  material  between  the  mother  and  child.  Whether 
by  osmosis  or  by  an  act  of  secretion,  the  nutritive  materials  of  the  maternal 
blood  pass  through  the  intervening  membrane  into  the  fetal  blood  on  the 
one  hand,  while  waste  products  pass  in  the  reverse  direction  into  the  maternal 
blood  on  the  other  hand.  Inasmuch  as  oxygen  is  absorbed  and  carbon 
dioxid  exhaled  by  the  same  structures,  the  placenta  is  to  be  regarded  as 
both  an  absorptive  and  a  respiratory  organ.  So  long  as  these  exchanges  are 
permitted  to  take  place  in  a  normal  manner  the  nutrition  of  the  embryo  is 
secured. 

The  Nutritive  Supply  of  the  Embryo.— The  growth  and  development 
of  the  embryo  from  the  period  of  fertilization  to  the  period  of  birth  require 
a  continuous  and  ever  increasing  supply  of  food  materials.  This  is  derived 
from  several  sources  and  requires  for  its  utilization,  the  development,  in 
different  classes  of  animals,  of  specialized  forms  of  the  circulatory  apparatus, 
the  relative  importance  of  with  varies  in  accordance  when  the  source  of 
the  food  supply.  These  are  known  as  the  vitelline,  the  allantoic,  and  the 
placental  circulations.  All  these  forms  are  present  at  successive  stages  in 
the  development  of  the  human  embryo  but  only  the  last  is  of  major 
importance. 

As  the  ovum  passes  down  the  oviduct  it  imbibes  its  nutritive  material 
from  the  mucosa.  When  it  lodges  itself  in  the  uterus  it  probably  receives 
additional  material  in  the  same  way.  The  period  during  which  it  does  so 
is,  however,  very  limited. 

The  Vitelline  Circulation. — The  vitelline  circulation,  which  in  oviparous 
animals,  e.g.,  the  chick,  is  of  primary  importance  because  of  the  large 
amount  of  food  stored  in  the  vitellus  or  yolk,  is  in  mammals  of  relatively 
slight  importance  because  of  the  Hmited  supply  of  food  in  the  vitellus.  It 
is  nevertheless  present  in  early  stages. 

The  Allantoic  Circulation  which  in  oviparous  animals  is  also  of  primary 
importance  in  the  latter  half  of  the  incubation  period  both  as  an  absorption 
and  respiratory  apparatus  is  also  present  in  mammals  to  a  slight  extent, 
but  it  is  merely  a  transition  stage  in  the  development  of  placental  circulation. 

The  Placental  Circulation. — The  development  of  the  fetal  or  placental 
circulatory  apparatus  by  which  the  fetus  obtains  its  food  supply  and  neces- 
sary oxygen  and  frees  itself  from  carbon  dioxid  has  been  alluded  to  in  a 
foregoing  paragraph  relating  to  the  formation  of  the  placenta.     After  the 


700  TEXT-BOOK  OF  PHYSIOLOGY. 

blood-vessels  of  the  embryo,  the  umbilical  arteries  and  vein  have  come  into 
histologic  and  physiologic  relations  with  the  uterine  blood-vessels,  the 
nutritive  materials  and  the  oxygen  are  derived  entirely  from  the  maternal 
blood-stream  which  at  the  same  time  receives  carbon  dioxid  and  perhaps 
other  waste  products  from  the  fetal  blood-stream.  The  placenta  thus 
serves  as  a  digestion  and  respiratory  organ.  The  blood  having  undergone 
these  changes  now  leaves  the  placenta  and  returns  to  the  fetus  by  the  um- 
bilical vein.     This  blood   is  relatively  rich  in  nutritive  material  and  of  a 


a  iju 
y.pii 


Fig.  353. — The  Fetal  Circulation,  ao.  Aorta,  a.pu.  Pulmonary  artery,  au.  Umbilical 
artery,  da.  Ductus  arteriosus,  dv.  Ductus  venosus.  int.  Intestine,  vci  and  vcs.  Inferior  and 
superior  venae  cavas.  vh.  Hepatic  vein.  vp.  Venaportas.  v.  pu.  Pulmonarv  vein.  vii.  Umbilical 
vein. —  {Front  Kollntann.) 

scarlet  red  color  by  reason  of  the  presence  of  an  increased  amount  of  oxygen. 
As  it  passes  into  the  abdominal  cavity  a  portion,  about  one-half,  of  the  blood 
is  directed  by  the  ductus  venosus  into  the  inferior  vena  cava,  while  the  re- 
mainder is  emptied  into  the  portal  vein,  by  which  it  is  distributed  to  the 
liver  and  from  which  it  emerges  by  the  hepatic  veins  and  is  poured  into  the 
inferior  vena  cava.  The  blood  in  the  vena  cava  is  thus  a  mixture  of  venous 
blood  from  the  lower  extremities  and  liver,  and  oxygenated  blood  from  the 
placenta.  After  its  discharge  into  the  right  auricle  the  blood  is  directed  by 
a  fold  of  the  lining  membrane,  the  Eustachian  valve,  through  an  opening  in 


REPRODUCTION.  701 

the  interauricular  septum,  the  foramen  ovale,  into  the  left  auricle.  It  then 
flows  through  the  auriculo-ventricular  opening  into  the  left  ventricle,  thence 
into  the  aorta,  and  by  its  branches  is  distributed  to  all  parts  of  the  body. 

The  blood  from  the  head  and  upper  extremities  is  emptied  by  the  superior 
vena  cava  into  the  right  auricle,  but  as  it  passes  in  front  of  the  Eustachian 
valve,  it  flows  directly  into  the  right  ventricle  and  then  into  the  pulmonary 
artery.  On  account  of  the  unexpanded  condition  of  the  lungs  and  the 
almost  impervious  condition  of  the  pulmonary  capillaries,  but  a  small 
portion  of  the  blood  passes  through  them,  while  the  larger  portion  by  far 
passes  into  the  aorta  directly  through  a  duct,  the  ductus  arteriosus,  which 
enters  at  a  point  below  the  origin  of  the  left  carotid  and  subclavian  arteries. 
A  comparison  of  the  blood  distributed  to  the  head  and  upper  extremities, 
with  that  distributed  to  the  lower  extremities,  will  show  a  larger  percentage 
of  nutritive  material  and  oxygen  in  the  former  than  in  the  latter,  a  fact 
which  has  been  offered  as  an  explanation  of  the  more  rapid  growth  of  the 
liver  and  upper  half  of  the  body.  As  the  blood  passes  through  the  aorta, 
a  portion  is  directed  from  the  main  current  by  the  hypogastric  and  umbilical 
arteries  to  the  placenta,  where  it  loses  carbon  dioxid  and  gains  oxygen,  and 
changes  in  color  from  a  bluish  red  to  a  scarlet  red. 

Parturition. — At  the  end  of  gestation — approximately  280  days  from 
the  time  of  conception — a  series  of  changes  occur  in  the  uterine  structures 
which  lead  to  an  expulsion  of  the  child,  the  placenta,  and  decidua  vera.  To 
this  process  in  its  entirety  the  term  parturition  is  given.  At  this  time,  from 
causes  not  clearly  defined,  the  uterine  walls  begin  to  exhibit  throughout  their 
extent  a  series  of  slight  contractions  which  are  somewhat  peristaltic  in  char- 
acter; these  contractions,  which  gradually  increase  in  frequency  and  vigor, 
bring  about  a  dilatation  of  the  internal  os  and  a  descent  of  the  membranes 
into  the  cervical  canal.  The  pressure  exerted  by  these  membranes  during 
the  time  of  the  contraction  materially  assists  in  the  relaxation  of  the 
circular  fibers  and  a  dilatation  of  the  external  os.  When  the  dilatation  has 
so  far  advanced  that  the  diameter  of  the  external  os  attains  a  measure  of 
7  or  8  cm.,  the  tension  of  the  membranes  becomes  sufficiently  great  to  lead 
to  their  rupture  and  to  a  partial  escape  of  the  amniotic  fluid.  With  this 
event,  the  presenting  part  of  the  child,  usually  the  head,  descends  into  the 
vagina.  After  a  short  period  of  rest  the  uterine  contractions  return  and 
rapidly  increase  in  vigor  and  duration.  As  a  result  of  the  pressure  thus 
exerted  from  all  sides  on  the  body  of  the  child,  the  head  gradually  descends 
still  further  into  the  vagina  and  finally  emerges  through  the  vulva  to  be 
followed  in  a  short  time  by  expulsion  of  the  trunk  and  limbs,  and  a  discharge 
of  the  remaining  amniotic  fluid.  With  the  expulsion  of  the  child  the  uterine 
contractions  cease  for  a  period  of  ten  or  fifteen  minutes,  when  they  again 
recur,  with  the  result  of  detaching  the  placenta  and  expelling  it  into  the 
vagina.  It  is  then  removed  by  the  cooperative  action  of  the  abdominal 
and  perineal  muscles.  The  hemorrhage  which  would  naturally  occur  with 
the  detachment  of  the  placenta  and  the  laceration  of  the  maternal  vessels 
is  prevented  by  the  firm  continuous  contraction  of  the  uterine  walls,  by 
which  the  vessels  are  compressed  and  permanently  closed. 

The  Establishment  of  Inspiration  and  the  Adult  Circulation.^ 
After  the  birth  of  the  child  and  the  detachment  of  the  placenta,  there  speedily 


702  TEXT-BOOK  OF  PHYSIOLOGY. 

occurs  a  decrease  in  the  quantity  of  oxygen  and  an  increase  in  the  quantity 
of  carbon  dioxid  in  the  blood,  a  condition  which  causes  a  discharge  of  nerve 
energy  from  the  inspiratory  center,  a  contraction  of  the  inspiratory  muscles, 
an  expansion  of  the  thorax,  and  an  inflow  of  air  into  the  lungs. 

In  addition  it  is  very  probable  that  the  stimulation  of  the  inspiratory 
center  is  also  occasioned  by  the  arrival  of  nerve  impulses  from  the  skin, 
developed  by  the  cooling  of  the  skin  due  to  the  vaporization  of  the  amniotic 
fluid. 

In  the  later  months  of  intrauterine  life  the  vascular  apparatus  undergoes 
certain  anatomic  changes  which  favor  the  transition  from  the  placental  to 
the  adult  circulation.  Thus  the  ductus  venosus  contracts,  and  sends  a 
a  larger  volume  of  blood  into  and  through  the  liver;  the  Eustachian  valve 
diminishes  in  size  and  at  the  time  of  birth  has  almost  disappeared;  a  mem- 
branous fold  grows  upward  and  backward  from  the  edge  of  the  foramen 
ovale  on  the  left  side;  the  ductus  arteriosus  also  contracts.  With  the  first  in- 
spiration and  the  expansion  of  the  lungs,  the  blood  which  enters  the  pul- 
monary artery  passes  through  the  pulmonary  capillaries  in  large  volume  and 
is  returned  by  the  pulmonary  veins  to  the  left  auricle.  The  entrance  of 
the  blood  into  this  cavity  presses  the  membranous  fold  against  the  margins 
of  the  foramen  ovale  and  thus  prevents  the  further  flow  of  blood  from  the  right 
auricle.  The  blood  entering  the  right  auricle  by  the  inferior  vena  cava  now 
flows  into  the  right  ventricle,  which  is  favored  by  the  small  size  of  the  Eu- 
stachian valve.  The  foramen  ovale  is  permanently  closed  at  the  end  of  a 
week  or  ten  days;  the  ductus  arteriosus  at  the  end  of  four  days.  The  um- 
bilical vein  and  ductus  venosus,  at  the  end  of  four  or  five  days,  have  also 
become  almost  impervious  from  the  contraction  of  their  walls.  The 
proximal  ends  of  the  hypogastric  arteries  remain  open  and  carry  blood  to  the 
walls  of  the  bladder.  The  distal  ends  of  the  arteries  are  converted  into  im- 
pervious cords. 

Lactation.- — As  pregnancy  advances  the  mammary  glands  increase  in 
size,  partly  from  a  deposition  of  fat  and  connective  tissue  and  partly  from 
a  multiplication  of  the  secreting  acini.  The  lining  epithelial  cells  at  the  same 
time  increase  in  size,  and  toward  the  end  of  pregnancy  begin  to  exhibit 
functional  activity.  At  the  time  of  birth,  or  within  a  day  or  so  after  birth, 
the  acini  are  filled  with  a  fluid  which  in  its  qualitative  composition  resem- 
bles milk  and  is  known  as  colostrum.  It  is  distinguished  from  milk  more 
especially  in  the  fact  that  it  contains  in  large  quantity  a  proteid  which  coag- 
ulates on  boiling,  and  certain  inorganic  salts  which  have  a  laxative  efi^ect 
on  the  new-born  child.  Normal  lactation  and  the  phenomena  which 
accompany  it  are  fully  established  by  the  end  of  the  second  or  third  day. 

The  composition  of  milk  and  the  mechanism  of  its  production  have  been 
stated  in  the  chapter  on  Secretion. 

Physiologic  Activities  of  the  Embryo. — During  intrauterine  life  the 
evolution  of  structure  is  accompanied  by  an  evolution  of  function.  The 
relatively  simple  and  uniform  metabolism  of  the  undifferentiated  blastoder- 
mic membranes  gradually  increases  in  complexity  and  variety,  as  the  individ- 
ual tissues  and  organs  make  their  appearance  and  assume  even  a  slight 
degree  of  functional  activity.  As  to  the  periods  at  which  different  organs 
begin  to  functionate,  but  little  is  positively  known. 


REPRODUCTION.  703 

The  primitive  heart,  in  all  probability,  begins  to  pulsate  very  early,  as  in 
an  embryo  from  fifteen  to  eighteen  days  old  and  measuring  but  2  mm.  in 
length,  Coste  found  the  amnion,  the  allantois,  the  omphalo-mesenteric 
vessels,  and  the  two  primitive  aortae  developed.  In  the  earlier  weeks,  all 
products  of  metabolism  are  doubtless  eliminated  by  the  placental  structures; 
but  as  metabolism  increases  in  complexity  the  liver  and  kidney  assume  ex- 
cretory activity.  Thus,  at  the  end  of  the  third  month  the  intestine  contains 
a  dark,  greenish,  viscid  material — meconium — composed  of  bile  pigments, 
bile  salts,  and  desquamated  epithelium;  the  amniotic  fluid,  as  well  as  the 
fluid  within  the  bladder,  contains  urea  at  the  end  of  the  sixth  month,  in- 
dicating the  establishment  of  both  hepatic  and  renal  activity.  Contractions 
of  the  skeletal  muscles  of  the  limbs  begin  about  the  fifth  month,  from  which 
it  may  be  inferred  that  the  mechanism  for  muscle  activity,  viz.,  muscles, 
efi'erent  nerves,  and  spinal  centers,  has  become  anatomically  developed  and 
associated,  and  capable  of  coordinate  activity.  These  contractions  are,  in  all 
probability,  automatic  or  autochthonic  in  character  due  to  stimuli  arising 
within  the  spinal  centers.  The  remaining  organs  remain  more  or  less 
inactive. 

After  birth,  with  the  first  inspiration  and  the  introduction  of  food  into  the 
alimentary  canal,  the  physiologic  mechanisms  which  subserve  general 
metabolism  begin  to  functionate  and  in  the  course  of  a  week  are  fully  estab- 
lished. At  this  time  the  cardiac  pulsation  averages  about  135  a  minute;  the 
respiratory  movements  vary  from  30  to  35  a  minute,  and  are  diaphragmatic 
in  type;  the  urine,  which  was  at  first  scanty,  is  now  abundant  and  propor- 
tional to  the  food  consumed;  the  digestive  glands  are  elaborating  their  re- 
spective enzymes,  digestion  proceeding  as  in  the  adult.  The  hepatic  secre- 
tion is  active  and  the  lower  bowel  is  emptied  of  its  contents;  the  coordinate 
activites  of  the  nerve-,  muscle-,  and  gland-mechanisms  are  entirely  reflex  in 
character.  Psychic  activities  are  in  abeyance  by  reason  of  the  incomplete 
development  of  the  cerebral  mechanisms. 


APPENDIX. 


PHYSIOLOGIC  APPARATUS. 

The  study  of  the  physical  and  physiologic  properties  of  muscles  and 
nerves  necessitates  the  employment  of  some  stimulus  which,  when  applied  to 
either  tissue,  will  call  forth  a  contraction  of  the  muscle,  or  the  development  of 
a  nerve  impulse  in  the  nerve.  The  most  convenient  stimulus  is  electricity, 
for  the  reason  that,  with  appropriate  apparatus,  its  intensity  and  duration 
can  be  graduated  with  the  utmost  nicety.  Moreover,  it  does  not  destroy 
the  tissues,  as  do  many  chemic,  physical,  and  mechanic  stimuli. 

It  is  therefore  necessary  that  the  student  should  have  a  practical  ac- 
quaintance with  those  appliances  by  means  of  which  electricity  is  generated, 
applied  and  controlled. 

The  electric  cell  is  an  apparatus  composed  of  different  elements,  which, 
by  virtue  of  chemic  actions  taking  place  among  them,  generate  and  conduct 
electricity.  In  its  simplest  form  an  electric  cell  consists  of  two  metals — 
zinc  and  copper,  or  carbon,  or  platinum,  etc.,  immersed  in  an  exciting  fluid, 
usually  dilute  sulphuric  acid  (Fig.  354). 

The  zinc  element  is  the  one  acted  on  chemically  by  the  sulphuric  acid, 
and  at  the  expense  of  which  the  electricity  is  maintained.  It  is  known  as  the 
generating  element.     The  copper  is  the  collecting  and  conducting  element. 

With  the  immersion  of  these  elements  in  a  solution  of  H2SO4  a  chemic 
action  at  once  takes  place  between  the  zinc  and  the  acid,  with  the  formation 
of  zinc  sulphate  and  the  liberation  of  hydrogen,  as  expressed  in  the  following 
formula : 

Zn  +  H2SO,  =  ZnS04  +  H2. 

The  zinc  sulphate  passes  into  the  solution,  while  the  hydrogen  accu- 
mulates on  the  surface  of  the  copper  element. 

As  all  chemic  action  is  accompanied  by  the  development  of  electricity, 
it  can  be  shown  by  appropriate  means  that  this  is  the  case  at  the  surface  of 
the  zinc.  Such  a  combination  is  the  means  of  establishing  a  difference  oj 
potential  between  two  points;  the  point  of  highest  potential  being  the  surface  of 
the  zinc  or  the  positive  element,  the  point  of  lowest  potential  being  the  copper 
or  the  negative  element.  So  long  as  the  elements  remain  unconnected 
there  is  no  movement  of  electricity,  no  current. 

If  the  ends  of  the  elements  projecting  beyond  the  fluid  are  connected  by 
a  copper  wire,  a  pathway  or  circuit  is  established,  and  a  movement  of  the 
electricity  takes  place.  As  electricity  flows  from  the  point  of  high  to  the 
point  of  low  potential,  it  follows  that  inside  the  cell  the  current  flows  from 
the  zinc  to  the  copper,  and  outside  the  cell  from  the  copper  to  the  zinc. 
Such  a  current  is  termed  a  continuous,  a  galvanic  or  a  voltaic  current.  Inas- 
much as  there  is  a  progressive  fall  in  potential  between  the  highest  and 

704 


PHYSIOLOGIC  APPARATUS. 


705 


lowest  points,  it  follows  that  any  two  points  in  the  circuit  will  exhibit  a 
similar  difference  of  potential.  For  this  reason  the  projecting  end  of  the 
copper  element  is  at  a  higher  potential  than  the  projecting  end  of  the  zinc 
element.  The  end  of  the  copper  is,  therefore,  termed  the  positive,  +  pole  or 
anode,  the  end  of  the  zinc  the  negative, — pole  or  kathode. 

Electric  Units. — Owing  to  the  difference  of  the  electric  potential  in  the 
cell,  the  electricity  leaves  the  cell  under  a  certain  degree  of  pressure,  termed 
the  ''electro-motive  force."  As  it  passes  through  the  circuit  it  meets  with 
resistance,  the  amount  of  which  will  depend  on  the  nature  of  the  circuit 
material,  its  length,  and  the  area  of  its  cross-section.  In  accordance  with 
the  resistance  will  depend  the  quantity  of  electricity  that  a  given  electro- 
motive force  vvill  press  through  in  a  unit  of  time.     The  strength  of  the 


Fig.  354._Ax 
Electric  Cell. 


T  + 


FiG.  355. — Two  Simple  Electric  Cells  Jolned 
IN  Series.     C.  Copper.     Z.  Zinc. 


current  will  therefore  not  depend  entirely  on  the  electro-motive  force,  but, 
rather  on  the  ratio  between  the  electro-motive  force  and  the  resistance. 

For  the  measurement  of  electric  quantities,  a  system  of  units  has  been 
devised.  The  unit  of  electro-motive  force  is  the  volt;  the  unit  of  resistance 
is  the  ohm,  i.e.,  the  resistance  offered  by  a  column  of  mercury  106.3  cm.  long 
and  I  sq.  mm.  section  at  0°  C;  the  unit  of  quantity  is  the  coulomb; the 
unit  of  time  is  one  second.  One  volt  is  the  electro-motive  force  which, 
when  steadily  applied,  will  press  through  a  resistance  of  the  ohm,  one  coul- 
omb of  electricity  in  one  second  of  time  yielding  a  current  strength  of  one 
ampere. 

The  relation  may  be  expressed  in  the  following  fonr^ula.  Ohm's  law: 


C  (current  strength) 


Electro-motive  force  (E.  M.  F.) 
Resistance  (R) 


or  Ampers  ^ 


Volts 
'  Ohms 


In  practical  work  it  is  often  necessary  to  increase  the  strength  of  the 
current.  This  is  done  by  uniting  two  or  more  cells  in  series,  i.e.,  uniting  the 
copper  of  one  cell  to  the  zinc  of  a  second,  and  so  on  (Fig.  355).  If  the 
resistance  remains  the  same  the  total  voltage  and  current  are  those  of  one 
cell  multipHed  by  the  number  of  cells  united. 

The  cell  as  above  described  cannot  maintain  a  current  of  constant 
strength  for  any  length  of  time,  for  the  following  reasons: 

I.  The  sulphuric  acid  solution,  in  consequence  of  its  chemic  action, 
soon  becomes  nothing  more  than  a  saturated  solution  of  zinc  sulphate,  after 
45 


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which  its  chemic  activity  ceases.     The  current,  therefore,  soon  diminishes  in 
strength. 

2.  The  accumulation  of  hydrogen  bubbles  on  the  surface  of  the  copper 
hinders  the  passage  of  the  electricity.  In  a  short  time  they  develop  a  current 
in  the  opposite  direction,  which  also  tends  to  weaken  the  original  current. 
This  action  is  termed  polarization  of  the  elements. 

Cells  of  this  character  are  not  suited  for  physiologic  work,  in  which 
constancy  in  the  strength  of  the  current  is  absolutely  necessary.  To  over- 
come these  disadvantages,  cells  have  been  devised  which  are  less  violent  in 
action,  which  prevent  polarization,  and  which  maintain  a  current  of  constant 
strength  for  a  long  period  of  time.  One  of  the  most  generally  used  for 
physiologic  purposes  is — 

The  Daniell  cell.  This  consists  of  a  porous  cup  containing  a  saturated 
solution  of  CuSO^,  copper  sulphate,  in  which  is  immersed  a  copper  plate  or 
rod.  This  combination  is  placed  in  a  glass  vessel  containing  a  solution  of 
HjSO^  (I'^S)-     III  this  solution  is  immersed  a  roll  of  sheet  zinc  (Fig.  356). 

Each  of  the  plates  is  provided  with 
a  binding  screw.  When  the  cell  is  in 
action  the  sulphuric  acid  attacks  the 
zinc,  forming  zinc  sulphate,  and  liber- 
ates hydrogen;  the  cup  being  porous, 
the  hydrogen  passes  into  the  copper 
sulphate  solution,  where  it  combines 
with  the  sulphuric  acid  radicle,  and 
liberates  metallic  copper.  Polariza- 
tion of  the  copper  is  thus  prevented. 
%  The  metalHc  copper  is  deposited  on 
the  copper  plate,  which  is  thus  kept 
bright.  The  copper  sulphate  solution 
is  kept  at  the  point  of  saturation  by 
a  packing  around  the  copper  cylinder  a 
quantity  of  the  crystals  of  the  salt. 
The  sulphuric  acid  passes  back  into 
the  porous  cup,  to  take  the  place  of  that  used.  This  cell  is  remarkably 
constant  for  these  reasons,  and  well  adapted  for  physiologic  as  well  as  other 
purposes  where  a  current  of  uniform  strength  is  necessary. 

The  projecting  ends  of  the  copper  and  zinc  plates  are  termed  respectively 
the  positive  pole  or  anode,  and  the  negative  pole  or  kathode.  The  electro- 
motive force  of  a  Daniell  cell  is  practically  i  volt;  but  when  the  two  poles 
are  connected  by  a  wire  of  i  ohm  resistance,  the  current  strength  will  be  less 
than  I  ampere,  possibly  only  0.7,  owing  to  the  resistance  offered  to  the  flow  of 
electricity  by  the  fluids  between  the  zinc  and  the  copper.  In  all  measurements, 
the  internal  resistance  of  the  cell  must  be  taken  into  consideration. 

The  Dry  Cell. — The  commercial  dry  cell  is  a  convenient  source  of 
electricity  for  general  laboratory  work.  It  consists  of  a  cup  of  zinc,  the  inner 
surface  of  which  is  covered  over  with  a  thick  layer  of  a  paste  of  plaster  of 
Paris,  saturated  with  ammonium  chlorid.  In  the  center  of  the  cup  there  is 
a  rod  of  carbon.  Surrounding  this  rod  and  occupying  the  space  between  it 
and  the  plaster-of-Paris  paste,  is  a  mixture  of  manganese  dioxid  and  charcoal. 


il. ..    ;S(J-       iJASii.LL 


PHYSIOLOGIC  APPARATUS. 


707 


The  upper  surface  of  the  cell  is  sealed  to  prevent  evaporation.  The  electric- 
ity is  generated  at  the  surface  of  the  zinc  cup  by  the  chemic  action  of  the 
chlorin  which  arises  from  the  dissociation  of  the  ammonium  chlorid.  When 
the  plates  are  united  by  a  conjunctive  wire  the  current  within  the  cell  flows 
from  the  zinc  (the  positive  element)  to  the  carbon  (the  negative  element), 
and  without  the  cell  from  the  carbon  (the  positive  pole)  to  the  zinc  (the 
negative  pole). 

Leads. — By  means  of  insulated  wires  attached  to  the  poles  of  a  cell,  the 
electricity  may  be  conducted  from  the  cell  and  used  for  exciting  or  stimulat- 
ing purpose.  As  the  wires  thus  become  practically  prolongations  of  the 
plates  their  ends  become  the  corresponding  poles.  In  experimental  work 
the  ends  of  the  wires  are  pro\dded  with  special  de\dces,  termed — 

Non-polarizable  electrodes. — The  necessity  for  the  employment  of 
such  electrodes  arises  from  the  fact  that  when  the  ends  of  the  wires  from  a 
cell  are  placed  in  direct  contact  with  the  tissues  chemic  changes  are  produced 
in  a  short  time,  which  lead  to  their  polarization.  As  a  result,  a  current 
opposite  in  direction  to  that  of  the  cell  is 
developed,  which  tends  to  weaken  or  neu- 
tralize it.  This  polarization  current  vitiates 
the  result  of  many  experiments  made  with 
highly  irritable  tissue  such  as  nerA-e-tissue. 
Whether  for  stimulating  purposes  or  for  the 
purpose  of  detecting  the  existence  of  electric 
currents  in  living  tissues,  it  is  essential  that 
the  electrodes  used  shall  be  non-polarizable. 
The  earliest  electrodes  of  this  character  were 
made  by  du  Bois-Reymond  and  were  based 
on  the  fact  discovered  by  Regnault  that  a 
strip  of  chemically  pure  zinc  or  amalga- 
mated zinc  (Matteucci)  immersed  in  a  satu- 
rated solution  of  zinc  sulphate  would  not 
polarize.  One  form  made  by  du  Bois-Rey- 
mond is  shown  in  Fig.  357.  It  consists  of 
a  flattened  glass  tube  attached  to  a  universal 
joint  and  supported  by  an  insulated  brass 
stand.  The  lower  end  of  the  tube  is 
closed  with  kaolin  or  China  clay  made  into 
a  paste  with  a  0.6  per  cent,  solution  of  sodium  chlorid.  It  can  be  moulded 
into  any  desired  shape.  The  interior  of  the  tube  is  partially  filled  with  a 
saturated  solution  of  sulphate  of  zinc  in  which  is  immersed  the  strip  of  amal- 
gamated zinc.     To  the  upper  end  of  the  zinc  the  conducting  wire  is  attached. 

The  V.  FleischI  brush  electrode  is  similar  to  the  preceding  except  that 
the  end  of  the  tube  is  closed  by  the  brush  of  a  camel's-hair  pencil. 

The  d'Arsonval  electrode  consists  of  a  glass  tube  containing  a  silver  rod 
coated  with  fused  silver  chlorid.  The  interior  of  the  tube  is  filled  with 
normal  salt  solution  0.6  per  cent,  and  the  end  closed  with  a  thread  or  plug 
of  asbestos  which  is  made  to  project  beyond  the  tube  for  a  short  distance. 

Any  one  of  these  three  electrodes  is  suitable  for  physiologic  experimenta- 
tion, as  their  free  ends  neither  corrode  the  tissues  nor  develop  electric  currents. 


Fig.  357. — NoN-POLARiz.\BLE  Elec- 
trodes. I.  Du  Bois-Reyraond's.  2. 
Von  Fleischl's.     ^.  d'Arsonval's. 


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TEXT-BOOK  OF  PHYSIOLOGY. 


Keys. — Muscle  and  nerve-tissues  are  conductors  of  electricity.  When, 
therefore,  the  terminals  (the  non-polarizable  electrodes)  of  the  wires  of  a  cell 
are  placed  in  contact  with  either  a  muscle  or  a  nerve  a  circuit  is  made  through 
which  a  current  of  electricity  flows;  when  one  or  both  are  removed,  th'e 
circuit  is  broken  and  the  current  ceases.  In  practical  work  it  is  often  neces- 
sary to  keep  the  electrodes  in  contact  with  the  tissues  for  a  variable  length 
of  time.  The  circuit,  however,  may  be  alternately  made  and  broken  at  will 
by  interposing  along  the  return  wire  a  mechanic  contrivance  known  as  a  key, 
of  which  there  are  many  forms. 

The  du  Bois-Reymond  Friction  Key. — This  consists  of  a  plate  of 
vulcanite  attached  to  a  screw  clamp  by  which  it  can  be  fastened  to  the  edge 
of  a  table  (Fig.  358).  The  surface  of  the  vulcanite 
plate  carries  two  rectangular  blocks  of  brass,  each 
of  which  has  two  holes  drilled  through  it,  for  the 
insertion  of  wires,  which  are  held  in  position  by 
small  screws.  A  movable  bridge  of  brass,  provided 
with  an  ebonite  handle,  serves  to  make  connection 
between  the  blocks.  There  are  two  ways  of  in- 
terposing this  key  in  the  circuit. 

1.  As  a  Simple  Key. — ^For  this  purpose  one  of  the 
wires,  usually  the  negative,  is  carried  from  the 
cell  to  one  block  and  then  continued  from  the 
second  block.  When  the  bridge  is  down,  the 
circuit  is  made  and  the  current  passes;  when 
it  is  up,  the  circuit  is  broken. 

2.  As  a  Short-circuiting  Key. — When  used  for  this 
purpose,  the  wires  of  the  cell  are  carried  to 
the  inner  holes  of  each  block  and  then  con- 
tinued from  the  outer  holes  to  the  tissues  or 
to  some  form  of  apparatus  which  it  is  desired 
to  actuate.  When  the  key  is  closed,  i.e.,  when 
the  bridge  is  down,  the  current  on  reaching 
the  key,  will  divide,  one  portion  passing  across 
the  bridge  and  so  back  to  the  cell,  the  other 
portion  passing  to  the  tissue  or  apparatus. 
The  amount  of  the  current  which  is  returned 

to  the  cell  through  the  short  circuit  will  be  proportional  to  the  resistance 

of  the  longer  circuit.     As  the  latter  is  usually  great  in  comparison  with 

the    former,  practically  all  the  current  is  short-circuited.     When  the 

bridge  is  lowered,  therefore,  the  current  is  short-circuited;  w^hen  it  is 

raised,   the  current  flows  into  the  longer  circuit  through  the  tissue  or 

apparatus. 

The  Mercury  Key.- — In  this  form  the  connection  is  established  by  means 

of  mercury.     It  consists  of  a  circular  block  in  the  center  of  which  there  is  a 

cup  containing  mercury  (Fig.  359).     At  opposite  points  there  are  binding 

posts,  one  of  which  is  provided  with  a  rigid  fixed  copper  rod  passing  into  the 

mercury;  the  other  is  provided  with  a  movable  bent  rod  which  may  be  made 

to  dip  into  or  be  withdrawn  from  the  mercury  by  the  ebonite  handle. 

The  effect  of  a  constant  or  galvanic  current  on  a  muscle  or  nerve  will 


Fig.  358. — Du  Bois-Rey- 
mond Friction  Key. 


PHYSIOLOGIC  APPARATUS. 


709 


depend  to  some  extent  on  its  strength.     This  may  be  accurately  regulated 
by  means  of  an  apparatus  known  as — ■ 

The  Rheocord. — With  this  apparatus  an  electric  current  may  be  divided, 
one  portion  continuing  through  a  conductor  back  to  the  battery,  the  other 
portion  being  sent  off  through  the  nerve.  The  strengths  of  these  two  currents 
are  inversely  proportional  to  the  resistances  of  their  circuits.  A  simple  form 
of  rheocord  (Fig.  360)  consists  of  a  long  wire  arranged  for  convenience  in 
parallel  lines  on  a  small  wooden  base  and  connected  at  its  two  ends  with 
binding  posts  A  and  B.  The  resistance  of  this  wire,  1.6  ohms,  can  be 
increased  by  the  introduction  of  small  resistance  coils,  between  D  and  B, 
varying  from  5  to  20  ohms. 

The  two  binding  posts  A  and  B  are  connected  with  the  positive  and 
negative  poles  of  an  electric  cell  re- 
spectively.    A  simple  key  is  placed  in 
the  circuit. 

From  A,  a  wire  passes  to  one  of 
the  electrodes  on  which  the  muscle  or 
nerve  rests.  A  second  wire  passes  from 
the   second  electrode  to  a  clamp  S, 


Fig.  359. — A  INIercury  Key. 


Fig.  360. — Rheocord. 


by  way  of  the  binding  post  C,  which  can  be  fastened  to  the  long  wire  at  any 
given  point.  The  current,  on  reaching  A,  will  divide  into  two  branches,  one 
of  which  will  pass  along  the  wire  A,  B,  and  thence  back  to  the  cell;  the  other 
will  pass  through  the  nerve  and  back  to  S  and  thence  also  to  the  cell.  The 
amount  of  current  passing  through  the  nerve  circuit  will  be  inversely  pro- 
portional to  the  resistance  of  the  nerve  and  directly  proportional  to  the 
difference  of  potential  between  A  and  S.  If  S  is  close  to  A,  the  difference  of 
potential  is  slight.  If  S  is  removed  from  A  toward  B,  the  dift'erence  of 
potential  is  increased  and  the  current  sent  through  the  nerve  circuit  is 
increased. 

In  many  experiments  it  is  necessary  to  reverse  the  direction  of  the  current, 
in  other  experiments  to  defied  it,  without  changing  the  position  of  the  elec- 
trodes.    Both  these  results  may  be  accomplished  by  the  use  of — 

Pohl's  commutator.  This  is  a  round  block  of  wood  with  six  cups, 
each  of  which  is  in  connection  with  a  binding  post  (Fig.  361).  In  each  of  the 
two  cups  marked  i  and  2,  -f  and  — ,  is  inserted  one  end  of  a  copper  wire 


yio 


TEXT-BOOK  OF  PHYSIOLOGY. 


bent  at  right  angles.  The  other  ends  of  the  wires  are  supported  and  in- 
sulated by  a  hard-rubber  handle.  To  the  top  of  each  wire  is  soldered  a 
semicircular  copper  wire.  This  arrangement  permits  of  a  rocking  move- 
ment, whereby  the  opposite  ends  of  the  semicircular  wires  can  be  made  to  dip 
into  cups  3  and  4,  and  into  cups  5  and  6  alternately.  Two  wires  crossed  in 
the  middle  of  the  block  serve  to  connect  opposite  pairs  of  cups.  When  in 
use,  the  cups  are  filled  with  clean  mercury.  The  method  of  using  the  com- 
mutator is  as  follows : 

I.  As  a  Current  Reverser. — The  positive  and  negative  poles  of  the  electric 
cell  are  connected  by  wires  with  binding  posts  i  and  2  respectively. 
A  key  is  interposed  in  the  circuit.  Wires  are  then  carried  from  binding 
posts  3  and  4  to  the  electrodes  in  connection  with  the  muscle  or  nerve. 
The  rocker  of  the  commutator  is  so  turned  that  the  ends  of  the  semicir- 
cular wires  dip  into  cups  3  and  4.  The  direction  of  the  current  will  be 
on  the  closure  of  the  circuit  from  i  to  3,  then  from  3  along  a  wire  to  and 
through  the  tissue  and  back  to  4,  and  thence  to  the  cell.  If  the  position  of 
the  rocker  be  now  reversed  so  that  the  opposite  ends  of  the  semicircular 


Fig.  361. — Pohl's  Commutator.     A.  Arranged  as  a  current  reverser;  B,  as  a  cur- 
rent deflector. 

wires  dip  into  cups  5  and  6,  the  direction  of  the  current  through  the  tissue 
will  be  reversed.     The  positive  current,  after  entering  binding  post  i, 
will  flow  to  5;  then  along  one  of  the  cross  wires  to  4;  then  along  a  wire 
to  and  through  the  tissue  and  back  to  3,  along  the  opposite  cross  wire 
to  6,  thence  to  2  and  so  back  to  the  cell. 
2.  As  a  Current  Deflector. — When  it  is  desirable  to  deflect  the  current  to  two 
pairs  of  electrodes  differently  situated,  wires  are  carried  from  binding 
posts  3  and  4  to  one  pair,  and  from  5  and  6  to  the  other  pair.     The  cross 
wires  are  then  removed.     According  to  the  position  of  the  rocker  the 
current  will  be  deflected  to  one  or  the  other. 
The  Inductorium.— This  is  an  apparatus  designed  for  the  purpose  of 
obtaining  single  or  rapidly  succeeding  electric  currents  by  induction.     Its 
construction  is  based  on  facts  discovered  by  Faraday,  some  of  which  are  the 
following: 

If  two  circuits,  a  primary  and  a  secondary,  are  placed  parallel  to  each 
other,  the  former  connected  with  a  galvanic  cell,  the  latter  with  a  galvan- 
ometer, it  is  found  that,  at  the  moment  the  primary  circuit  is  made,  and  at  the 


PHYSIOLOGIC  APPARATUS. 


711 


moment  it  is  broken,  a  current  is  induced  in  the  secondary  circuit,  as  shown 
by  a  momentary  deflection  of  the  galvanometer  needle.  During  the  con- 
tinuous flow  of  the  current  through  the  primary  circuit  there  is  no  evidence 
of  a  current  in  the  secondary  circuit.  The  induced  current  is  but  of  momen- 
tary duration.  The  current  flowing  through  the  primary  circuit  is  termed 
the  inducing,  the  current  flowing  through  the  secondary  circuit  the  induced 
current. 

The  induced  current  is  opposite  in  direction  to  that  of  the  inducing  cur- 
rent when  the  circuit  is  made  or  closed;  it  is  the  same  direction,  however, 
when  the  circuit  is  broken  or  opened. 

If  the  circuits  are  arranged  in  the  form  of  coils,  it  is  found  that,  other 
things  being  equal,  the  strength  of  the  induced  currents  will  be  proportional 
to  the  number  of  turns  in  the  coils. 

If  the  coils  are  placed 
at  varying  distances 
from  each  other,  the 
strength  of  the  induced 
current  varies,  increas- 
ing as  the  coils  are  ap- 
proximated, decreasing 
as  they  are  separated. 

Approximation  or 
separation  of  the  coils 
while  the  current  is  flow- 
ing through  the  primary 
circuit  develops  'an  in- 
duced current,  which 
disappears,  however,  the 
moment  the  movement 
of  the  coil  ceases.  A 
sudden  increase  or  de- 
crease in  the  strength  of  the  inducing  current  also  develops  an  induced 
current. 

When  the  coils  are  approximated  or  the  primary  current  increased  in 
strength,  the  induced  current  is  opposite  in  direction  to  that  of  the  inducing 
current;  with  the  reverse  conditions,  the  induced  current  has  the  same 
direction. 

The  induced  currents  have  been  termed,  in  honor  of  their  discoverer, 
Faradic  currents. 

The  du  Bois-Reymond  inductorium,  based  on  the  foregoing  facts, 
consists  essentially  of  two  coils  of  insulted  copper  wire,  termed  primary  and 
secondary  (Fig.  362). 

The  primary  coil,  R^  consists  of  thick  copper  wire  wound  around  a 
wooden  spool  attached  to  a  vertical  support.  The  beginning  of  this  coil  is  at 
the  binding  post  S",  its  termination  either  at  binding  post  P"  or  S'".  In  the 
course  of  this  primary  wire  or  circuit,  there  are  placed  two  vertical  bars  of 
soft  iron,  B',  connected  at  their  bases  to  form  a  horseshoe  magnet,  around  the 
ends  of  which  the  wire  is  coiled.  The  object  of  this  device  will  be  explained 
later. 


Fig.  362. — Inductorium  of  du  Bois-Reymond.  R',  Pri- 
mary, R",  secondary  spiral.  B.  Board  on  which  R"  moves. 
I.  Scale.  -1 — -.  Wires  from  batten,'.  P',  P".  Pillars.  H. 
Neef's  hammer.  B'.  Electro-magnet.  S'.  Binding  screw- 
touching  the  steel  spring  (H).  S"  and  S'".  Binding  screws  to 
which  to  attach  wires  where  Neef's  hammer  is  not  required. 


712  TEXT-BOOK  OF  PHYSIOLOGY. 

Inside  the  primary  coil  there  is  placed  a  bundle  of  soft  iron  wires,  which, 
as  soon  as  the  circuit  is  made,  become  magnetized,  with  the  efTect  of  in- 
creasing the  action  of  the  inducing  current. 

The  secondary  coil,  R",  consists  of  a  much  greater  number  of  turns  of 
a  finer  copper  wire,  the  ratio  being  about  40  to  i,  also  wound  around  a  spool, 
having  a  tunnel  sufficiently  large  to  enable  it  to  slide  over  the  primary.  By 
these  means  the  strength  of  the  induced  current  is  increased.  As  a  result  of  the 
construction  of  the  inductorium,  the  low  electro-motive  force  of  the  cell  is  trans- 
formed into  the  high  electromotive  force  characteristic  of  the  induce  d  current. 
As  the  number  of  turns  of  wire  in  the  secondary  coil  is  to  the  number  in  the 
primary,  so  are  the  electro-motive  forces  in  the  secondary  coil  to  those  in  the 
primary  coil. 

The  secondary  coil  slides  along  a  track,  B,  which  permits  it  to  be  moved 
toward  or  away  from  the  primary.  The  distance  between  the  two  coils  can 
be  measured  and  the  strength  of  the  induced  current  again  reproduced, 
other  things  being  equal,  by  means  of  a  centimeter-millimeter  scale  pasted  on 
the  edge  of  B. 

The  ends  of  the  wire  of  the  secondary  coil  are  fastened  to  two  binding 
posts  to  which  conducting  wires  provided  with  hand  electrodes  can  be 
attached. 

The  inductorium  may  be  used  for  obtaining  either  a  single  current  or  a 
series  of  rapidly  repeated  induced  currents. 

The  Single  Induced  Current. — On  account  of  its  high  electro-motive  force, 
its  penetrative  power,  and  short  duration,  the  single  induced  current  is  a 
most  convenient  and  suitable  form  of  stimulus  for  many  purposes.  In 
order  to  obtain  such  a  current,  the  positive  wire  of  the  cell  is  carried  to 
binding  post  S",  and  the  negative  wire  either  to  S'"  or  P".  A  key  is  placed  in 
the  primary  circuit.  The  course  of  the  current  will  then  be  on  the  closure  of 
the  circuit  from  the  cell  to  S",  thence  around  R'  to  S'",  and  so  back  to 
the  cell;  or  if  the  negative  wire  is  connected  with  P",  the  course  of  the  cur- 
rent on  leaving  R'  will  be  through  the  coils  surrounding  the  two  vertical  bars 
B',  thence  to  P",  and  so  back  to  the  cell.  It  the  secondary  coil  be  placed 
close  to  the  primary  and  the  wires  of  the  secondary  brought  into  contact 
with  a  muscle,  it  will  be  found  that  with  both  the  make  and  the  break  of 
the  primary  circuit  a  current  is  induced  in  the  secondary,  as  shown  by  a 
short  quick  pulsation  of  the  muscle;  but  during  the  time  of  closure  of  the  cir- 
cuit the  induced  current  is  wanting,  as  shown  by  the  quiescent  condition  of  the 
muscle.  It  w^ill  be  apparent,  however,  from  the  energy  of  the  contraction 
that  the  break-induced  current  is  a  more  efficient  stimulus  than  the  make- 
induced  current.  •  That  this  is  the  case  is  made  evident  by  removing  the 
secondary  to  the  end  of  the  slide-way  and  then  gradually  bringing  it  toward  the 
primary  half  a  centimeter  at  a  time,  making  and  breaking  the  circuit  after  each 
movement  until  a  pulsation  of  the  muscle  occurs.  It  will  be  found  to  occur 
first  on  the  break  of  the  circuit.  As  the  secondary  approaches  the  primary  a 
position  will  be  reached  when  a  pulsation  occurs  on  the  make  as  well  as  on 
the  break  of  the  circuit,  though  it  will  be  less  pronounced. 

The  explanation  offered  for  this  difference  in  the  strength  of  the  two  induced  currents  is  as 
follows:  With  the  make  of  the  circuit  and  the  passage  of  the  battery  current  through  the  primary 
coil  there  is  induced  in  the  neighboring  and  parallel  turns  of  the  wire  and  extra  current  opposite  in 


PHYSIOLOGIC  .\PPARATUS.  713 

direction  to  the  the  primary  current.  This  extra  or  self-induced  current  antagonizes  and  prevents 
the  current  from  attaining  its  maximum  development  as  quickly  as  it  otherwise  would,  and  therefore 
its  efficiency  as  an  inducer  of  a  current  in  the  secondary  is  diminished.  On  the  break  of  the  cir- 
cuit the  primary  current  disappears  quickly,  and  as  there  is  nothing  to  retard  its  disappearance  its 
efficiency  as  an  inducer  of  a  current  in  the  secondary  coil  is  not  diminished.  It  is  not  infrequently 
stated  that  the  disappearance  of  the  primary  current  induces  in  the  neighboring  coils  a  break  extra 
current  corresponding  in  direction  which  assists  in  the  development  of  the  induced  current.  This 
is  not  the  case,  however,  as  no  break-extra  current  is  developed  in  the  inductorium  as  ordinarily 
used  when  actuated  by  a  batter}'  current  of  moderate  strength. 

As  it  is  not  so  much  the  intensity  of  the  current  as  it  is  rapid  variations  in  intensity  that  produce 
effects,  it  is  readily  apparent  why  the  induced  current  developed  at  the  break  of  the  primary  is 
more  effective  as  a  stimulus  than  the  induced  current  developed  at  the  make  of  the  primary  circuit. 
The  quantitv  of  electricity  is,  however,  the  same  in  both  cases. 

If  the  secondary  be  pushed  further  along  the  slideway  until  it  largely 
covers  the  primary  coil,  a  position  will  be  reached  when  the  make-induced 
current  equals  in  its  efficiency  as  a  stimulus  the  break-induced  current; 
and  if  the  secondary  be  yet  further  advanced,  a  position  is  reached  when  the 
make-induced  current  becomes  more  powerful  and  efficient  than  the  break- 
induced  current,  as  shown  by  the  greater  contraction  of  the  muscle.  This 
result  is  explained  by  the  fact  that  the  make-extra  current  is  now  able  of 
itself  to  induce  a  current  in  the  secondary  coil,  on  account  of  its  proximity, 
which,  added  to  that  induced  by  the  battery  current,  produces  a  current, 
greater  than  that  induced  on  the  break  of  the  circuit/ 

Rapidly  Repeated  Induced  Currents. — As  the  single  induced  current  is  of 
extremely  short  duration,  it  is  inefficient  as  a  stimulus  in  the  conduct  of 
many  experiments.  It  is  necessary,  therefore,  to  develop  it  with  a  frequency 
that  is  sufficient  to  give  rise  to  a  summation  of  effects.  The  duration  of  the 
stimulation  may  be  thus  considerably  prolonged.  This  is  accomplished 
by  introducing  in  the  primary  circuit  close  to  the  primary  coil  an  automatic 
interrupter,  usually  Neef's  modification  of  \Vagner's  hammer  (Fig.  371). 
This  consists  of  a  vertical  post,  P',  to  the  top  of  which  is  fastened  a  metallic 
spring  carrying  at  its  opposite  end  a  steel  or  iron  hammer,  H,  which  hangs  over, 
but  does  not  touch,  the  two  vertical  bars  of  soft  iron  around  which  the  wire 
of  the  primary  coil  is  wound.  About  the  middle  of  the  spring  on  its  upper 
surface  there  is  a  small  plate  of  platinum  which  is  in  contact  with  an  adjust- 
able, platinum-tipped  screw,  S',  carried  by  a  plate  of  brass  in  connection  with 
binding  post  S". 

For  the  purpose  of  interrupting  the  primary  circuit  frequently  in  a  unit 
of  time,  and  thus  developing  induced  currents  in  quick  succession,  the 
apparatus  is  arranged  in  the  following  way:  The  positive  and  negative 
poles  of  the  electric  cell  are  connected  by  wires  with  binding  post  P'  and  P", 
a  key  being  interposed  in  the  circuit.  If  the  screw  S'  is  in  contact  with 
the  spring,  the  current  on  the  closure  of  the  circuit  will  enter  P',  pass 
along  the  spring  to  S',  thence  into  and  through  the  primary  coil  R',  to  the 
coils  surrounding  the  vertical  bars  B',  then  to  P",  and  so  back  to  the  cell. 

As  the  current  passes  around  the  vertical  bars,  they  are  magnetized. 
The  magnetization  draws  down  the  hammer,  and,  in  so  doing,  breaks  the 
circuit  at  the  tip  of  the  screw,  S'.  The  vertical  bars  are  at  once  demagnetized, 
and  the  hammer  is  restored  to  its  original  position  by  the  elasticity  of  the 
spring.     The  circuit  is  thus  reestablished,  the  current  flows  through  the 

'  "On  certain  peculiarities  of  the  inductorium,"  Prof.  Colin  C.  Stewart,  "Univ.  Pa.  Medical 
Bulletin,"  Feb.,  1904. 


714 


TEXT-BOOK  OF  PHYSIOLOGY. 


coils,  the  bars  are  again  magnetized,  the  hammer  is  drawn  down,  to  be 
followed  by  a  second  break  of  the  circuit. 

The  number  of  times  the  circuit  is  thus  7nade  and  broken  per  second  will 
vary  with  the  length  of  the  spring. 

As  each  interruption  of  the  primary  circuit  develops  an  induced  current, 
it  follows  that  the  latter  must  succeed  each  other  with  a  frequency  corres- 
ponding with  the  frequency  of  the  former.  If  while  the  primary  circuit 
is  thus  being  interrupted  the  wires  of  the  secondary  coil  be  placed  in  contact 
with  a  muscle,  the  induced  current  will  give  rise  to  contractions  which  will 
succeed  each  other  so  rapidly  that  they  fuse  together,  producing  a  spasm 

or  tetanus  of  the  muscle.  For  this  reason 
these  currents^  are  frequently  spoken  of  as 
tetanizing  currents,  and  the  procedure  as 
tetanization  or  Faradization.  These  cur- 
rents also  increase  in  strength  as  the 
secondary  approaches  the  primary. 


Fig.  363. — Helmholtz's  Modifi- 
cation or  Neef's  Hammer.  As 
long  as  c  is  not  in  contact  with  d, 
g  h  remains  magnetic;  thus  c  is  at- 
tracted to  d  and  a  secondary  circuit, 
a,  b,  c,  d,  e,  is  formed;  c  then  springs 
back  again,  and  thus  the  process 
goes  on.  A  new  wire  is  introduced 
to  connect  a  with  /.     K.  Battery. 


Helmholtz's  Modification  of  the  Inductorium. — 

With  a  view  of  equalizing  the  strengths  of  the  induced 
currents,  Helmholtz  suggested  a  device  the  adoption  of 
which  accomplishes  this  to  a  certain  extent.  It  con- 
sists (Fig.  362)  in  connecting  vnth  a  wire  binding  posts 
P'  and  S",  and  in  providing  binding  post  P"  with  an 
adjustable  screw  which  can  be  raised  until  the  spring 
comes  in  contact  with  it,  when  the  hammer  is  drawn 
down  by  the  electromagnet  B'.  This  latter  arrange- 
ment is  practically  a  short-circuiting  key  by  which  a 
portion  of  the  current  is  returned  to  the  cell  without 
ever  entering  the  primary  coil.  The  same  arrange- 
ment, though  differently  lettered,  is  shown  in  Fig.  363. 
By  the  use  of  the  entire  device  the  changes  in  the 
primary  coil  are  made  not  by  making  and  breaking  the 
primary  current,  but  by  alternately  long-  and  short- 
circuiting  the  current.  "When  the  short-circuiting  key 
is  opened,  the  full  volume  of  the  primary  current  flows 
through  the  primary  coil.  When  the  short-circuiting  key  is  closed,  most  of  the  current  fails  to 
enter  the  coil,  taking  the  easier  path  through  the  key.  Some  of  the  current,  however,  always  flows 
through  the  coil  and  is  never  diverted.  The  cycle  of  changes  in  the  electric  condition  of  the 
primary  coil  is  thus  altered  for  two  reasons: 

"First,  we  no  longer  have  an  alternation  between  a  full  primary  current  and  none  at  all — rather 
an  alternation  between  a  full  primary  current  and  a  weaker  one.  The  difference  in  the  phases  is 
thus  lessened,  the  extent  of  the  change  on  making  and  breaking  is  lessened,  and  correspondingly 
the  efficiency  of  the  make  and  break  currents  induced. in  the  secondary  coil  is  sUghtly  decreased. 
"  Second,  on  making  the  primary  current,  as  in  the  ordinary  coil,  the  sudden  appearance  of  the 
primary  current  is  antagonized  by  the  opposing  make  extra  current,  with  the  result  that  the  make 
induced  current  is  still  further  reduced;  while  on  breaking  the  current  the  break  extra  current  can 
now  flow  through  the  primary  coil  across  the  short-circuiting  key.  This  current,  trailing  behind 
the  disappearing  primary  current  in  the  same  direction,  produces  the  same  effect  as  if  the  primary 
current  itself  were  to  disappear  slowly.  As  a  result  the  disappearance  of  the  primary  current  loses 
its  former  efficiency  as  an  inducer  of  secondary  currents,  and  the  break  induction  current  is  reduced 
to  about  the  efficiency  of  the  make. 

"This  so-called  'equalizing'  of  the  make  and  break  induced  currents  is  never  perfect,  if  for  no 
other  reason,  because  the  make  extra  current  must  take  the  long  circuit  through  the  battery,  while 
the  break  extra  current  has  an  easier  path  through  the  short-circuiting  key,  and  is  thus  greater  than 
the  make  extra  current."     (C.  C.  Stewart.) 


THE  GRAPHIC  METHOD. 


The  term  graphic  is  applied  to  a  method  by  which  curves  or  tracings  are 
obtained  which  represent  the  extent,  duration,  and  time  relations  of  the 


PHYSIOLOGIC  APPARATUS. 


715 


Fig.  364. — A  Receiving  Tambour. 


movements  accompanying  physiologic  processes.  If  these  movements 
can  be  translated  in  one  direction,  they  may  be  recorded  in  different 
ways : 

1.  By  attaching  the  moving  structure — e.g.,  heart,  muscle,  etc. — to  a  delicate 

lever  the  free  extremity  of  which  is  provided  with  a  writing  point. 

2.  By  transmitting  the  movement  through  a  column  of  air  enclosed  in  a 

rubber  tube  the  two  ends  of  which  are  attached  to  a  metallic  capsule, 
covered  by  a  rubber  membrane,  termed  a  drum  or  tambour.  When 
the  membrane  of  the 
first  tambour  is  pressed 
or  driven  inward,  the 
air  is  forced  through 
the  rubber  tube  into 
the  second  tambour 
and  its  membrane  is 
pushed  outward.  As 
soon  as  the  primary 
pressure  is  removed, 
the  membranes  return 
to  their  former  con- 
dition. If  the  mem- 
brane of  the  first  tam- 
bour is  drawn  outward,  the  air  in  the  system  is  rarefied  and  the  mem- 
brane of  the  second  tambour  is  pressed  inward.  For  the  purpose  of 
registering  the  movement  transmitted  by  the  column  of  air,  the  second 
tambour  is  provided  with  a  light  lever  supported  by  a  vertical 
bearing  resting  on  a  small  metallic  disc.  The  membrane  of  the  first 
tambour  is  frequently  provided  with  a  button,  which  is  placed  over  the 
moving  structure.  The  inward  movement  of  the  membrane  of  the 
first  tambour  produces  an  outward  movement  of  the  membrane  of  the 
second  tambour,  indicated,  though  magnified,  by  the  rise  of  the  free 

end  of  the  lever.     The 
reverse    movement   of 
the    membrane   is  at- 
tended by  a  fall  of  the 
lever.     The  first  tam- 
bour is  termed  the  re- 
ceiving, the  second  the 
recording     tambour 
(Figs.  373'  374)- 
By  enclosing  an  organ — e.g.,  kidney,  spleen,  arm,  finger,  etc. — in  a  rigid 
glass  or  metal  vessel  which  at  one  point  is  in  communication  with  a 
recording  apparatus — e.g.,  (i)  a  piston  provided  with  a  lever  (page  482) ; 
or  (2)  a  tambour  and  lever  (page  347);  or  (3)  a  mercurial  manometer 
carrying  a  float  and  pen   (page  333).     The  space  between  the  part 
investigated  and  the  vessel  is  filled  with  fluid.     The  variations  in  volume 
of  the  organ  cause  a  displacement  of  the  fluid  and  give  rise  to  a  to-and- 
fro  movement  which  is  taken  up  and  reproduced  by  the  recording 
apparatus. 


Fig.  365. — A  Recording  Tambour. — (Marey.) 


7x6 


TEXT-BOOK  OF  PHYSIOLOGY. 


The  writing  point  may  be  (i)  some  form  of  pen  carrying  ink  which  records 
the  movement  on  a  white  paper  surface,  or  (2)  a  piece  of  metal,  glass,  or 
paper  which  records  the  movement  on  smoked  paper  or  glass. 

The  Recording  Surface. — The  surface  which  receives  and  records  the 
movements  of  a  pen  or  lever  is  usually  that  of  a  cylinder  which  is  covered 
with  glazed  paper  and  coated  with  a  thin  layer  of  soot,  obtained  by  passing 
the  cylinder  through  the  flame  of  a  gas  burner.  The  axis  of  the  cylinder  is 
su-pported  by  a  metal  framework.  If  the  writing  point  of  the  lever  be  placed 
against  the  cylinder  and  a  movement  be  imparted  to  it,  a  portion  of  the  soot  is 
rubbed  ofT,  leaving  a  white  line  behind.  If  the  cylinder  be  stationary,  the 
rise  and  fall  of  the  lever  are  recorded  as  a  vertical  line.     Such  a  record  shows 

only  the  extent  of  a  movement. 
If  the  cylinder  is  traveling,  how- 
ever, at  a  uniform  rate,  the  rise  and 
fall  of  the  lever  are  recorded  in  the 
form  of  a  curve  the  width  of  the 
two  arms  of  which  will  .depend 
partly  on  the  rapidity  of  the  move- 
ment of  the  lever  and  partly  on  the 
rate  of  movement  of  the  cylinder. 
The  cylinder  movement  is  initiated 
and  maintained  by  clock-work  or 
by  the  transmission  of  power  by 
belting  to  a  system  of  pulleys  in 
connection  with  its  axis.  As  the 
tracing  is  wave-like  in  form,  the 
cylinder  is  frequently  spoken  of  as 
a  kymograph  or  wave  recorder 
(Fig.  366). 

From  the  record  thus  obtained 
it  is  possible  to  determine  not  only 
the  extent  but  also  the  duration,  the 
form,  and  the  rate  of  recurrence  of 
any  given  movement. 

The  Extent  of  a  Movement. — As 
the  lever  not  only  takes  up  and  re- 
produces a  movement,  but  at  the 
same  time  magnifies  it,  it  is  essential 
that  the  degree  of  magnification  be 
known,  in  order  to  determine  the  actual  extent  of  the  movement.  The 
magnification  of  the  lever  is  readily  determined  by  dividing  the  distance 
between  the  axis  of  the  lever  and  its  writing  point  by  the  distance  between 
the  axis  and  the  point  of  attachment  of  the  structure,  and  then  dividing 
the  height  of  the  tracing  by  this  quotient.  The  final  quotient  represents 
the  extent  of  the  movement. 

The  Time  Relations  of  a  Movement. — When  recorded  in  the  form  of  a 
curve,  the  duration  of  the  entire  movement,  or  of  any  one  portion  of  it,  can 
be  determined  by  means  of  a  time  marking  or  chronographic  apparatus,  con- 
sisting of  (i)  a  small  signal  magnet  provided  with  a  movable  armature,  to 


Fig.    366. — Kv.M(j(;KAPii. 
zold,  Leipzig.) 


!  Boruttau's,    Pet. 


PHYSIOLOGIC  .APPARATUS. 


717 


which  is  attached  a  writing  style;  (2)  an  automatic  interrupter;  and  (3)  an 
electric  cell. 

The  Signal  Magnet. — The  magnet  (Fig.  367)  is  actuated  by  the  electric 
current  made  and  broken  at  regular  and  known  intervals  by  an  automatically 
acting  interrupter  placed  in  the  circuit.  With  each  make  and  break  of  the  cir- 
cuit the  armature  and  style  move  alternately  downward  and  upward.  The 
excursion  of  the  style  can  be  readily  recorded  on  a  traveling  surface.  The 
character  and  number  of  the  interruptions  per  second  will  determine 
the  character  of  the  tracing.  If  they  occur  in  a  rhythmic  manner,  the  tracing 
will  be  sinusoidal  or  wave-like 
in  form.  If  the  time  of  inter- 
ruption is  of  short  duration  as 
compared  with  the  time  of 
closure  of  the  circuit,  the  trac- 
ing will  be  a  horizontal  line 
with  short  vertical  elevations  at 
regular  inten'als. 

The  Automatic   Interrupter 


Fig.  367. — Signal  Magxet. 


-The   circuit 


may  be  interrupted  by 
vibrating  reeds,  tuning-forks,  metronomes,  etc.  A  well-known  form  of 
vibrating  reed  is  shown  in  Fig.  368.  This  consists  of  a  metallic  frame 
carrying  a  coil  of  wire  in  the  center  of  which  there  is  a  core  of  soft  iron.  To 
the  vertical  part  of  the  frame  there  is  fastened  the  reed,  the  distal  end  of 
which  is  bent  to  dip  into  an  adjustable  mercury  cup.  When  in  circuit  the 
current  enters  the  coil,  then  flows  into  and  through  the  frame  and  the 
reed  to  the  mercury,  and  thence  back  to  the  cell.  On  the  closure  of  the  cir- 
cuit and  the  magnetization  of  the  iron  core  the  reed  is  withdrawn  from  the 
mercury,  the  circuit  broken,  and  the  core  demagnetized.     The  elasticity  of 

the  spring  returns  it  to  the  mer- 
cury, when  the  circuit  is  again  re- 
stored. The  reed  may  be  so  con- 
structed that  it  will  be  raised  and 
lowered  50,  100,  or  200  times  a 
second.  The  armature  of  the 
signal  magnet  undergoes  a  corre- 
sponding number  of  elevations 
and  depressions.  If  the  reed  vi- 
brates 100  times  in  a  second,  the 
distance  from  crest  to  crest  of  the 
wave  tracing  will  represent  j^  of 
a  second.  Interrupters  of  various 
kinds  have  been  devised  which  make  and  break  the  circuit  from  i  to  250 
times  a  second. 

Moist  Chamber. — In  many  experiments,  it  is  necessary  to  keep  the  nerve 
or  muscle  preparation  in  a  uniformly  moist  atmosphere.  To  secure  this,  a 
moist  chamber  is  employed  (Fig.  369).  This  consists  of  a  hard-rubber 
platform,  supported  by  a  piece  of  brass,  which  slides  up  and  down  a  vertical 
rod,  and  which  can  be  clamped  at  any  height.  By  means  of  a  short  lever 
the  vertical  rod  can  be  turned,  carrying  the  platform  from  side  to  side. 
The  rod  is  secured  to  a  firm  iron  base. 


Fig.  368. — Page's  \'ibr.a.tixg  Reed 
modification.) 


(Reichert's 


7i8 


TEXT-BOOK  OF  PHYSIOLOGY. 


Six  double  binding  posts  for  the  attachment  of  wires  pass  through  the 
platform.  Near  the  side  of  the  upper  surface  of  the  platform  there  rises  a 
vertical  rod,  carrying  a  clamp  for  holding  the  femur  of  a  nerve-muscle  prep- 
aration, as  well  as  a  horizontal  rod  for  supporting  at  least  three  pairs  of 
non-polarizable  electrodes.  A  groove  around  the  outer  edge  of  the  platform 
receives  a  glass  shade,  which  covers  the  whole.  The  air  of  the  chamber  is 
kept  moist  by  placing  in  it  pieces  of  blotting-paper  saturated  with  water. 

From  the  under  surface  of  the  platform  there  descends  a  rod,  which,  by 
means  of  a  double  binding  screw,  supports  a  horizontal  rod,  modified  at  one 
end  to  carry  the  delicate  axis  of  a  light  stiff  recording  lever.  The  end  of  this 
lever  is  pointed,  to  enable  it  to  write  on  a  smoked  glass  or  paper.  Beneath 
the  axis  is  a  strip  of  brass,  carrying  a  screw,  which  gives  support  to  the  lever 


Fig.  369, — Moist  Chamber. 

until  the  instant  the  contraction  of  the  muscle  begins.  This  "screw,  the 
after-loading  screw,  also  enables  the  lever  to  be  placed  in  a  horizontal  position. 
The  portion  of  the  lever  near  the  axis  is  provided  with  a  double. hook,  the 
lower  portion  of  which  serves  for  the  attachment  of  the  weight  byj^which  the 
muscle  is  counterpoised. 

In  some  experiments,  as  in  the  registration  of  a  muscle  contraction  under 
varying  conditions,  it  is  necessary  to  give  the  lever  mass  by  attaching  weights 
directly  beneath  the  muscle.  This,  however,  introduces  certain  errors  in 
the  movements  of  the  lever,  which  somewhat  deform  what  would  otherwise 
be  the  normal  curve.  If  the  weight  be  attached,  not  opposite  to  the  muscle 
attachment,  but  close  to  the  axis  of  the  lever,  the  undesirable  acceleration 
of  the  lever  movement,  during  both  contraction  and  relaxation,  is  largely 
prevented.  The  lever  may  be  a  straw,  a  strip  of  celluloid  or  aluminium. 
It  should  be  as  light  as  possible.  The  writing  point  may  be  made  of  stiff 
paper,  a  piece  of  tinsel,  glass,  or  aluminium.     It  should  have  sufficient  elas- 


PHYSIOLOGIC  APPARATUS. 


719 


ticity  to  keep  it  in  contact  with  the  cylinder  during  the  excursion  of  the 
lever.  The  writing  point  should  be  placed  as  nearly  parallel  as  possible 
to  the  surface  of  the  cylinder. 

Normal  Saline  Solution. — To  prevent  drying  and  a  loss  of  irritability 
the  tissue  under  investigation  should  be  kept  moist  with  the  normal  saline 
solution  (NaCl,  0.6  per  cent.).  This  solution  very  largely  prevents  either 
absorption  or  extraction  of  water  from  the  tissues  and  thus  retards  chemic 
changes  in  their  composition. 

Ringer's  solution,  largely  used  for  the  same  purpose,  is  made  by 
saturating  0.65  per  cent.  NaCl  solution  with  calcium  phosphate  and  then 
adding  2  c.c.  of  a  i  per  cent,  solution  of  potassium  chlorid  to  each  100  c.c. 

The  Galvanometer  and  Capillary  Electrometer. — In  the  detection 
and  investigation  of  the  electric  currents  of  muscles,  nerves,  and  other 
tissues,  the  physiologist  is  limited  to  the  galvanometer  and  capillary  electro- 
meter. The  principle  of  the  galvanometer  is  based  on  the  fact  that  an 
electric  current  flowing  through  a  wire  parallel  in  direction  with  a  magnetic 
needle  will  tend  to  set  the  needle  at  right  angles  to  the  direction  of  the 
current.  The  essential  requisite  of  any  galvanometer  used  for  physiologic 
purposes  is  that  it  will  re- 
spond quickly  to  the  influ- 
ence of  extremely  weak 
currents.  This  is  realized 
by  the  use  of  small  light 
needles,  the  adoption  of 
the  astatic  system,  or  some 
similar  device  by  which 
the  directive  influence  of 
the  earth's  magnetism  is 
eliminated,  and  the  multi- 
plication of  the  number  of 
turns  of  the  wire  in  the  coils 
which  surround  the  needle. 

The  tangent  galvano- 
meter, or  boussole,  as  con- 
structed bv  Wiedemann, 
is  the  form  most  frequently  ^'^  37o--Wiedemanx's  Boussole. 

employed  in  physiologic  investigations  (Fig.  379).  It  consists  primarily 
of  a  thick  copper  cylinder,,  through  which  a  tunnel  has  been  bored. 
Within  this  tunnel  is  suspended  a  magnetized  ring,  just  large  enough 
to  swing  clear  of  the  sides  of  the  chamber.  The  object  of  making 
the  magnet  ring-shaped  is  to  increase  its  strength  in  proportion  to  its  size, 
and  to  get  rid  of  the  central  inactive  part.  Connected  wuth  and  passing 
upward  from  the  magnetized  ring  through  the  copper  cylinder  is  an  alumin- 
ium rod,  surmounted  by  a  circular  plane  mirror.  Above  the  mirror  rises  a 
glass  tube,  which  carries  on  top,  on  an  ebonite  support,  a  little  windlass,  cap- 
able of  being  centered  by  three  small  screws.  On  the  windlass  is  wound  a 
single  filament  of  silk,  which  passes  down  the  tube  and  is  attached  to  the 
mirror.  The  magnet  can,  by  this  contrivance,  be  raised  or  lowered  and 
centered  in  the  copper  chamber.     Deflections  of  the  mirror  from  currents 


720  TEXT-BOOK  OF  PHYSIOLOGY. 

of  air  are  prevented  by  inclosing  it  with  a  brass  cover  provided  with  a  glass 
window.  The  coils  are  placed  on  each  side  of  the  copper  chamber,  and 
supported  by  a  rod,  on  which  they  slide.  By  this  arrangement  they  can  be 
approximated  until  they  meet  and  completely  conceal  the  cylinder.  By 
varying  the  position  of  the  coils  the  influence  of  the  current  upon  the  needle 
can  be  increased  or  diminished.  An  advantage  which  this  galvanometer 
possesses  is  the  damping  of  the  oscillation  of  the  needle,  so  that  it  quickly 
comes  to  rest  after  deflection.  This  is  accomplished  by  the  development  of 
induction  currents  in  the  copper  cylinder,  the  direction  of  which  is  opposite 
to  that  of  the  movement  of  the  needle.  The  instrument,  therefore,  is 
aperiodic — that  is  to  say,  when  the  needle  is  influenced  by  a  current  it 
moves  comparatively  slowly  until  the  maximum  deflection  is  reached,  when 
it  comes  to  rest  without  oscillations.  When  the  circuit  is  broken  the  needle 
swings  slowly  back  to  zero,  and  again  comes  to  rest  without  oscillations. 

Inasmuch  as  the  needle  is  not  astatic,  it  is  rendered  so  by  the  use  of  an 
accessory  magnet — the  so-called  Hauy's  bar.  This  magnet,  supported  by  a 
rod  directed  perpendicular  to  the  coils,  is  placed  in  the  magnetic  meridian, 
horizontal  to  the  needle,  with  its  north  pole  pointing  north.  By  sliding  the 
magnet  toward  the  needle  the  directive  influence  of  the  earth's  magnetism 
is  gradually  diminished,  and  when  it  is  reduced  to  a  minimum  the  needle 
acquires  its  highest  degree  of  instability.  By  means  of  a  pulley  an  angular 
movement  can  be  imparted  to  the  end  of  the  accessory  magnet  in  the  direc- 
tion of  the  magnetic  meridian,  which  serves  to  keep  the  needle  on  the  zero  of 
the  scale.  The  deflections  of  the  needle  are  observed  by  means  of  an  astro- 
nomic telescope,  above  which  is  placed  a  scale  divided  into  centimeters  and 
millimeters,  and  distant  from  the  galvanometer  about  six  or  eight  feet.  As 
the  numbers  on  the  scale  are  reversed,  they  will  be  seen  in  the  mirror  in  their 
natural  position,  and  with  the  deflection  of  the  needle  the  number  will  appear 
as  if  drawn  across  the  mirror.  The  extent  of  the  deflection  is  readily 
determined  when  the  needle  comes  to  rest. 

The  reflecting  galvanometer  of  Sir  William  Thompson  is  also  used  for 
the  same  purposes. 

The  Capillary  Electrometer. — Not  withstanding  the  extreme  sensi- 
tiveness of  the  modern  galvanometer,  it  has  been  found  desirable,  in  the  in- 
vestigation many  of  physiologic  processes,  to  possess  some  means  which 
will  respond  even  more  promptly  to  slight  variations  in  electro-motive 
force.  This  has  been  realized  in  the  construction  by  Lippmann  of  the  capil- 
lary electrometer.  The  principle  of  this  apparatus  rests  upon  the  fact  that 
the  capfllary  constant  or  the  surface-tension  of  mercury  undergoes  a  change 
upon  the  passage  of  an  electric  current,  in  consequence  of  a  polarization  by  hy- 
drogen taking  place  at  its  surface.  If  a  capillary  glass  tube  be  filled  with  mer- 
cury and  its  lower  end  inserted  into  a  solution  of  sulphuric  acid,  and  the  former 
connected  with  the  positive  and  the  latter  with  the  negative  electrode,  it 
will  be  observed,  upon  the  passage  of  the  current,  that  a  definite  movement 
of  the  mercury  takes  place,  in  the  direction  of  the  negative  electrode,  in  conse- 
quence of  the  diminution  of  its  capillary  constant  or  the  tension  of  its  surface  in 
contact  with  the  acid.  As  a  reverse  movement  follows  a  cessation  of  the  current, 
a  series  of  oscillations  will  follow  a  rapid  making  and  breaking  of  the  current. 
If  the  direction  of  the  current  is  reversed,  the  capillary  constant  is  increased 


PHYSIOLOGIC  APPARATUS. 


721 


and  the  mercury  ascends  the  tube  toward  the  negative  pole.  From  facts 
such  as  these  Lippmann  constructed  the  capillary  electrometer,  a  con- 
venient modification  of  which  devised  by  M.  v.  Frey,  is  shown  in  Fig.  371. 
This  consists  of  a  glass  tube,  A,  forty  millimeters  in  length,  three  millimeters 
in  diameter,  the  lower  end  of  which  is  drawn  out  to  a  fine  capillary  point. 
The  tube  is  filled  with  mercury  and  its  capillary  point  immersed  in  a  10  per 
cent,  solution  of  sulphuric  acid.  The  vessel  containing  the  acid  is  filled  to 
the  extent  of  several  millimeters  with  mercury  also.  The  mercury  in  the 
tube  is  put  in  connection  with  a  platinum  wire  (a),  and  the  acid  in  the  vessel 
with  a  second  wire  (b).  When  a  constant  current  passes  into  the  apparatus 
in  the  direction  from  b  to  a  the  mercury  is  pushed  up  the  tube,  and,  upon 
the  breaking  of  the  current,  it  may  or  may  not  return  to  the  zero-point.     For 


Fig.  371. — Von  Frey's  Capillary  Electrometer. 


Fig.  372. — Capillary 
Electrometer.  R. 
Mercury  in  tube;  capil- 
lan'  tube.  s.  Sulphuric 
ac'id.  q.  Hg.  B.  Ob- 
server.    M.  Microscope. 


the  purpose  of  measuring  in  millimeters  of  mercury  the  pressure  necessary 
to  compensate  this  change  in  the  capillary  constant  produced  by  the  electro- 
motive force  of  polarization,  the  apparatus  is  provided  with  a  pressure- vessel, 
H,  and  a  manometer,  B.  This  electrometer  can  be  applied  to  any  microscope 
having  a  reversible  stage.  The  oscillations  of  the  mercury  can  then  be 
observed  with  the  microscope  provided  with  an  ocular  micrometer  (Fig.  372). 
The  special  advantage  of  the  electrometer  is,  that  it  will  respond  instantly 
to  any  variation  in  the  electro- motive  force  and  indicated  a  difference  of 
potential,  according  to  Lippmann's  observation,  as  slight  as  the  f  Q-^g-Q^  of  a 
Daniell.  These  rapid  oscillations  can  be  recorded  by  photographic  methods. 
In  using  either  the  galvanometer  or  the  electrometer  for  detecting  the 
existance  of  electric  currents  or  differences  of  potential  in  living  tissues,  it  is 
46 


722  TEXT-BOOK  OF  PHYSIOLOGY. 

absolutely  essential  that  non-polarizable  electrodes  be  employed  in  con- 
nection with  it. 

DISSECTION  OF  THE  HIND -LEG  OF  THE  FROG. 

Much  of  our  knowledge  of  the  physiologic  properties  of  muscles  and 
nerves  has  been  derived  from  the  study  of  the  muscles  and  nerves  of  the 
cold-blooded  animals,  especially  of  the  frog,  for  the  reason  that  in  these 
animals  the  tissues  retain  their  vitality  under  appropriate  conditions  for  a 
considerable  period  of  time  after  death  or  removal  from  the  body.  The 
muscles  generally  employed  for  experimental  purposes  are  the  gastrocnemius, 
the  sartorius,  the  semimembranosus,  the  gracilis,  and  the  hyoglossus.  The 
nerve  generally  employed  is  the  sciatic.  Both  muscle  and  nerv^e  piay  be 
studied  independently  of  each  other,  or  they  may  be  studied  together,  as 
when  in  their  usual  physiologic  relation.  For  this  latter  purpose  the  gas- 
trocnemius muscle  and  sciatic  nerv^e  are  employed,  constituting  the  so-called 
"nerve-muscle  preparation." 

For  these,  and  many  other  reasons,  the  student  should  familiarize  him- 
self with  the  general  anatomy  of  the  frog,  and  especially  with  the  anatomy 
of  the  posterior  extremities. 

Preparation  of  the  Frog. — Destroy  the  frog  by  plunging  a  pin 
through  the  skin  and  soft  tissues  covering  the  space  between  the  occipital 
bone  and  the  first  vertebra  until  the  point  is  stopped  by  the  vertebra.  Turn 
the  pin  toward  the  head  and  push  it  into  the  brain  cavity;  move  it  from  side 
to  side  and  destroy  the  brain.  Pass  the  pin  into  the  spinal  canal  and  destroy 
the  spinal  cord.  With  a  stout  pair  of  scissors  cut  off  the  body  behind  the 
fore-limbs.  Remove  the  viscera  and  the  abdominal  walls.  Draw  the 
hind-legs  out  of  the  skin.  Place  the  legs  on  a  glass  plate,  back  uppermost, 
and  moisten  them  freely  with  normal  saline  solution. 

Observe  on  the  outer  side  of  the  dorsal  surface  of  the  thigh  the  follow- 
ing muscles  (Fig.  373,  374).  The  triceps  femoris  (tr),  made  up  of  the 
rectus  anticus  (ra),  the  vastus  externus  (ve),  and  the  vastus  intemus  (vi), 
not  seen  from  behind;  on  the  inner  side,  the  semimembranosus  (sm)  and 
the  rectus  internus  minor  or  gracilis  (ri").  Between  these  two  groups, 
note  the  biceps  femoris  (b).  Above  the  thigh  observe  the  gluteus  (gl),  the 
ileococcygeus  (ci),  and  the  pyriformis  (p). 

In  the  leg  observe  the  gastrocnemius  (g)  with  its  tendon  (the  tendo 
Achillis),  the  tibialis  anticus  (ta),  and  the  peroneus    (pe). 

Turn  the  frog  on  its  back  and  note  the  muscles  on  the  ventral  surface 
of  the  thigh,  the  rectus  internus  major  (ri') ,  and  minor  (ri"),  the  adductor 
magnus  (ad")?  the  sartorius  (s),  the  adductor  longus  (ad'),  and  the  vastus 
internus  (vi).  In  the  leg,  in  addition  to  those  already  seen  from  behind, 
note  the  tibialis  posticus  (tp)  and  the  extensor  cruris  (ec). 

Note  in  the  abdominal  cavity  the  three  large  spinal  nerves,  the  seventh, 
eighth,  and  ninth. 

Dissection  of  the  Sciatic  Nerve. — The  sciatic  nerve  is  composed 
of  the  seventh,  eighth,  and  ninth  spinal  nerves.  After  its  emergence  from 
the  pelvic  cavity,  it  passes  down  the  thigh  between  the  semimembranosus 
and  the  biceps  muscles,  in  company  with  the  femoral  blood-vessels.     Below 


PHYSIOLOGIC  APPARATUS. 


723 


the  knee  it  divides  into  the  tibialis  and  peroneus  nerves;  the  former  sending 
branches  into  the  gastrocnemius.  In  its  course,  the  sciatic  sends  branches 
to  the  muscles  of  the  entire  leg,. 

Carefully  separate  the  biceps  and  semimembranosus  by  tearing  the 
connective  tissue  uniting  them.  The  sciatic  nerve  and  femoral  blood- 
vessels come  into  view;  with  a  bent  glass  rod  gently  separate  the  nerve 
from  its  surroundings  from  the  knee  to  the  thigh.  Begin  at  the  knee.  In 
order  to  expose  the  nerv-e  at  the  pelvis,  it  will  be  necessary  to  divide  the 
pyriformis  and  the  ileo-coccygeus  muscles.  Care  must  here  be  exercised, 
so  as  not  to  injure  the  nerve  which  lies  immediately  beneath.  Lift  up  the 
urostyle  with  the   forceps  and  separate  it  from  the  last  vertebra.     With 


ec 


Fig.  373. — Leg  Muscles  of  the  Frog. 
Ventral  Surface. — i^Ecker.) 


Fig.  374. — Leg  Muscles  of  the  Frog. 
Dorsal  Surface. — (Ecker.) 


the  scissors  cut  off  the  vertebral  column  above  the  seventh  vertebra.  Place 
the  legs  on  the  dorsal  surface  and  then  di\dde  the  seventh,  eighth,  and  ninth 
vertebras  lengthwise.  With  the  forceps  lift  up  one  lateral  half  of  the  vertebrae 
and  free  the  nerve  ar  far  as  the  knee  by  dividing  connective  tissue  and  nerve 
branches.     Be  careful  not  to  injure  the  nerve  with  scissors  or  forceps. 

The  Nerve-Muscle  Preparation. — Divide  the  tendo  AchilHs  just 
below  its  fibro-cartilaginous  thickening  at  the  heel,  and  detach  the  gastroc- 
nemius up  to  the  knee.  Cut  through  the  tibio-fibular  bone  just  below 
the  knee-joint.  Cut  the  femur  transversely  near  its  middle  and  remove 
the  muscles  from  the  lower  end,  carefully  avoiding  injury  to  the  nerve. 
The  completed  preparation  consists  of  the  gastrocnemius  muscle,  the  sci- 
atic nerve,  with  half  of  the  seventh,  eighth,  and  ninth  vertebrae  and  the  lower 
half  of  the  femur. 


724  TEXT-BOOK  OF  PHYSIOLOGY. 

THE  ANATOMY  OF  THE  FROG  HEART  AND  THE  VASCULAR 

APPARATUS. 

The  heart  of  the  frog  can  be  readily  exposed  after  the  animal  has  been 
made  insensible  by  destruction  of  the  brain.  The  sternum  is  divided  longi- 
tudinally and  each  half  drawn  outward  by  gentle  traction  of  the  anterior 
extremities.     The  pericardium  is   then  divided  and  turned   aside. 

When  viewed  from  the  ventral  surface,  Fig.  375,  the  heart  shows  two 
auricles,  a  right  and  left,  a  single  ventricle  and  a  more  or  less  conical  vessel, 
the  conns  arteriosus,  which  arises  from  the  right  side  of  the  base  of  the  ven- 
tricle. When  viewed  from  the  dorsal  surface,  Fig.  376,  it  presents  a  tri- 
angular-shaped vessel,  the  sinus  venosus,  formed  by  the  union  of  the 
terminations  of  the  two  superior  and  inferior  venae  cavae.  A  dissection  of 
the  heart  shows  that  the  cavity  of  the  sinus  venosus  communicates  with 


Fig.  375.— Ventral  Surface  of  the  Frog  Heart.  {After  Hmces.)  ra.  Right  auricle. 
la.  Left  auricle,  v.  Ventricle,  ca.  Conus  arteriosus,  p' .  Pulmo-cutaneous  trunk,  s' .  Sys- 
temic aortic  trunk,     c' .  Carotid  trunk,     ac.  Left  anterior  caval  vein. 

the  cavity  of  the  right  auricle  by  means  of  a  transversely  oval  foramen,  in 
the  posterior  wall  of  the  auricle.  This  opening  is  provided  with  two 
valves,  a  ventral  and  dorsal,  the  free  edges  of  which  are  directed  toward 
the  cavity  of  the  auricle.  Two  pulmonary  veins,  a  right  and  left,  penetrate 
the  dorsal  wall  of  the  left  auricle. 

A  longitudinal  section.  Fig.  377,  of  the  heart  shows  that  the  auricles, 
though  separated  by  a  septum,  communicate  below  by  a  common  orifice 
with  the  cavity  of  the  single  ventricle.  This  orifice,  the  auriculo-ventric- 
ular,  is  provided  with  two  valves  the  free  edges  of  which  are  directed  toward 
the  cavity  of  the  ventricle. 


PHYSIOLOGIC  APPARATUS.  725 

The  conus  arteriosus  is  separated  from  the  ventricle  by  three  semilunar 
valves.  The  interior  of  the  conus  is  traversed  by  a  longitudinally  disposed 
membranous  valve  attached  to  its  dorsal  surface;  the  ventral  edge  is,  however^ 
free.  The  upper  extremity  of  the  conus  passes  into  the  hulhus  aorta,  from 
which  it  is  separated  by  a  semilunar  valve  and  the  free  extremity  of  the 
longitudinal  valve.  From  the  bulbus  aortae  arise  two  large  branches,  a 
right  and  a  left,  each  of  which  is  subdivided  by  two  longitudinal  partitions 
into  three  vessels,  the  carotid  trunk,  the  aortic  arch,  and  the  pulmo-cutaneous 
trunk.  (See  Fig.  377.)  The  carotid  and  aortic  trunks  communicate 
separately  with  the  cavity  of  the  bulbus,  while  the  pulmo-cutaneous  trunk 
communicates  with  the  conus  arteriosus  by  a  single  orifice,  just  below  the 
free  end  of  the  longitudinal  valve.     After  pursuing  a  short  course  these 


.SV 


Fig.  376. — Dorsal  Surface  of  the  Frog  Heart,  {aft^  Howes.)  ra.  Right  auricle. 
la.  Left  auricle,  sv.  Sinus  venosus.  sv' .  Opening  of  sinus  venosus  into  right  auricle. 
pv.  Pulmonary  vein.  v.  Right  anterior  caval  vein,  p'  s'  c' .  The  pulmo-cutaneous,  aortic  and 
carotid  trunks  respectively. 

three  vessels  separate  from  one  another  to  distribute  blood  to  the  various 
organs  of  the  body.  The  two  aortic  trunks  wind  around  the  esophagus 
and  unite  posteriorly  to  form  the  dorsal  aorta;  the  pulmo-cutaneous  divides 
into  a  pulmonary  artery  which  is  distributed  to  the  lung  and  a  cutaneous 
branch  which  is  distributed  to  the  skin. 

The  course  of  the  blood  through  the  heart  cavities  is  therefore  as  follows: 
The  venous  blood  poured  by  the  venae  cavae  into  the  sinus  venosus  passes 
through  the  sino-auricular  foramen  into  the  right  auricle.  While  the 
right  auricle  is  being  filled  from  this  source,  the  left  auricle  is  being  filled 
by  blood  coming  through  the  pulmonary  veins.  "When  the  auricles  contract, 
which  they  do  simultaneously,  each  passes  its  blood  into  the  corresponding 
part  of  the  ventricle,  which  then  instantly  contracts  before  the  venous  and 
arterial  bloods  have  time  to  mix.     Since  the  conus  arteriosus  springs  from 


726  TEXT-BOOK  OF  PHYSIOLOGY. 

the  right  side  of  the  ventricle  it  will  at  first  receive  only  venous  blood,  which 
on  the  contraction  of  the  conus  might  pass  either  into  the  bulbus  aortse 
or  into  the  aperture  of  the  pulmo-cutaneous  trunks.  But  the  carotid  and 
systemic  trunks  are  connected  with  a  much  more  extensive  capillary  system 
than  the  pulmo-cutaneous  and  the  pressure  in  them  is  proportionally  great, 
so  that  it  is  easier  for  the  blood  to  enter  the  pulmo-cutaneous  trunks  than  to 
force  aside  the  valves  between  the  conus  and  the  bulbus.  A  fraction  of  a 
second  is,  however,  enough  to  get  up  the  pressure  in  the  pulmonary  and 
cutaneous  arteries,  and  in  the  meantime  the  pressure  in  the  arteries  of  the 
head,  trunk,  etc.,  is  constantly  diminishing,  owing  to  the  continual  flow  of 
blood  toward  the  capillaries.  Very  soon  therefore  the  blood  forces  the 
valves  aside  and  makes  its  way  into  the  bulbus  aortas.  Here  again  the 
course  taken  is  that  of  least  resistance;  owing  to  the  presence  of  the  carotid 


Fig.  377. — Frog  Heart  with  Ventral  Surface  Dissected  Away  to  Show  its  Struc- 
ture. {After  Parker  and  Haswelt).  ra.  Right  auricle,  /a.  Left  auricle,  ia.  Septum  between 
auricles,  sao.  Sino-auricular  opening,  ca.  Conus  arteriosus,  sv.  Semilunar  valves,  av. 
Auriculo-ventricular  valves.  Iv.  Longitudinal  valve,  sv' .  Semilunar  valve,  p'  s'  c'.  Pulmo- 
cutaneous  aortic  and  carotid  trunks  respectively,     cc.  Columns  carnese. 

gland,  the  passage  of  blood  into  the  carotid  trunks  is  less  free  than  into 
the  wide  elastic  systemic  trunks.  These  will  therefore  receive  the  next 
portion  of  blood  which,  the  venous  blood  having  been  mostly  driven  to  the 
lungs,  will  be  a  mixture  of  venous  and  arterial.  Finally  as  the  pressure 
rises  in  the  systemic  trunks  the  last  portion  of  blood  from  the  ventricle, 
which  coming  from  the  left  side  is  arterial,  will  pass  into  the  carotids  and  so 
supply  the  head"  (Parker  and  Has  well). 

The  muscle  fibers  composing  the  walls  of  the  heart  from  the  sinus  venosus 
to  the  conus  arteriosus  are  continuous,  though  at  the  sino-auricular,  the 


PHYSIOLOGIC  APPARATUS.  •       727 

auriculo-ventricular,  and  the  ventriculo-conic  junctions  the  continuity  is  to 
some  extent  interrupted  by  bands  of  circularly  disposed  fibrous  tissue,  serving 
for  the  support  of  the  valves,  which  momentarily  interfere  with  the  ready 
passage  of  the  contraction  wave  from  one  division  of  the  heart  to  another. 
The  frog  heart  receives  its  nutritive  material  from  the  blood  flowing  through 
its  cavities.  During  the  diastole  the  blood,  under  the  influence  of  the  slight 
pressure  developed,  passes  from  the  interior  of  the  heart  into  a  system  of 
irregular  passage-ways  or  channels  which  penetrate  the  heart-wall  in  all 
directions,  and  thus  comes  into  direct  contact  with  the  heart-cells.  With 
the  beginning  of  the  systole  the  blood  is  forced  out  of  these  channels  into  the 
interior  of  the  ventricle,  bringing  with  it  the  products  of  tissue  metabolism. 
The  Heart  Beat. — If  the  heart  while  beating  is  lifted  up  by  a  ligature 
attached  to  the  apex  it  will  be  observed  that  the  contraction  begins  in  the 
walls  of  the  sinus  venosus,  then  passes  to  the  auricles,  thence  to  the  ventricle 
and  finally  to  the  conus;  from  this  it  may  be  inferred  that  the  physiologic 
stimulus  acts  primarily  in  the  walls  of  the  sinus  from  which  its  effect,  viz., 
the  excitation  process,  is  conducted  from  one  cavity  to  another  in  quick 
succession. 


INDEX. 


Abducent  nerve,  58q 
Aberration,  chromatic,  665 

spheric,  665 
Absorption,  205 

by  epithelium  of  villi,  216 

of  foods,  214 
of  fat,  209 
of  proteins,  217 
of  sugar,  216 
of  water,  216 

of  lymph,  214 

spectra  of  blood,  248 
Acapnia,  424 
Accommodation  of  the  eye,  657 

convergence  of  eyes  during,  662 

force  of,  661 

mechanism  of,  658 

range,  660 
Acoustic  area,  554 

.  nerve,  595 
Action  currents  of  muscles,  79 
of  nerves,  104 

reflex,  113 

of  medulla  oblongata,  533 
of  spinal  cord,  505,  508 
Adrenal  bodies,  463 
Agraphia,  558 
Albuminoids,  16 
Albumins,  15 
Alcohol,  effects  of,  122 
Alimentan,'  canal,  133 
Animo-acids,  13 
Amnion,  697 
Amylopsin,  188 
Amylase,  149 
Amyloses,  7 
Animal  body,  structure  of,  2 

heat,  429 
Ankle  clonus,  510 

jerk,  510 
Anti-dromic  nerves,  370 
Aphasia,  557 

amnesic,  558 

ataxic,  557 
Apnea,  424 

chemica  or  vera,  424 

vagi  or  inhibitoria,  424 
Arterial  circulation,  319 

pressure,  333 
Arteries,  structure  and  properties  of,  319 
Articulate  speech,  620 
Asphyxia,  425 

Association  centers  of  cerebrum,  558 
Astigmatism,  665 


Autonomic  nerve  system,  608 

Basal  ganglia,  525 
Bile,  192 

composition  of,  192 
mode  of  secretion,  193 
physiologic  action,  194 
pigments,  193 
salts,  193 
Bilirubin,  193 
Biliverdin,  193 
Bioplasm,  28 

physiologic  properties,  28 
Blind  spot,  667 
Blood,  227 

changes  in,  during  respiration   403 
circulation  of,  261 
coagulation  of,  230 
chemistr}'  of,  258 
extra  vascular,  258 
intravascular,  259 
constituents  of,  227 
corpuscles,  234,  252 
defibrinated,  232 
general  composition  of,  257 
physical  properties  of,  228 
plates,  256 
pressure,  332 
arterial,  ^;^^ 

auscultatory  method,  348 
capillar}',  337 
causes  of,  338 

determination  of,  in  man,  343 
methods  of  estimation,  332,  345 
variations  in,  340 
venous,  337 
quantity  of,  257 
serum,  232 

velocity  of,  in  arteries,  350 
of,  in  capillaries,  352 
of,  in  veins,  353 
viscosity  of,  230 
Bronchial  innervation,  379 
Burdach,  column  of,  500 

Calcium  salts  of  the  body,  20 
Calorimeter,  433 
Capillar}'  blood-vessels,  321 
functions  of,  321 

circulation,  361 

electrometer,  720 
Capsule,  internal,  527 

functions  of,  535 
Carbohydrates,  7 


729 


73° 


INDEX. 


Carbon  monoxide  hemoglobin,  250 
Cardiac  cycle,  272 

impulse,  272 
Catalysis,  136 
Cardio-accelerator  center,  313 

factors  which  determine  its  activity,  314 
Cardio-inhibitor  center,  314 

factors  which  determine  its  activity,  315 
Cardio-pulmonary  vessels,  264 
Carotid  pulse,  358 
Caseinogen  16,  19 
Catalysis,  136 
Caudate  nucleus,  525 
Cells,  structure  of,  23 

chemic  composition,  24 

manifestations  of  life  by,  2  5 

reproduction  of,  27 
Central  organs  of  the  nerve  system,  491 
Cerebellar  tract,  499 
Cerebellum,  565 

functions  of,  567 

results  of  experimental  lesions,  568 
Cerebrum,  537 

convolutions  of,  539 

fissures  of,  537 

functions  of,  544 

localization  of  function  in,  546 

motor  area  of  the  chimpanzee  brain,  552 

motor  area  of  the  human  brain,  555 

motor  area  of  the  monkey's  brain,  549 

sensor  areas  of  the  human  brain,  554 

sensor  areas  of  the  monkey's  brain,  548 

structure  of  the  gray  matter,  541 

structure  of  the  white  matter,  542 
Chemic  composition  of  the  body,  6 
Cheyne-Stokes  respiration,  426 
Chimpanzee  brain,  motor  area  of,  552 
Cholesterin,  193 

Chorda  tympani  nerve,  150,  594 
Chorion,  697 
Chromo- proteins,  17 
Chyle,  219 
Ciliary  ganglion,  618 

movement,  86 

muscle,  644 

function  of,  660 
Circulation  of  blood,  261 

forces    concerned,  365 

hydrodynamic  considerations,  322 
Clark's  vesicular  column,  496 
Classification  of  food  principles,  118 
Coagulated  proteins,  18 
Cochlea,  680 

functions  of,  686 
Colostrum,  444 
Commutator,  709 
Complemental  air,  398 
Conjugated  proteins,  17 
Connective  tissues,  32 

physical  and  physiologic  properties  of,  37 
Coronary  arteries,  287 

vaso-motor  nerves  of,  288 

effects  of  ligation,  288 
Corpora  quadrigemina,  524 
functions  of,  534 

striatum,  525 
functions  of,  534 


Corpus  luteum,  692 
Cranial  nerves,  572 
Crura  cerebri,  523 

functions  of,  533 
Crystalline  lens,  649 

Defecation,  202 

nerve  mechanism  of,  203 
Deglutition,  156 

nerve  mechanism  of,  162 
Demarcation  current,  78 
Depressor  nerve,  316,  375,  602 
Dextrin,  8 
Dextroses,  8 
Diabetes,  452 

Diapedesis  of  leucocytes,  363 
Diaphragm,  383 
Diffusion,  222 
Digestion,  133 
Digestive  apparatus,  133 
Dilatator  pupillae  muscle,  644 
Direct  cerebellar  tract,  499 

pyramidal  tract,  498 
Ductless  glands,  454 
Ductus  arteriosus,  701 

venosus,  700 
Dyspnea,  425 

Electrodes,  non-polarizable,  707 
Electrometer  capillary,  720 
Electrotonic  alterations  in  excitability  of  nerves, 
106 

current,  105 
Electrotonus,  105 
Encephaio-spinal  fluid,  492 
Endocardium,  263 
Enterokinase,  200 
Enzymes,  135 
Epidermis,  487 
Epididymis,  693 
Epinephrin,  466 

Epithelial  tissues,  functions  of,  30,  31 
Equilibration,  mechanism  of,  567 
Erepsin,  189,  190 

Erlanger's  sphygmomanometer,  346 
Erythrocytes,  234 
Eupnea,  425 

Eustachian  tube,  677,  686 
Excretion,  470 

Expiratory  forces  and  muscles,  391 
Expired  air,  composition  of,  401 
Eye,  cardinal  points  of,  651 

dioptric  apparatus  of,  649 

muscles  of,  671 

physiologic  anatomy  of,  642 

reduced,  654 

schematic,  653 

Facial  nerve,  590 

paralysis  of,  591 
Fallopian  tube,  689 
Fat,  10 

absorption  of,  219 

digestion  of,  189  • 

emulsification  of,  12 

saponification  of,  11 
Feces,  201 


INDEX. 


731 


Fecundation,  695 
Fehling's  solution,  8 
Ferments,  135 
Fetal  circulation,  679 

membranes,  697 
Fibrin,  iS 
Fibrinogen,  233 
Fillet,  513,  521,  522 
Filtration,  225 
Follicle,  Graafian,  688 
Food, 115 

animal,  128 

cereal,  130 

composition  of,  128,  131 

disposition  of,  119 

heat  value  of,  123 

principles,  119 

quantities  required  daily,  116 

vegetable,  131 
Forced  expiration,  372 
Forces   aiding   the   movement   of   lymph   and 

chyle,  220 
Fovea,  648,  668 
Frog  heart,  anatomy  of,  724 

Galactose,  9 

Gall-bladder,  191 

Galvanic  current,  eflfect  of,  on  nerves,  105 

Galvanometer,  719 

Ganglia,  peripheral,  618 

Gaseous  exchange  in  lungs,  411 

in  tissues,  410 
Gases  of  blood,  relation  of,  403 
tension  of,  408 

carbon  dioxid,  407 

nitrogen,  407 

oxygen,  405 
Gastric  digestion,  163 

fistulae,  166 

glands,  165 

juice,  168 

mode  of  secretion,  170 
physiologic  action  of,  172 
Globulins,  15 

Glossopharyngeal  nerve,  597 
Glycogen,  8,  451 

Glycogenic  function  of  the  liver,  448 
Gluco-proteins,  17 
Gmelin's  test  for  bile  pigments,  193 
Goll,  column  of,  500 
Gowers'  antero-lateral  tract,  499 
Graafian  follicle,  688 
Graphic  method,  714 
Green  vegetables,  132 

Hairs,  489 

Hearing,  sense  of,  677 

Heart,  261 

action  of  sympathetic  nerve  on,  307,  310. 

of  vagus  nerve  on,  308,  311 
auriculo- ventricular  bundle,  268 
beat,  nature  of  the  stimulus,  297 
action  of  inorganic  salts,  298 
frequency  of,  286 
theory  of,  300 
of  the  excised  heart,  289 
blood-supply,  286 


Heart,  causes  of  the  variations  of,  286 

course  of  blood  through,  264 

cycle  of,  272 

idio- ventricular  rhythm,  294 

intracardiac  nerve-cells,  303 

intraventricular  pressure  curve,  278 

mechanics  of,  270 

modifications  of  beat  due  to  the  action  of 
drugs,  317 

muscle-band  of  His,  268 

muscle-fibers  of,  266 

negative  pressure  of,  281 

nerve,  mechanism  of,  303 

orifices  and  valves,  265,  266 

origin  and  distribution  of  the  sympathetic 
nerves  to,  395 

origin  and  distribution  of  the  vagus  nerve 
to,  395 

physiologic  anatomy  of,  261 

relative  function  of  auricles  and  ventricles, 
276 

sounds,  285 

synchronism  of  the  two  sides,  278 

valves,  action  of,  274 

work  done  by,  366 
Heart- muscle,  properties  of,  289 

automaticity,  297 

conductivity,  291 

irritability,  289 

response  to  action  of  an  artificial  stimulus, 
300 

rhythmicity,  296 

tonicit}',  296 
Heat  dissipation,  434,  436 

production,  435 

relation  to  work,  437 

rigor,  65 
Helmholtz's  theory  of  color  perception,  674 
Hematin,  251 
Hemianopsia,  549,  578 
Hemoglobin,  243 

absorption  spectra  of,  248 

chemic  composition  of,  244 

compounds  of,  250 

quantity  of,  244 
Hemoglobinometer,  Gowers',  246 
Hemometer,  v.  Fleischl's,  247 
Hering's  theory  of  color  perception,  675 
Histons,  14 
Hormone,  171 
Horopter,  670 
Hypermetropia,  664 
Hyperpnea,  423 
Hypoglossal  nerve,  606 

Incus,  679 

Indol,  474 

Induced  currents,  712,  713 

Inductorium,  710 

Infra-proteins,  18 

Insalivation,  143 

nerve  mechanism  of,  149 
Inspiration,  395 

movements  of  thorax,  389 

muscles,  388 
Insula,  541 
Intercostal  muscles,  384 


732 


INDEX. 


Internal  capsule,  527 

functions  of,  535 

secretion,  454 
Intestinal  digestion,  181 

fermentation,  201 

juice,  183 

physiologic  action  of,  igo 

movements,  196 

nerve  mechanism  of,  198 

rhythmic  segmentation,  196 
Intraauricular  pressure,  281 
Intracranial  circulation,  560 

mechanism  of,  561 
Intrapulmonic  pressure,  386 
Intrathoracic  pressure,  386 
Intravascular  coagulation,  259 
Intraventricular  pressure,  278 
Invertase,  190 
Iris,  643 

functions  of,  662 

nerve  mechanism  of,  580,  583 
Iron  of  the  body,  19 
Irritability  of  muscles,  54 

of  nerves,  98 
Island  of  Langerhans,  185 

of  Reil,  541 
Isometric  myogram,  66 
Isotonic  myogram,  61 
Isthmus  of  encephalon,  521 

functions  of,  527 

Jacobson's  nerve,  598 
Joints,  45 

classification  of,  45 

Keith-Flack  node,  276 
Kidney,  473 

histology  of,  476 
Knee-jerk,  509 
Kymograph,  716 


Labyrinth  of  ear,  679 
Lacrimal  glands,  676 
Lactation,  702 
Lacteals,  219 
J^actose,  10 
Language,  556 
Larynx,  620 

nerve  mechanism  of,  628 

structure  of,  620 
Lateral  columns  of  the  spinal  cord,  499 
Law  of  contraction,  108 
Lecithin,  193 
Lemniscus,  513,  521,  522 
Lens,  crystalline,  649 
Lenticular  nucleus,  525 
Leukocytes,  252 

chemic  composition  of,  250 

classification  of,  254 

functions  of,  255 

number  of,  282 

origin  of,  255 

physiologic  properties,  253 
Levers,  81 
Levulose,  9 
Limbic  lobe,  540 


Liver,  191,  444 

elaboration  of  bile  by,  447 

formation  of  urea  in,  453 

functions  of,  447 

influence  of  the  nerve  system  on,  447,  450 

production  of  glycogen,  448 
Localization  of  functions  in  cerebrum,  546 
Lungs,  structure  of  the,  379 
Lymph,  210 

absorption  of,  214 

composition  of,  2 1 1 

functions  of,  213 

movement  of,  220 

physical  properties,  210 

production  of,  211 
Lymph-capillaries,  206 
Lymph-glands,  207 
Lymph-nodes,  206 
Lymph-vessels,  206 
Lymphocytes,  209,  254 

Macula  lutea,  645 
Malleus,  679 
Maltose,  10 
Mammary  glands,  441 
Mastication,  138 
muscles  of,  180 
nerve  mechanism  of,  141 
Meats,  composition  of,  128 
Medulla  oblongata,  519 

reflex  activities  of,  533 
Meibomian  glands,  676 
Membrana  tympani,  678 

function  of,  684 
Menstruation,  691 
Metabolism  on  protein  diet,  127 

on  fat  and  carbohydrate  diet,  127 
Methemoglobin,  251 
Migration  of  leukocytes,  254,  363 
Milk,  442 

composition  of,  129,  442 
mechanism  of  secretion,  443 
Moist  chamber,  718 
Mosso's  plethysmograph,  360 

spygmomanometer,  344 
Motor  area  of  chimpanzee  brain,  552 
of  human  brain,  554 
of  monkey  brain,  549 
oculi  nerve,  579 
Mouth  digestion,  138 
Movements  of  the  eyeball,  671 
of  the  intestines,  196,  200 
of  the  lower  jaw,  140 
of  the  lungs,  392 
of  the  stomach,  177 
Muscle,  action  currents  of,  79 
contraction,  58 

chemic  phenomena  of,  73 

electric  phenomena  of,  76 

graphic  representation  of,  60 

modifying  influences  of,  63 

phenomena  following  stimulation,  58 

physical  phenomena  of,  58 

rigor  mortis,  74 

summation  effects,  68 

tetanus,  69 

thermic  phenomena  of,  75 


INDEX. 


733 


Muscles,  electric  currents  from,  76 

electric  currents,  negative  variation  of,  77 

energj',  source  of,  74 

fatigue,  65 

groups,  special'  action  of,  80 

sense,  637 

sound,  73 

spindle,  637 

stimuli,  56 

tissue,  48 

chemic  composition  of,  51 
elasticity,  53,  59 
histology  of,  49,  83 
irritability,  54 
physical  properties  of,  52 
physiologic  properties  of,  52 
tonicity,  54 
Myenteric  plexus,  183,  198 
Myopia,  663 
Myosinogen,  15,  52 
Myxedema,  456 

Nerve,  abducent,  589 

acoustic,  595 

facial,  590 

glosso-pharyngeal,  597 

hypoglossal,  606 

irritability,  98 

oculo-motor,  579 

olfactory,  574 

optic,  575 

pneumogastric,  vagus,  599 

spinal  accessory,  604 

stimuli,  99 

trigeminal,  585 

trochlear,  584 
Nerve  impulse,  100 
Nerve-muscle  preparation,  loi,  723 
Nerve  system,  functions  of,  493 
Nerve  tissue,  87 

histology  of,  87 
Nerves,  autonomic  system  of,  60S 

chemic  composition  and  metabolism  of,  92 

classification  of,  97 

degeneration  of,  95 

development  of,  95 

effects  of  galvanic  current  on,  105 

electric  currents  of,  102 

electric  currents  of,  negative  variation  of, 
103 

electric  excitation  of,  loi 

electric  phenomena  of,  102 
action  currents,  104 
diphasic  action  currents,  104 

peripheral  endings  of,  93 

physiologic  properties  of,  98 

pilo-motor,  97 

polar  stimulation  of,  108,  109 

relation  of,  to  central  ner\e  system,  92 

stimuli  of,  99 
Neuron,  87 

Nicotin,  actions  of,  317,  368 
Nucleo-proteins,  17 
Nucleus  caudatus,  525 

cuneatus,  521 

gracilis,  521 

lenticularis,  525 


Nutritive  supply  of  the  embrj-o,  699 

Oculo-motor  nerve,  579 
Ohm's  law,  722 
Olein,  II 

Olfactory  nerve,  574 
Oncograph,  482 
Oncometer,  482 
Ophthalmic  ganglion,  618 
Optic  constants,  650 

nerve,  575 

thalamus,  526 
functions  of,  534 
Optogram,  669 
Organ  of  Corti,  682 
Osazones,  10 
Osmometer,  224 
Osmosis,  222 
Osmotic  pressure,  223 
Ossicles  of  ear,  679 
Otic  ganglion,  619 
Ovary,  688 
Ovulation,  691 
Ovum,  689 
Oxygen  in  blood,  405 

in  tissues,  409 

quantity  absorbed  daily,  401,  413 
Ox)-hemoglobin,  250 

Pacinian  corpuscle,  634 
Palmitin,  11 
Pancreas,  184 
Pancreatic  juice,  186 

mode  of  secretion,  186 

physiologic  action  of,  1S8 
Parathyroids,  458 

effects  of  removal,  458 
Partial  pressure  of  gases,  405 
Parturition,  701 
Pepsin,  168     . 
Peptones,  174 

Peripheral  organs  of  the  nerve  system,  491 
Peristalsis,  158,  196 
Perspiration,  486 
Petrosal  nerves,  91,  593,  594 
Pettenkofer-Voit  respiration  apparatus,  412 
Pexin,  169 
Phagocytosis,  256 
Phloridzin  diabetes,  453 
Phonation,  620 

mechanism  of,  626 
Phospho-proteins,  16 
Physiology  of  the  cell,  23 

of  movement,  38 
Pilo-motor  nerves,  97,  489 
Pituitar)'  body,  458 

effects  of  total  removal,  460 
of  anterior  lobe  removal,  461 
of  posterior  lobe  removal,  462 
of  injection  of  extracts,  463 
Placenta,  698 

Plasma  of  blood,  composition  of,  232 
Pleura,  384 
Pneumatograph,  399 
Pneumogastric  nerve,  599 
Pneumograph,  396 


734 


INDEX. 


Polar  stimulation,  io8 

of  human  nerves,  109 
Pons  varolii,  522 

functions  of,  533 
Portal  vein,  216 
Postures,  82 
Presbyopia,  663 
Prosecretin,  187 
Protamins,  14 
Proteins,  11 

chemic  composition,  11 

color  reactions,  19 

physical  properties,  13 

precipitation  tests,  19 

structure  of,  13 
Ptyalin,  149 
Pulmonary  blood-vessels,  381 

vascular  apparatus,  364 

ventilation,  402 
Pulse,  354 

frequency,  357 

volume,  281 

wave,  velocity  of,  356 
Punctum  proximum,  661 

remotum,  661 
Pyramidal  tracts  of  spinal  cord,  498,  499 

Reaction  of  degeneration,  112 
Red  corpuscles,  234 

chemic  composition  of,  243 

effects  of  reagents,  239 

function  of,  241 

life  history  of,  242 

number  of,  237 

of  vertebra  ted  animals,  240 
Reduced  hemoglobin,  249 
Reflex  action,-  112 

laws  of,  508 
Refractory  period  of  the  heart,  301 
Regnault's  and  Reisset's  respiration  apparatus, 

412 
Relation  of  gases  in  the  blood,  404 
Rennin,  169 
Reproduction,  688 

Reproductive  organs  of  the  female,  688 
Reproductive  organs  of  the  male,  692 
Reserve  air,  398 
Residual  air,  398 
Respiration,  377 

changes  in  composition  of  air  during,  400 

changes  in  composition  of  blood,  403 

changes  in  tissue^,  408 

chemistry  of,  400 

expiratory  forces  and  muscles,  391 

first  inspiration,  418 

mechanism  of  gaseous  exchange,  411 

nerve  mechanism  of,  415 

number  per  minute,  396 

reflex  stimulation  of,  418 

effects  of  a  change  of  pressure  of  the  blood 
gases  on,  422 

total  respiratory  exchange,  412 

volumes  of  air  breathed,  397 
Respiratory  apparatus,  677 

movements,  388 

muscles,  388 

effects  of,  on  arterial  pressure,  427 


Respiratory  muscles,  effects  of,  on  the  flow  of 
blood  through  the  thoracic  vessels,  426 
of  upper  air  passages,  395 
pressures,  413 
quotient,  401, 
rhythm,  396 

modification  of,  423 
Cheyne-Stokes,  426 
sounds,  399 
.  types,  395 
Retina,  645 

functions  of,  667 
Retinal  image,  649 
eize  of,  655 
Rheocord,  709 
Rhythmic  segmentation,  196 
Rigor  mortis,  75 
Rima  glottidis,  620 
respiratoria,  625 
vocalis,  625 
Routes  of  tlae  absorbed  food,  220 
Rush  peristalsis,  197 

Saccharose,  9 
Saliva,  145 

physiologic  action  of,  148 
Salivary  glands,  143 

histologic  changes  in,  during  secretion, 

147 
nerve  mechanism  of,  149 
Sebaceous  glands,  490 
Sebum,  489 
Sclero-proteins,  16 
Secretin,  187 
Secretion,  438 

internal,  454 
Semen,  694 

Semicircular  canals,  680 
Sensor  areas  of  human  brain,  553 

of  monkey  brain,  547 
Serum,  232 

Setchenow's  center,  512 
Sight,  sense  of,  642 
Sino-auricular  node,  270 
Skatol,  474 

Skeleton,  physiology  of,  44 
Skin,  486,  632 

nerve  endings  in,  632 

reflexes,  508 
Sleep,  563 
Smell,  sense  of,  640 
Sodium  glycocholate,  192 

taurocholate,  192 
Special  senses,  631 
Spectroscope,  248 
Speech,  628 
Spermatozoa,  694 
Spheno-palatine  ganglion,  619 
Sphygmograph,  357 
Sphygmomanometer,  344,  346 
Spinal  accessory  nerve,  604 

cord,  494 

encephalo-spinal  conduction,  516 
functions  of,  503 
as  a  conductor,  512 
as  an  independent  center,  503 
nerve-cells,  classification  of,  496 


INDEX. 


735 


Spinal  cord,  nerve-fibers  of,  498 
classification  of,  498 
reflex  actions  of,  505 
reflex  irritability  of,  510 
relation  of  spinal  nerves  to,  501 
segmentation  of,  502 
spinal  nerve  roots,  functions  of,  501 
spino-encephalic  conduction,  512 
structure  of  gray  matter,  494 
structure  of  white  matter,  498 
tracts  of,  499 
Spirometer,  397 
Splanchnic  nerves,  617 
Spleen,  467 

functions  of,  467 
Stanton's  sphygmomanometer,  345 
Stapes,  679 
Starch,  7 

digestion  of,  148 
Starvation,  123 
Stearin,  11 

Stereognostic  area,  555 
Stomach,  163 

movements  of,  177 
nerve  mechanism  of,  179 
Suprarenal  capsules,  463 
Sv/eat-glands,  487 
Sweat,  influence  of  nerve  svstem  on  production 

of,  488 
Sympathetic  nerve  system,  608 

functions  of  the  cervical  portions,  616 
fimctions  of  the  lumbosacral  portions, 

617 
functions  of  the  thoracic  portion,  616 
functions  of  peripheral  ganglia,  618 
ganglia,  relation  to  visceral  structures,  612 
relation  to  the  central  nerve  system,  614 

Taste  buds,  639 

nerve  of,  638 

sense  of,  638 
Teeth,  138 
Tegmentum,  522 
Temperature  of  body,  429 

regulation  of,  434 

sense,  635 
Tendon  reflexes,  509 
Tension  of  gases  in  blood,  408 

tissues,  410 
Tensor  tympani  muscle,  679 

functions  of,  684 
Testicles,  692 
Tetanus,  69 

experimental,  72 

pathologic,  72 

pharmacologic,  72 

physiologic,  71 
Thoracic  duct,  209 
Thorax,  381 

d)mamic  condition  of,  387 

mechanic  movements  of,  3S5 

static  condition  of,  385 
Thyroid  gland,  455 

effects  of  removal,  445 

cretinism,  446 
Tidal  air,  398 
Tissue  spaces,  205 


Tongue,  638 

Total  carbon-dioxid  exhaled,  413 

oxygen  absorbed,  413 

respiratory  exchange,  412 
Touch,  sense  of,  632 
Trachea,  378 

Traube-Hering  waves,  428 
Trochlear  nerve,  584 
Trypsin,  188 
Turck,  column  of,  498 
Tympanum,  677 

Upper  air-passages,  respiratory-  movements  of, 

395 
Urea,  453 

seat  of  formation,  453,  472 
Uric  acid,  473 
Urine,  470 

composition  of,  471 
mechanism  of  secretion,  479 

influence  of  blood  composition,  483 
influence  of  nerve  system,  483 
of  blood-pressure,  480 
Urination,  484 

nerve  mechanism  of,  485 
Uterus,  690 

Vagina,  691 
Vagus  nerve,  606 

influence  on  heart,  308,  311 

seat  of  action. 
Valves  of  heart,  265 
Vas  deferens,  693 
Vascular  apparatus,  319 

glands,  454 

hydrodynamic  considerations,  322 

stream  bed,  328 

nerve  mechanism  of,  366 
Vaso-motor  center,  371,  372 

direct  stimulation,  373 

nerves,  367 

reflex  stimulation,  373 
Veins,  322 

structure  and  function,  322 
Velocity-  of  blood,  349 
Venous  circulation,  363 

pulse,  358 
Vertebral  column,  47 
Vesiculae  seminales,  693 
Villi,  214 

functions  of,  216 
Visceral  muscle,  83 

functions  of,  84 

properties  of,  84 
Viscosit}'  of  blood,  230 
Vision,  649 

accommodation,  657 

astigmatism,  665 

binocular,  670 

color  perception,  673 

fimctions  of  retina,  667 

hypermetropia,  664 

myopia,  663 

presbyopia,  663 
Visual  angle,  655 
Vital  capacity  of  lungs,  398 


736 


INDEX. 


Vocal  bands,  623 
sounds,  627 
Voice  and  speech,  628 
Volume  pulse,  360 

Wernicke's  pupillary  reaction,  583 
White  blood-corpuscles,  252 

function  of,  256 

migration  of,  369 

origin  of,  255 


White  physiologic  properties,  253 

varieties  of,  254 
Wrisberg,  nerve  of,  592 

Yellow  spot,  645 

Zymogen,  149 

pepsinogen,  168 
ptyalogen,  149 
trypsinogen,  190 


^P3-f  JB85 


m 


